Thread: Fluctuating RPMs 05 GP

Hello everyone, I've used the boards many times for my GP needs and I have a problem I can't seem to pinpoint. First off I have an '05 Base with 89k miles and I am getting inconsistent RPMs while doing down the road. When I am going down the road without a load on the gas pedal or with my cruise set the RPMs will bounce up from anywhere around 200 to 600 revolutions higher then back down to normal RPMs. Example to be a little more clear, say im cruising and I'm running around 1,600 RPMs, the tach will jump up to 2,200 RPMs then back down to 1,600 constantly until I accelerate but then it starts again. This is terribly killing my gas mileage. Anyway, I had my buddy whose a mechanic who kinda helps me on the side hook it up to a diagnostic tool. I had 2 trouble codes, one was the upstream o2 sensor wasn't heating properly in order to collect data properly, and the other being something along the lines of it couldn't display the trouble engine light on the dash. We replaced the o2 sensor and still had the same problem. We hooked it up again with no trouble codes, and he even checked for misfiring at idle and under load and nothing abnormal struck his attention. I read on other threads to look at the throttle body inlet and I did and had some carbon buildup. I did not manually scrub away the buildup but shot carb cleaner in the vac return in the side of the air intake duct a few times. That didn't seem to help. I checked the plug wires at the plugs and coil packs and looked very nice with no corrosion. I replaced the plugs and air filter about 20,000 miles ago. Just replaced the o2 sensor a few days ago, and today I put a bottle of iso-Heet in the tank to get rid possible water in the tank/lines. This has been going on for about a month and with rather cold weather, if water was present it wouldn't be suitable climate to evaporate the water. And the only real mod I have done is a resonator and muffler cut and put in a high flow flowmaster super 44 single to dual on which has been on 7+ months. I also read possible CPS sensor but I don't think it would relate to my situation as there has been no trouble codes set for it. Any one have any suggestions? Should I physically scrub/clean the carbon buildup on the throttle body inlet or wait and run the iso-Heet run through? Thanks in advance for assisting.

Still having same issue, more fluctuating at lower RPMs now. I'm gonna try seafoaming it and check to make sure the spark plug wire isnt touching the o2 sensor. I read in another thread possible FPR or head gaskets? I'm gonna have it hooked up to a scan tool and run it down the road and watch it. Anyone have any ideas what to look for? Irregularities?

Sounds like a vacuum leak to me.

You need fresh spark plugs by now, fresh plug wires. Use ONLY MAF cleaner on your MAF.

Fuel filter replacement is like every 30,000 miles... but that should be unrelated.

Where should I look for the vacuum leak at? And I did plugs about 20k ago but not wires. And I should replace fuel filter as well.

I would start by looking at all the vacuum hosing and the rubber connectors for easily visible flaws. Failing that, you may need to pray some carb cleaner at it while the engine is running to see if the idle changes when the area is sprayed. A very common spot is right off the lower intake manifold, passenger side - there is a rubber T there that dry rots easily.

When you changed the spark plugs, did you make sure they were gapped properly?

Yep, I checked them before I installed them.

Finally got it hooked up to scan tool going down the road and found out the o2 sensor I just put is going ape crazy. We're gonna check wiring and if thats not it get a warranty on the sensor. But, never threw any codes?

It's a good idea to make sure you use Delco sensors. Cheaper ones can cause issues.

Alright, kind of an update. I did spray carb cleaner around the intake manifold while running and around the T and no variation in engine speed. We took it down the road while hooked up to the scan tool and there was nothing that stuck out. The upstream was bouncing around 200ish to 800ish (not sure but some wide span) and the downstream had slight variation but was fairly constant. My friend looked at the stored codes and there was 3 counts of the upstream sensor failed to heat properly. Now, thinking back about a year and a half ago, I had the tranny replaced. After I got it back it seemed like it would not build up heat properly. They screwed the tranny up, ended up having to bring it back 2 more times cause they didnt fix it right, then 15,000 miles later of a 12,000 warranty, tranny goes out again. But, when they first replaced it the first time, I thought it might of been a bad thermostat. Put in a fail-safe... ran about 3 months, popped, got a warranty put another in, month later that one popped so I said screw it and put OE spec back in it. Still have problems with temp building up. When its snowy and cold now, its lucky to barely make it past the 1/4 hash on the gauge. So what im wondering is if when they did my tranny maybe they screwed up the wiring or CTS? And when I get done driving and shut it down it registers as the engine not running normal and when I start up it throws that error. So I did a new OE thermo less than a year ago, I just did a coolant flush before winter so 4 months ago maybe. Think that the CTS could be causing my o2 to be throwing a code and unstable RPMs?

So... I'm pretty sure we found the culprit. Torque converter. Drove around for a while hooked to scan tool and found nothing. I told him I seen issues similar to mine with the torque converter not fully locking in. So we went back out for a drive and it showed the computer allowing the torque converter to engage but only had a duty cycle of around 58/60ish even down hills. He locked it via the scan tool and the fluctuating RPMs quit, he dis-engaged and went back to what it was doing. Got a 30,000 mile 3 year warranty with only 6 months and 7k on the new one.

If you were able to manually lock it and it locked solid how does this point to the TCC being the problem? Sounds to me like whatever controls it isn't doing it right.

On a side note my wife drives an 06 Saturn vue with a Honda engine and trans and they are known for the same exact symptoms. Oddly enough changing the ATF fixes the problem 98% for quite a while. I didn't believe it until I did it my self. I did 3 drain / refills and the problem vanished for the most part for 10,000 miles now.

Being low on gas makes the compressor cycle which will change the load on the engine abruptly.

Thats completely different then what Negative is talking about.

P.S. R134A's name is Suva. Freon was Dupont's name for CFC refrigerants such as R-12 and R-22. Yes, I'm full of useless information.

It ended up being the pedal position sensor. They had a tech bulletin on them and they are blamed for the duty cycle/oscillating RPMs. I replaced it with a few bolts and the part was like 65 bucks. It helped iron out RPM bumps going down a flat stretch but I still have slight RPM jumps still on inclines. Does anyone know if you have to calibrate them or reset the values on the cars computer? It helped a lot but it seems like it needs fine tuned still. Or maybe my maybe my tc is still bad.