LT1 Erratic Idle
Been chasing this issue for more than a year now, hopefully someone has some insight ! I will try to provide as much info as possible, mods in sig below.
While driving / car moving, my rpms in neutral will sometimes climb or stick to (after pulling out of a gear) 2000 rpms with no throttle input. Putting it back in car the car will maintain speed around 2k rpm in 1st to 4th (instead of DFCO). When I come to a COMPLETE stop, the rpms will drop back to normal idle (750rpm). I also had an issue where the idle would hunt on cold start, but it hasn't don't that in a while. It was acting this way last night, but I didn't have anything to scan/ log with.. Potentially related issues include: long starts after car is left alone for more than 10 minutes, feathering the throttle seems to help a lot; surging/bucking at low rpms, and sometimes requires fair amount of throttle to start in first or it dies / bucks like crazy with not enough throttle (makes starting from a stop unpredictable). I think I've ruled out vacuum leaks since it's intermittent, happens hot or cold, and spraying carb cleaner around didn't make a difference. Parts I've replaced since my engine rebuild: CTS ICM Coil Plugs Wires Fuel pressure regulator I also pulled the IAC out and sprayed it with MAF cleaner and WD40, didn't look any cleaner.. this part is currently at the top of my list cause it could affect all of the above. However, the logs I have seem to suggest the IAC is working. Any ideas?? |
I had similar problems on my S4 and it ended up being the IAC. For the price it cost just throw one in before you start going really crazy.
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No SES or stored codes. Briefly engaging a higher gear will lower the rpm (obviously) but pushing clutch back in and the rpms climb back up. Real annoying getting into the driveway like that :-? |
Check the port where the IAC sits as well. Carbon can build up and cause the pintle of the IAC to not seat properly. IACs can be funny critters.
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Stock throttle body?
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is your car tuned? can always try an IAC first.
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Vacuum leaks can be intermittent. A gash in a hose might just need the right twist of the motor in order to spring a small leak. Did you do a thorough check of all the vacuum lines? Wiggle and spray lol
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I should also note on one occasion coasting on the highway at 60mph in neutral same thing happened BUT the rpms bounced between 2000-2300 which is odd. |
i think our car had brake booster issues? i cant remember. maybe something with that nipple on the booster. doesn't hurt to check ;)
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What are the iac counts hot idle? Have you tried a relearn yet?
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Yeah, stick a new one in and do the idle relearn.
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New IAC seems to have helped with erratic idle on cold start, hanging rpms, and accelerating from a stop.
Still has extended crank hot starts and bucking in the lower rpms. I installed an AEM wideband a few weeks ago and noticed when it's having trouble with hot starting the guage reads lean to full lean, is this tune related? Otherwise it reads around 14-15 afr normal driving and around 12 WOT... |
Check fuel pressure when its showing full lean?
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+1 to FP. does it crank normal speed too?
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Engine cranks normal (LT4 starter). |
You would have to isolate different sections of the fuel system after pressurizing to find the drop off, if you want to pursue it. But for now, just check it when the issue occurs to see if its a potential cause.
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Took a quick video of warm start, did not feather gas. Fuel pressure dropped from 44 to 10psi, but looks fine, wideband still reads 16+ for a few minutes. Pump is an airtex from 2013 (stranded away at school, had no choice and worked fine). |
Fuel pressure dropped to 10psi and wideband maxed out lean? No, don't think its vapor lock.
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https://youtu.be/4E8i55YenH4 |
Where does it drop in the video? I'm an idiot haha. Definitely sounds like your issue though. You'll have to rig up a shut off so you can prime the system, close it off, and see where the pressure bleeds off from.
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Does anyone make a fuel filter with a check valve built in? It seems like it has to be either check valve in the pump or maybe a leak in the in-tank lines/connection.. |
Ohhhhh. Yeah its either pump, in tank line, injectors, or regulator. I don't know if anything exists but I would say buy/make a testing setup with a shut off so you can find the actual problem instead of doing a band aid. Something like a quick connect to go on the feed line with a shut off and gauge ports on either side of it. Should help isolate where the issue is and be somewhat inexpensive. Someone will probably have a better idea or an amazon link shortly :lol:
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usually its the one-way valve in the pumps. vapor lock is virtually impossible in fuel injection systems due to the pressure the fuel is under.
What if you prime it 4 times when its warm, does it start normal? |
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