Gas and Oil Mileage

[william myers]

>From my recent 900 mile round trip to the Texas Coast rally, I had
posted a report of 5.9 MPG towing a Cavalier and 300 miles/qt on Mobile
1. You might remember my having to change a power steering pump at a
rest stop. One good suggestion was to check the plugs. The plugs
checked as all light gray with no sign of oil or soot. Two had a black
shadow on 1 side of the insulator, does that indicate anything?

No sign of oil leaks, at least while parked.

Also checked the compression and got 5 cylinders between 142 and 152, 2
cylinders 130 and 135 and 1 cylinder at 100 pounds pressure. I shot
some oil in the top of that low cylinder and when rechecked only went
up to 110. Maybe a burnt valve. Could that 1 cylinder have caused my
poor gas mileage?

Any ideas appreciated,
Bill
'74 Glacier ... on the digest ...
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[bright walter]

Hi Bill;

Your problem has had me wondering. Since you had to get 3 tanks of
gas at least, its probably a accurate mpg #. You don't notice the engine
running rough on low speed acceleration due to the low compression in that
one cylinder? I would think the lack of power output at 100lb compression
would be noticeable on acceleration. Before this latest post, I was going
to suggest with the toad, 3.07 axel, etc, that maybe the vacume was
extremely low, causing the carb to respond with too much enrichment but this
was a bit of a long shot. Now that you have added that one cylinder is
flying so close to the ground, this seems more plausible to me. Low vacume
also could account for the low oil mileage, especially in conjunction with
worn valve guides. BTW what carb do you have? If a Holly, you may have a
blown power valve. This is what controls enrichment on those carbs. When
they go bad, the fuel economy usually goes down sharply. Hope this helps,
walter bright 76GB.

 

[william myers]

Thanks for responding Walter. I have the Holley TBI with closed loop.
About the same age as Emory's apparently. Your suggestion and Emory's
post about how to adjust it with the light tells me that's something to
check. The tail pipe is fairly black. Also get HEI advance weights
someone posted. It actually runs fairly smooth, so I was surprised to
see that 100 lb cylinder. Acceleration is something tht I don't have a
reference for. Maybe someone has a 0 - 40 time. Not sure what a
normal vacuum is for the 3.07 either, anyone got a reading at some
speed there?

Thanks,
Bill ... on the digest ...

- -----
>>From: "Bright, Walter"
Subject: RE: GMC: Gas and Oil Mileage
Hi Bill;

Your problem has had me wondering. Since you had to get 3 tanks of
gas at least, its probably a accurate mpg #. You don't notice the
engine running rough on low speed acceleration due to the low
compression in that one cylinder? .....
>>
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[heinz wittenbecher]

Just curious, you folks using a heated O2 sensor?

Heinz
getting ready for the southward (home) part of this trip.
Departing Terrace as soon as Coffee finished brewing.

>

>
> >
>
> Bill - when you have it in closed loop does the red light flash on and off
at
> idle and at about 1500-2000 rpm and at highway speeds when you are
driving?
> If not, perhaps you oxygen sensor is bad. You could also have a "lazy" O2
> probe. They do wear out and need replacement once in a while. I replaced
> mine last year as the red light was very slow to respond and I was getting
a
> lot of soot from my exhaust pipe which indicated a rich mixture. Worked
well
> with the new probe and the soot went away. I bought a Bosch at the
AutoZone
> store. I can find the number if you need it.
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM
>

 

[wayne newland]

Emery

If you look in a junk yard, What are you looking for (make & model)? vac pump.

Wayne Newland

>
> traveling in the mountains, when vacuum dropped below 4, the outside heater
> vents would open even though the AC was on. Then when vacuum got back up to
> 6, they would close. I need to figure out how to correct that problem.
> Justin >>
>
> Justin -- this is a common problem that I had suffered with for many years.
> When I put in the electric vacuum pump to provide a good vacuum to the brakes
> I disconnected the small tube that goes from the manifold to the plastic
> vacuum tank and hooked it to the input of my vacuum pump. I no longer have
> the heater problem.
>
> If you get your pump from the junkyard get the hoses with it. Included is a
> tee fitting with just the right size for the 1/8" tube.
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM
>
> Emery Stora

 

[wayne newland]

Emery

Thanks. I've been to this site many times, but I missed this. I can't remember
everything, everywhere, but you should see my GMC file. As I have time, I'm
reviewing the things that I thought were important (at the time) and beginning to
make notes in my shop manuals & parts book. I think I have 4 notebooks full of
suggestions and sources.

Wayne Newland

>
> pump.
>
> Wayne Newland
> >>
>
> Wayne - look at Gene Fisher's site. If I remember he has the various autos
> listed that use the vacuum pump. Just use the mouse key on this: http://www.california.com/~eagle/ t=_self] HREF="http://www.california.com/~eagle/">GMC MH INFO - Eugene Fisher[/url]
> or if that isn't highlighted on your internet service type in
> http://www.california.com/~eagle/
>
> Gene has gone to a lot of work to put most of the common and some not so
> common modifications and other info on his site. it is a good place to check
> for answers to your questions. There are several other good sites. I don't
> remember if Gene has Links to other sites but I or someone else can provide a
> good list of many of the sites.
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM