Mystery Over-revving High Idle - JeepForum.com
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post #1 of 20 Old 01-11-2019, 08:11 PM Thread Starter
Gypsy_Botanist
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Mystery Over-revving High Idle

Hello all,
I am in need of some help. Been doing some updates/maintenance on my 93 YJ 2.5L. Changed the fuel pump and drove her over to my friends garage (it's heated) to change the water pump and fan clutch. Got everything back together and when I started her up the RPMs shot up to 2900-3000. We started to look at vacuum lines. We went through all the vacuum lines coming off the throttle body. We found some leaks, fixed them and used a vacuum pump to check the repairs. However when we started her back up the RPMs are still at 2500. She seemed like the RPMs calmed down a little but, still really high. Ran the self diagnostic and didn't come up with any codes. I am totally out of ideas for what the problem could be, please help and advise.

Thanks,
Ian.

I can also post pictures of things.

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post #2 of 20 Old 01-12-2019, 08:19 AM
Waternut
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So even after driving to your friends house, the idle is still way up? You didn't pull the intake or exhaust manifold did you?

There are a ton more more vacuum ports under the throttle body. You've got fuel pressure regulator that's a vacuum line, brake booster, MAP sensor on the bulkhead, valve cover breather, pcv valve, and even the charcoal canister has lines that tie into the manifold...or could tie into the manifold. Mine even had some kind of check valve on the back that I have no idea what it does but if the hose was cracked, it would certainly cause problem. Any one of those could be causing a vacuum leak. Check all of the connections and those are good, check the hoses. I don't know that spraying fluids around the engine bay would help with the revs being that high.


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post #3 of 20 Old 01-12-2019, 08:45 AM Thread Starter
Gypsy_Botanist
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Waternut, no the high idle started when we turned her on after we got the water pump in. The idle was normal after the fuel pump replacement. On the throttle body i only saw 4 vacuum lines. I'll take a picture.
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post #4 of 20 Old 01-13-2019, 02:38 PM
Boojo35
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How did you check for vacuum leaks with a vacuum pump? This is not conclusive. Never been able to prove a intake gasket vacuum leak with a vacuum pump for example.

If you cannot fix it with a hammer then it has to be an electrical problem.
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post #5 of 20 Old 01-13-2019, 09:46 PM Thread Starter
Gypsy_Botanist
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Boojo I just tested each line, plugging one end and applying vacuum to the other and reading the gauge to see if it is holding a vacuum. Is there a better way? I'm a novice so i really don't know too well what I'm doing all the time, unless I've done it before. I have a Chilton's for my jeep but it's not the best at explaining how to test everything. I just switched out the TPS, the old one was giving me some funny voltage (could have been the ground I was using the new one tests well, at rest 0.75 v to all out at 2.5 v) and when we started her up she was sitting at 2k (normal for the cold) but jumped back up to except it's at 4k instead of 3. She is really acting like the gas pedal is always all the way down all the time. I drove her from the garage, my buddy and I've been working on it, home. All the time i was driving her home i never touched the gas pedal, just let what ever is wrong keep her going. It really seems like the gas pedal isn't working, when i was driving in 4th, she would slowly top out (55ish) but could go faster(65-70ish). When i press on the pedal all the linkage underneath the hood looks like its working fine. When i applied pressure on the gas pedal its like it didn't work at all. Had to shut her off at stop lights and sharp turns because idle was not an option, i didn't wana blow the engine up sitting at 4k rpms.
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post #6 of 20 Old 01-14-2019, 08:55 AM
Boojo35
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I get now what you tested for vacuum. All things that the vacuum lines go to hold vacuum. There still can be a vacuum leak at the cylinder head to intake mating surface for instance. This can be checked by spraying that area with carb cleaner and listening for RPM changes. Keep in mind that carb cleaner is flammable when you use it. You can catch stuff on fire.

When you checked the gas pedal, did you look to see if the throttle valve actually closes?

If you cannot fix it with a hammer then it has to be an electrical problem.
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post #7 of 20 Old 01-14-2019, 03:17 PM
Waternut
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Quote:
Originally Posted by Gypsy_Botanist View Post
Boojo I just tested each line, plugging one end and applying vacuum to the other and reading the gauge to see if it is holding a vacuum.
That only confirms that the vacuum lines you know of aren't broken. What's more likely is you've accidentally pulled a vacuum line off and don't realize it's disconnected somewhere. Especially near the front of the engine near the water pump where you were working. Unfortunately, there isn't a good way to detect this without spraying a flammable liquid all over the engine compartment and listening for rpm changes. Based on the 2500rpm idle that you're currently getting, slight rpm changes will be tough to detect. It may be better to try and listen around the engine bay for overly loud air intake noise. Feel around the fittings on the intake manifold as well. If you feel suction, that's it. It won't be a dangerous level of suction but probably best to do this while the engine is still fairly cool.

Does your jeep have the vacuum disconnect on the axle or vacuum lines running to the transfer case? Check those if it does.

If all of that fails, start plugging things the engine absolutely doesn't need with your hands for a few seconds and see if the revs drop. Things like any port on the manifold, brake booster line, and any other hose that looks like it's not held on by a clip or retainer.


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post #8 of 20 Old 01-14-2019, 04:33 PM
Luuca
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do the simple things first.

1. Get some Throttle Body Cleaner (NOT Carb Cleaner) and remove and clean out your Throttle Body. A stuck or sticking IAS will cause high idle. Make sure the gasket below your TB is intact and in place.
2. Once the TB is cleaned out, run the engine. If it still has high idle, with the engine running, disconnect the IAS plug. if it continues to idle, you have a leak.

more than likely you cracked a rotted vacuum line somewhere while replacing the water pump. We have all had this type of "I checked it twice" moments and found that something was not as we assumed.
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post #9 of 20 Old 01-15-2019, 03:56 PM
blake989
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Quote:
Originally Posted by Waternut View Post
Unfortunately, there isn't a good way to detect this without spraying a flammable liquid all over the engine compartment and listening for rpm changes.
I use a plumber's soldering torch with a propane bottle, not the self-igniting torch, the cheap one with only a valve. Crack the valve open and move it around potential leaks...works like a champ and has about the same odds of burning your face off.
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post #10 of 20 Old 01-15-2019, 06:29 PM
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A simple way to test vacuum and fuel pressure. You said you replaced the throttle position sensor, was this before or after the high idle ? If it was before you could have gotten a bad TPS..Put the old one back on and see if it idles down.. I've gotten a lot of bad sensors from the parts stores in the last few years





Edit : you might have gotten the spring on the wrong side of the arm on the tps...check and correct if necessary
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post #11 of 20 Old 01-15-2019, 09:23 PM
jsawduste
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Besides the TPS question and assuming the IAC is plugged in and the pintle seat is clean.


Take the intake plumbing off and slide a business card down over the air bleeds in the throttle body. These bleeds feed air to the IAC which in turn feeds air to the engine at idle. If blocking the bleeds off doesn't reduce the RPM than the engine is getting air from somewhere else.


If it does (you should be able to kill the engine by choking off the air supply) you know you don't have a vacuum leak.
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post #12 of 20 Old 01-22-2019, 09:13 AM
stephencbrinson
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Quote:
Originally Posted by Gypsy_Botanist View Post
Hello all,

I am in need of some help. Been doing some updates/maintenance on my 93 YJ 2.5L. Changed the fuel pump and drove her over to my friends garage (it's heated) to change the water pump and fan clutch. Got everything back together and when I started her up the RPMs shot up to 2900-3000. We started to look at vacuum lines. We went through all the vacuum lines coming off the throttle body. We found some leaks, fixed them and used a vacuum pump to check the repairs. However when we started her back up the RPMs are still at 2500. She seemed like the RPMs calmed down a little but, still really high. Ran the self diagnostic and didn't come up with any codes. I am totally out of ideas for what the problem could be, please help and advise.



Thanks,

Ian.



I can also post pictures of things.
Did you find a resolution... I have the exact t same issue after changing my water pump. IAC issues would be a random coincidence for me because I did not remove the air intake.

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post #13 of 20 Old 01-22-2019, 03:38 PM
bharris68
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To stephen and the OP - the Water pump on the 2.5 and the Power steering pump share a bolt or two from memory and I'm trying to remember if they hang off of the intake manifold?


If they do it's possible there is an intake system leak in Stephens case and in the case of the OP there is a block of Vacuum ports on the passenger side firewall.


OP: Use the UNLIT propane torch trick and wave it around the vacuum port block there on the firewall and if you don't see anything obvious unplugged, then you may come across cracked or compromised vacuum lines.


stephen - similar trick for you, but you'll be moving it near the intake manifold gasket very closely in order to get unlit propane near the gasket even as the fan blades are doing their darndest to blow the propane away! If you come across the leaking space, you'll need a set of feeler gauges to determine where the leak is. I would recommend against "Just tightening everything" because it's unknown if / where the issue could be. You don't want to overtorque the bolts and either break them off or wreck the gasket.
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post #14 of 20 Old 01-22-2019, 03:57 PM
stephencbrinson
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@bhar ris....I have a known exhaust leak at the manifold. I will be replacing the gasket this weekend. Since the exhaust and intake are the same gasket...I will update with my results.

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post #15 of 20 Old 01-22-2019, 05:19 PM
bharris68
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Stephen - good deal - when you do, there are alignment dowels and it's really critical to get everything lined up before and as you torque it down. I left a gap and the over-revving and TBI trying to compensate for air/fuel mix made a hell of a mess...
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fuel assembly , high idle , throttle body , vacuum advance

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