A new problem developed in my wife's Fiero (the nice lo mileage one of course) this weekend. Driving on the interstates, under part throttle cruising conditions, the engine really stumbles, but no backfiring. Under WOT and 3/4 throttle, idle- the engine seems to run normally. I'm thinking its not ignition because it does not get worse under load and it will rev to redline without a problem. Any ideas Fiero guys??

Thanks!

Mark

Is the SES light comming on and setting a code 44?? Is you gas milage good??

I have been getting sometimes this same symtom but have the code 44 set. Possible that it can be an O2 sensor, but can't confirm this. I will getting an AC delco sensor soon. I don't intend to use other aftermarket parts due to the quality. Even when it comes to spark plugs, I use AC Delco.

The AC delco O2 sensor is about 65$ CAD here in Canada. I don't find it excessive, because it should be replaced every 2-3 years. (That's my opinion)

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fierogt28

88 GT, loaded, 5-speed

quote Originally posted by fierogt28: Is the SES light comming on and setting a code 44?? Is you gas milage good?? I have been getting sometimes this same symtom but have the code 44 set. Possible that it can be an O2 sensor, but can't confirm this. I will getting an AC delco sensor soon. I don't intend to use other aftermarket parts due to the quality. Even when it comes to spark plugs, I use AC Delco. The AC delco O2 sensor is about 65$ CAD here in Canada. I don't find it excessive, because it should be replaced every 2-3 years. (That's my opinion)

I've seen the the light come on only briefly, I don't think it was on long enough to even set a code. The O2 sensor is a newer Bosch I think.

Thanks,

Mark

Bad Throttle Position Sensor will do that. They get a bad "spot" usually where they stay allot like at idle. I had one get a bad spot that equated to about 45 mph. Above and below it ran fine but hold the throttle in that certain spot and it would miss and buck. Wouldn't set a code either.

quote Originally posted by Hudini: Bad Throttle Position Sensor will do that. They get a bad "spot" usually where they stay allot like at idle. I had one get a bad spot that equated to about 45 mph. Above and below it ran fine but hold the throttle in that certain spot and it would miss and buck. Wouldn't set a code either.

OK, I'll check the TPS. I think I have one around here, easy enough to swap it out.

Thanks,

Mark

Well, took the car out today for a drive as the weather is getting better around here. Boy, did this problem come back big time. The engine really stumbles around 2-2.5K RPM under cruise conditions. As mentioned before, the engine pulls hard to redline and no backfiring when it is stumbling, no codes. I did switch out the TPS but apparently, this was not the problem. Fierogt28 mentioned the O2 sensor, it is newer as the original failed. Could a bad O2 cause this? Makes sense that it is sensor of some sort. It also has an off idle stumble thats been there longer than the 2K RPM issue. Hmmm could a wonky MAP sensor cause this? Maybe time for a scan tool.

Thanks,

Mark

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86 SE V6 4 speed
86 SE V6 Auto
2008 G6 GT "Street" Coupe
2005 Buick 3.6 Rendezvous
2001 Olds Silhouette (AKA The Band Van)

Since it's not the TPS, then I'd move to the EGR system given that the problem occurs while cruising, and that you've seen an intermittent SES light. The EGR is about the only fault that will turn on the light and not trip codes. In particular, I'd look at the vacuum line running between the EGR solenoid at the pulley-end of the engine and the EGR diaphram. If it's broken or leaking, then when the ECM calls for EGR, the solenoid will open allowing unmeasured air into the engine, leaning out the mixture.

If that's not it, then I'd check the ignition system components. Look for:

1. corrosion on the terminals inside the distributor cap;
2. spark plug wires that are chafed, cut, or rubbing on heat shields;
3. spark plug gaps and condition; and
4. corrosion on the distributor pick-up.

Good luck.

If your test involved using a used TPS, you could very well have two bad TPS's

Have you checked your fuel pressure?

quote Originally posted by phonedawgz: If your test involved using a used TPS, you could very well have two bad TPS's Have you checked your fuel pressure?

The TPS is an NOS part, not used. Haven't checked the fuel pressure yet but I will along with looking at the EGR system and vacuum in general.

Thanks,

Mark

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86 SE V6 4 speed
86 SE V6 Auto
2008 G6 GT "Street" Coupe
2005 Buick 3.6 Rendezvous
2001 Olds Silhouette (AKA The Band Van)

Well, the stumble is fixed. More offen than not, when something like this pops up, go back to the last thing replaced, in this case the oxygen sensor. The oxygen sensor was mentioned as a possible problem in the thread. Pulled the old one from my 3.8 swap engine (in pretty good shape) and put it in place of the new one I bought last year- bingo, problem gone and it runs great. Even went right through inspection. Tried to probe the defective one with my Fluke meter but only got some intermittent resistance readings, which could be if an O2 sensor works on capacitance. I think I may beat the defective unit with my 6 lb mal just because- nah! Its a Bosch if anyone wonders. The original failed by the seal rusting out and the element falling out of the body. Happened on a trip of course; an instant exhaust leak and trouble codes 8>( . Well, anyway, back to the 3.8 now...

BR's,

Mark

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86 SE V6 4 speed
86 SE V6 Auto
2008 G6 GT "Street" Coupe
2005 Buick 3.6 Rendezvous
2001 Olds Silhouette (AKA The Band Van)