I hope someone can shed light on this problem cause it is driving me crazy! I have searched and read the many other stumbling problem posts others have had but none seem to match mine.

I have a 91 GTA 5.0 TPI completely stock. Once the car has warmed up, I have a stumble between idle and 1200 RPM while doing a slight brake rev. The stumble is there also while free revving but it is hard to tell at what RPM it ends. Doing the break rev seems to "amplify" it by putting a load on the engine. Anything above 1200 RPM the stumble mysteriously disappears.

At idle the car will stumble, almost stall, then the engine rev's up to catch itself from stalling and that would keep happening until eventually it will stall. Important fact: the car takes the gas perfectly fine with no hesitation whatsoever and goes from 0-redline without a hitch. Like if I am sitting at a red light and it's stumbling, I can just floor it and gets up and goes without a hitch.

I personally have done a major tune up on the car shorty after I got it due to it failing emissions. Parts changed: oxygen sensor, distributor cap, rotor, plugs, wires, thermostat, air cleaner, serpentine belt, pcv valve, and fuel filter. One of my problems is this - I don't remember if I had this problem before I did the tune up. I can't pinpoint when it started happening. I have rechecked things so I am assuming the tune up work was okay though.

Now in chasing this problem, I have gone out and replaced the IAC valve, TPS, and both the ECM and PROM with all factory OEM parts but nothing has helped.

Since day one of getting the car (last February), whenever I set the cruise and drove on the highway for a little while my check engine light would come on. I never knew what code it was throwing till recently I got it hooked up to a computer and it was diagnosed as a bad EGR valve. So I started poking around in that area and noticed my vacuum hose coming from the throttle body going to the EGR valve was melted! I changed that and made sure I was getting vacuum to the EGR and I was. Still didn't fix the stumble. I have confirmed that my EGR solenoid is working. However, there is a possibility that the EGR valve could be bad. My father said he manually applied vacuum to it figuring it would open and the engine should bog down. That didn't happen. We have even gone as far as taking the EGR off the car, making sure everything looks good and is clean and then put it back on.

Here's my theory on all this. I am not totally ruling out that the EGR could be bad, but I didn't think that could cause the exact stumbling symptoms that I am experiencing. I would think that the engine would show signs of hesitation if the EGR was bad and in a stuck open position. I have no hesitation whatsoever.

Could a bad EGR valve really cause this problem? If so why I am I only getting a stumble up to 1200 RPM? I am so boggled at this point. I was also thinking it could be something fuel related: injectors or fuel pressure, etc. What else could it be?

Anyone at all....please, please, please....any thoughts and opinions would be greatly appreciated! I desperately need HELP! lol

Thanks,
Rob

PS. Sorry for the long post.

 

look for vacuum leak, double check your work and make sure you didn't mess up somewhere. check or change the filters, check and set idle, timing, tps, or iac.

 

Ede, thanks for the response. I will no doubt have to double and triple check things.

I thought most of the settings are controlled by the ECM (timing, idle, tps, and iac)? The tps can't be adjusted on mine. The idle I know there is a torx screw that can change the idle setting but I still though the ECM varies the idle as necessary to whatever it is programmed at. I am not sure about setting the iac and timing though.

I can understand if I was just having and idling problem and that could be caused by something minor just set wrong. But my idling problem is due to the stumbling issue. How many things could cause a stumble from 0-1200 rpm only with no hesitation or noticable loss of power? Very weird if you ask me. Someone out there must have experienced this same problem with the number of these engines out there. There were tons of other stumbling posts but unfortunately none seemed to match my exact symptoms.



Are there any ALDL computer programs out there that are for TPI engines? I have seen a couple but they were for TBI only. Something like this might help out a lot.

 

You have a 1227730 ecm with a speed density fuel management system... You can use Craig Moates GMECM program to scan your car... you can find it here: http://www.moates.net

 

No one else with any ideas what could cause this problem? EGR valve, ignition module, coil, map sensor, timing, fuel pressure, injectors, etc???? To me I wouldn't think it is ignition related but I am just guessing at this point. And there is no hesitation so I wouldn't think it was fuel related either - but again just an educated guess.

But don't these symptoms narrow it down some and give more of an idea what it could be:

* Seems like the stumble is not there when engine is cold. It gets progressivly worse as the engine reaches operating temperature.

* Stumble is only between idle-1200 rpm under load (slight break rev). No stumble above 1200 rpm.

* No hesitation whatsoever - free revving to redline.


Anyone else with any thoughts, comments, theories, ideas...anything???

 

Wow, I can't believe no one else has any ideas.

 

I take it you have cleared the codes and re-checked to see what the 'puter is saying?

Sounds awfully lot like EGR....but if the codes don't say so then I have no clue.

 

Hi Rob,

It seems that you are being confounded by a very similar, if not exactly the same, problem that I am having with my Formula. I have the L98 engine, but being an '89 it has the old 160 ECM and uses Mass Air Flow instead of Speed Density. The car has a stumble between about 900-1500 RPMs and only after it has warmed up. I also get terrible fuel mileage -- are you noticing any fuel consumption weirdness?

I am still in the process of troubleshooting my problem. If you have not done so already, I recommend downloading Craig Moates' ALDL data monitoring software and having a look at what your ECM is seeing while you are experiencing the stumbling. Go here and click the ECM852.ZIP link near the top of the page. Of course this is all requisite on you having an ALDL interface cable and a laptop (or desktop) handy.

In my case, scanning the ECM wasn't conclusive, but I learned that my BLMs are way high so the ECM is adding fuel to compensate for what it perceives to be a lean condition. I believe it is not actually lean, so personally I'm inclined to believe that my O2 sensor is at fault. But since you said you replaced yours, it would certainly be worth checking the fuel pressure as well to see if there is a fuel delivery issue. Other than that, I have heard that a faulty EGR can cause this problem as well. You might also try unplugging your O2 sensor entirely to force the ECM into open loop and see how it performs then. Unplugging your O2 should throw a code as I understand it.

Let me know if you find anything; it might help me too.

 

Thanks for your replies!

Bandit... I haven't noticed any fuel consumption weirdness as far as all of a sudden. I don't know what the gas mileage should be but knowing it's a V8 I never expected it to be all that good. I average about 11-13 mpg doing mostly city driving. Doing highway I think I have hit around 17 mpg. That sounds about right to me. What do you get for gas mileage?

I have thought about connecting my laptop to the ALDL to see if I can spot anything. I just have to get a cable and download that software. I am still considering doing this.

I have replaced most of what could have been wrong. Who knows, maybe even when I did a tune up I got a defective part or something. But I will try those couple of things that you mentioned like unplugging the O2 sensor. I can't test the fuel pressure unless I buy a gauge. It would be awesome if I could figure this out myself but at this point I am getting the feeling it is above our (my dad and I) head. I am afraid that I'll end up keep purchasing parts that I may not need and then if nothing works I'll still have to send the car out. But I'll try a couple more things before throwing in the towel. If I find anything I will let you know. Thanks for you help!

-Rob

 

My gas mileage is terrible...something like 7-8 MPG city. I've managed to hit around 15-16 highway when I first got the car (this past October), but the problem didn't seem so bad then, so whatever it is, it's been getting worse. Sounds like your ECM isn't dumping excessive fuel with numbers like that...that's pretty close to normal.

Sounds like you're in a very similar situation, all right. I'm a total newbie to DIY car repair, so I've been spending most of my time loading up on tools and reference manuals (and of course, advice on this board, which is free.... ) I don't have a fuel press gauge yet either; it is something I need to get, but something on a long list of stuff that all costs money. I just outfitted my garage with tools and storage stuff...lots more still to get...

If you're interested in an ALDL cable, though, I just ordered one from MaStach.com that was a good price and worked just fine. Doesn't have the pricey molded connector (it has four wires with pins on the end instead) but it works just as well.

Good luck -- if I discover anything, I'll post back here!