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Car started doing this recently. Only with AC on, car will randomly attempt to 'idle' the engine about 2500 rpm. Car is very loud (built ls2 427, headers, BB PRT etc) so it's kinda embarrassing to sit at a light with the car randomly going ROAAARRRRRRR!!! haha
I did the clutch recently so plenty of opportunities to pinch or knock something.
More info:
- no codes, zero, none
- Will do the high idle thing stopped or rolling, clutch in or out, no difference
- engine/tune has been the same for years, no mechanical or time change other than new clutch is a bit lighter
- does it only with AC on
yea i was going to type more, but accidently touched send. It could be a dirty throttle body too. Since the ac loads the engine idle has to be increased. The fuel trims need to be looked at to help with diagnosis, though. If t is a vacuum leak, it will be running lean
Okay, I had discounted that, as I haven't touched anything really that could contribute to a vacuum leak, and it happens only with AC on. but worth looking into, that would be the simplest and easiest fix! Thanks! Will keep the thread updated!
I I do not have tuning software which I assume is necessary to look at fuel trims, however I do have wideband 02 meter, and in the high idle situation it is lean, around 16 to 1.
Throttle throttle body is very clean, I inspected that recently.
Sprayed around carb cleaner while idling and could not find anything ...
But MAF sensor is wiggly and wonky, could be that. Car has a Fast92 so mani has to come off to fix. Looking at relocating it.
Drove the car yesterday and it behaved, so that's good
Sorry I didn't reply sooner, I have cancer, and some days it is hard to go to the garage and open the FSM. Without being able to look at fuel trims, MAF and MAP, it will be hard to troubleshoot what you have. First, using your DIC pull the PCM code and see if you have P0174,P0174 codes. This is a sign of a lean condition, but if you don't have those codes it does not mean you are not running lean. All it means is that the lean condition is below the threshold. Now if the engine idles at a correct RPM with no AC on, but runs consistently higher when you turn on AC, then that means you likely have a source of air being drawn in to the intake when AC is on. The MAF measures air going through it and the PCM adjusts long term fuel trims based on this air flow (unless you have a "tune"). If you have un-metered air coming in from a vacuum leak, then the PCM corrects this with short term fuel trim based on feedback from the front O2 sensors.
If your idle is erratic ONLY when AC is on, then there really can be no other source than the vacuum actuator for the mode, defroster, or recirc. The vacuum connection for these devices connects to the intake right at the MAP sensor which is at the back of the intake. It usually is a small purple plastic tube a little less than 1/8" dia. This purple tube goes to a vacuum accumulator (just a chamber to hold a volume in vacuum). Then there are 2 hoses leaving the accumulator and going inside the cockpit.
Now depending on whether you have the manual or the "automatic" system, the actuator control is slightly different. Since it only happens when AC is turned on, then that seems to tell us that the problem is not the tube from the engine to the accumulator, not the accumulator, not the tubes going to the actuator, but probably on the "down stream side of one of the actuators. You can confirm this by pulling the passenger front wheel, the RF inner fender and finding the accumulator and the little tubes. Then, you can pull the purple tube that routes back to the intake, start the engine, block it off. If the engine idles smooth, then turn on AC with that tube blocked off, and see if idle changes.
If it does not, then you have now determined that the vacuum leak causing high idle is something in the AC vacuum circuit. If it does , then MAYBE you have a dirty Throttle body, that is not responding properly to the PCM command to increase idle when AC comes on. If you have a low-range lack of throttle response, that could be another clue something ain't quite right with the TB. I don't think your MAF has anything to do with this, but without looking at scan tool data, MAF, MAP, fuel trims, etc, all this is just SPECULATION.
Below are the vacuum schematics from the 2002 FSM. Note the first is for the manual control, the second two for the automatic control. I've had 3 C5's andthey all had the automatic style.
you do not need tuning software. 16/1 is 8% lean, and you should be seeing significant loss of performance, or the way your meter is reading is wrong. If that lean condition is true and short term trims are not making corrections, then you are headed for trouble with the engine. The C5 O2’s are not WB,so I’m not sure what/how is being measured. There are some inexpensive OBD2 plugins that will give you decent but limited ability to look at the parameters you need.
Exactly what has been done to the engine in terms of tunes and mods?
I would bet on unmetered air going in, aka vacuum leak. Only reason I asked if it was retuned right before the issue started is perhaps someone messed with the AC torque airflow compensation settings. I just read from OP that the tune hasn't been touched for years, so discard that.
I would bet on unmetered air going in, aka vacuum leak. Only reason I asked if it was retuned right before the issue started is perhaps someone messed with the AC torque airflow compensation settings. I just read from OP that the tune hasn't been touched for years, so discard that.
I’m with you, It has to be unmetered air, AC torque corrections would be metered air, so even with a tune you would still think unmetered air.
Sorry I didn't reply sooner, I have cancer, and some days it is hard to go to the garage and open the FSM. Without being able to look at fuel trims, MAF and MAP, it will be hard to troubleshoot what you have. First, using your DIC pull the PCM code and see if you have P0174,P0174 codes. This is a sign of a lean condition, but if you don't have those codes it does not mean you are not running lean. All it means is that the lean condition is below the threshold. Now if the engine idles at a correct RPM with no AC on, but runs consistently higher when you turn on AC, then that means you likely have a source of air being drawn in to the intake when AC is on. The MAF measures air going through it and the PCM adjusts long term fuel trims based on this air flow (unless you have a "tune"). If you have un-metered air coming in from a vacuum leak, then the PCM corrects this with short term fuel trim based on feedback from the front O2 sensors.
If your idle is erratic ONLY when AC is on, then there really can be no other source than the vacuum actuator for the mode, defroster, or recirc. The vacuum connection for these devices connects to the intake right at the MAP sensor which is at the back of the intake. It usually is a small purple plastic tube a little less than 1/8" dia. This purple tube goes to a vacuum accumulator (just a chamber to hold a volume in vacuum). Then there are 2 hoses leaving the accumulator and going inside the cockpit.
Now depending on whether you have the manual or the "automatic" system, the actuator control is slightly different. Since it only happens when AC is turned on, then that seems to tell us that the problem is not the tube from the engine to the accumulator, not the accumulator, not the tubes going to the actuator, but probably on the "down stream side of one of the actuators. You can confirm this by pulling the passenger front wheel, the RF inner fender and finding the accumulator and the little tubes. Then, you can pull the purple tube that routes back to the intake, start the engine, block it off. If the engine idles smooth, then turn on AC with that tube blocked off, and see if idle changes.
If it does not, then you have now determined that the vacuum leak causing high idle is something in the AC vacuum circuit. If it does , then MAYBE you have a dirty Throttle body, that is not responding properly to the PCM command to increase idle when AC comes on. If you have a low-range lack of throttle response, that could be another clue something ain't quite right with the TB. I don't think your MAF has anything to do with this, but without looking at scan tool data, MAF, MAP, fuel trims, etc, all this is just SPECULATION.
Below are the vacuum schematics from the 2002 FSM. Note the first is for the manual control, the second two for the automatic control. I've had 3 C5's andthey all had the automatic style.
That is a lot of badass information sir and I greatly appreciate it! prayers for your health. I have the FSM and that is how lazy I am - I know people much smarter / more experienced than me are on the forum and can point me in the right direction! Thanks!!
Originally Posted by k24556
you do not need tuning software. 16/1 is 8% lean, and you should be seeing significant loss of performance, or the way your meter is reading is wrong. If that lean condition is true and short term trims are not making corrections, then you are headed for trouble with the engine. The C5 O2’s are not WB,so I’m not sure what/how is being measured. There are some inexpensive OBD2 plugins that will give you decent but limited ability to look at the parameters you need.
Exactly what has been done to the engine in terms of tunes and mods?
There is a dual-channel Wideband 02 meter installed (one for each bank, i am leery of something going wrong with 1 injector and not seeing it). The AFR's are around 13:1 at 'normal' idle, the correct 14.7:1 at cruise, I don't know what at WOT as I've not been there since installing the WB02 and even if I had, I would not be looking at the gauge at WOT though i could log it. I've not been to WOT in some time as I recently had the diff rebuilt and am in the final few heat cycles of breaking it in gently.
The AFR goes to 15-16:1 only when it's doing it's 'high idle' thing.
The engine is a custom LS2 based 427 built by MTI maybe 10yrs ago. The PO had the tune touched up about 3-4yrs ago IIRC.
The only semi-major change recently was the lighter clutch; 35lb total mass vs 55lb total mass IIRC. I understand this can cause idle low-speed drive ability issues though this idle issue seems too random for that.
Erratic high idle, only with AC on
though i could log it. I've not been to WOT in some time as I recently had the diff rebuilt and am in the final few heat cycles of breaking it in gently.