Brent 89-GT

I am having a problem with my car. It is missing pretty bad at light throttle. This makes the car "buck" back and forth even though you hold the pedal steady. Even a medium application of power smooths the car right out. Full throttle, it pulls like the beast it is It seems to idle just fine, the only time it bucks is in, say 3rd or 4th gear cruising at 30-40 mph under very light throttle. It also was doing it at 55-60 in top gear. There is no dull thud of an ignition miss, there is no real change in the exhaust note like a heavy ignition miss either.

Any guesses? Could it be a failing O2 sensor? Isn't the O2 sensor out of the loop under acceleration? Could it be symptoms of a failing brain?(the cars, not mine). It just seems like a lack of fuel or good spark would get worse, not better, with heavy power applications.

MikeN

Good one, are you sure it is actually "missing".......as in ignition. Maybe it's a MAF starting to go bad? Try and disconnect it and see if it acts differently.
Might be the brain, as one that is going south can do all sorts of strange things, but your symptom is not that common for a bad one.

Scott G

Sounds like the LH controller. Mine was doing the same thing. When the LH failed completely, the car wouldn't start. My LH was rebuilt and I haven't had any surging since.

Others have reported similar problems that turned out to be the LH controller. Try a search.

Brent 89-GT

Well..... I found a possible cause. There was a HUGE mouse nest in the center of the air filter just above the MAF sensor. They had even chewed through the filter a little. I hate to think about how much crap I have sucked through the engine I am a little hesitant to drive it but I am just dying to know if that was the problem. I think I will take a short drive just to see IF I was lucky enough to have a cheap fix

John Struthers

I might be outside the realm here, or, perhaps I'm just unfamiliar with 5 speeds.
But, is there any chance this is simply 'lugging'?
Try second or third at a 10 mph slower road speed at same rpm if the buck is still there you can either adjust the clutch -don't think so -.
Or you can simply shift at a higher rpm & road speed.
If this is happening after a down shift -coasting and jabbing- down shift again , your rpm shouldn't be very high from what I'm reading.
I'd hate to be in a bucking GT in wet/slick conditions.
BTW that bucking as I'm sure you know is very unhealthy for the drivetrain,
so you want to cure whatever the problem is fairly soon.
To be sure I'm no expert on GT shift points standard or auto.
But using the 82' 928/928 S for an example -yeah, I know 2 different animals-:
Even if you granny shift at 2000 rpm>
from 2nd to 3rd you should be at 22-30 mph, (redline 62mph approx)
from 3rd to 4th you should be at 30-40 mph, (redline 86mph approx)
Even in Pattycakes (3spd auto) I seldom shift out of 1st before 50-ish.

Brent 89-GT

Thanks for the idea John. It actually does it at 2000 rpms and at a light load at 3000 rpms, so I kind of tried that already. The bucking isn't too severe usually, probably not near enough to do any damage. I am trying to cure it before that

Since I have obviously sucked some crud through the MAF, I wonder if that is what is failing? I did drive it a little and the problem is still there with no air filter and no mouse house present

I have heard about people unplugging the MAF, how will it run with it unhooked? I just don't want to unhook it, condenm to sensor, pay to replace it and find out it was still the brain.

John Struthers

Brent,
I believe from the past post's on this subject ( check archives ) the disconnect was just a method of determining if there was any difference between the MAF being hooked -up or not.
If no difference the MAF has failed. But to be sure ....
You might want to contact John Speake by post or PM.

MikeN

Make sure everything is cleaned out.......you might want to actually pull the
MAF if possible and check the inside......make sure no crud is stuck anywhere......then replace.

Another no cost option is to disconnect the battery for about 30 minutes to see if the computer "learned" a crappy reading with the crud in place.

Can't say what it should run like with the MAF disconnected......all I know is it goes into some sort of "limp home" mode.......whatever that is like.

John Speake

Hi Brent,
If the MAF wire hasn't been actually broken, it should be OK. The burn off with get rid of any dust. It doesn't sounds as though it has broken.

Having said that, if the MAF is getting very aged, the O2 loop will start having problems following it under all conditions.

The O2 lopp has an adaptation range of about 22%. because the MAF error is multiplied in the ECU, the allowable MAF ageing is only about 4-5%.
Once it gets outside the range of the loop, all hell lets loose !

That's still much better than for a non-cat car, which is why most Euro S2's need a refurbed MAF by now.

The cause of the problem could be MAF ageing, but also an old O2 sensor has a much slower response time than a new one. So this is could also be another possible cause.

If you can borrow a MAF from a local enthusiast, try that first. If you can't borrow one, I can "lend" you one on a sale or return basis. If you return it unmolested, it will cost you about $30 to refund my out of pocket expenses. My US agent has a unit in stock.

John/Mike - the limp home mode is really only just for diagnosing a completely u/s MAF. The car will run quite badly, but will "get you home"

Regards,

Brent 89-GT

Mike, I already pulled the MAF out and cleaned it. There was minor debris in there that I cleaned out. There did not appear to be any damage. I had unhooked the battery earlier(prior to mouse house discovery) but I will do it again. Do these computers learn? Will the computer set a check engine light if something like the MAF or O2 sensor goes bad?

Brent 89-GT

Thanks for the offer John! The cost of these parts is looking pretty intimidating The O2 sensor is by far the cheapest of the three (LH brain, MAF O2, sensor). I may try that first. I wasn't really planning on an electronic engine control restoration program just yet. I will let you know if I need to try a new MAF.

Bernie

Have you thought about possible clutch problems?

I say this because I am having a very similiar problem with my 4-speed Fiero.
Car bucks if I try to granny around the 2-2500RPM mark.
Do you have any problems with your clutch when the car is cold? What I mean is after initial startup, do you have to slip the clutch for a minute or two to prevent the clutch from being "grabby".

I am pretty sure I have a rear, main seal leak on my Fiero. Not similiar I know but I have heard that there could be a possibility of oil splashing onto my clutch disc and contaminating it.

Not trying to redirect your approach but I think its worthy of mention....

MikeN

Brent -

Yes, I do believe the computers have a limited amount of "learn" ability, not very much by today's standards, but some. Don't know too much about the check engine light.......doesn't seem to get posted on a lot here and there seems to be many problems on the board that would have/should have triggered it.......so I wouldn't put much stock in it.

John -

thanks for the info on the "limp home" mode.......never had to disconnect mine to find out........fingers crossed!

Jack Riffle

Brent,
Possibly you could have one or more fouled plugs due to running too rich with the obstruction in the filter housing. I would check that first before the more expensive options.