Transmission vacuum modulator leaking?

G

gordon gibson

New member
Oct 3, 2012
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Hello all

In the process of putting the coach to bed for winter, a new problem seems to have surfaced. I am getting white smoke (sometimes a lot of it) coming
out the tail pipe intermittently. First time I noticed it was after stopping for lunch on a 200 mile drive. Starting back up I had white smoke
blowing for longer than I would expect from condensation. It stopped fairly quickly and I did another 100 miles with no reoccurrence. Then yesterday
I drove it to the gas station to fill the tanks up - just far enough so that the temp gauge reached its normal operating level. White smoke on
starting up, and all the way home.

Today I started the coach and had white smoke for the first 5 - 10 minutes, then it stopped. I checked fluid levels - coolant level seems normal,
but ATF was a bit low. I added a half a quart then started up to measure the level again....and the white smoke really came billowing out.

My guess is a leaking diaphragm in the transmission vacuum modulator that is allowing ATF to be sucked into the intake manifold. Does this sound
right? The tranny was rebuilt about 8,500 miles ago so the modulator should have been replaced at that time. But the engine seems to be running too
smoothly for it to be a cracked head or leaking head gasket. Besides, the coolant level is holding steady. No coolant sludge on the engine oil
dipstick either.

I know the right thing would be to lift the coach, pull the rubber hose off the modulator and check to see if ATF drips out - but I just have dinky
car axle stands and am unwilling to crawl under the coach in my city driveway just supported by the bottle jack. Building ramps is a project for next
summer.

So if this diagnosis sounds right to the group I will head to a tranny shop Monday morning. Looks to me like the modulator should be very quick to
R&R. at a garage with a good floor jack and a pair of good axle stands. They are forecasting 8 inches of snow for Tuesday so I'd really like to get
this fixed and the coach put away beforehand....
--
Gordon Gibson
1976 23" Norris Upfit
Montreal West, Quebec, Canada
 
Just disconnect the Vac hose at the intake and see.

On Sat, Dec 9, 2017 at 9:15 PM, Gordon Gibson
wrote:

> Hello all
>
> In the process of putting the coach to bed for winter, a new problem seems
> to have surfaced. I am getting white smoke (sometimes a lot of it) coming
> out the tail pipe intermittently. First time I noticed it was after
> stopping for lunch on a 200 mile drive. Starting back up I had white smoke
> blowing for longer than I would expect from condensation. It stopped
> fairly quickly and I did another 100 miles with no reoccurrence. Then
> yesterday
> I drove it to the gas station to fill the tanks up - just far enough so
> that the temp gauge reached its normal operating level. White smoke on
> starting up, and all the way home.
>
> Today I started the coach and had white smoke for the first 5 - 10
> minutes, then it stopped. I checked fluid levels - coolant level seems
> normal,
> but ATF was a bit low. I added a half a quart then started up to measure
> the level again....and the white smoke really came billowing out.
>
> My guess is a leaking diaphragm in the transmission vacuum modulator that
> is allowing ATF to be sucked into the intake manifold. Does this sound
> right? The tranny was rebuilt about 8,500 miles ago so the modulator
> should have been replaced at that time. But the engine seems to be running
> too
> smoothly for it to be a cracked head or leaking head gasket. Besides, the
> coolant level is holding steady. No coolant sludge on the engine oil
> dipstick either.
>
> I know the right thing would be to lift the coach, pull the rubber hose
> off the modulator and check to see if ATF drips out - but I just have dinky
> car axle stands and am unwilling to crawl under the coach in my city
> driveway just supported by the bottle jack. Building ramps is a project
> for next
> summer.
>
> So if this diagnosis sounds right to the group I will head to a tranny
> shop Monday morning. Looks to me like the modulator should be very quick to
> R&R. at a garage with a good floor jack and a pair of good axle stands.
> They are forecasting 8 inches of snow for Tuesday so I'd really like to get
> this fixed and the coach put away beforehand....
> --
> Gordon Gibson
> 1976 23" Norris Upfit
> Montreal West, Quebec, Canada
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.appliedgmc.com
1-800-752-7502
 
Can you check it for fluid at the carb/manifold end of the vacuum line? That should be accessible from the top.

>
> Hello all
>
> In the process of putting the coach to bed for winter, a new problem seems to have surfaced. I am getting white smoke (sometimes a lot of it) coming
> out the tail pipe intermittently. First time I noticed it was after stopping for lunch on a 200 mile drive. Starting back up I had white smoke
> blowing for longer than I would expect from condensation. It stopped fairly quickly and I did another 100 miles with no reoccurrence. Then yesterday
> I drove it to the gas station to fill the tanks up - just far enough so that the temp gauge reached its normal operating level. White smoke on
> starting up, and all the way home.
>
> Today I started the coach and had white smoke for the first 5 - 10 minutes, then it stopped. I checked fluid levels - coolant level seems normal,
> but ATF was a bit low. I added a half a quart then started up to measure the level again....and the white smoke really came billowing out.
>
> My guess is a leaking diaphragm in the transmission vacuum modulator that is allowing ATF to be sucked into the intake manifold. Does this sound
> right? The tranny was rebuilt about 8,500 miles ago so the modulator should have been replaced at that time. But the engine seems to be running too
> smoothly for it to be a cracked head or leaking head gasket. Besides, the coolant level is holding steady. No coolant sludge on the engine oil
> dipstick either.
>
> I know the right thing would be to lift the coach, pull the rubber hose off the modulator and check to see if ATF drips out - but I just have dinky
> car axle stands and am unwilling to crawl under the coach in my city driveway just supported by the bottle jack. Building ramps is a project for next
> summer.
>
> So if this diagnosis sounds right to the group I will head to a tranny shop Monday morning. Looks to me like the modulator should be very quick to
> R&R. at a garage with a good floor jack and a pair of good axle stands. They are forecasting 8 inches of snow for Tuesday so I'd really like to get
> this fixed and the coach put away beforehand....
> --
> Gordon Gibson
> 1976 23" Norris Upfit
> Montreal West, Quebec, Canada
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
There will be evidence there.

> Just disconnect the Vac hose at the intake and see.
>
>
> On Sat, Dec 9, 2017 at 9:15 PM, Gordon Gibson

>
>> Hello all
>>
>> In the process of putting the coach to bed for winter, a new problem
>> seems to have surfaced. I am getting white smoke (sometimes a lot of it)
>> coming
>> out the tail pipe intermittently. First time I noticed it was after
>> stopping for lunch on a 200 mile drive. Starting back up I had white smoke
>> blowing for longer than I would expect from condensation. It stopped
>> fairly quickly and I did another 100 miles with no reoccurrence. Then
>> yesterday
>> I drove it to the gas station to fill the tanks up - just far enough so
>> that the temp gauge reached its normal operating level. White smoke on
>> starting up, and all the way home.
>>
>> Today I started the coach and had white smoke for the first 5 - 10
>> minutes, then it stopped. I checked fluid levels - coolant level seems
>> normal,
>> but ATF was a bit low. I added a half a quart then started up to measure
>> the level again....and the white smoke really came billowing out.
>>
>> My guess is a leaking diaphragm in the transmission vacuum modulator that
>> is allowing ATF to be sucked into the intake manifold. Does this sound
>> right? The tranny was rebuilt about 8,500 miles ago so the modulator
>> should have been replaced at that time. But the engine seems to be running
>> too
>> smoothly for it to be a cracked head or leaking head gasket. Besides,
>> the coolant level is holding steady. No coolant sludge on the engine oil
>> dipstick either.
>>
>> I know the right thing would be to lift the coach, pull the rubber hose
>> off the modulator and check to see if ATF drips out - but I just have dinky
>> car axle stands and am unwilling to crawl under the coach in my city
>> driveway just supported by the bottle jack. Building ramps is a project
>> for next
>> summer.
>>
>> So if this diagnosis sounds right to the group I will head to a tranny
>> shop Monday morning. Looks to me like the modulator should be very quick to
>> R&R. at a garage with a good floor jack and a pair of good axle stands.
>> They are forecasting 8 inches of snow for Tuesday so I'd really like to get
>> this fixed and the coach put away beforehand....
>> --
>> Gordon Gibson
>> 1976 23" Norris Upfit
>> Montreal West, Quebec, Canada
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>
>
>
>
> --
> Jim Kanomata
> Applied/GMC, Newark,CA
> jimk
> http://www.appliedgmc.com
> 1-800-752-7502
>

--
Jim Kanomata
Applied/GMC, Newark,CA
jimk
http://www.appliedgmc.com
1-800-752-7502
 
If your engine was consuming enough coolant to make white smoke, you would likely detect a sweet odour to the exhaust.

I think you'll be lucky if a trans shop here in Montreal has a modulator in stock. No demand up here due to salt & rust taking out our vintage vehicles prematurely.

I would disconnect the vacuum line at the carb, and verify for presence of fluid. If the problem is related to the modulator, I'd be inclined to leave it disconnected and park the coach as is for the winter, making note that the modulator needs replacement. It will be much nicer to work on the coach in the warmer spring weather.

Les Burt
Montreal
'75 Eleganza 26'

>
> There will be evidence there.
>

>>
>> Just disconnect the Vac hose at the intake and see.
>>
>>
>> On Sat, Dec 9, 2017 at 9:15 PM, Gordon Gibson

>>
>>> Hello all
>>>
>>> In the process of putting the coach to bed for winter, a new problem
>>> seems to have surfaced. I am getting white smoke (sometimes a lot of it)
>>> coming
>>> out the tail pipe intermittently. First time I noticed it was after
>>> stopping for lunch on a 200 mile drive. Starting back up I had white smoke
>>> blowing for longer than I would expect from condensation. It stopped
>>> fairly quickly and I did another 100 miles with no reoccurrence. Then
>>> yesterday
>>> I drove it to the gas station to fill the tanks up - just far enough so
>>> that the temp gauge reached its normal operating level. White smoke on
>>> starting up, and all the way home.
>>>
>>> Today I started the coach and had white smoke for the first 5 - 10
>>> minutes, then it stopped. I checked fluid levels - coolant level seems
>>> normal,
>>> but ATF was a bit low. I added a half a quart then started up to measure
>>> the level again....and the white smoke really came billowing out.
>>>
>>> My guess is a leaking diaphragm in the transmission vacuum modulator that
>>> is allowing ATF to be sucked into the intake manifold. Does this sound
>>> right? The tranny was rebuilt about 8,500 miles ago so the modulator
>>> should have been replaced at that time. But the engine seems to be running
>>> too
>>> smoothly for it to be a cracked head or leaking head gasket. Besides,
>>> the coolant level is holding steady. No coolant sludge on the engine oil
>>> dipstick either.
>>>
>>> I know the right thing would be to lift the coach, pull the rubber hose
>>> off the modulator and check to see if ATF drips out - but I just have dinky
>>> car axle stands and am unwilling to crawl under the coach in my city
>>> driveway just supported by the bottle jack. Building ramps is a project
>>> for next
>>> summer.
>>>
>>> So if this diagnosis sounds right to the group I will head to a tranny
>>> shop Monday morning. Looks to me like the modulator should be very quick to
>>> R&R. at a garage with a good floor jack and a pair of good axle stands.
>>> They are forecasting 8 inches of snow for Tuesday so I'd really like to get
>>> this fixed and the coach put away beforehand....
>>> --
>>> Gordon Gibson
>>> 1976 23" Norris Upfit
>>> Montreal West, Quebec, Canada
>>>
>>> _______________________________________________
>>> GMCnet mailing list
>>> Unsubscribe or Change List Options:
>>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>>>
>>
>>
>>
>> --
>> Jim Kanomata
>> Applied/GMC, Newark,CA
>> jimk
>> http://www.appliedgmc.com
>> 1-800-752-7502
>>
>
>
>
> --
> Jim Kanomata
> Applied/GMC, Newark,CA
> jimk
> http://www.appliedgmc.com
> 1-800-752-7502
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
Thanks for the answers guys. But one more question - If I disconnect the modulator vacuum line from the intake manifold, will the coach still be
drivable? It's a fairly short distance to the storage place, but once there, the coach will be moved from time to time And if I do this, I assume
that I should be capping the vacuum port on the manifold. Should I also be capping the line to the tranny vacuum modulator?
--
Gordon Gibson
1976 23" Norris Upfit
Montreal West, Quebec, Canada
 
es, drivable in first gear only. W

With vacuum removed the modulator thinks that you have your foot full throttle and it delays shifting to second until you reach some high RPM. Maybe
4500 or 5000 RPM. I have never tried it.

I fail to understand your hesitancy to fix the problem if it is indeed leaking. It is a simple one bolt removal to change it. You can slide under it
and remove the bolt and hose. Then pull the modulator out. If you need more room drive up on something with the right front wheel only. I removed
one in a parking lot in Nevada one time by simply driving up on a curb in a store parking lot.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Gordon,Leave it hitched up until Spring. It not going to hurt
anything.
JR WrightMichigan

-----------------------------------------From: "Gordon Gibson"
To:
Cc:
Sent: 10-Dec-2017 15:16:28 +0000
Subject: Re: [GMCnet] Transmission vacuum modulator leaking?

Thanks for the answers guys. But one more question - If I disconnect
the modulator vacuum line from the intake manifold, will the coach
still be
drivable? It's a fairly short distance to the storage place, but once
there, the coach will be moved from time to time And if I do this, I
assume
that I should be capping the vacuum port on the manifold. Should I
also be capping the line to the tranny vacuum modulator?
--
Gordon Gibson
1976 23" Norris Upfit
Montreal West, Quebec, Canada

_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
/>
 
If it appears that the modulator is defective. Take the money you'd spend at a shop to replace the modulator. ( It's super simple to do.) And invest in a jack, and jack stands. Then do it yourself now. Or in the spring when it's warm. Then you'll have it fixed, and the jack, and stands for the future. Bob Dunahugh
 
Ken,
I get it, but...first I need to confirm the problem. Then I have to source a modulator. As Les Burt mentioned, that will probably involve getting it
shipped from somewhere- most probably the US, which always takes twice as long as one would hope. Just this morning we got several inches of wet white
fluffy stuff, and there's another 10 or so on the way on Tuesday. I've changed enough starters and water pumps lying under a car in the snow and slush
to find procrastination an appealing course of action.

Also - I thought I read somewhere that the modulator needed to be adjusted by turning an internal set screw a given number of turns then fine tuning
form smooth shifts. Is this the case?
--
Gordon Gibson
1976 23" Norris Upfit
Montreal West, Quebec, Canada
 
Requires no adjustment.
http://www.buyautopartsonline.ca/catalog-1/itemdetail/acdelco-professional-c
anada/89057789

-----Original Message-----
From: Gmclist On Behalf Of Gordon Gibson
Sent: Sunday, December 10, 2017 11:18 AM

Ken,
I get it, but...first I need to confirm the problem. Then I have to source
a modulator. As Les Burt mentioned, that will probably involve getting it
shipped from somewhere- most probably the US, which always takes twice as
long as one would hope. Just this morning we got several inches of wet white
fluffy stuff, and there's another 10 or so on the way on Tuesday. I've
changed enough starters and water pumps lying under a car in the snow and
slush
to find procrastination an appealing course of action.

Also - I thought I read somewhere that the modulator needed to be adjusted
by turning an internal set screw a given number of turns then fine tuning
form smooth shifts. Is this the case
 
Gordon,
Once you have determined if the modulator is at fault, I suggest you make a few calls first thing Monday morning to see if a local auto parts or trans shop can supply the modulator quickly. The answer to that quest will quickly help you decide whether to procrastinate.

Les Burt
Montreal
'75 Eleganza 26'

>
> Requires no adjustment.
> http://www.buyautopartsonline.ca/catalog-1/itemdetail/acdelco-professional-c
> anada/89057789
>
> -----Original Message-----
> From: Gmclist On Behalf Of Gordon Gibson
> Sent: Sunday, December 10, 2017 11:18 AM
>
> Ken,
> I get it, but...first I need to confirm the problem. Then I have to source
> a modulator. As Les Burt mentioned, that will probably involve getting it
> shipped from somewhere- most probably the US, which always takes twice as
> long as one would hope. Just this morning we got several inches of wet white
> fluffy stuff, and there's another 10 or so on the way on Tuesday. I've
> changed enough starters and water pumps lying under a car in the snow and
> slush
> to find procrastination an appealing course of action.
>
> Also - I thought I read somewhere that the modulator needed to be adjusted
> by turning an internal set screw a given number of turns then fine tuning
> form smooth shifts. Is this the case
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
Gordon,Your looking for a modulator for a TH-425 GM transmission out
of a 1975 Oldsmobile Toronado 7.5L 455cid V8. This is a good starting
point to get you into most of the autoparts computers systems.
I will give you some part numbers here, you should be able to cress
them to a Canadian number at Canadian Tire Store:
AutoZone part# MV105 and I have used this several times on number of
transmission.
Others that are available from RockAuto.com are:
ATP JX67Fram part Delco 89057785 fits both the TH400 & TH425
transmissions as do all of these modulators.Airtex/Wells
7R1003Standard Motor Products TM15
JR Wright
Michigan

-----------------------------------------From: "Les Burt"
To:
Cc:
Sent: 10-Dec-2017 17:45:48 +0000
Subject: Re: [GMCnet] Transmission vacuum modulator leaking?

Gordon,
Once you have determined if the modulator is at fault, I suggest you
make a few calls first thing Monday morning to see if a local auto
parts or trans shop can supply the modulator quickly. The answer to
that quest will quickly help you decide whether to procrastinate.

Les Burt
Montreal
'75 Eleganza 26'

>
> Requires no adjustment.
>
http://www.buyautopartsonline.ca/catalog-1/itemdetail/acdelco-professional-c
/> > anada/89057789
>
> -----Original Message-----
> From: Gmclist On Behalf Of Gordon Gibson
> Sent: Sunday, December 10, 2017 11:18 AM
>
> Ken,
> I get it, but...first I need to confirm the problem. Then I have to
source
> a modulator. As Les Burt mentioned, that will probably involve
getting it
> shipped from somewhere- most probably the US, which always takes
twice as
> long as one would hope. Just this morning we got several inches of
wet white
> fluffy stuff, and there's another 10 or so on the way on Tuesday.
I've
> changed enough starters and water pumps lying under a car in the
snow and
> slush
> to find procrastination an appealing course of action.
>
> Also - I thought I read somewhere that the modulator needed to be
adjusted
> by turning an internal set screw a given number of turns then fine
tuning
> form smooth shifts. Is this the case
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
/>
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
/>
 
Thanks to all for your helpful comments.

But....(sigh)....looks like the diagnostic for an easy fix is incorrect. I pulled the modulator vacuum hose off the manifold and it was bone dry.
Also, disconnecting it made no difference to the white smoke billowing out the exhaust pipe. The final "nail in the coffin" is probably that the low
coolant warning light lit up after 15 minutes of idling.

So I guess I have to accept that Prestone is finding its way into the combustion chambers somehow. AAAARGH I guess there is no quick fix and that
the problem is either a blown head gasket, or a crack in the block or a cylinder head.
--
Gordon Gibson
1976 23" Norris Upfit
Montreal West, Quebec, Canada
 
Gordon,

Since you are certain that it is consuming coolant, you will need to protect the engine from the possibility of coolant sitting in the cylinders and causing rust over the winter storage period.

If procrastination wins, at a minimum, you need to drain the rad and engine block of coolant, and preferably fog the engine to protect the cylinder walls.

Draining just the rad won't totally eliminate the risk of coolant migrating into the cylinders. You need to pull the block drains as well.

Les Burt
Montreal
'75 Eleganza 26'

>
> Thanks to all for your helpful comments.
>
> But....(sigh)....looks like the diagnostic for an easy fix is incorrect. I pulled the modulator vacuum hose off the manifold and it was bone dry.
> Also, disconnecting it made no difference to the white smoke billowing out the exhaust pipe. The final "nail in the coffin" is probably that the low
> coolant warning light lit up after 15 minutes of idling.
>
> So I guess I have to accept that Prestone is finding its way into the combustion chambers somehow. AAAARGH I guess there is no quick fix and that
> the problem is either a blown head gasket, or a crack in the block or a cylinder head.
> --
> Gordon Gibson
> 1976 23" Norris Upfit
> Montreal West, Quebec, Canada
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
To reinforce the previous posting about antifreeze in the cylinders. I am working on one that has 2.9% antifreeze in the oil. The pistons rings
were rusted to he cylinder walls. Looking at it with a borescope (camera) showed just slight traces of rust on the walls right next to the pistons.
I removed all plugs and squirted PB Blaster all around the piston tops 3 times in 7 days. It did not help. So I mixed up some Acetone and PSF and
filled all cylindesr to the bottom of the spark plug holes and let it soak. I tried several times a week to turn the engine and nothing. Finally
after three weeks of soaking and applying air pressure to the cylinders that had valves closed, I got the engine freed up.

My point out of all of this is If there is any chance of getting anti-freeze (and water) in those cylinders, I would make a special effort to soak
down the tops of those pistons as much as possible. You could also go through removing plugs and filling cylinders with something like ATF or very
thin oil. That is your choice but if it is going to be parked for an extended period of time, put something in it. I do not know if just fogging it
is enough.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Once again, thanks for the helpful comments guys. Given what I think to be the nature of the problem, procrastination has ceased to be an option and
I am trying to find a a garage to fix it. My preference would be to bring it back to Borrmann's Garage in Bluevale ON (500 miles away) but that's
probably not a wise idea.

My experience is that it's hard to find good garages to work on GMCs. The truck places don't like to work on motorhomes and their preferences go to
diesel motors. Garages focused more on cars often don't have service bays big enough and they don't like tying their lifts up for days while
searching for parts or having machine work done.
--
Gordon Gibson
1976 23" Norris Upfit
Montreal West, Quebec, Canada
 
Would it be advisable to insert some radiator stop leak and head out for
Boremanns at first light? Only slowing down for fuel and fluids.

bdub

On Tue, Dec 12, 2017 at 10:49 AM, Gordon Gibson
wrote:

> Once again, thanks for the helpful comments guys. Given what I think to
> be the nature of the problem, procrastination has ceased to be an option and
> I am trying to find a a garage to fix it. My preference would be to bring
> it back to Borrmann's Garage in Bluevale ON (500 miles away) but that's
> probably not a wise idea.
>
> My experience is that it's hard to find good garages to work on GMCs.
> The truck places don't like to work on motorhomes and their preferences go
> to
> diesel motors. Garages focused more on cars often don't have service bays
> big enough and they don't like tying their lifts up for days while
> searching for parts or having machine work done.
> --
> Gordon Gibson
> 1976 23" Norris Upfit
> Montreal West, Quebec, Canada
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
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