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Old 06-13-2010, 09:08 AM   #1
wesleyn
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Default Can a bad IAC cause a misfire while driving and at idle?

My 02 GT auto is still missing around 1500 RPM, and has a slight miss every few seconds at idle. I cleaned the IAC, MAF, and intake, replaced the plugs with OE Motorcraft platinums, and checked for vacuum leaks/bad hoses. I also bought a new COP and swapped them out one at a time, making no difference. I also left the battery disconnected for 2 hours and cleared the KAM. I still have the miss under load, between 1100-1600 rpm, and a intermittant miss when idling. I am hoping someone else has experienced this and can offer assistance. I am to the point with this that I am starting to consider getting rid of the car. I have searched, and I am not finding a definite answer. Thanks in advance!
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Old 06-13-2010, 11:06 AM   #2
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At idle a problematic IAC could possibly cause a miss, however it is quite unlikely--at speed is would be nearly impossible for the IAC to be the culprit.

How many miles on the car? If the stumble happens under light loads, and lower rpms, then I would suspect the EGR system; particularly the EGR valve on the upper plenum.

The EGR valve can get sticky and sluggish with age and not respond quickly enough to open/close commands from the PCM which can cause the stumble at light loads and idle.
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Old 06-13-2010, 11:31 AM   #3
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Thanks for the quick reply! The car has 55k on it. It is stored in the winter. Is there a way to test the EGR valve to see if it is bad? You are talking about the round EGR with the vacuum line on top that has the tube run to it from the exhaust right?
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Old 06-13-2010, 11:55 AM   #4
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Yup, that's the guy...

You need a vacuum source to test it, one of those hand-held vacuum pumps will work nicely.

There is no need to remove it for testing, just disconnect the vacuum line and connect the pump, then pull a vacuum of 20-25inHg and hold it for a while--the vacuum should not drop at all. If it does then the diaphragm is ruptured and you'll need a new valve, there is no repair for a bad diaphragm.

Next, release the vacuum quickly. The valve should close rapidly in one motion (this can be clearly heard). When the valve is stuffed/coked up with exhaust crap it will close somewhat slowly (½ to ¾ second) and unevenly--again you can gauge this by listening to the valve. If this is the case you need a new valve, unless you want to try to clean it by scraping out the crud through the inlet and outlet ports.
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SOLD! - 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum,
Delta Force tuned, Steeda UDPs, Ralco flywheel,
RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.

New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq


Engineering is, at its base, making what you want from what you have.
At its extreme it is making what you want from what you can get.

Last edited by cliffyk; 06-13-2010 at 11:58 AM.
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Old 06-13-2010, 12:00 PM   #5
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Awesome. I will get a vacuum pump and test it out. I will post the results shortly. Thanks again!
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Old 06-13-2010, 07:59 PM   #6
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Okay, I tested the EGR and it did not leak at 25inHG. It also returns quickly and smoothly when I release the vacuum so I think it is good. I also went over the whole engine checking for vacuum leaks, and found nothing. I am beginning to think this car wants traded in. :-)
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Old 06-14-2010, 03:57 AM   #7
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Does the miss happen under light, medium, or heavy loads? Are there any intake mods, or is the car basically stock?
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SOLD! - 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum,
Delta Force tuned, Steeda UDPs, Ralco flywheel,
RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.

New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq


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Old 06-14-2010, 09:58 AM   #8
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The only mods that I have woud be a catback and X pipe, and a BBK CAI. I pulled the plugs again last night and noticed that the porcelain insulator had a small chip out of it on cylinder #6. These plugs are only a week old and this one was not like that when I gapped it and put it in. No damage otherwise, but I am rather concerned by it none the less. I replaced that plug again, but still the same missing. It misses at idle and under medium/heavy load (like 35-40 mph) cruising down the road. It is worse with the A/C on. If I hold my foot on the gas and keep the RPM's right around 1400-1500 rpm in neutral, I can hear and feel it missing as well. Is there a possibility the PCV is causing this? It moves when you shake it, but thats all I know as far as that. Thanks again for all the help!
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Old 06-15-2010, 02:24 PM   #9
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So my check engine light came back on after I replaced the bad plug and gave everything another once-over. I have P0205 Injector Circuit open cylinder 5 and P0206 Injector Circuit open cylinder 6. When I listen to thees injectors with a hose, they sound the same as the others with the car running. I am stumped........
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Old 06-15-2010, 07:30 PM   #10
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Quote:
Originally Posted by wesleyn View Post
So my check engine light came back on after I replaced the bad plug and gave everything another once-over. I have P0205 Injector Circuit open cylinder 5 and P0206 Injector Circuit open cylinder 6. When I listen to thees injectors with a hose, they sound the same as the others with the car running. I am stumped........
The shop manual says this about P0205:

Quote:
Faulty fuel injector driver within the PCM.
Can you read the freeze frame data with your code scanner? That will tell you quite a bit about the load, rpm, etc, at which the codes were thrown.

I suspect when you were listening to them with the hose the car was stopped and the engine was idling--unless you have more arms than an average human, two of which are rather long.

It is possible that those two injectors are not behaving properly at speed.

I am not a big believer in miniature mechanics that come in bottles, nonetheless I do run Techron injector cleaner through one or twice a year--I don't know if actually fixes anything because I've never had fuel injector problems, but it might be worth a try to run a bottle or two through...
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SOLD! - 2003 GT, UPR X, FRPP 24lb/h, Magnaflow, PP 70mm TB & plenum,
Delta Force tuned, Steeda UDPs, Ralco flywheel,
RAM HDX clutch, 3.73s, 262 rwHP/305 lb-ft.

New ride (7/1/2013) 1998 Mercedes SL500-5.0L 32V VVT 326/347 HP/tq


Engineering is, at its base, making what you want from what you have.
At its extreme it is making what you want from what you can get.
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Old 06-15-2010, 07:30 PM
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