robsjeep

Got a 1990 XJ 6 cylinder Renix. Intermittantly upon start up she will idle at 2000 to 2200 rpm. I turn off the key, stop motor, restart, and most times it will again fast idle, sometimes it will go to normal idle speed, then I can drive away. . I may have to try the start, off, start.... cycle several times until finally it idles correctly, usually less than half a dozen restarts in a row are required before idle is normal.

When does this happen? First start of the day,, cold motor, or after a brief shut down after a hours long highway trip, at full operating temperature... I.E.... it is not a temperature driven issue.
How often does this happen? On a recent road trip, it did it about maybe 10 percent of the start ups, Id get the fast idle on first attempt, then it may take one to maybe up to six restart attempts in a row before idle is normal, then it may not happen again for two starts, or 20 starts, may happen again that same day, or not again for a week.., seemingly random.

Engine runs very nicely otherwise, engine system well maintained otherwise. Just this seemingly random high idle that persesists for several restarts, then it is fine again. All my manifold vacuum hoses are in good condition, Engine has good manifold vacuum at normal idle speed.

I have swapped out three different used idle speed air valves. Problem persists. I am stumped!

Any suggestion on a direction in my hunt for the solution?

Many Many Thanks in Advance!

cruiser54

Generally a TPS failure. You've refreshed the sensor grounds, right?

robsjeep

The throotle switch has been adjusted, using volt meter as per factory book. problem did not change. my snap on scanner says that switch is correct too. still stumped.

Saudade

If your tps is good, then try tip 11 on Cruiser's site. Cleaning the TB and IAC.

robsjeep

Done that already, problem persists.

cruiser54

done Tip 5, right?

robsjeep

yes

cruiser54

Dipstick ground?

robsjeep

Yes.

robsjeep

How is the idle valve controlled? what sensor inputs are used to determine the idle valve set point?

cruiser54

ECU controlled. I'd have to dig out the REnix manual to answer the second question.

Ohms reading between battery negative terminal and dipstick ground?

Think your IAC is hanging up?

Saudade

The ECU controls the it. The two regulate the amount of air going through the bypass passage to control the idle speed. When the engine is cold, the ECU raises the idle to allow faster warm up. As the engine warms up, it lowers the idle.

Also your AC has an impact. Turn on AC and the ECU will raise the idle to accommodate the extra load.

There's also the Power Latch Relay (PLR). When you turn your key off, the PLR remains energized for a few seconds so the ECU can reposition the IAC for the next startup. Perhaps this might be a suspect?

robsjeep

thanks the power latch, that is interesting, but keep in mind, upon shut down normally the idle is right, and often it is at operating temperature meaning the idle valve is set at a low value upon shut down, So I am think that the power latch is not the issue, as that would imply that immediatly prior to shut down that the idle valve was left in a high speed condition. But that is not the case, upon shut down idle has been correct, (except times that I have to cycle the engine several times to "fix" the 2000 rpm idle upon start up.

this high idle is much more than is used by the air conditioner. AC is off, but even when on, 2000 rpm is much to much.

recall this high idle happens randomly upon start, be the engine stone cold, or at operating temp. it will remain at high idle until I turn it off, it may take several start stop cycles of the motor before the idle is normal. This only has happened on start up, never during driving. when jeep comes to a stop after driving the idle is normal, this 2000 rpm high idle only happens once and a while upon start, never after the jeep has started normally and has been running. only on a start does this happen. never once upon coming to an idle after driving.

Dan Tracz

Hello I have the same issue exactly with my 90 jeep Cherokee 4.0 renix , I have replaced the tps,iac, cleaned the TB. It's getting worse, happens 25% of startups. Did you figure it out? Thanks Dan

cruiser54

Have you visited www.cruiser54.com and completed Tips 1,3,4, and 5 yet?