The RenX Files: High idle problems

letank said:
Thanks Mike, I'll play w it manana.

BTW ground tested according to specs < 1ohm, I have .5 and .3 ohm. Reading low values is always a challenge.

The diagnostic procedure (Chrysler Powertrain diagnostic procedures) that i have does not list any specific voltage, if it is below 1 volt, then test ground resistance from B terminal of TPS to vehicle ground to be less than 5 ohms, then adjust voltage to attain idle speed.

The few other thinks that i gather from web searches:
PCV, EGR, the usual vacuum leak at intake, and brake booster grommet

Michel
90 wranwran 2.5, AX15 193Kmiles
74 wagoneer 340Kmiles
Click to expand...

IIRC the 2.5 L engine has a different idle control scheem than the 4.0! Different parts, no IAC, more like an old style throttle plate external valve?
 
Ecomike said:
IIRC the 2.5 L engine has a different idle control scheem than the 4.0! Different parts, no IAC, more like an old style throttle plate external valve?
Click to expand...

Correct, it is called Idle Speed Actuator (ISA), there is no side port.

I found another link which states that the TPS voltage should be 0.2 for M/T, and throttle plate closed, BUT (yes big but) 4.6 to 4.7 volts for the M/T with the throtttle plate should wide open WOT. I have 0.23volts, did not measue the WOT value. Too much to do at this time. I am back north, jeep is south.

I am probably chasing something.... the idle speed will gradually come down. I need to digest what i found. Apparently the PS circuit has a pressure sensor that will kick the idle up while the PS pressure is high. At least i do not have A/C which has also a feedback. The ISA is also a partner responding for all the feedback.

At least i found out that the thermostat was busted.... very weak spring, the temp never went over 155F (use an infrared thermometer). I put a 180F,which allow me to calibrate the temp gauge, outside temp is in the 90ish. Let the kid drive it around town for a while, and will put a 195 later.

Michel
90 YJ 2.5l M/T 193Kmiles
74 wagoneer 340Kmiles
 
slowjeep said:
my 90 4.0 is hard at startup when cold and extremely hard when warm, if I manage to start it when warm as soon as I go to gear rpms fall to 0!!!!, what 's going on?!?!?!?!?!
Click to expand...

Stick, auto trany? How many miles.

Could be many thinks. We are more on the high idle issue. But hard starting can be so many things. We can start w the easy/inexpensive work: the fact that it is harder to start when hot is the same: weak spark is weaker when electronics heat up, weak battery, not enough juice to power relays/solenoid, corroded battery terminal or inner cable.

Check your grounds, clean up all your connectors is they are covered w oil. Time to get familiar w using a good voltmeter.

Michel
90 YJ 2.5l M/T 193Kmiles
74 wagoneer 340Kmiles
 
I may be reviving a slightly dead thread, but this one is so darn helpful I have a 88' Pioneer 5 speed 4.0, was having this high idle problem since I bought it (About 3 months) and I found this beast of a thread.

I checked my TPS, and sure enough, there was a big hunk of carbon or something on the electrode. Picked it off, plugged it back in, haven't have a crazy, wide open 3k throttle for 4 days.

On top of that, it idles a hell of alot better, like closer to 1k rpm instead of 650 or even lower, like the thing is getting ready to kick out.

So, just wanted to say thanks for this thread.
 
so it's taken me two days of reading this thread and fartin around with the heep. I've checked, cleaned, and tightened the ground on the block, and on the firewall. I have set my tps at .83v. i have checked and replaced vaccum lines, i have checked and tightened manifold bolts. the throttle body was removed and cleaned, and the problem still persists. when i start it up, the heep idles perfect right around 700-750. once i start driving the idle sticks up at 1500-2000 rpm. this actually all started when the throttle body was removed and cleaned and reinstalled. so at this point i'm pulling my hair out and i'm stressed out over it so any help would be great. I'm pretty sure that for some reason the iac is not closing. the iac was removed, cleaned, and reinstalled. i even tried swapping it out with another jeep. i tried disconnecting the battery and leaving it disconnected. when i take the air tube off of the throttle body and the engine is running, if i put my thumb over the hole that the iac sucks air thru it idles down to about normal. so i'm pretty sure something is telling it to stay open. any thoughts?
by the way, this is an 88 4.0 with m/t. it's a 4dr laredo.
thanks in advance
 
You must have an intake manifold or vacuum line leak somewhere that you have missed.

When the IAC air inlet on the top of the throttle body is covered & blocked, the engine should die if there are no vacuum leaks!

How is your CCV vacuum system set up? Does it have the 2.2 to 2.8 mm orifice in the back connection line at the rear valve cover entry.
 
mntllycrzy said:
i don't know what ccv it has. what is ccv? all i know is that it started acting up after i cleaned the t/b
Click to expand...


Crank Case Ventilation, CCV, the controlling element is an orifice about 2.5 mm inside diameter that restricts the crankcase air flow rate to the intake manifold. If it is gone and if a 1/4 ID hose or larger has been used on the back side of the valve cover connection to the intake manifold the engine gets to much air idle, and IAC becomes ineffective.
Did you use a new throttle body to intake manifold gasket?
Could some trash be stuck in the IAC port, holding the valve open?

I have occasional high idles when my floor mat gets stuck, shoved up under the gas peddle!
Be sure and check the linkage and butterfly to see if it is closing al the way!
 
butterfly is closing, nothing in iac port. i had the iac out to check. i didn't change anything in regard to hose size or anything. the lines on the tb were good. all i did was take the tb off, clean it, and reinstall it. cant be the linkage, the butterfly is shut when the idle is way up. why would the engine die when i plug the iac port? there is another hole in the tb where the engine can get air. it also has an adjuster in it.
 
mntllycrzy said:
butterfly is closing, nothing in iac port. i had the iac out to check. i didn't change anything in regard to hose size or anything. the lines on the tb were good. all i did was take the tb off, clean it, and reinstall it. cant be the linkage, the butterfly is shut when the idle is way up. why would the engine die when i plug the iac port? there is another hole in the tb where the engine can get air. it also has an adjuster in it.
Click to expand...

Not sure about another air supply route other the IAC for idle air supply. Where is the second path you are seeing?

All the ones I have worked on that worked properly will die if you seal off the IAC air supply on the top of the TB. The only other path I know of is the CCV line with a 2.2 mm orifice, and that air is under some vacuum already and is not enough to keep the engine at idle.
 
i'm holding the tb in my had as i'm typing this. if the big spring is in the 12 position, the other orifice is in the 3 position. i'm going to get another new tb gasket tomorrow and try it again, it's just driving me nuts at this point
 
The other port you are looking at is likely the fine adjustment for the idle (factory set, emission thing, likely minimum idle). There is a metal plug/cover that you can pry off and see the torx end of an air bypass adjustment screw. The adjustment screw is most often all the way closed from the factory. If the IAC is working properly this adjustment does very little.
You can try blowing through the orifice, though it's likely screwed all the way in from the factory or plugged with junk.
Plug off all the vacuum lines to the intake manifold, use short pieces of tubing with a screw in the end. The only vacuum line you actually need to keep the XJ running is the MAP.
If this doesn't cure your problem, I'd look at solvent or water in the TPS. The TB gasket or the intake gasket for vacuum leaks.
The TB seats itself into the gasket, if you reuse the old TB gasket you have to be careful that it seats into the indentations pressed into the gasket.
 
here's another question.... this truck is a 5spd. there is a trans fuse in the fuse block. what is this fuse for? the fuse is blown and when i replaced it it blew again. what does this do and where do the wires go so i can try to trace down the short?
 
mntllycrzy said:
here's another question.... this truck is a 5spd. there is a trans fuse in the fuse block. what is this fuse for? the fuse is blown and when i replaced it it blew again. what does this do and where do the wires go so i can try to trace down the short?
Click to expand...

It may have been an automatic, later converted to a 5 speed.
 
i kind of doubt it was converted, it was bone stock when i got it, and the one owner before me didn't say anything about changing it over. either way, where do the wires go from the fuse block?
 
i have an 89 5-speed and it also has a trans fuse. im not sure, but i think its for the back up lights. check the back up light switch on the tranny or if your back up lights are working.

as for the high idle, mine will idle to around 1100rpms somtimes when its warmed up, but not always. sometimes at a stop light when its idling high, i put it in first gear with the clutch in and with my other foot on the brake and slowly release the clutch until it slightly grabs and the idle will drop to 750 and push the clutch back down.
 
the back up switch in the trans is not plugged in so i know the lights do not currently work. I am wondering why this fuse is blown and opens as soon as a new one is installed
 
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