fixing tip-in stumble diagnosis

[Jim Bell]

I have a totally stock 87 GN w/40k miles and the seemingly common stumble problem in two conditions. #1 occasional stumble/driveline jerk at highway speed 70-75. #2 Serious stumble with excessive driveline jerk coming off idle until I can get the engine up to about 2000 rpm, then it is generally fine. Much worse hot than cold. Nearly impossible to drive hot.

What should I be checking? I searched for vacuum leaks, nothing visible and no rev changes using a propane torch (off) gently moved about the top of the engine. MAF sensor is spotless as is the screen. Throttle body and stainless inlet pipe are clean.

When the stumble is not present the car accelerates very smoothly and I have never felt a miss/stumble under boost.

Appreciate any advice you may have. After 27 years I have to believe we have figured out the cause of this GN stumble issue

Thks

JIm

 

[earlbrown]

Ifter 27 years I have to believe we have figured out the cause of this GN stumble issue

Yep, and you're not going to like the answer(s).

On the interstate cruise if the engine cutting out for a second feels electrical, it might be the TCC disengaging and reengaging. The test is easy, jump out the TCC wire on the ALDL plug and force it to stay locked up (that will test the ECM side and rule out the 4th gear pressure switch)

Since it's worse and undrivable hot, you might be looking at a new ignition module


Now for the part that's really going to suck (and not in the good way)

Every vac line and check valve under the hood needs to be replaced (strictly due to age), A scantool will tell you how to get the IAC set to that the throttle blade still gets to do most of the air metering at idle. That way there's not a big transition and a few milliseconds of dry manifold when the throttle is cracked.

That MAF is most likely flakey and need to be replaced with a translator and modern MAF,



Now here's the part that really hurt$

You need a new fuel pump (with hangar mods), a hot wire kit, a modern chip, new injectors, valve springs, timing set, scan tool, fuel filter(s), rocker shaft buttons, (I also HIGHLY recommend rocker shaft supports), plugs, wires, and weld up the crack in the DS header.

Oh yeah, and all that crap has go to be installed too.


There's also a good chance the DS motor mount has given up do to the years and all the oil from the leaky valve cover gasket and front seal.


After that ENTIRE list is taken care of you'll be able to more accurately troubleshoot what the car needs to be running 98% perfect and go from there.

 

[Jim Bell]

thanks, I suspected TCC might have been part of the problem. dropping into 3rd gear is a smoother drive and eliminates the stumble at highway speed. I did a lot of analysis and simulation of TCCs for GM many years ago.

I suspect I will put the old GN in the front yard and let it go to the first gear head with 20k. My C6 is much more reliable anyway.

 

[earlbrown]

If you're familiar with the TTC circuit it's easy to diagnose. My problem was a flaky 4th gear switch and running 90 on the interstate it felt EXACTLY like an oddball misfire. A simple flick of my manual lockup switch would make it stop. Granted, in my case the hot side on the brake pedal was functioning normally.



If you decide to take care of the preventative maintenance items on that list I posted you might want to keep it. That GN in a fairly stock state of tune will mop the floor with a C6.

 

[Razor]

Its a 27 year old car with 27 year old parts. When that Vette gets to be 27 years old.. it will have its drama.

Everything Earl has listed is common replacement. Why I hate museum cars nobody ever drives or wants to do proper upgrades. It starts with a simple tuneup and goes from there.

 

[earlbrown]

^and goes, and goes, and goes, and goes.....

 

[Razor]

Funny how you get a car thats "all original" and buyers go awww .. and the ones all the updates are done are now molested and modified...

 

[earlbrown]

And then they get pissed they have to spend $1000 to end up with a reliable 5-seat 11sec street car with A/C on pump gas.

 

[GNRick]

Ok so you've owned the car since new and just now looking at turbobuick.com? Where have you been hiding? LOL....anyway just start with a modern chip and your troubles will disappear. You might also want to do the spring cleaning procedures.

 

[Nascar83Fan]

From the sounds of it, a tt chip that's burnt for todays gas and for those injectors with alittle spring cleaning stuff done will help cure the issues. Having at least a scanmaster to see what it's doing would help too.


Posted from the TurboBuick.Com mobile app

 

[7007v6buickman]

@earlbrown how do youi jump the TCC using the ADLA PORT?

 

[whitehot1]

Here you go 7007v6buickman.

http://www.gnttype.org/techarea/electrical/switches.htm

There is a lot of good info on that site.

 

[davelostmissing]

IMHO I wish people would spend more time doing analysis and less time claiming a chip will fix everything.
stock car - stock boost levels? I know for a fact cars run fine this way. And get good gas mileage to boot.
Stock MAFS on the other hand....

 

[GNRick]

IMHO I wish people would spend more time doing analysis and less time claiming a chip will fix everything.
stock car - stock boost levels? I know for a fact cars run fine this way. And get good gas mileage to boot.
Stock MAFS on the other hand....

I had a car that came with the stock chip and had the tip in stumble. A new chip from TT fixed it.

 

[7007v6buickman]

@davelostmissing that links no good. I'm gonna try to find it thought thanks.

 

[Nascar83Fan]

The one posted came up with a 404 for some reason, but it was whitehot that posted it not someone else.


Posted from the TurboBuick.Com mobile app

 

[Jerryl]

I suspect I will put the old GN in the front yard and let it go to the first gear head with 20k. My C6 is much more reliable anyway.

Don't take this the wrong way, but these cars are not for everyone. I even told my buddy not to buy one of these cars because he lacks the skills, the patience and the facilities to be a happy owner, seriously.

 

[standupjskiguy]

I don't believe one needs to throw a whole bunch of parts at the car. Without determining what is actually needed.
I don't agree with the implication that an original car needs $1000 worth of parts just cause it's old.
I have an 85 T with 140 k and I still have the majority of original parts on it.
Maf, TPS, IAC, ignition module, coils, most of the vacuum lines, original injectors, etc. Yes,
I have a collection of spare parts because I know something will go bad eventually.
That car was the wife's daily driver and it has been very reliable.
My original 87 with 13k runs fine, it's all original.
I think, a turbo tweak chip will help a lot to clear up most of your problems.
I put one in because I had some annoying drive-ability issues and they went away.
If you were near Chicago, I'd let you try my chip.
An all original car may or may not need a bunch of parts. It depends on a lot of variables.

 

[Chuck Leeper]

Its a 27 year old car with 27 year old parts. When that Vette gets to be 27 years old.. it will have its drama.

Everything Earl has listed is common replacement. Why I hate museum cars nobody ever drives or wants to do proper upgrades. It starts with a simple tuneup and goes from there.

Added to what Julio said: It won't be long B4 the C6 follows the C5 in parts supply issues. We are constantly looking for used items such as pcm's, steering shaft sensors, tac modules, etc, etc.

 

[standupjskiguy]

Upon thinking a little more about this. The MAF might be dirty or went bad causing a stumble.
Try cleaning it with the special cleaner they make for this purpose.
In addition, if the cam sensor is out of adjustment, bucking and even backfiring at highway speeds
is possible. I had this on the 85 when I had an idiot work on the car and he never adjusted the cam sensor
after putting a new timing chain set on the car.
Otherwise, on my 87 I had mild stumble and mild low speed bucking which were resolved with a
Turbo Tweak chip for my stock setup.