88 Xj: IAC Excess Engine Speed @ Start

More than likely not the IAC. We just went through all of this on Cherokee Forum. It was the sensor ground circuit and C101 issues. It can also be the TPS failing on the engine side. Do the sensor ground test I'm adding below. Then you know where to start.



The C101 cleaning is ALWAYS a good idea BTW.

Here's a link:
http://www.cherokeeforum.com/f2/1988-cherokee-idle-issues-138153/

Cruiser’s Renix Sensor Ground Test
Set your meter to measure Ohms. Be sure the key is in the OFF position. Using the positive (red) lead of your ohmmeter, probe the B terminal of the flat 3 wire connector of the TPS . The letters are embossed on the connector itself.
Touch the black lead of your meter to the negative battery post. Wiggle the wiring harness where it runs parallel to the valve cover and also near the MAP sensor mounted on the firewall. If you have an 87 or 88 with the C101 connector mounted on the firewall above the brake booster, wiggle it, too.
You want to see as close to 0 ohms of resistance as possible. And when wiggling the harnesses/connectors the resistance value should stay low. If there is a variance in the values when wiggling the wires, you have a poor crimp/connection in the wiring harness or a poor ground at the engine dipstick tube stud. On 87 and 88 models, you could have a poor connection at the C101 connector as well.
Revised 05/03/2012

RAVC1 said:

cruiser54,

Many thanks for the response. I'll look into this tomorrow.

Rick

Click to expand...

Keep us posted please.

A high idle at startup is also caused by the latch relay. One of its purpose is to give the computer time to put the IAC in the start up position. If it doesn't get set, your startup rpms will soar but then back down to normal.

old_man,

This is precisely how I would describe my starts.

Rick

RAVC1 said:

old_man,

This is precisely how I would describe my starts.

Rick

Click to expand...

So the idle does come back down? It doesn't stick there til you shut it off and restart?

If you take the inlet tube off the top of the TB, you can look down the bypass bore where the IAC sits and observe it's behavior.

When running warmed up, the pintle should be nearly fully extended blocking off the bypass. When you shut down, the B latch engages to retract the pintle in preparation for the next start up. Once you start, the pintle should extend to reduce the idle to normal.

Sometimes the bore is a little coked up causing the pintle to stick. A little TB cleaner usually does the trick. Or remove the IAC and use TB cleaner and a bottle brush to thoroughly clean the bore and seat out.