1999 GTP Surging

I have a 1999 Grand Prix GTP (3.8 S/C) with roughly 112k miles on it. Recently my car has been intermittently surging about 100-250 RPMs at speeds between 40-65 MPH. If I give it some gas and get on the boost it drives normally without a surge or hesitation. I would say the car only does it about 20% of the time I drive it and is flawless the other 80%....however this has happened enough times to cause concern. It seems to still be shifting as smoothly as it always has, as I have no noticed any hard shifts or jerking. Even worse the SES light is not illuminated and they're aren't any stored codes.

The fluid level in the Tranny is fine, and the fuild itself still seems to be fresh. (The color is right, never can tell with the funny smell or trans. fluid).

Anyone else having or had similar problems that could point me in the right direction? The problem does not seem to be severe, but it's noticeable enough to be annoying. Thanks for any help.

- Justin -

It sounds like you are getting "fishbite/chuggle" which is the torque converter locking/unlocking when it shouldn't. When was the last transaxle service (screen filter and ATF)? Also, a secondary ignition issue like spark plugs or a flaky TPS can cause the issue.

Thanks BNaylor for the reply. It has been less than 10k miles since my transmission service. I believe they did a fluid exchange so the filter could be an issue. I bought this car with 93k miles and just hit 112k in 2.5 years of driving, and don't really know much of the service history before I owned it. I have already added some seafoam trans. tune and plan to drop the pan and replace the filter soon. (Is there a recommened amount of time to run the Seafoam before changing the fluid?)

As for the plugs I have had those replaced within the last year however do not know what brand were installed. I had my downpipe break causing the back 3 wires to start melting so I had the plugs and wires replaced together. I've also been thinking the TPS as well because today while I was test driving the car I seemed to get a little RPM jump after letting my foot off the gas. It was only about 200 RPM, but I don't ever remember the car doing that in the past. Thanks again for your time.

- Justin -

Just a little update and some bad news. I went and got a new Filter to do a pan drop only to find two of my pan bolts have been snapped off. Luckily there is no fluid leak. Now I'm afraid to remove the pan... I'm thinking I may not be so lucky after the change and I will have a leak. Has anyone else had this happen? What's the chance I can keep going with the other 18 bolts?

Also I don't know if anyone can tell me. What should the readings be if I test my TPS? I'd like to at least try to test it before buying a new one.

Thanks,
- Justin -

Using a multimeter (DMM) the TPS output to the PCM should be around .4 volts dc at idle and 4.5 volts at WOT. Reading should be linear and progressive from idle to WOT as you move the accelerator. No erratic reading. Check the latter with engine off not running, ignition to ON.

I finally was able to test the TPS and it was bad. So I bought a new one and installed it. I haven't noticed any surges so far however after a few drives I had the SES light come on with a P0122 code. What is the definition of the code and what does it generally take to fix?

I resolved the P0122 issue and so far so good! I'll post back soon to let everyone know if the TPS solved the surging for good.

I would still appreciate if someone could point me in the right direction as to how long to leave that Seafoam running in my tranny. Thanks again Bnaylor for all your help.

-Justin-

I wouldnt leave the seafoam in there for more than a week or so.