1991 Pontiac 6000 running rough

[Patriot_RAM]

Hey Guys,



I have a 1991 Pontiac 6000 with the 3. 1 v6 and 3sp auto transmission. The past year, it has been running rougher and rougher. When cruising through town or at hwy speeds (between 35 and 55), it jerks back and forth. From what I understand, this is because the engine is running rough when torque converter is locked up, causing the "roughness" to transfer straight to the driveline. The other symptoms are a slight hesitation when you press the throttle, rough idle after warmed up, and sometimes when you let off the throttle in stop and go traffic, it nearly dies before sputtering and regaining its idle.



When the engine is cold, it isn't an issue. When it gets up to about operating temp after 8 to 10 minutes of driving, then it starts to run rough. I put an inline spark plug light on each plug wire while it experienced the miss, but I never noticed the inline light "missing" when the engine did.



I put a dollar bill behind the tail pipe. It flutters a little as expected when the engine is running. When it starts running rough, however, it doesn't suck the dollar bill back in, it actually "puffs" it out away from the tail pipe.



I'm not a mechanic, but here's what I tried. I already mentioned the in-line light on the spark plug wires seemed to look okay. I replaced the plug wires with new ones from O'Rielly. I replaced the plugs with new AC Delco plugs, gapped them properly. This didn't change anything. I replaced the original factory coils with three new BWD Select coils from O'Rielly. If anything, that made it worse. I ran seafoam through the crank case two times and the fuel tank three times.



The oil stays clean, doesn't burn a drop, no smoke at all. The engine runs very strong with the exception of this problem. It runs good cold, but stumbles on itself after it warms up. The only other thing I can think of is that the car did have a new fuel filter installed about three years ago.



Ideas?



Thanks!

 

[fest3er]

You seem to have stable supplies of all four required components: fuel, air, compression, spark.

Offhand ideas: EGR, O2 sensor, ignition timing, cam/timing chain/belt and, if it has one, idle air control. I'd start with putting an old-time timing light on it; the ignition timing should be steady, consistent.

 

[TBurow]

the older 3. 1 gm engines had bad fuel injectors that would short out when hot, the easiest way to test them is get the engine hot, pull the upper intake and test the resistance with an ohm meter, ther should all be within 10% of each other, usually 16 ohms. the only other possible causes could be a failed coolant temp sensor causing the car to run rich when it warms up, or a sticking open egr valve, but it would be more noticable at idle or light throttle and not affect full throttle power.

 

[Patriot_RAM]

the older 3. 1 gm engines had bad fuel injectors that would short out when hot, the easiest way to test them is get the engine hot, pull the upper intake and test the resistance with an ohm meter, ther should all be within 10% of each other, usually 16 ohms. the only other possible causes could be a failed coolant temp sensor causing the car to run rich when it warms up, or a sticking open egr valve, but it would be more noticable at idle or light throttle and not affect full throttle power.

The symptoms make me think it may be the EGR valve not closing all the way. I tested the resistance on the EGR over the weekend and it was perfect. I'm going to pull it off the car tonight or tomorrow and use some carb cleaner to see if it's leaking.



Thanks for the suggestions! I'll let you know what I find.



Thanks!