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Cleaned TB now a very high idle

M

MudfrogK5

NAXJA Forum User
Location
Roanoke VA
Ok, I was having issues with my 90 cherokee (4.0L) stalling after first starting and figured out the fix was to clean the TB (idle was fine). At the same time I sprayed some TB cleaner on the IAC. Put everything back together and now I have a high idle. The only thing that comes to mind is the vacuum line coming from the MAP sensor. I could not for the life of me get the figure 8 gromet pushed all the way in to the TB. It's about 1/2 - 3/4 of the way in and it's tight so it's not loose. Anyway, today I pulled the IAC and completely cleaned it and I still have a high idle. I tried unplugging the IAC, the first time the idle dropped down, then went back up after plugging it back in. The second time it made no difference in the idle, was still idling high.

If I unplug the vacuum line from the MAP sensor the idle will significantly drop, almost to the point of stalling. Would this indicate a bad MAP sensor? The only vacuum leak I have found is where the EGR valve's nipple snapped off so the hose will not stay on. I have tried closing the line and holding the vacuum line to the EGR valve and this made no difference.

Help would be greatly appreciated. Oh I did see on LungHD about testing the MAP sensor but to be honest my voltmeter has so many settings I don't have a clue which one to use lol.
 
Ok. I would suspect a faulty IAC, but we will get back to that.

There is a small screw, 3/32 hex, sometimes referred to as a "base idle screw" but it is actually the "keeping the steel throttle place from slamming closed against the alloy throttle body screw". Remove the intake tube so you can observe the throttle plate in its natural setting. Turn out the hex screw until it no longer has any effect on the throttle plate--so that it is fully closed. Then, turn the hex screw in until you can perceive the smallest bit of movement of the throttle plate and then STOP. Now, put things back together.

Ok, you read lunghd.com on the IAC. There is no way to check its function w/o the factory "exerciser" tool. You can check continuity between the ECU and the IAC, and that is about all. lunghd.com says about $65 for the IAC--well, rockauto.com is asking $91 currently. That is a lot of money to blow on a crap shoot.

I would consider replacing BOTH of the vacuum harness pieces, they will run about $22 ea at NAPA, cheaper from rockauto.com until you add in the shipping.

Note, when you want to put something like the MAP line back into the throttle body a little petroleum jelly goes a long way in helping things go together.

You can try the salvage yards for a Renix with an IAC, but don't give them more than $5 for it if you can find one--still a crap shoot if it will work.
 
If idle was fine BEFORE the work performed, gotta go back and look at work performed.

The #1 cause of high idle is a vacuum leak. Try spraying some carb cleaner all around the TB area. If the idle changes when doing this, you'll know what to suspect.

#2 is the idle air control. Cleaning MAY restore function but not always. How did you clean it? Here is a procedure....

The Idle Air Control (IAC) is mounted on the back of the throttle body. The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body.

Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

"Gently" wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Reinstall IAC and check idle quality
　
 
Last edited:
MudfrogK5 said:
If I unplug the vacuum line from the MAP sensor the idle will significantly drop, almost to the point of stalling. Would this indicate a bad MAP sensor? .
Click to expand...

that would indicate the MAP is working. if you unplugged it and there was no change, that would indicate it is bad.

on my 89, it will barely idle if the MAP is disconnected.
 
MudfrogK5 said:
The vacuum harness your referring to.. is that the big block that all of the vacuum lines go in to?
Click to expand...

That would be it, the "big block" at the front of the valve cover is the junction of the two pieces.
 
If you sprayed the throttle position sensor, it is hosed.
 
birchlakeXJ said:
If idle was fine BEFORE the work performed, gotta go back and look at work performed.

The #1 cause of high idle is a vacuum leak. Try spraying some carb cleaner all around the TB area. If the idle changes when doing this, you'll know what to suspect.

#2 is the idle air control. Cleaning MAY restore function but not always. How did you clean it? Here is a procedure....

The Idle Air Control (IAC) is mounted on the back of the throttle body. The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body.

Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

"Gently" wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Reinstall IAC and check idle quality
　
Click to expand...

That is exactly what I did however there was so much buildup I did do some twisting, etc.. It's fairly clean now but maybe I damaged it?

I'm guessing this is probably the culprit. I checked all of my vacuum lines and as I stated the only one not hooked up is the one for the EGR valve and I have read this will not change the idle.

89xj said:
that would indicate the MAP is working. if you unplugged it and there was no change, that would indicate it is bad.

on my 89, it will barely idle if the MAP is disconnected.
Click to expand...

This is good to know.

joe_peters said:
That would be it, the "big block" at the front of the valve cover is the junction of the two pieces.
Click to expand...

I found these on rockauto, I will check with Napa and some other local parts stores.

old_man said:
If you sprayed the throttle position sensor, it is hosed.
Click to expand...

I have no idea if I did or not. I only recall spraying the TB and IAC, I don't even recall seeing an opening to the TPS.
 
MudfrogK5 said:
I have no idea if I did or not. I only recall spraying the TB and IAC, I don't even recall seeing an opening to the TPS.
Click to expand...

the tps is on the throttle body and unless your careful, its very easy to spray the tps when cleaning the throttlebody. the tps is a variable resistor and when it gets wet it is possible to damage it as it can "short out" when electricty is applied(from the constant 5v input).
 
89xj said:
the tps is on the throttle body and unless your careful, its very easy to spray the tps when cleaning the throttlebody. the tps is a variable resistor and when it gets wet it is possible to damage it as it can "short out" when electricty is applied(from the constant 5v input).
Click to expand...

I used TB cleaner which dries quickly. I sprayed all over but feel sure that if the TPS was sprayed it would have been dry by the time I plugged everything back up.
 
I've been doing some digging and I'm leaning towards it being the IAC, I have found one local in the upper $30's so I'm going to try changing this. From what I have read the TPS will cause erratic acceleration, etc but I have not seen people claim it will cause a high idle.
 
Do you have the MAP sensor vac hose fitting ("figure 8 gromet") upside down in the TB. Is that even a possibility? Don't have Jeep handy.
 
I tested the TPS sensor and it looks fine, I used the procedure I found on Youtube.

Trying to put it back together but the TB gasket broke.. can you believe out of 2 Advance Autos and 1 Autozone nobody had this in stock?!?!
 
Try CAP or NAPA as well? I have CAP, NAPA, AZ, and AAP all within about two miles of me, CAP is open the latest and NAPA/AAP seem to have the best stock usually.
 
Advance will have it to me today as their main warehouse is in my city.. just a pain to go out to get one, then have to come back a few hours later when I could have been done.
 
i made my own throttlebody gasket. i just assumed that the parts store would not have it, so i bought the gasket material that comes in a sheet and used the old gasket as a template.

as for the tps not having anything to do with idle, thats not true. it may not be your problem, but a wet tps can cause an extremely high idle. 2000-3000rpms are not uncommon from a wet or misadjusted tps.
 
Well my purchase of the IAC for $38 fell through so I bought one for $62 from advance.. didn't fix the issue. now to top it off coolant is leaking from an unknown location..
 
No tach so I dunno.. I'd guess 1500-2000.. high enough that I can release the clutch and go through 1st and 2nd gear without touching the gas..
 
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