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Broadcastmatt 05-06-2009 12:38 PM

1998 Wrangler High Idle, w/ New TPS
I have a 98 2.5L 4cyl. Wrangler that has a high idle. When I start it cold it is pretty normal, around the 750-850 rpm area, but as the engine warms and/or I drive it the idle rises to upwards of 2000 rpm, and would likely keep going if I let it get higher. When I start it warm it will spike up to 2000 or so and then come back down to 1000 area.

A couple weeks ago my check engine light went on and the code was for the TPS (Throttle Position Sensor), so I just replaced that hoping it would solve the problem. I also used STP throttle body cleaner spray to clean the TB while I have the air duct off.

I was told by a local mechanic who just took a look at it and he didn't think it was a vacuum leak because the idle would be high even when starting it cold.

Kicker- My water pump was leaking for a while and I had just replaced it. I did not have the idle problems until after replacing the water pump, but the check engine light with the TPS code occured prior to replacing the pump.

Any Ideas?? I am not vert versed in doing things on my own, but I can hanlde many things.

stevens243 05-06-2009 12:41 PM

If you put a new TPS on and aren't getting a code anymore, disconnect the battery for 10 minutes, reconnect it and start again. The computer may need the raytheon reset so it can adjust for the new sensor.

Good (easy and cheap) place to start.

Broadcastmatt 05-06-2009 12:46 PM

I will give that a shot, I have to return a torx socket set and will let it reset while I'm gone. Will, update you as soon as I try it again.

Thanks Larry

Jerry Bransford 05-06-2009 01:30 PM

Usually a high idle is caused by a vacuum leak but it could also be caused by a hanging IAC (idle air controller). The IAC is what provides all of the engine's air at idle and if it is dirty enough to hang up, it might feed too much air into the throttle body.

Check out this thread for info on how to clean it... idle speed help

Just make sure there's no reason for a vacuum leak too... like if the intake manifold had been removed/replaced in recent history. :)

Broadcastmatt 05-06-2009 02:00 PM

Alright, I tried disconneting the battery but still, the gradual increase of idle speed (kind of like the seconds hand of an analog clock) gets up to 1750 when the temp gage hits mid-way.

Now, on the thread that Jerry directed me to, it says to use the throttle body cleaning spray by concentrating into the AIC hole within the throttle body. How much spray? Is there such thing as too much? And if it gets to the point where I must disconect it and clean the plunger, will i know it when i see it?

Unfortunetly, I don't know if the intake manifold has been replaced before, it doesn't appear to have been. Also, I do think I hear a slight whistle coming from the passenger side of the engine, which would indicate a vacuum leak, but not sure which hoses to check. Sorry, I know I must be ignorant.

azlenny 05-06-2009 02:29 PM

use a good amount of cleaner the better thing is to pull the IAC and clean it off the tb and than reinstall but can very easly be done on the tb

Broadcastmatt 05-06-2009 03:32 PM

I sprayed a good 4-5 seconds on two seperate occasions of the cleaning spray into the inlet in the throttle body, but no improvement on the high idle speed. I tried letting it run for a little why to see if it would eventually correct, but to no avail.

Broadcastmatt 05-06-2009 04:43 PM

I removed the IAC, disconnected it to get my rachet around the wiring, and cleaned the plunger with the spray cleaner and a cloth, also used some more spray in the area between the inlet and the IAC; still no improvement:banghead:

I took it for a ride and when I would shift out of gear to another, the RPMs would drop drastically down to 850-1100 area but then spike right back up to 1500-2000 rpm.. perhaps that is a clue. Another observation is I would be driving and then drop out of gear into nuetral, and that same drop in RPM to say the 'normal" idle speed - then back up to the excessive speed.


Jerry Bransford 05-06-2009 06:52 PM

All I can think of now is vacuum leak.

Broadcastmatt 05-06-2009 07:00 PM

whats a good way to pinpoint the leak? I have heard of spraying brake cleaner and looking for bubbles? Also a small propane source over areas until the vehicle idles normally (sounds risky?)... whats a good way?

Jerry Bransford 05-06-2009 07:41 PM

The purpose of spraying TB cleaner, brake cleaner, WD-40, etc. is to allow the vacuum leak to suck it into the engine which will cause a sudden rpm change. Same thing with propane... using a common propane torch like any hardware store sells, turn the gas on fairly low and without lighting it, point the propane gas flow at the likely points of leak (including the where the intake manifold is bolted to the head. An rpm jump will confirm the propane (or whatever) has been sucked into the engine and you have found the leak. Some people swear by propane but I'd rather use an aerosol can full of cleaner myself. :)

meyers 05-07-2009 06:54 AM

I wouldn't use propane, I know some do but what if you got an errant spark. IMHO pumping flammable gas into the air anywhere near a running engine could be very bad.

Carb cleaner does the same thing, as well as WD-40

Broadcastmatt 05-07-2009 08:28 AM

ok, i will try with WD-40. Will spry near the intake manifold and around hoses. Anywhere esle I should target?

Broadcastmatt 05-07-2009 11:16 AM

Alright, so i took WD-40 and sprayed around the TB near all the hose connections, and where the intake manifold connects to the engine.. bu again.. no changes in the idle... I might not be spraying the right areas though..

could the drive belt be causing this? too tight? Ummm I just had the thermostat changed while I was doing the water pump... anything there? I also flushed the coolant after the water pump change, maybe water somewhere?... distributor cap wet?

Just throwing as much out there to maybe get a better idea; vaccuum still seems most likely?

meyers 05-07-2009 11:46 AM

HMM I just reread this could you have a bad coolant sensor, causing the computer to spaz out? You said it started around the time you had the water pump done I would check all the connections for the coolant and other sensors in that general area.

Broadcastmatt 05-07-2009 12:10 PM

Any way I could figure out if that is the actual cause before I buy it? NAPA priced me out a CTS at $44.

rrich 05-07-2009 04:02 PM

You had the water pump done? Isn't the temp sensor in the thermostat housing, right by the water pump?

Take a real close look at the temp sensor's wire and the crimped end. Since they probably took it off to get it out of the way, they may have broken the wire, or it's connection is intermittent.

Brake cleaner - (if there is a vacuum leak it gets in the combustion chamber) when brake cleaner burns it creates a very deadly gas.

WD-40, most carb and TB cleaners don't burn like gasoline, you might as well use water.
It takes a lot of it to affect the idle by flooding the cylinder, or it may temporarily plug the leak by surface tension.

The best is a burnable cleaner, like Gumout.

But - the problem with a liquid is it can wash dirt into the leak temporarily. Plus if it is burnable and ignites, the puddles continue to burn, possible melting things or even burning the vehicle down.

The best way is gasoline - but that's way to dangerous - plus it ruins paint.

Propane can ignite, but it has to be in the right proportion of air to fuel. Plus if it ever does ignite - it flashes, scares hell out of you, then it's out. It doesn't puddle and continue to burn. Just don't spray it on the distributor. Even if it doesn't ignite, it's gone within seconds.

foxinthemudd 05-07-2009 04:34 PM

if you don't find a vacuum leak try completly replacing the IAC. I cleaned mine out that had the same problem a few times with no luck. replaced the TPS, still had the same problem. But when I replaced the IAC, everything went back to normal.

1998 TJ 4cyl...seems like they had the exact same issue...

Broadcastmatt 05-07-2009 08:03 PM

I had it suggested to me that the connection between the rise in idle speed and the vehicle temperature may suggest that it would be related to the Coolant Temperature Sensor... seems like they would go hand in hand.

Any takers?

stevens243 05-07-2009 09:05 PM

Maybe, but you reset your computer and aren't getting a code for a bad sensor...

I'd try the new IAC (they are cheap) and most likely the cause...

I'm also wondering just how wet things got in the engine compartment when you did the water pump?

Broadcastmatt 05-07-2009 09:11 PM

It is a possability that there may have been a decent amount of rouge water... I started it when it was dry out and did have a slight drip of what appeared to be water coming from the the tail-pipe. How/What might the water effect the idle if it could?

Change any of your thoughts?

stevens243 05-07-2009 09:17 PM

What area do you live?

Is there any water in/around your air cleaner? Is the filter dry?

Did any of the other sensors get really soaked (especially the higher ones)?

I don't know if it applies, but I had a similar thing happen for a couple days after I powerwashed my firewall...

Check for dryness and if you can, give it a good day with the hood open in the sunshine..

Broadcastmatt 05-08-2009 10:42 AM

It didnt get too wet during the flush, most of the water that went rouge stayed at the front of the vehicle near the radiator. Everything in that area stayed pretty dry. I'm starting to think it may be the IAC from what everyone is telling me. Now I just have to find one and see if thats the case i guess

Broadcastmatt 05-08-2009 12:10 PM


Boys, so i think I broke the code, but I stopped by a local shop (one man run, pretty cool guy) to spend the $80 bucks to find out if he could find something I couldn't with a vaccuum problem.

Sure enough, there was a leak. He said on the intake manifold there was a bolt that appeared to have been missing for years, was pretty impossible to notice though. He said I would never have found it with brake/throttle cleaner because its not as flamable these days as it used to be, and everything else is very dangerous.

He used a smoke machine, let the engine take it in and was able to pinpoint the leak. He said the warmer the engine got the more the gasket would expand, thus the reason for the increase in idle speed as the temp went up. There was another bolt that was loose as well because of the missing bolt. He put in a new one and tightened the old bolt, and now my TJ sounds and drives great!

Thanks everyone for all your help.. in the end, between replacing the water pump, TPS, cleaning the IAC, TB, and buying a few supplies, I spent $305. The water pump labor alone was going to be no less than $240. So with your help I saved some $$, learned a lot, and did some good things to the Jeep. Thanks again. And if I wasn't such a newb, I might of found the vaccuum leak.


Koshon 05-20-2009 05:50 PM

Ok so I found someone with the same problem i have been fighiting with... wistle and all!! sooooo can you tell a bit more on where this bolt was or what it was to?

Broadcastmatt 05-20-2009 09:07 PM

The Mechanic said it was on the underside of the intake manifold. It was an open shop and I had got there to see him just putting things back together, he had a few crazy ratchet extensions to get to it. I never saw exactly where it was, but like I said, he used a smoke machine to find the exact spot and took care of it within an hour.

Once he replaced it with a new one and tightened the existing bolts it ran great.

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