99 Cavalier engine "Stumbles" at 55mph

I have a 1999 Chevy Cavalier 180,000 miles of mixed city/highway driving. I am a pretty laidback driver, so those haven’t been very hard driving miles and the car has been wonderfully reliable.

Recently, it began seriously stuttering, it feels more like it’s suddenly stumbling when it’s at highway speeds 55-60mph.


-This only occurs at 50+ mph and will stop if I drop below 45mph or so.

-This only occurs after the car has been going for 5-10minutes on the highway. Even if the car is fairly warm, it doesn’t immediately start stumbling when I get on the highway.

-This occurs at cruising load on the engine, not just pushing hard on the accelerator (that suddenly makes it way worse)

-No starting problems

-No problems accelerating at lower speeds, including up an onramp

-I’ve replaced the spark plugs and wires

Note: When I changed the plugs they were fouled with what looks kind of like wet oil. But the car has no smoke out the exhaust and doesn’t seem to require much oil replacement, so I don’t think I’m losing/burning oil.

-I’ve run some gas additive that removes water through it to no avail

-I filled it with Ultra94 (I thought regular was expensive) gasoline, no change

-When I listen/feel the exhaust from the tailpipe it’s “puttering” at an even but rapid pace. I never noticed this before, so it may be normal. The engine doesn’t sound like it’s gotten louder.

I’ve brought up these symptoms to car people and I’ve mostly gotten puzzlement and different hypothesis. The fact that it starts and runs fine is confounding. The fact that even if the car is already warm or hot it still takes a few minutes of highway speed for this to occur.

What I have not done, but when I mentioned it to my auto advisers didn’t produce much faith in them as a solution:

-Not changed the fuel filter

-Not changed the oxygen sensors

-Not changed the air filter, but I’m about to do so.

-Not changed any part of the distributor

-Not changed the battery

Sounds like a classic fuel filter problem.

I’m now thinking it’s a problem with the automatic transmission, I’m still not sure. It only happens in the highest gear.

If the stuttering starts, I tried jamming down the accelerator, this drops it down a gear to speed up and the stuttering goes away. It comes back when I let off the accelerator and the car upshifts to keep cruising at highway speeds.

Does this sound logical? I’m still not sure.

My vote is for a clogging catalytic converter. (or other restricted exhaust).

Replaced the air filter on it last night
I drove it today at highway speeds and the behavior repeated when at 55mph+

I jumped off the highway and drove on Rt 1 in Mass along the North Shore from NewHampshire to Boston. Speeds around 35-45mph most of the way, no problems at all including hills and acceleration.

Funny enough, the stretch closer to Boston gets back up to 55mph speed limit (although Boston drivers take that as permission to do 70mph) and it seemed okay. I’m now pretty confused.

I’m going to take it to have the transmission looked at for possible problems. If the transmission is bad then I’m going to have to have the car euthanized.

The correct answer was torque converter…torque converter.

This is according to Aamco of Malden (great friendly service, and felt very honest) . My answer, fill it with dirt and grow something in it. Just under two grand to replace the torque converter and do a basic transmission rebuild.

Thanks for the advice and help