97 5-speed XJ Fast Idle when warm

[joe_peters]

The idle should be completely dependent on the IAC. The "base idle screw adjustment" isn't really there for that purpose, it is just commonly referred to that way. A better description would be "keep the steel throttle plate from slamming closed in the alloy throttle body and wearing bad spots in it" screw adjustment.

What happens is somebody thinks the idle is off, and the first thing they do is crank the "base idle screw" in. And then all those who follow aren't aware that the IAC now has to work not only to adjust as real time conditions require, but it is also starting off from a failed premise that the data the PCM/ECU is receiving is correct.

Your idle should be 700-750 rpms, in drive, with the engine and transmission fully warmed up--after about 20 minutes of operation. There is no specification for P or N.

It is the job of the IAC to provide the correct idle.

 

[1987Comanche]

Adjusting the idle set screw had an effect on the idle speed...finally something did! I backed it way off at first and she struggled to stay running. I finally found a happy medium around 750-900 RPM (it's not too stable and hunts...at least the IAC works). I haven't driven it much so it will take a few days to see what happens. I think the PO adjusted the set screw because the IAC plugged up and stopped working. I also found a 160 deg thermostat in place of the stock 195 thermostat. I corrected that so we'll see what happens next.

 

[joe_peters]

Adjusting the idle set screw had an effect on the idle speed...finally something did! I backed it way off at first and she struggled to stay running. I finally found a happy medium around 750-900 RPM (it's not too stable and hunts...at least the IAC works). I haven't driven it much so it will take a few days to see what happens. I think the PO adjusted the set screw because the IAC plugged up and stopped working. I also found a 160 deg thermostat in place of the stock 195 thermostat. I corrected that so we'll see what happens next.

Sounds like progress has been made!

If the idle settles down (speed-wise) but still tends to "hunt", then try adjusting the TPS again (probably needs a little tweak since adjusting the "base idle" screw anyway.

 

[1987Comanche]

The idle has settled in and is pretty steady but I've now discovered a new problem. If I do a cold start and leave the truck alone she'll idle fine for just over a minute and then stall. Also if I cold start and begin driving she'll try to stall at the first stop sign (0.5mi from the house). I'll feather the throttle at the 1st stop sign and, by the time I get to the second stop sign (another 0.5mi) she's fine. I did readjust the TPS recently and there was no difference.
I'm wondering if this has anything to do with oil being ingested through the intake manifold. I upgraded to a later model valve cover ('96 with the baffle plate) and this cut the oil in the air cleaner by 60%. I still have a fairly decent amount coming through the breather tube.

 

[hawk_sixsix]

Removed the IAC and WOW! there was a lot of carbon caked onto it. Spent 20 minutes shaving it off with my knife. Polished it clean with TB cleaner and used cleaner-soaked Q-tips inside the IAC housing and the TB. I had a ridge of carbon around the butterfly.

Held off on replacing the TPS. Didn't want to violate the repair Commandment of Thou Shalt Not Shotgun and lose my base. :lecture:

Idles smooth now, no warm fast-idle, and a happy side effect is noticably better running in all gears.

Mission accomplished. :smoker:

Then again, maybe not. Months later, it's back: idle soaring to 1500 or 2000 RPMs. I had a code 25 come up; I replaced the IAC. Old IAC that I had cleaned in the above quoted post was still as clean as the day I put it in.... so it's not a carbon buildup problem. New IAC didn't solve the problem. In fact, it might be getting worse (higher and higher "peak" idle each day). A quick stab of the gas pedal brings it back down - reminds me of the old carburetor kickdown.

TB looks clean. IAC is new. TPS is new (last month). Code 25 CEL went away. Idle still fast. :smsoap:

 

[joe_peters]

Try cleaning and lubricating the throttle mechanism. Check for intake leaks around the throttle plate shaft.

 

[hawk_sixsix]

Try cleaning and lubricating the throttle mechanism. Check for intake leaks around the throttle plate shaft.

What's that? Send it in as a core exchange for a 62mm throttlebody? :laugh:

 

[joe_peters]

What's that? Send it in as a core exchange for a 62mm throttlebody? :laugh:

Well, that would be a good plan too!

Somebody posted up recently they installed a new bored TB and had a high idle--air was being sucked in around the throttle plate shaft and the shaft was hanging up and not closing fully. Little grease never hurts.

 

[hawk_sixsix]

Removed original TB. The two bolts nearest to the head let out a hiss and tiny bubbles when I broke them loose. (No, it was not my opening another beer). Except for sun beating down on it, engine was otherwise cold.

Original throttlebody showed no obvious evidence of damage. Installed new 62mm Fastman TB. Had to gasket-match: a 62mm TB, 62mm intake opening, and 62.000mm gakset will NOT line up perfectly and without a ridge. Trimmed about 1 mm all around, used spray copper to keep it in place (it's all I had) and abracadabra - no more fast idle. Runs great.

I'm sure there was a cheaper way to fix it, but that's how I roll. :roflmao: