Engine Misfire

[mark sawyer]

So I took the coach out for a final run earlier today after doing some pre-trip maintenance and I noticed the engine not running quite as smooth as
normal... It was minor, and I had not run the coach for a few weeks, so thought it just needed to run a bit. Getting it out on the highway, under
heavy load I got a decent backfire, which the coach has never really done before.... My AF meter also showed a lean condition after the backfire. I
don't believe it was showing lean before. As a limped it back home, the backfires started getting worse, though with smooth, light throttle inputs it
would run ok, though still lean per the afm

My coach runs a Howell system, but no EBL... Standard valvetrain, and the o2 sensor for my afm is on the left bank. I started poking around, one of
the things I did was to pull the fuse to the engine computer so I could turn the coach over without it firing. When I did this, I heard was what
sounded like compression puffs coming back up through the intake. Opening the throttle butterflies I could also feel the puffs and hear them get
louder the more I opened the throttle. I also have an engine vacuum gauge, and I could see small jumps on the gauge that tied to the puffs in the
intake.

I started pulling plugs one at a time to see which cylinder made the puffs quieter, and I believe the problem is number 2 cylinder. (Puffs got
quieter when I pulled this plug)
Assuming my understanding of how the Howell fuel injection works is correct, my assumption is if #2 is not firing, and dumping raw fuel into the
exhaust, the Howell will try to lean out the engine. Since my afm is on the left bank, I would just see a lean reading, since the raw fuel was going
to the right bank.

So does anyone have any idea what would could cause compression puffs to go into the intake? The only thing I could think of was a broken intake
valve spring, or maybe a dropped valve? I would have thought if it were a dropped valve I would hear the valve banging around... But the only thing
odd I am hearing out of the ordinary are the compression puffs.

About to go out and pull the valve covers, but does anyone have any ideas for other areas for me to check? The kids are bummed as we were supposed to
get on the road today, and I kinda doubt this is going to be something I can fix quickly and salvage the trip, so any help, thoughts, etc. would be
very appreciated by me and the kids
--
Mark S. '73 Painted Desert,
Manny 1 Ton Front End,
Howell Injection,
Leigh Harrison 4bag and Rear Brakes,
Fort Worth, TX

 

[Ken H.]

Mark,

If you have the suspected sticking valve, you'll probably still need
to remove the valve cover, but before doing so, I'd do a compression check.

Ken H.

On Sat, Aug 1, 2020 at 6:15 PM Mark Sawyer via Gmclist <

> So I took the coach out for a final run earlier today after doing some
> pre-trip maintenance and I noticed the engine not running quite as smooth as
> normal... It was minor, and I had not run the coach for a few weeks, so
> thought it just needed to run a bit. Getting it out on the highway, under
> heavy load I got a decent backfire, which the coach has never really done
> before.... My AF meter also showed a lean condition after the backfire. I
> don't believe it was showing lean before. As a limped it back home, the
> backfires started getting worse, though with smooth, light throttle inputs
> it
> would run ok, though still lean per the afm
>
> My coach runs a Howell system, but no EBL... Standard valvetrain, and the
> o2 sensor for my afm is on the left bank. I started poking around, one of
> the things I did was to pull the fuse to the engine computer so I could
> turn the coach over without it firing. When I did this, I heard was what
> sounded like compression puffs coming back up through the intake. Opening
> the throttle butterflies I could also feel the puffs and hear them get
> louder the more I opened the throttle. I also have an engine vacuum
> gauge, and I could see small jumps on the gauge that tied to the puffs in
> the
> intake.
>
> I started pulling plugs one at a time to see which cylinder made the puffs
> quieter, and I believe the problem is number 2 cylinder. (Puffs got
> quieter when I pulled this plug)
> Assuming my understanding of how the Howell fuel injection works is
> correct, my assumption is if #2 is not firing, and dumping raw fuel into the
> exhaust, the Howell will try to lean out the engine. Since my afm is on
> the left bank, I would just see a lean reading, since the raw fuel was going
> to the right bank.
>
> So does anyone have any idea what would could cause compression puffs to
> go into the intake? The only thing I could think of was a broken intake
> valve spring, or maybe a dropped valve? I would have thought if it were a
> dropped valve I would hear the valve banging around... But the only thing
> odd I am hearing out of the ordinary are the compression puffs.
>
> About to go out and pull the valve covers, but does anyone have any ideas
> for other areas for me to check? The kids are bummed as we were supposed to
> get on the road today, and I kinda doubt this is going to be something I
> can fix quickly and salvage the trip, so any help, thoughts, etc. would be
> very appreciated by me and the kids
> --
> Mark S. '73 Painted Desert,
> Manny 1 Ton Front End,
> Howell Injection,
> Leigh Harrison 4bag and Rear Brakes,
> Fort Worth, TX
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[jim kanomata1]

Mark,
Since it is a throttle body, one cylinder is not effected.
Check all your vacuum lines and mabe increase the pressure.

On Sat, Aug 1, 2020 at 3:15 PM Mark Sawyer via Gmclist <

> So I took the coach out for a final run earlier today after doing some
> pre-trip maintenance and I noticed the engine not running quite as smooth as
> normal... It was minor, and I had not run the coach for a few weeks, so
> thought it just needed to run a bit. Getting it out on the highway, under
> heavy load I got a decent backfire, which the coach has never really done
> before.... My AF meter also showed a lean condition after the backfire. I
> don't believe it was showing lean before. As a limped it back home, the
> backfires started getting worse, though with smooth, light throttle inputs
> it
> would run ok, though still lean per the afm
>
> My coach runs a Howell system, but no EBL... Standard valvetrain, and the
> o2 sensor for my afm is on the left bank. I started poking around, one of
> the things I did was to pull the fuse to the engine computer so I could
> turn the coach over without it firing. When I did this, I heard was what
> sounded like compression puffs coming back up through the intake. Opening
> the throttle butterflies I could also feel the puffs and hear them get
> louder the more I opened the throttle. I also have an engine vacuum
> gauge, and I could see small jumps on the gauge that tied to the puffs in
> the
> intake.
>
> I started pulling plugs one at a time to see which cylinder made the puffs
> quieter, and I believe the problem is number 2 cylinder. (Puffs got
> quieter when I pulled this plug)
> Assuming my understanding of how the Howell fuel injection works is
> correct, my assumption is if #2 is not firing, and dumping raw fuel into the
> exhaust, the Howell will try to lean out the engine. Since my afm is on
> the left bank, I would just see a lean reading, since the raw fuel was going
> to the right bank.
>
> So does anyone have any idea what would could cause compression puffs to
> go into the intake? The only thing I could think of was a broken intake
> valve spring, or maybe a dropped valve? I would have thought if it were a
> dropped valve I would hear the valve banging around... But the only thing
> odd I am hearing out of the ordinary are the compression puffs.
>
> About to go out and pull the valve covers, but does anyone have any ideas
> for other areas for me to check? The kids are bummed as we were supposed to
> get on the road today, and I kinda doubt this is going to be something I
> can fix quickly and salvage the trip, so any help, thoughts, etc. would be
> very appreciated by me and the kids
> --
> Mark S. '73 Painted Desert,
> Manny 1 Ton Front End,
> Howell Injection,
> Leigh Harrison 4bag and Rear Brakes,
> Fort Worth, TX
>
> _______________________________________________
> 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.gmcrvparts.com
1-800-752-7502

 

[john r. lebetski]

If there is stiction on the valve stem holding it open, there would be one heck of a valve tick.
--
John Lebetski
Woodstock, IL
77 Eleganza II

 

[mark sawyer]

> Mark,
>
> If you have the suspected sticking valve, you'll probably still need
> to remove the valve cover, but before doing so, I'd do a compression check.
>
> Ken H.

Thanks Ken... Not sure why I did not think of putting a compression gauge on it... John: No ticking I can hear from the valvetrain...

Here's what I got:

1: 166
2: 157
3: 162
4: 162
5. 165
6. 88
7. 155
8. 162

Gonna pull the rockers on #6 and see if I can see anything from the top, but not holding my breath on that...

Also, I was thinking of putting a few squirts of oil in #6 and running the compression test again. I want to say I remember someone telling me that
if you do that and the #s come up, you have a bottom end issue... #s the same, top end...

--
Mark S. '73 Painted Desert,
Manny 1 Ton Front End,
Howell Injection,
Leigh Harrison 4bag and Rear Brakes,
Fort Worth, TX

 

[mark sawyer]

So the wet compression test did not change the numbers at all, thank goodness.... I'm in the middle of rebuilding the engine on our truck, so I was
not looking forward to the prospect of doing another motor any time soon... Especially on the motorhome.

I pulled the rockers and did not see anything unusual, so our trip is definitely off, and will use some of my time off to pull the head instead...

So what would you guys think about me starting it the way it is and driving it from the front of the house to the shed out in the back yard so I can
do the work out of the Texas summer sun...

Would that be a boneheaded move? Having nightmares of dropping a valve or a valve seat coming loose and taking out a piston....
--
Mark S. '73 Painted Desert,
Manny 1 Ton Front End,
Howell Injection,
Leigh Harrison 4bag and Rear Brakes,
Fort Worth, TX

 

[mark sawyer]

> Would that be a boneheaded move? Having nightmares of dropping a valve or a valve seat coming loose and taking out a piston....

Would it make sense to leave the rockers off and pull the pushrods and drive it to the back yard on 7 cylinders? Or would the lifters bounce around
on the cam and cause damage? The GMC is currently in the driveway right in front of the garage as well... So the vehicles inside are trapped as well
if I don't move her...
--
Mark S. '73 Painted Desert,
Manny 1 Ton Front End,
Howell Injection,
Leigh Harrison 4bag and Rear Brakes,
Fort Worth, TX

 

[mark sawyer]

> Would it make sense to leave the rockers off and pull the pushrods and drive it to the back yard on 7 cylinders? Or would the lifters bounce
> around on the cam and cause damage? The GMC is currently in the driveway right in front of the garage as well... So the vehicles inside are
> trapped as well if I don't move her...

Thought about this some more and that's a bad idea... Lifter shoots out and I would have no oil pressure...

So first question still stands... Drive it as is with whatever is wrong with the valves/head or pull the head where it sits?

--
Mark S. '73 Painted Desert,
Manny 1 Ton Front End,
Howell Injection,
Leigh Harrison 4bag and Rear Brakes,
Fort Worth, TX

 

[Ken H.]

Drive it.

Ken H.

On Sun, Aug 2, 2020 at 2:18 AM Mark Sawyer via Gmclist <

> > Would it make sense to leave the rockers off and pull the pushrods and
> drive it to the back yard on 7 cylinders? Or would the lifters bounce
> > around on the cam and cause damage? The GMC is currently in the
> driveway right in front of the garage as well... So the vehicles inside are
> > trapped as well if I don't move her...
>
> Thought about this some more and that's a bad idea... Lifter shoots out
> and I would have no oil pressure...
>
> So first question still stands... Drive it as is with whatever is wrong
> with the valves/head or pull the head where it sits?
>
> --
> Mark S. '73 Painted Desert,
> Manny 1 Ton Front End,
> Howell Injection,
> Leigh Harrison 4bag and Rear Brakes,
> Fort Worth, TX
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[johnny]

Crank it twice (once to let the trapped cars out, once to pull in the garage) and run it 200 feet? Certainly.

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell

 

[mghamms]

I'd drive it to where i could work on it.

--
1977 Kingsley 455 as stock as it gets except lots of Ragusa parts

 

[Matt Colie]

Aw Chit.....

#6 is on the A/C side.

Unless valve seats were put in that engine (not a common thing) there is not any chance that one came loose. If a valve broke, you won't even get to
88.

88# is not completely dead. What you might have is just a chip of carbon under the valve seat. Unfortunately, a lot of heavy work needs to get done
just to prove the point. But, there is a way (I don't know it) to move the A/C compressor out of the way without breaking the system open. You need
to learn that.

A cheap and dirty diagnosis at this point would be get the cylinder head cover off and look at the valve stem heights. If one of the valves is
visibly height or low, that is an indicator of a problem. If one looks low, then put shop air on #6 with a small regulator and tap the valve. If it
is carbon on the seat. you might knock it loose. (This is almost a sucker bet, but it is still worth the effort (imho&e). At this point, that head
has to come off.

When you have to pull a head, do not try to believe that you can wrestle the intake and head out by hand and alone. There isn't room for two and
everything to too heavy, so plan to rig lifting gear.

You will have to drain the cooling system, but that will not get you clear. There are drain plugs in both banks and at least the right has to come
out. As I think you would do a better job by removing the intake too, draining both sides adds only a little difficulty to the operation.

What ever happens, please keep us in the loop.

Best of luck

Matt - had to have his whole engine out twice. Once for the engine and once for the frame.

--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit

 

[mark sawyer]

> Crank it twice (once to let the trapped cars out, once to pull in the garage) and run it 200 feet? Certainly.
>
> --johnny

Actually, just crank it once... Run it up a small hill, turn it around and straight into the shed out back... 200 feet is about right.

--
Mark S. '73 Painted Desert,
Manny 1 Ton Front End,
Howell Injection,
Leigh Harrison 4bag and Rear Brakes,
Fort Worth, TX

 

[mark sawyer]

> Aw Chit.....
>
> #6 is on the A/C side.
>
> Unless valve seats were put in that engine (not a common thing) there is not any chance that one came loose. If a valve broke, you won't even
> get to 88.
>
> 88# is not completely dead. What you might have is just a chip of carbon under the valve seat. Unfortunately, a lot of heavy work needs to get
> done just to prove the point. But, there is a way (I don't know it) to move the A/C compressor out of the way without breaking the system open.
> You need to learn that.
>
> A cheap and dirty diagnosis at this point would be get the cylinder head cover off and look at the valve stem heights. If one of the valves is
> visibly height or low, that is an indicator of a problem. If one looks low, then put shop air on #6 with a small regulator and tap the valve. If
> it is carbon on the seat. you might knock it loose. (This is almost a sucker bet, but it is still worth the effort (imho&e). At this point, that
> head has to come off.
>
> When you have to pull a head, do not try to believe that you can wrestle the intake and head out by hand and alone. There isn't room for two and
> everything to too heavy, so plan to rig lifting gear.
>
> You will have to drain the cooling system, but that will not get you clear. There are drain plugs in both banks and at least the right has to
> come out. As I think you would do a better job by removing the intake too, draining both sides adds only a little difficulty to the operation.
>
> What ever happens, please keep us in the loop.
>
> Best of luck
>
> Matt - had to have his whole engine out twice. Once for the engine and once for the frame.

THAT is an interesting idea, Matt... Definitely worth a shot. I'll that a try right now.

If not, yeah, AC side... But, I'm running a modern Sanden SD7... Much smaller than the A6. I think I may be able to get her moved enough to clear
the head. However, I did not change the condenser out for a parallel flow unit when I updated the comp... (Which I feel is needed to do a 134a
conversion correctly here in TX), So it IS charged with R12, so I definitely want to do my best not to have to crack the system...

--
Mark S. '73 Painted Desert,
Manny 1 Ton Front End,
Howell Injection,
Leigh Harrison 4bag and Rear Brakes,
Fort Worth, TX

 

[mark sawyer]

You know, this engine uses quite a bit of oil.... it definitely could be carbon.... Any reason not to give an intake valve cleaner like GM Top
Engine Cleaner a try? Not talking the additives u put in the gas... but one of the ones you run directly into the intake with the engine running....
--
Mark S. '73 Painted Desert,
Manny 1 Ton Front End,
Howell Injection,
Leigh Harrison 4bag and Rear Brakes,
Fort Worth, TX

 

[mark sawyer]

Pretty sure I figured out the issue....

So the previous owner installed the fuel injection kit, and he used this part as the top hat for the throttle body:

https://www.gmcrvparts.com/product-p/g10351.htm

So far so good.

Where it was not so good, was that instead of using a 25 cent rubber grommet to seal the crankcase breather where the hose went into the top hat, in
an exceptional piece of decision making, decided to use what looks like JB Weld to seal around the hose on the INSIDE of the top hat. I apparently
have never flipped the thing over in the 5 years I've owned the coach, (When I've had it off in the past I just set it aside with the hose still
attached) as I'm pretty sure I would have had a heart attack if I had seen that.

Anywho, half of the blob of JB weld is not present and accounted for... And the rest was loose enough it scrapped off with my finger. Guessing the
missing piece decided to exit stage right through #6 piston. Probably bending a valve... I would also assume this is why there does not appear to be
any lower end damage... (Knock on wood... at least so far) I would imagine JB weld would be pulverized and burned up inside the combustion chamber.
At least I hope that is the case.

Thank you all for the input on troubleshooting this issue.... When I get the head off I'll post up some pics in case some are curious about what
dried jb weld will do to the inside of the engine.

--
Mark S. '73 Painted Desert,
Manny 1 Ton Front End,
Howell Injection,
Leigh Harrison 4bag and Rear Brakes,
Fort Worth, TX

 

[mark sawyer]

Matt: Thanks for the advice on pulling the head as well... I will definitely rig up some sort of mini-gantry to help lift stuff off... Younger me
would have scoffed at that idea... Older me thinks younger me was pretty dumb sometimes...

On pulling the head, I did more or less plan on pulling the intake. I was actually thinking of bringing it along with the head to the machine shop
have them check it over for cracks, clean it etc. Is this worth the time and expense do you think?

can anyone thing of anything else I should address while the top end is removed? the engine was supposedly rebuilt 20k miles or so ago, but after the
JB weld thing, I'm taking everything I was told with a grain of salt...

I was actually half thinking of pulling the other head as well and having it gone through. It would not be too much more work, and like I said, the
engine does use some oil. Since the compression seems pretty good, I would think it is maybe leaking past the guides or possibly on a poorly sealed
intake manifold?
--
Mark S. '73 Painted Desert,
Manny 1 Ton Front End,
Howell Injection,
Leigh Harrison 4bag and Rear Brakes,
Fort Worth, TX

 

[Jim Hupy]

The real drill is as follows.
1. Drain the coolant, including the plug on the side of the crankcase
where the knock sensor is.
2. Remove the upper radiator hose and thermostat housing along with all
the heater hoses that attach to the intake manifold.
3. Drill a 1/4" hole in the hatch cover flange so that you can thread an
eye bolt with a piece of cable or chain attached to surround the A/C
compressor. I use a turnbuckle on the other end of the cable with another
eye bolt screwed into it. Stretch the cable around the compressor, run a 1"
wood screw through the eye and screw it into the floor. Remove all the
bolts from the compressor, slide it towards the passenger tire, and tighten
the turnbuckle until the compressor is tight against the floor. DO NOT
REMOVE OR LOOSEN ANY A/C HOSES. It can stay right there until you have
finished with the head and manifold.
4. Remove all the sensor wiring, zip tie it back out of your way.
5. Remove the intake manifold after you have removed the EFI injector body
and it's hoses. Label which one is which. Same with vacuum hoses. Put your
lifting device in place, unbolt the intake manifold, and get it out of your
way.
6. Remove the passenger side exhaust manifold, followed by the cylinder
head.
There you go! Be sure to get rid of every last drop of spilled
anti-freeze. Now, before you do anything else. It's important.
Jim Hupy
Salem, Oregon

On Sun, Aug 2, 2020, 11:09 AM Mark Sawyer via Gmclist <

> Pretty sure I figured out the issue....
>
> So the previous owner installed the fuel injection kit, and he used this
> part as the top hat for the throttle body:
>
> https://www.gmcrvparts.com/product-p/g10351.htm
>
> So far so good.
>
> Where it was not so good, was that instead of using a 25 cent rubber
> grommet to seal the crankcase breather where the hose went into the top
> hat, in
> an exceptional piece of decision making, decided to use what looks like JB
> Weld to seal around the hose on the INSIDE of the top hat. I apparently
> have never flipped the thing over in the 5 years I've owned the coach,
> (When I've had it off in the past I just set it aside with the hose still
> attached) as I'm pretty sure I would have had a heart attack if I had seen
> that.
>
> Anywho, half of the blob of JB weld is not present and accounted for...
> And the rest was loose enough it scrapped off with my finger. Guessing the
> missing piece decided to exit stage right through #6 piston. Probably
> bending a valve... I would also assume this is why there does not appear
> to be
> any lower end damage... (Knock on wood... at least so far) I would
> imagine JB weld would be pulverized and burned up inside the combustion
> chamber.
> At least I hope that is the case.
>
> Thank you all for the input on troubleshooting this issue.... When I get
> the head off I'll post up some pics in case some are curious about what
> dried jb weld will do to the inside of the engine.
>
> --
> Mark S. '73 Painted Desert,
> Manny 1 Ton Front End,
> Howell Injection,
> Leigh Harrison 4bag and Rear Brakes,
> Fort Worth, TX
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>

 

[billvv]

Jim,
I am impressed with your patient and thorough answers on this forum. Some enterprising soul should compile and publish then as 'The Hupy Method -
"Just What I Did".

--
Bill Van Vlack
'76 Royale; Guemes Island, Washington; Twin bed, full (DS) side bath, Brazilian Redwood counter and settee tops,455, 6KW generator; new owner a/o mid
November 2015.

 

[LarryW]

> Jim,
> I am impressed with your patient and thorough answers on this forum. Some enterprising soul should compile and publish then as 'The Hupy Method
> - "Just What I Did".

Me too. Jim is on my bucket list to meet before I..........
--
Larry
78 Royale w/500 Caddy
Menomonie, WI.