My '85 MGM, "Maisa"

[Arquemann]

chuff.. chuff.. chuff" noise from the exhaust

Sounds kinda familiar, but it could also be a misfire.
I did a compression test, cyls 1-7 are all right around 145-155 psi (10-10,5 bar) and I can't get the compression tester to cyl 8. The no. 8 plug wasn't fouled atleast, quite the opposite...





I ran out of time today, I'll get her back together tomorrow. Tomorrow might bring more surprises.
I don't know what the actual fuck is going on anymore...

[DerekTheGreat]

Those numbers sound good to me. The plugs look good too, how many kms/miles are on 'em? The ground electrode on #1 & #7 (possibly # 4 too, possibly just the lighting) look a little wet compared to the others, but none are the smoking gun I was hoping to see. With the engine that came out of my truck, #1 was CAKED with carbon/oil all in the spark plug gap.

[Arquemann]

Here's the problem with those plugs:
The shop receipt from a couple years back says 8pcs of NGK UR45. Which those are NOT, those are some rusty old ACDelcos. I have no clue how old those are or how many miles are on them.
It seems I was bullshitted on those plugs, or the mechanic put the old ones back in if the NGKs made no difference.
To me most of those plugs look way too white, which would indicate those getting hot.

I THOUGH I HAD NEW-ISH PLUGS.

Yesterday I put in completely new Champion coppers and new 8mm plug wires, and today I took her for a rip. The bucking on decel is pretty much GONE. Idle sounds smoother, while it doesn't really feel that different.
-There's still some... ehm "inconsistency" slowing down from about 25 to 15 mph.
-Also some bucking on very light throttle at about 45 mph.
-Driving at very low throttle the mixture goes lean, to about 16 AFR at worst, doesn't affect anything much. Mixture is instantly fine if more or less throttle is applied. Is 16 AFR harmful at such low load situation?
Just a thought, but my transfer slots might be too closed, which might cause this.

At this point, I need to drive it more to know how things stabilize and to see how things act consistently.
PLUGS, PLUGS FOR FUCK'S SAKE!

[friskyfrankie]

Did I not mention a misfire? Good job on NOT assuming anything and checking thew plugs and wires!!

[Arquemann]

Did I not mention a misfire? Good job on NOT assuming anything and checking thew plugs and wires!!

Yeah I was thinking it was a misfire aswell, but it took me long enough to question the shop receipt that says I have new plugs. I have no idea how much money and time I've wasted due to a bullshitting mechanic.

[friskyfrankie]

Yeah I was thinking it was a misfire aswell, but it took me long enough to question the shop receipt that says I have new plugs. I have no idea how much money and time I've wasted due to a bullshitting mechanic.

Unfortunately, it is the folks we trust that disappoint us. You did ALL the right things, checked even things you thought were fine and narrowed down the problem (or at least part of it). I congratulate your persistence and hope you have many years of enjoyable driving on this car. Having an older car is a challenge - something always requires repair but that is part of the "charm" of older car ownership.

[Tiggie]

My experience with the AOD leads it to be a bit clunky on decel around 15-20mph as it pulls out of locked third and into second. Especially true if the TV pressure is up a bit.

I think 16:1 is okay if it is low load. It doesn't take much throttle to load up a lugging 302 hanging out in 3rd at 20-25ish mph. If it pings, it might be something to worry about.

[Arquemann]

My experience with the AOD leads it to be a bit clunky on decel around 15-20mph as it pulls out of locked third and into second. Especially true if the TV pressure is up a bit.

I think 16:1 is okay if it is low load. It doesn't take much throttle to load up a lugging 302 hanging out in 3rd at 20-25ish mph. If it pings, it might be something to worry about.

Yeah the downshift could be the cause for that issue. My TV pressure is up from factory aswell. That issue was the least of my worries anyways.

I'll fiddle around to figure out the tendency to lean out. Initially I set the transfer slots to squares, but I've had to slow down the idle speed quite a bit from that. Currently the idle speed fine, but I wonder if the transfer slots aren't exposed enough so it leans out at low throttle before the accelerator pump hits. I'll check for vacuum leaks aswell, I know for sure the automatic e-brake release leaks 9/10 times.

[DerekTheGreat]

Here's the problem with those plugs:
The shop receipt from a couple years back says 8pcs of NGK UR45. Which those are NOT, those are some rusty old ACDelcos. I have no clue how old those are or how many miles are on them.
It seems I was bullshitted on those plugs, or the mechanic put the old ones back in if the NGKs made no difference.
To me most of those plugs look way too white, which would indicate those getting hot.

I THOUGH I HAD NEW-ISH PLUGS.

Yesterday I put in completely new Champion coppers and new 8mm plug wires, and today I took her for a rip. The bucking on decel is pretty much GONE. Idle sounds smoother, while it doesn't really feel that different.
-There's still some... ehm "inconsistency" slowing down from about 25 to 15 mph.
-Also some bucking on very light throttle at about 45 mph.
-Driving at very low throttle the mixture goes lean, to about 16 AFR at worst, doesn't affect anything much. Mixture is instantly fine if more or less throttle is applied. Is 16 AFR harmful at such low load situation?
Just a thought, but my transfer slots might be too closed, which might cause this.

At this point, I need to drive it more to know how things stabilize and to see how things act consistently.
PLUGS, PLUGS FOR FUCK'S SAKE!

White also means a lean mixture, which is what I'd be leaning towards and what I'd prefer.

Perhaps there is some bit of crossfire going on, if changing the plugs and wires made a big difference with your issue. At night, try spritzing some water around all the plugs, coil, dizzy cap and along the wires as it runs. If anything changes, investigate further.

16:1 at cruise is great, I wouldn't change it. It's only lean on acceleration/load one needs to worry about, as that's what'll cause combustion temps to increase and all that. General Motors TBI stuff has a lean cruise feature built in. I'm not sure what regions had it enabled, but it's mostly disabled. Reenabling it is something a lot of folks do to increase their cruise mpg, it's worth about 10-20% depending on the application. I want to say 16 or 16.5:1 was what the ratio GM targeted was. Downside (if you care) is increased NOx emissions. Should also be noted there is a better pay off with EFI, as I believe it also runs the EGR for combustion chamber fill and advances the timing to compensate for the lean mixture.

[matth825]

Nice job getting to the root of it. I’m surprised plugs would cause a misfire on decel due to the extremely low load the engine is under. I would have thought the miss would have been more noticeable under hard acceleration.


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[Arquemann]

Regarding the leaning out, going as far as 16 AFR but only at very light throttle, not really representative of cruising speed. I adjusted the accelerator pump arm since it had a bit of play in it. It might fix it, might not. Not too worried at this point, since it's seemingly not affecting anything.
I'll have play around with this thing, get to know the behaviour first, then I'll continue tuning it.

Shot around some bräkleen aswell, no obvious vacuum leaks to be found. Also I did plug the e-brake release, so it won't leak in any position. With the brake release plugged and the AC in off position, there's genuinely zero vacuum leaks from the vacuum distribution block!

I'll drive it around more tomorrow, try to figure out if the 45mph bucking is gone or what's it doing then.
But dang, on cold start this thing sounds good! The cold start and idle sound changed quite dramatically with all 8 cylinders firing well lol.

[DerekTheGreat]

You're getting there, and learning a lot in the process. Good stuff.

Careful with the brake clean, a lot of that stuff isn't flammable any more so it won't aid with finding leaks. Plus, if thats the stuff which has chlorine or whatever in it, bad news bears. Phosgene gas isn't very good for the respiratory system. Oh and uh, it's a "parking" brake.

[Arquemann]

Drove around some more, a bit of backroads and city driving running errands. I couldn't replicate the 45mph bucking at all, which is good I guess?
Still leans out very light throttle, but again it doesn't seem to affect driveability in any way. AFR on normal cruise speed is around 13,5-14. I've got main jets one size smaller which are going in at some point.

I guess throttle tip-in could be a bit crispier, since it goes kinda rich. The idle is sometimes a bit rough feeling, the bumpiness comes and goes. The mixture isn't quite spot on yet, ~14AFR.
Overall though, the car behaves very nicely.

[Arquemann]

You're getting there, and learning a lot in the process. Good stuff.

Careful with the brake clean, a lot of that stuff isn't flammable any more so it won't aid with finding leaks. Plus, if thats the stuff which has chlorine or whatever in it, bad news bears. Phosgene gas isn't very good for the respiratory system. Oh and uh, it's a "parking" brake.

Yeah the stuff I have isn't flammable, but if it sucks it in it bogs real hard, so I guess it works too. Maybe, maybe not.
PaRkiNg BrAke, nyeeeh It's an emergency brake because you never ever use unless your brake pedal goes to the floor and then you're experiencing an "emergency".

[DerekTheGreat]

Not according to OEM literature Owner's manuals usually describe using the parking brake any time you park the car, especially on hills before shifting into park, so you don't damage the parking pawl when trying to shift out of park. I think the days of losing brake function entirely went away for us around '68, when dual channel braking systems became standard.

Don't you guys drive lots of stickshifts in Finland? I wish the parking brake in my truck worked, I'm tired of having to shut it off every time I want to park and get out for just a few moments. I'm waiting to find a 14 bolt rear end for it though, soon as I do that swap I'll put a new cable in it and be good to go.