This is for my 95 Formula 6 speed. The car has 106000 miles and the only mods are the K and N and SLP loudmouth. This week it started to run REAL bad, lots of stumble and hesitation especially at anything over 2k RPM. Took it to the mechanic who is a freind with a 1995 Impala SS. He checked the battery, real week. Alternator showed fine. Replaced the battery with a quality new one specified for the car and it ran GREAT all day yesterday. I ran it out in the country for about an hour and 45 miles. Get in it today to go see the folks in Orlando and it is running like crap again. Anything over 2000 RPM and the engine/trans is shuddering very bad, wouldnt go over 60 mph or so. I had the Opti, coil, plugs, wires all done in 2005 at the local Pontiac dealer, all of whom I know and trust to do it right. Also had the tank dropped and the fuel pump done at the same time, all new GM parts. (yes, the bill was considerable). It had about 80000 miles on it then. Have always used 93 octane from major oil companies and the oil is, and has been Mobil 1.
Does anyone have any ideas? Also-NO trouble lights on dash are coming one and all gauges are in the normal range.
Thanks, Bret

 

Check the fuel pressure, scan it for codes (some serious codes do not turn on the SES light), check for a plugged cat.

What does the dash gauge indicate sytem voltage is when it's running poorly?

 

The volt gage, along with all the others, look fine, it is over the 13 area. Didnt think of the cat since it is intermittant, comes and goes.

 

OK- the dealership mechanic drove it with the latest computer they have for about 20 miles and it did the rough running a little, and never threw a code. He ran the history and found the number 7 cylinder was mis firing. Inspection found "carbon tracking"? on the plug and the exhaust manifold had a broken bolt at the flange to the exhaust downpipe right next to #7 plug. Good exhaust shop replaced the bolt and found no other issues/leaks on either side. Go to drive it home and same problem. Everything was dry, but did drive about 10 miles in TS Debby that is drenching Gainesville right now.
Does anyone have any ideas, I am not totally great on these modern engines, but it is starting to frustrate me very much. Stock car except the K/N filter and SLP exhaust. No codes, gauges all in normal range, no dash lights coming on. And I trust the dealer guys, have known the service writer 25 years, back before he went to work there and there was NO BILL from them for the time they spent and the exhaust shop only charged me $42 for the bolt replacement.
Any help is appreciated.Love the car, want to keep it forever.

 

i would go with wet opti from the rain, and check the mas sensor see if theres any debri from the k&n filter on the the sensor its self , i have had a little piece cause mine to run bad . if the filter got wet it can cause it

 

OK- here is the final. Took the car back to the dealer. Mechanic checked it again and said it was the wire. They don't sell the single wires. Service manager called GM rep and since I had the plugs. wires, opti, coil, etc done there in 2005 the rep warranteed the wires and cut the charge for labor. 8 plugs, a wire set and labor was $325 out the door and the wires are warranteed again. I know that is alot, but I am not good enoguh mechanically to have done that myself, just being honest. Picked it up on last Saturday, pointed it south on I75 to Orlando and she ran like a scalded dog all the way past 100. ( then yes, I backed down to close to legal) Hasent had a single problem since.