2005 Intake/fuel/idle/misfire problems

I have a 2005 Grand prix with the 3.8L series 3 in it. I had a Missfiring sensation at around 1800-2000rpm that would not show up at higher rpms or at idle. I was told to replace the plugs and wires and clean out the intake...... So i did that and now the car idles high (1000-1200rpm) and throws the p0507 (idle air control system higher than expected) code with a check engine light that just appeard.... also my message ceter says engine performance reduced and I have absolutly no throttle responce when this happens. I have taken it to a shop and they can't find anything on the scanners asfar as a bum sensor and the car still acts as it did before when driving until you put it in park and it revs out. The car also ocasionaly stalls when you come to a stop (how i first noticed this) and to get it to re-start you have to give it some throttle. Someone please help I just bought this car 4 days ago and its being a pain.

**BTW it has 126 000km on it if it matters

Welcome to AF.

What shop did you take it to? Was it a GM dealership? The throttle system is "drive by wire" and a qualified tech should properly diagnose or check it out.

So, prior to you replacing the plugs/wires & throttle/intake cleaning it had no service codes or "Performance Reduced" issues, just the miss?

If this is the case, you need to go over what you've done. Carefully inspect all the wiring and harness at the throttle body. You need to use caution around the throttle actuator control (TAC) and throttle plate when cleaning it. That code is set when the PCM sees the actual idle speed not matching its set idle. Even a simple vacuum leak like a line left off can cause this to happen.

When the PCM sees a problem with the TAC, it enters into this reduced power mode. Levels of pedal input filtering can range from reduced top speed to limiting maximum throttle plate opening to forcing idle only to complete engine shut down.

Good news is that after your fault is corrected and code cleared, the throttle will automatically recalibrate itself. You'll be good to go.

Some cars, like Toyotas require a micky mouse PITA reset procedure.

Yes correct prior it has no fault codes, not an idiot when it comes to automotive and everything is done to manufactures spec.... BTW the car doesn't have vacume lines to leak and the electronic gas pedle thing was untouched.

Maybe I misunderstood when you said you cleaned out the intake. I took that as cleaning the throttle body, so how was it cleaned?

Sprayed that intake cleaner in it, cleaned throttle body yes and cleaned out with a rag, doesn't do much for actual intake manifold.... Also i should mension that it only started the high idle after I held it at the missfire point for a few minutes trying to get a check engine lite so I could scan it.

Ensure there is no air leak at the air intake duct between the MAF and throttle body, also that the hard plactic pipe is well seated in the duct. Even a small intake leak will throw off the initializing of the throttle at start up.

Every time this car's started, the PCM resets the minimum throttle and gas pedal positions. For high idle I would try this next: Clear the code and key off. Restart the engine ensuring you don't touch the gas pedal. If there's no issues then it shoul be good to go.

The misfire-surge, maybe someone else could chime in.

I checked the best I could, the car without touching the gas will start and idle around 900-950 rpm and wont throw anything on but if you rev it (drive it type of thing) it only returns to about 1100rpm which then throws the reduced engine performance thing in my message center a the p0507 code out, you have to reset it in order to get it drive again. I took the MAF off and sprayed it with break cleaner in hopes of cleaning it out, put it back together not hooked to the air filter and it was idleing at 800rpm which i think is acceptable, shut the car off and put it all back together and took for a drive and the same shit happens again with the high idle and the code, still has the missfire in park around 1800-2000rpm. could it be anything to do with the tube that runs from the plenum to the air intake tube after the MAF becuase if I pull it out and plug it so it cant suck air in it will idle at the 800rpm i think it should idle at. Right now its just a piss off.

I have an 05 Grand prix, was experiancing problems with what i thought was a mid RPM missfire and a high idle causeing the reduced engine power message to come on in my message center. With my cheap scanner it would come up with a p0507 code I would clear it and be good to go. I took it to the shop today and with his scanner it threw p0068, p0101, p1626, u1064 and u1301. The car drives fairly good, has rpm fluxuation while driving though and when you put it in park revs at 1100rpm causeing the reduced engine power, tried new battery with same resutls....... VERY FRUSTRATING!

New update, unplugged MAP and runs good with no miss but stil idles high a kinda all over the place between 800 and 1400rpm but the miss is gone and wont give me reduced engine power anymore which is nice.

New update, unplugged MAP and runs good with no miss but stil idles high a kinda all over the place between 800 and 1400rpm but the miss is gone and wont give me reduced engine power anymore which is nice. Im going to swich out the one from my buddies car and see..... its a 2001 though.

I did not know 05's still had MAF's with the drive by wire system.... HAHA!

It may idling all over the place because the maf is unplugged. If it were my car and unplugging the MAF solved the missfire, I'd start with replacing it and see if the other issues clear up. You know its bad, why over diagnose if you don't have to?

Unplugged MAP (mass air pressure) not MAF (mass air flow). If you unplug the MAF it doesn'tn at all lol. It was not a typo.

Sorry, Brain Fart! It has been a long day. (sorry Bob, please no more demerit points...)

krazykyle2,

Your new thread was merged with the previous thread which is based on the same issue(s). Please do not start new post(s) on the same issue(s). All it causes is confusion for members to include Moderators and possible redundant answers.

I was screwing around with it again today and discovered that unplugging the EGR valve gave me the same outcome as the MAP sensor, Still idles high but doesn't go into limp mode and doesn't have the miss to it. Also hooked up a good snap-on scanner and checked voltage to sensors for fluxuation and nothing at all even while it was doing its 2000rpm miss.... also noticed im starting to get a rougher idle and sometimes when accellerating its very choppy. I also tried a map sensor off my buddies car with same gm part number and it worked the same as mine and mine worked in his car so that should tell me that the sensor is good, everything else on his car is differant cause its not a drive by wire setup.

To start disconnecting key engine management components does not prove anything and IMO a poor way to troubleshoot especially if there are no DTCs showing the component like a MAF sensor, MAP sensor or EGR valve is bad.

How is the engine temperature or have you checked the ECT for proper calibration? For the P0507 DTC (high IAC counts/high idle) the PCM references the ECT - Engine Coolant Temperature sensor. RPM > (Desired RPM + value from look up table based on ECT). Otherwise it will go into "Limited Authority Mode" aka reduced power mode.

This issue is starting to sound like a defective throttle (TAC) or even APP (accelerator position) module or possibly PCM module.

A Snap On scan tool cannot effective check operation between the TAC, APP and PCM on GM electronic throttle body systems.


Note: To get a P0507 DTC a prerequisite is no bad MAP, MAF, EGR components to include any DTCs these sensors could give off when bad.

Car needs a thermostat I think, It warms up to just past the 1/4 mark and stays there while driving (gotta remember -10 tempuratures), but if you let it sit and idle for a minute or two it jumps up to the half mark until rad fans kick in. Where is My IAC on this car with drive by wire, cant even find the part listed anywhere telling me its either built into the throttle body or non existant.

I should also say the warmer the car gets the higher the idle gets, idles normal when cold.

I just read on another site that moving the butterfly manually in the drive by wire setup will cause the high idle and only way to fix in ecm re-program..... when I cleaned my throttlebody i definatly moved it and held it open with a screwdriver, this was a nissan forum i read but could it be the same for pontiac?

The TAC which takes the place of the older style idle air controller (IAC) found on '97 - '03 GPs is the black module on the throttle body with the electrical connector. Behind that is the servo motor that opens and closes the throttle plate. The APP which sends signals to the TAC is at the accelerator pedal in the cabin. I have a similar electronic throttle body and drive by wire on my GXP. There is a calibration procedure but a GM Tech 2 scan tool must be used and the values stored in the PCM.

Even at your temps the engine coolant temperature should get to the rating of the thermostat which is 195 degrees stock sooner or later. If not then try the thermostat. The P0507 DTC appears to be the main one to resolve first.

sounds like the throttle positioner is haveing problems,

Anyone else have any ideas on a idle re-learn, there has to be a way without paying gm to do it.

Problem solved: Its always the cheap and easy stuff we bypass.

Took it to GM today and they had there look at it, everything was cause by......... (the suspence) a crack in a $10 plastic and an o-ring part causing an intermitent vacume leak. The peice that holds down the PCV valce and the MAP sensor in place had a crack in it (seen it with my eyes) and the bottom 0-ring was missing, this caused a leak when pressure built up causing my surges and stuff.... Needless to say $10+ a few other tiny thing and paying the guy for figuring it out I was only down $100 so all good.

P.S. Its also important to not that there are I beleive 2 o-rings in with the pcv and map sensor from my understanding, not taking it apart now that its fixed.

Good stuff! Glad you posted back what happened.