3800Impala

Hi. I've been having this strange problem with my Impala for the past couple of weeks. After the transmission shifts, the RPM drops as normal, but if I let it go below 1900 RPM, I get an odd "jitter" in the RPM. I can also hear the engine revving up and down. The car also wont accelerate any more unless I put the pedal down more. (Not like WOT, or anything, though.) When it first started, it was only by about revving by about 100 RPMs. Last week, though, it started doing this by 500 RPM. I cleaned the mass airflow sensor and replaced the throttle position sensor with a spare I had from another 3800 engine. (Although this engine had even more miles on it than my Impala does). After I replaced it, I test drove it and for the first 5 miles it continued to have this problem. After that, though the problem went away for about a week. Now it's back to jittering by 100 RPM. I'm afraid it's going to get worse, like last time. I have no SES light, either. Does this sound like a TPS issue? I'm worried that it's the torque converter or something else that requires transmission removal....

Other details:
Car has 170,000 miles.
I bought the car two years ago @ 155,595 miles. It was in bad shape. Needed head gaskets, plastic intake had exploded, needed battery, mirror, radiator, etc minor. I fixed every issue, and it ran great for 15,000 miles. The one mistake I made was when I replaced the radiator, I lost a lot of transmission fluid. I added some back in but not enough. I drove it for about a mile with about 1.5 quarts low, until I realized why it wouldn't shift. I added the fluid and it started shifting, except it was kinda rough. So I added the trans-go shift kit and it severally reduced the hard shifts, but didn't make it go away 100%. It's been like this for 2 years with no issues shifting, except for a slight "bumpy" feeling going from 3 to 4. I'm worried this moment of stupidity from 2 years ago is coming back to bite me.

Frank3

How's the fuel filter? I had a similar varying of RPM with my 2002 Tahoe. That turned out to be the fuel pressure regulator. Any codes or check engine lights?

3800Impala

The fuel filter has about 15k on it. I replaced the pressure regulator 2 years ago, since that is apparently what caused those plastic intakes to explode. I have a fuel pressure guage so I can check that. I'm not getting a check engine light, but I can check the computer and see if there is any pending codes. I took it to two local transmission shops, both of which people around here say good thngs about. Yet, the second place I took it to, the guy claimed that well the TPS needed to be "reprogrammed" in the computer and it costs $45 for that and to come back next week. Well, thats not happening because I know there is nothing to program on this engine, regarding the tps. The first place, is kinda interesting. A couple months ago I stopped in there asking about the same transmission, but for a different car (a project I was working on). First guy I asked had know clue about anything I asked. Second guy, though, he was an expert and really knows what he's doing. Unfortunately, the one who test drove my car yesterday was the "less knowledgable" one. He "confirmed" my worry about it being the torque converter, suggesting an $1800 rebuild. Another bothersome thing was I asked if it could be the TCC or PCS solenoid, since I know those fail alot, and he says he doubted it, but he could pull the pan and look. Only problem is the solenoids are in the valve body, not the pan.

On your Tahoe, was the rpm fluctuating all the or just during shifting? Was there a certain RPM range it was more noticeable in?

Update: I checked the fuel pressure regulator and it's not leaking. I got 39 PSI, which apparently is normal. Guess it's not fuel related. Thanks for the suggestion, though.

Frank3

The fluctuating RPM would start at the 2-3 shift point then continue on after that. At a lower rpm range and gear range it wasn't as noticeable, maybe because the engine wasn't loaded up as much?