RPM Surging Between 2000 - 2500 RPM

[Enzy]

2000 Grand Prix GT 3.8.
I have had an annoying problem with this car for quite awhile.
When travelling at betwen 2000 - 2500 rpm and the load increases, either by gentle acceleration or climbing a hill, the engine continuously surges up and down about 500 rpm in about 1 second cycles. The only way to stop it is to let off the accelerator or shift down either manually or by pressing the accelerator. Otherwise, the car generally runs well.

I have replaced EGR valve, PCV valve, throttle position sensor, mass airflow sensor, fuel filter, plugs and wires. Fuel pressure tests normal.

I would appreciate any suggestions.

Thanks

[richtazz]

Welcome to AF

Does it do it only in OD while cruising at a steady speed? If so, it sounds like a TCC solenoid locking and unlocking the torque converter. If it does it in lower gears at lower speeds, I'd say you have a vacuum leak somewhere.

[Enzy]

Vacuum tests OK, with consistent 17-18 in. at idle and at 3000 rpm. I thought of the TCC solenoid, but the surging can happen in OD and 3rd for sure. Anybody know if the TC would normally lock in 3rd?

[BNaylor]

Engine vacuum looks good. The TCC will lock in 4th - OD only above 45 mph. When was the last tranny service? ATF and screen filter? What brand spark plugs to include heat range/numbers and ignition wires?

Also, it would help to post your mileage.

[goldberg_52403]

Wow, I just logged in to start a thread about the same thing. I have a 99 GTP, and it is basically the same situation. Under gentle acceleration, the revs will hop up and down about 250 rpms (maybe it will soon be worse and 500 rpms like yours), just enough to be incredibly annoying. Now I am worried from what is reported so far, as when I first got the car (~2 months ago), the check engine light came on regarding the TCC something. The shop replaced a wire for the TCC, and the light went away. Anymore input on this case would he highly appreciated. Other than this issue, the car runs great and continually does with this problem if I just give it a little more gas.

Thanks for starting this thread!

[BNaylor]

Originally Posted by goldberg_52403

Wow, I just logged in to start a thread about the same thing.

The fishbite or chuggle effect is caused by two possible problem areas. A bonifide transmission problem related to the torque convertor (TCC) and control circuits which could include throttle body components such as the TPS, etc. Or a secondary ignition problem not severe enough to be detected as a true misfire condition by the PCM module. The proper spark plugs and good quality ignition wires is critical or you'll get these anomolies.

As suggested to the OP always post your mileage.

[Enzy]

Wow, good suggestions! And nice to hear someone else has the same problem.
Mileage on the car is 142,500 miles. I have 71,000 on the plugs and 43,000 on the wires. ATF and filter were changed 31,000 ago.
The plugs are Bosch platinum #HR9DPY. They are the number recommended by the Bosch book. The wires are Niehoff Premium Silicone.
I see a fuse in the main fuse box labelled TCC. Tomorrow I will pull this to try and isolate the TCC as the cause. I'll also ohm the coil secondaries.
I'll report the results.

[BNaylor]

Originally Posted by Enzy

Wow, good suggestions! And nice to hear someone else has the same problem.
Mileage on the car is 142,500 miles. I have 71,000 on the plugs and 43,000 on the wires. ATF and filter were changed 31,000 ago.
The plugs are Bosch platinum #HR9DPY. They are the number recommended by the Bosch book. The wires are Niehoff Premium Silicone.
I see a fuse in the main fuse box labelled TCC. Tomorrow I will pull this to try and isolate the TCC as the cause. I'll also ohm the coil secondaries.
I'll report the results.

Pulling the TCC fuse will not do any good. You will lose the 12 volts to all shift, PCS, and TCC-PWM solenoids. The shifting will default to second gear and it will feel like the brakes are applied when trying to accelerate.

Do you have the same symptoms? The best plugs to use if you are stock are the AC Delco double platinum or Iridium or NGKs like TR55IX and AC Delco Premium Silicome 7mm igntition wires. Good idea to check coil resistance. .05-.09 ohms for primary and resistance 5K to 10K ohms for secondary.

[goldberg_52403]

Thanks for the input BNaylor. I just rolled over 103k, and actually just put in new TR55IX's last weekend, the wires seemed to be near new. Driving home today I realized that the surge is really only 100 rpm at the most, just enough to notice it. Could that perhaps be a symptom of a faulty TPS? I will continue to search the forums.

Thanks again.

[BNaylor]

Originally Posted by goldberg_52403

Thanks for the input BNaylor. I just rolled over 103k, and actually just put in new TR55IX's last weekend, the wires seemed to be near new. Driving home today I realized that the surge is really only 100 rpm at the most, just enough to notice it. Could that perhaps be a symptom of a faulty TPS? I will continue to search the forums.

Thanks again.

TPS is possible but you can check the output with a DMM at ignition to ON not START. The input which is PCM reference is around 5 volts dc. On the output you'll get around .4 volts at idle and 4.5 volts at WOT. While moving the accelerator upwards the voltage should progress up linearly and smoothly. No erratic indications.

[goldberg_52403]

I will try to check the output of the TPS for any erratic behavior. I suppose I am leaning against this because the car runs very smooth up until I get to highway speeds. Let's say the TCC solenoid is bad, would that cause the car to shift bad, or just have trouble accelerating in OD?

Once again, thank you BNaylor for you valuable advice.

[richtazz]

The most common symptom of a bad TCC is sluggish acceleration/hesitation in OD, and if they get worse, bucking/stalling as you slow down due to the torque converter not releasing and the trans not downshifting.

[goldberg_52403]

Ok, that explains exactly what is occurring, only under gentle acceleration in OD. Would it be recommended to have a tranny shop scan the transmission? I read somewhere that transmission issues dont alway flag the SES light, and a tranny scanner would be needed.

Thanks again to both richtazz and BNaylor, and sorry for all the questions.

[richtazz]

The TPS could still cause this too if it's erratic, as the PCM wouldn't be getting a signal you're rolling on throttle thus commanding the TCC to release the torque converter. If the TPS checks out, then it would be advisable to have the car scanned with a trans capable scan tool. Never apologize for asking questions, that is why we're here and we're glad to help.

You can also do a resistance test on the wires. Acceptable resistance is 500 ohms per inch. Just because the wires look new doesn't mean much. Misting them with water at night and watching for cross-firing would also be a good test for the wires.

[69ELCamino]

I have same symptoms/problem. guess what? it's the TCC and a rebuilt 4t65e.........