high idle(this one is different)

i have searched and searched for a fix to my somewhat unique high idle prob.
when it first started it was the usuall run of the mill high idle on start up and if i turned it off and waited a minute or two to restart it would fix itself.(yea right)
over the past few months it has become gradually worse.
SCENARIO: driving along road ,engine at operating temp. a/c on (or off) slow to a red light and the damn idle is at 2-3000 and will not come down!it takes all i have to get it to stop BEFORE i hit the person infront of me. i have read all of the search results for high idle and i have yet to come accross one that addresses an issue similar to mine.
thing that confuses me is that prior to slowing at the stop light is that the idle was FINE!

i have a new tps/adjusted to 83% of source voltage,clean iac motor,new cps,new o2 sensor,clean throttle body,have attached another ground to pin"B"of the tps flat connector(ECM pin "D"3)

any one here care to give this a shot?

Staff Sergeant Diaz
United States Marine Corps 1995-present

Sometimes a vacuum leak can cause high idle. Does the idle go up when you apply the brake? If so, it's a bad booster.

I am really NOT trying to insult you or anything by saying this, but sometimes the easy fixes are the hardest to find.

Mine was doing this one day... a little WD-40 on the throttle pulley where the cable comes in and it turns fixed it. Just a thought....

Andrew

Andrew T said:

I am really NOT trying to insult you or anything by saying this, but sometimes the easy fixes are the hardest to find.

Mine was doing this one day... a little WD-40 on the throttle pulley where the cable comes in and it turns fixed it. Just a thought....

Andrew

Click to expand...

ANYTHING is worth a shot at this point
ECHO
SIX
DELTA
OUT!

Frsno,

I had my XJ into the dealer to have the throttle body cleaned and the tech hooked up the test set and made sure everything was working fine. After her confirmed that he decided to fix my idle speed. There is an adjustment for the throttle body but dont' think you need to do that. It might be worth an hour of labor to have them check it out, you could be suffering from a dirty throttle body.

I don't know how long you have had this XJ but have you replace the Crankcase Vent (CCV) system components which show as 50,000 mile replacements? This solved alot of vacuum leaks on my rig. It the big and little tube connected to the valve cover. The large tube also has alot of small vacuum lines attached. You get the parts at the dealer, it will take the better part of a $100 bill to buy all 3 components.

Martin

When this happens pull the plug off of the IAC motor. If when you pull it off it doenst do anything then it is bad. Replace it. When I umplug mine the idle stays at 3000rpm's.
Just my 2 cents
HTH

I've had the same problem for a couple of years on my '88. I've noticed that if I shut it down (usually at a red light) and cycle the ignition 4 times w/o starting; the idle will return to normal after a brief run up. I know for sure it's not a vacuum leak or any of the other usual stuff.... I think it's electrical or something with the ECU. I've removed my 101 connector, so it's not that either. I'm willing to try just about anything to get rid of this. Please let stay in touch with me if you figure it out... I'll do the same. Thanks.

have you tried shifting into neutral when stopping (?) or even just cruising - with your foot off of the gas to see if your idle will be more normal?

Also try shifting to park (once stopped) and take your foot off of the brakes (power brake booster diaphragm or check-valve?)

Can you repeat this condition in a location where you can open the hood and work the 'standard' mechanical checks (throttle & tranny linkage not binding, checkin IF the idle air is entering via the ICS port, etc...)?

BenXJCA said:

When this happens pull the plug off of the IAC motor. If when you pull it off it doenst do anything then it is bad. Replace it. When I umplug mine the idle stays at 3000rpm's.
Just my 2 cents
HTH

Click to expand...

if i unplug it, how would it be able to change idle with no power?
am i missing something? i think im more confused now then when i started! hahahha



moconnor I've had the same problem for a couple of years on my '88. I've noticed that if I shut it down (usually at a red light) and cycle the ignition 4 times w/o starting; the idle will return to normal after a brief run up. I know for sure it's not a vacuum leak or any of the other usual stuff.... I think it's electrical or something with the ECU. I've removed my 101 connector, so it's not that either. I'm willing to try just about anything to get rid of this. Please let stay in touch with me if you figure it out... I'll do the same. Thanks


Will do




SATAN: i have tried shifting to neutral(as this is how i can stop at engine idle being stuck at 2000) and the idle will continue to rise to about 3000 and never above.
YES i have tried to continue with it in gear to see if it will come back down.... to no avail i end up doing about 40 mph with out touching to gas pedal. Feels kinda weird
I have yet to get it to act up when i am not in traffic, and im not to thrilled about the idea of putting in park and getting out to open the hood with the idle stuck at 2500,however, this looks to be the plan of attack at this point.as i understand it now . i would get out, and unplug the iac connector?



on a side note, this is an 89 laredo 4x4 4.0 auto/ i have attempted to resolv this issue previously and i ended up unpluging the connector to the iac,she has preformed with flawless results.i now have time to give it another shot.




Why in hell can't the Army do it if the Marines can. They are the same kind of men; why can't they be like Marines.
Gen. John J. "Black Jack" Pershing, US Army; 12 February 1918

On unplugging the Idle stepper motor -- some folks have been unplugging the motor when the vehicle is at an acceptible idle -- since the motor (a stepper motor operating a jack-screw air pintle valve) can no longer operate the idle kinda stays near where it was (if you allow for engine loading, temperature variations, etc) -- starting may be a little more difficult with less air passing into the intake (IF the ICS is the problem). Once this you can determine is that if the ICS is NOT the problem and you unplug it to "lock-in" a particular idle, and if you idle-problem reoccurs, the the ICS is NOT the issue and you can prolly reconnect it .. (NOTE: OK for the renix folks , but OBDI and OBDII will show errors - OBDII may show a MIL)

You mentioned trying all the "usual stuff" and all -- but is the Idle Control Slenoid in good working order and clean (they do wear out -- more frequently on automatics since the pintle operates more regularly in a limited area of the jack-screw) - they do tend to get "sticky" like you're describing. Then act-up by not moving as anticipated.

satan: i have had the iac motor unplugged for about 2 years,and i have NOT had a single high idle issue.
my next purchase will be a iac motor, however i have read numerous posts where the same things have been done and the high idle remains.

does anyone foresee a problem if i installed a rocker switch inside the cab that would enable me to manually control the iac motor?
this would involve a bit of wiring, but i really need to have this fixed.

the switch may be the ticket,as if i unplug the iac connector and i use the a/c my engine bogs down when at a stop

how would the iac be moving at cruise? and why as high as 2000rpms sometimes 3000 ?

ok
i got it to act up where i could open the hood and fool around.2000 rpms in park/1500 in drive

i unpluged the 3 wire connector to the tps. and the idle immediatly came down to normal.

can anyone confirm i have a bad tps BEFORE i go spend the 90+$ ?
this one is only about 6 months old

Yeah - there are a few controllers out there or it's easy enough to make your own (Renix Years) -- the IAC is a stepper motor and all you need is a driver and something so you won't mess-up the ECU (mine switches in dummy loads and isolates the ECU's output, but I'm sure that there's really no reason for the loads...

The IAC stuff usualy isn't active at cruise since the TPS is not providing "foot off gas" kinda data (the cruise cable is operating the throttle linkage).

The difference in the idle is a function of where your IAC is sticking and the engine's state -- the airflow through a wide-open IAC on a Renix TB can support about 3.25K no-load rpm in a warm 4.0 in a reasonable state of tune. With additional load (cold oil, auto-tranny in gear, alternator, A/C, etc) the nuber will be somewhat less.


Seems like a lot of hassle for a $55 part (Wells ac301, GP Sorensen 77910012)

frsno jeeper said:

satan: i have had the iac motor unplugged for about 2 years,and i have NOT had a single high idle issue.
my next purchase will be a iac motor, however i have read numerous posts where the same things have been done and the high idle remains.

does anyone foresee a problem if i installed a rocker switch inside the cab that would enable me to manually control the iac motor?
this would involve a bit of wiring, but i really need to have this fixed.

the switch may be the ticket,as if i unplug the iac connector and i use the a/c my engine bogs down when at a stop

how would the iac be moving at cruise? and why as high as 2000rpms sometimes 3000 ?

Click to expand...

frsno jeeper said:

ok
i got it to act up where i could open the hood and fool around.2000 rpms in park/1500 in drive

i unpluged the 3 wire connector to the tps. and the idle immediatly came down to normal.

can anyone confirm i have a bad tps BEFORE i go spend the 90+$ ?
this one is only about 6 months old

Click to expand...

Can't configrm the TPS is bad without first looking at the output voltages -- could be something as easy as an adjustment or as bizzarre as a wiring harness or even ECU voltage regulator issue.

Since the ECU reads it's data (to drive the IAC) from the TPS's flat connector, it's important to look at the voltage available at that that output to see if it's smooth and relatively linear.

Backprobe pin C (key in run) and measure it's output (compared to ground) as you slowly move the throttle plate -- you should see it start at some value less than 0.8v (at idle) and go up SMOOTHLY- NO JUMPS to something more than 4 (at WOT) -- Pin "B" should be at ground potential and PIN "A" should have a steady 5VDC

Does anyone think the EGR valve might be the cause of the intermittant high idle? Do they wear out, stick, carbon build up causing a vacuum leak? PS. I don't know anything about the EGR system but I've checked everything else and I'm running out of ideas.

I dont think it could be the EGR valve. when the EGR valve sticks open you get the oppiset problem of a ruff low idle.

Cam 90XJ

With the danger of being called "stupid", I had the exact same problem with my 1984 2.5lt XJ.

I know it is a carb engine, but the problem might very well apply to the TBI one.

The carb's axle had developed a significant play in the carb's body, thus sticking open or leaving air leak below the butterfly (or whatever you call it). That was creating a high idle situation, as you were rolling to a stop light, the RPMs remained at whatever your engine speed was before letting up on the right pedal.

Maybe this is what's causing your problem as well?

Just a though.