LT1 Erratic Idle

[FlyingDutchman]

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??

[LS1ow]

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.

[FlyingDutchman]

Originally Posted by LS1ow

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.

Considering I'm still throwing money at an LT1, most would call me crazy lol I'll probably go ahead and replace it and rule it out.

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

[unstable bob gable]

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.

[sweetbmxrider]

Stock throttle body?

[ThoR294]

is your car tuned? can always try an IAC first.

[FlyingDutchman]

Originally Posted by sweetbmxrider

Stock throttle body?

Yes

Originally Posted by ThoR294

is your car tuned? can always try an IAC first.

Yeah, tuned via email with street log. IAC is on order, I'll try that and clean the TB realllll good.

[ThoR294]

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

[FlyingDutchman]

Originally Posted by ThoR294

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

True.. but then if it's stuck and I'm in neutral and do nothing but brake smoothly to a stop, why would it come all the way back down to a normal idle?

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.

[ThoR294]

i think our car had brake booster issues? i cant remember. maybe something with that nipple on the booster. doesn't hurt to check

[sweetbmxrider]

What are the iac counts hot idle? Have you tried a relearn yet?

[FlyingDutchman]

Originally Posted by sweetbmxrider

What are the iac counts hot idle? Have you tried a relearn yet?

15-20 counts based on most recent log. Max I see is 130. I see its very active at all times while, so maybe it's just not moving quick enough due to age (20 years + 125k miles)?

[sweetbmxrider]

Yeah, stick a new one in and do the idle relearn.

[FlyingDutchman]

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...

[sweetbmxrider]

Check fuel pressure when its showing full lean?

[ThoR294]

+1 to FP. does it crank normal speed too?

[FlyingDutchman]

Originally Posted by sweetbmxrider

Check fuel pressure when its showing full lean?

Haven't done that yet, will try tmr. No vacuum ref it's 44psi, w/ vacuum 38psi. Pump seems to work fine. New AC DELCO regulator last spring. Rebuilt ford redtops. I did notice the pressure drops, like 20psi in 15 minutes. Most of the pressure is gone within an hour.

Engine cranks normal (LT4 starter).

[sweetbmxrider]

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.

[FlyingDutchman]

Originally Posted by sweetbmxrider

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.

Primed it like 4 times, same result, pretty much 0psi in about 20 minutes. Fires right up on cold start, cranks for a bit on warm start, maybe vapor lock in the rails from heatsoak and low pressure?

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).

[sweetbmxrider]

Fuel pressure dropped to 10psi and wideband maxed out lean? No, don't think its vapor lock.

[FlyingDutchman]

Originally Posted by sweetbmxrider

Fuel pressure dropped to 10psi and wideband maxed out lean? No, don't think its vapor lock.

Meant after warming it up the pressure was at 44 and dropped down to 10 while I waited to restart, see video

https://youtu.be/4E8i55YenH4

[sweetbmxrider]

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.

[FlyingDutchman]

Originally Posted by sweetbmxrider

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.

Lol just prior to the video I ran it to operating temp, turned it off and the pressure bled off from 44 to 10 as I waited about 15 minutes to restart, fuel rails went from cold to same temp as intake during that time.

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..

[sweetbmxrider]

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

[ThoR294]

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?