RPM Wavering, surging/hesitation

[sharpie22]

Hi all,

I have a 2000 Grand Prix GT with about 125,000 miles on it. Whenever I accelerate gently on the pedal the rpm will waver back and forth, especially between 2000 and 3000 rpm's. It seems to hesitate as the rpm goes down and then surges back and forth almost on one second intervals until I either let go of the pedal or punch it down to kick it into overdrive.

I have had a code in the past that the o2 sensor by the cat convertor was getting messed up readings, can't recall the specific number off hand though. Do you think that these problems could be caused by a bad catalytic convertor? I have smelled the classic rotten egg smell before from my car too, but it didn't seem to smell when I smelled the exhaust tonight.

How would I test this myself easily? I am a very poor college student and can not afford the equipment or the cost to get it tested if that was not the problem


Thanks

[BNaylor]

If the old code was a P0420 DTC then the converter is a possibility based on the symptoms. Get the converter tested or run an engine vacuum test which is inexpensive if you have access to an automotive vacuum gauge (0-30 in-hg) type.


Also, when was the last basic tune up. Spark plugs and ignition wires? Could be a misfire caused by a secondary ignition issue even without a SES/CEL light.

[sharpie22]

Hey -

I was driving to the store today and my check engine light went on . I went to Autozone and they pulled up the P0141 code for a failure in the circuit of the o2 sensor.

Guessing that this means that it is the thing at fault then right?? This could cause my rpm to surge at points, correct?

Also which one is this. Is it one of the o2 sensors before the cat converter, or is it after it?

Thanks all

[BNaylor]

It is the post CAT 02 sensor. Heater circuit malfunction. I doubt that 02 sensor is causing your issue. The sensor is only used to monitor CAT converter efficiency. To start check the electrical connector to it.

[sharpie22]

From researching this a little bit more, i have come to the conclusion that the cat converter is more than likely not at fault for this. My main question now is, would a o2 sensor behind the cat cause my rpm issues as described above? or do you think that that is a separate issue entirely.

[BNaylor]

It could be the converter. How is moderate to full acceleration? Does it feel like hitting a brick wall at higher rpms. On the post CAT 02 sensor you can disconnect it and it should not cause your issue. Still good to get that issue resolved since it monitors the converter. Also, you could have a secondary ignition issue causing the rpm bounce.

[sharpie22]

No, i definitely don't feel the brick wall affect that you are describing. The problem only arises under light acceleration from 2000 to 3000 rpm's. It really doesn't feel like a misfire, it is just like a surge and hesitation issue. I assume i would be getting codes for those anyways if that was the problem. I am driving it about 6 hours away today so I will let you know if i have any other issues or codes, i will probably get it scanned again after i get up there.

Is it easy to change plugs on this thing or am I looking at a pro job? I'd really love to get a good look at them

[BNaylor]

It doesn't sound like the converter. Back to the basics. Fuel and ignition. I would check fuel pressure to see if it is in specs (48-55 psi) and start replacing secondary ignition components like spark plugs, ignition wires or coil packs. On coil packs you can swap them out with each other or at least check primary and secondary coil resistance with a multimeter (DMM).

The plugs (rear bank) are not easy but DIY doable. Just take your time and have patience. Use the right spark plug removal tools.

[sharpie22]

Changing plugs/wires today. Didn't seem to be too bad driving up, the problem occurs more during day to day driving. We will see what this does, i think that it is due for a tune up anyways.

One other thing seemed odd today. I was driving and my thermostat gauge seemed to change very quickly from around 160 to 180 and then sometimes it would be at one, and switch to the other, not moving gradually at all. Is that something i need to worry about do you think?

[CrazyHorst]

My recommendation for the rear bank is to disconnect the torque strut "dogbones" and use a 1" wide ratchet-strap tie-down to roll the engine forward for more clearance. L67 is pictured by L36 will work in similar fashion.

In the pic I have the radiator out of the vehicle but same deal applies. Just check clearance from the front cast iron manifold to the heatshield on the back of the fan motor(s) as that's the usual point of contact and you don't want to damage anything. You don't have to move the upper-rad support wiring harness either...it was that way from the radiator removal.

The #6 plug requires a normal 5/8" spark plug socket and an extension to clear the O2 sensor which is nearly directly inline!

[BNaylor]

Originally Posted by CrazyHorst

My recommendation for the rear bank is to disconnect the torque strut "dogbones" .

Nah! That is really not necessary. You do know that is a good way to screw up the two lower engine/tranny mounts?


The biggest issue most of us have is getting the rear bank spark plug boots off and/or not damaging the metal heat shields. Good idea to use dielectric silicone grease during the install or replace the ignition wires in their entirety.

[sharpie22]

Hahahah didn't need to do any of that luckily. I managed to put an extension on the #6 and pulled it out no problem. as for the other two, putting a 2" extension and getting a shorty socket on there did the trick. I did manage to pop one plug putting it in #6, but I drove to the store and got another and put it in. Overall wasn't that big of a deal...probably took me around 2 hours.

Driving back home tonight, I will let you know if the rpm issue is still there.

Thanks

[sharpie22]

Well, took the ride home and my rpm issue is still very apparent, it didn't seem to change at all after doing plugs and wires.

What do you think is the next step? I will probably get it scanned to see if it popped any more codes tomorrow, but if it hasn't I have no idea where to go from there.

[doctorhrdware]

Could it be the ICM causing the problems?

[tblake]

Whats the condition of your MAF screen in your throttle body?

I had a slight surge/TCC Lock-up indecision at 45-55mph when going into overdrive a while back. At the time I had my MAF screen removed. Once I put it back in, problem solved.