Idle speed surges when engine is cold
Thread Starter
Team Owner
Joined: Sep 2003
Posts: 20,179
Likes: 673
From: Freedom is never more than one generation away from extinction.
St. Jude '03 thru '24
Idle speed surges when engine is cold
Seems lately the idle on my L98 will surge and hunt when the engine is cold. The cold RPM used to hold at 1000-1100 RPM and now its running 7-800 RPM. Once warmed, up the symptom disappears. The speed variation is minor since the digi-dash tach does not register the speed variations.
Anyone have any suggestions as to where a good place to start looking? I'm guessing a vacumm leak at this point. But where?
Anyone have any suggestions as to where a good place to start looking? I'm guessing a vacumm leak at this point. But where?
Team Owner
Joined: Dec 1999
Posts: 34,873
Likes: 487
From: Lacey WA RVN 68-69
NCM Sinkhole Donor
My 87 does the same thing except there is a good 300-400 RPM swing between 700 and 1100 RPM at startup and until after about a minute of idle (no driving). Once the engine gets some temp built up, everything is fine and the idle is stable at 800 RPM.The car has a older ZZ3 cam but I don't think this cam profile by itself would be enough to cause this when cold.
Possible dirt/crud in the TB butterfly?
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
The IAC is self regulating. it has a proper position for installation, but it never "goes out of adjustment" to where there is anything to be gained by "resetting" it. You might take a look to ensure that the IAC passages and valve are clean and carbon free.
RACE ON!!!
RACE ON!!!
Thread Starter
Team Owner
Joined: Sep 2003
Posts: 20,179
Likes: 673
From: Freedom is never more than one generation away from extinction.
St. Jude '03 thru '24
Anyone know what the preferred IAC Position is?
I'm reading this number from the DataMaster viewer.
Before setting the idle/IAC this value was 64-68.
After the reset, this value is at 23-27.
I did have to adjust the TB position up about 200 RPM to maintain the 450RPM idle with the IAC disconnected.
I'm reading this number from the DataMaster viewer.
Before setting the idle/IAC this value was 64-68.
After the reset, this value is at 23-27.
I did have to adjust the TB position up about 200 RPM to maintain the 450RPM idle with the IAC disconnected.
Racer
Joined: Jul 2001
Posts: 359
Likes: 0
From: Russellville TN
St. Jude Donor '06
I've always been told 25 is a good number to stay around on a stock L98. Pull the TB off and give it a good cleaning. Reset your cold idle speed and IAC and I bet everything will be G to G.
Intermediate
Joined: Feb 2003
Posts: 44
Likes: 0
From: Mesa AZ
Same issue but when the engine is warmed up
Guy's,
I have the same issue but when the engine is hot or warmed up. The shop I have it in is going crazy trying to resolve this and at present has replaced the EGR, Plugs and wires, and last of all the cap and rotor. They are in the process of reburning the prom but ran into to an issue after burning the prom. They could not access it after reburn and are going to try another program tomorrow morning. It has been observed that all sensors are reading ok, there are no vacume leaks and the IAC is good but when the surge happens the accelorator peddls is depressing by itself. Right now it appears to be a computer issue but we won't know until tomorrow when we reburn or put another computer in. Any inputs from you guy's would be greatly appreciated
I have the same issue but when the engine is hot or warmed up. The shop I have it in is going crazy trying to resolve this and at present has replaced the EGR, Plugs and wires, and last of all the cap and rotor. They are in the process of reburning the prom but ran into to an issue after burning the prom. They could not access it after reburn and are going to try another program tomorrow morning. It has been observed that all sensors are reading ok, there are no vacume leaks and the IAC is good but when the surge happens the accelorator peddls is depressing by itself. Right now it appears to be a computer issue but we won't know until tomorrow when we reburn or put another computer in. Any inputs from you guy's would be greatly appreciated
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