95 GP 3.1L stumble

[lcalzadilla]

Hello,

My name is Livan and I have a question regarding my mother's 95 Gran Prix SE with the 3.1L V6 and 199K.

We are having a horrible problem with hesitation and/or stumbling at freeway speeds under light to medium throttle inputs. Both my father and I are certified mechanics and we have both tossed our hands up.

Some additional history: The problem began as a light to almost intermittent stumble under light throttle at freeway speeds. The problem then evolved to turning the check engine light on. The scanner indicated an EGR problem (sorry can't remember code of the top of my head).

1. The EGR, spark plugs, fuel & air filter, oil & filter, trans service, O2 sensor, Map sensor, thermostat, coolant, temp sensor were all changed, additionally added a fuel injector cleaner slight improvement but still illuminated the check engine and reported the same code.

2. Checked all hose and wiring, replaced all vacuum hoses, no change.

3. Removed the intake plenum and throttle body -- thoroughly cleaned the plenum and throttle body, as well as the EGR pipe and all EGR portals and of course replaced the gaskets, in addition to replacing O2 and EGR sensors again (under warranty). Check engine light no longer illuminates, no codes are stored. Hesitation and stumbling remain. Please help!

Anxiously waiting reply,


Livan C.

[GTP Dad]

Bad fuel injectors. The injectors on these particular engines are prone to failure in high mileage conditions. If the injector cleaner helped, it probably cleaned them but didn't change anything. They are expensive but I think a complete replacement will correct your problem.

[lcalzadilla]

Originally Posted by GTP Dad

Bad fuel injectors. The injectors on these particular engines are prone to failure in high mileage conditions. If the injector cleaner helped, it probably cleaned them but didn't change anything. They are expensive but I think a complete replacement will correct your problem.

We have considered the injectors but the car idles perfectly and starts immediately.

[GTP Dad]

I had a similar issue with a 2.8, ran fine, idled like a champ but after it heated up it would stumble badly and almost die. Try this the next time it does this drop it into a lower gear like OD to D. If the problem goes away the injectors are bad and need replaced, if nothing happens then we'll have to look elsewhere. You can also check the resistance on the injectors since it will be relatively high if they are bad. Check both cold and hot!!

[richtazz]

I agree with GTP, that you have an injector problem. There is a small chance that you have a TCC solenoid that is slow to unlock the converter, if the stumble only happens at speeds of 40-55mph(that's the lock-up threshold speed range).

[lcalzadilla]

Originally Posted by richtazz

I agree with GTP, that you have an injector problem. There is a small chance that you have a TCC solenoid that is slow to unlock the converter, if the stumble only happens at speeds of 40-55mph(that's the lock-up threshold speed range).

I don't think it's the lock up (TCC) because the stumble also occurs if you brake load the vehicle. If the TCC would be stuck the vehicle would turn off if I'm not mistaken. But there's a simple test for that, disconnect the connector. At this point I'm open to any suggestions that are resonable and thought out.

[clutch492]

It is now 2010...... Did you ever figure out what was causing the hesitation? I am going through the same thing with my car.

I have since discovered that my catalytic convertor is plugged. I drilled a hole in the exhaust inlet to the cat convert. and looked inside with my bore-scope. It is almost totally plugged