RPMs stay high when clutch is pressed-in and in neutral

Lilbomb

Lilbomb

TJ Enthusiast
Joined
Oct 10, 2018
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155
Location
Reno, NV
Ok so a new one this morning, driving at say 40 mph in 3rd gear or 4th gear, coming up to a light I put the clutch in and put it in neutral, RPM'S drop from 2500ish to 1500 and stay there as I'm coasting. Will drop to 1250 and then go back up. If I leave it in gear and let the motor bring the RPM's down to say 800 but then put the clutch pedal in, it goes back up to 1500 RPM's. Thoughts?

Thanks!
 
Higher than normal rpms are caused by a vacuum leak. Your IAC (idle air control) might be dirty and hanging up so it stays open too far which is letting extra air into the engine to cause the extra-high rpms. Or you could have a leak in a vacuum line somewhere between the engine and either HVAC system or the power brake booster which is vacuum power.

Try cleaning your IAC first. Buy an aerosol can of throttle body cleaner and spray it into the throttle body while the engine is running at idle rpms. If you spray the cleaner into the throttle body at IDLE rpms the IAC will suck the cleaner in through itself. Doing so will flood the IAC with cleaner and stall the engine. That's normal and ok, just restart the engine and spray it some more. A fine mist instead of a heavy spray may help keep the engine running. Just don't raise the engine rpms above idle, that will shut the IAC off and the IAC will stop pulling the cleaner through the itself.

If that doesn't help it's time to look for a leak in the vacuum system. Spraying the IAC hoses while the engine is running can help locate the leak. If the spray finds a leak the cleaner will get sucked into the engine through the leak and you'll hear the engine rpms change.

This shows the vacuum system. The vacuum system has a line that passes through the firewall at '2' to connect to the HVAC system. The power brake booster gets its vacuum via larger diameter rubber hose between it and the intake manifold.

81745
 
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Ok so I cleaned the IAC and nothing changed. I replaced the IAC and still nothing changed. What it's doing is when I start the Jeep it runs at 1000 rpm's for 30 seconds or so and then drops down to the 600-7-- range and will just sit there and idle fine. As soon as I give it some throttle it jumps up to 2K rpm's and then down to 1500 rpm's and will stay there and fluctuate up and down but never drops below 1500rpm's until I shut it off or put it in gear and let the clutch out. Every time I start it back up it idles normal until I give it some throttle, but it never goes back down. I've sprayed for leaks and nothing. I'm wondering if the throttle body just needs to be cleaned? Could the TPS or MAP sensor cause this?

Also, I replaced the exhaust manifold a few months ago so the intake manifold and everything was off. It went back on fine and has been good for 3-5K miles. I wouldn't think there is a leak there but I guess it's possible. I replaced the fuel injector o-rings when I did that also and sprayed there and they seem fine.

At a loss here. Thanks for any input!!
 
Use this cheap setup to check for a vacuum leak. ..
20190310_114248.jpg

@Lilbomb Stick a shop rag in the tail pipe, block the throttle body and get a $10 hand operated fluid transfer pump from Harbor Freight . Now get a cigar.
Pump the cigar smoke into the intake manifold as pictured, I bet you find a vacuum leak.
 
My bet is the leak is from not having gotten the intake manifold perfectly seated which is common if that part of the job is rushed. And it can't be fixed by tightening the bolts more.
 
I checked the intake manifold and it's flush. I re-tightened the bolts anyway and it still does it. Must be a vacuum leak somewhere.
 
I am throwing this out there because this happened to me not too long ago and my jeep looks a lot like yours.

My brake booster went bad and when I was sitting at a light with my brake on the idle would raise. I also felt like it took more to get the Jeep to stop. I mean the brakes feel normal just took more space to get it to actually stop. It got so bad I could hear the leak when I pressed the brake pedal. I had a lot of miles on it. I replaced it with a mopar booster and it has been fine ever since. Not saying that is your problem just something easy and free to rule out.
 
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Jerry Bransford said:
Higher than normal rpms are caused by a vacuum leak. Your IAC (idle air control) might be dirty and hanging up so it stays open too far which is letting extra air into the engine to cause the extra-high rpms. Or you could have a leak in a vacuum line somewhere between the engine and either HVAC system or the power brake booster which is vacuum power.

Try cleaning your IAC first. Buy an aerosol can of throttle body cleaner and spray it into the throttle body while the engine is running at idle rpms. If you spray the cleaner into the throttle body at IDLE rpms the IAC will suck the cleaner in through itself. Doing so will flood the IAC with cleaner and stall the engine. That's normal and ok, just restart the engine and spray it some more. A fine mist instead of a heavy spray may help keep the engine running. Just don't raise the engine rpms above idle, that will shut the IAC off and the IAC will stop pulling the cleaner through the itself.

If that doesn't help it's time to look for a leak in the vacuum system. Spraying the IAC hoses while the engine is running can help locate the leak. If the spray finds a leak the cleaner will get sucked into the engine through the leak and you'll hear the engine rpms change.

This shows the vacuum system. The vacuum system has a line that passes through the firewall at '2' to connect to the HVAC system. The power brake booster gets its vacuum via larger diameter rubber hose between it and the intake manifold.

View attachment 81745
Click to expand...
Can you tell me what #8 is and what it does? Thank you!
 
To put it in simple terms it is a vacuum storage tank or big plastic empty box. It allows things like your brake booster and heater controls to function when the engine is not providing vacuum. For example, when you are at wide open throttle or the engine is off.
 
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pagrey said:
To put it in simple terms it is a vacuum storage tank or big plastic empty box. It allows things like your brake booster and heater controls to function when the engine is not providing vacuum. For example, when you are at wide open throttle or the engine is off.
Click to expand...
X2, good description of what #8 does.
 
Solved!

Ok, so I haven't driven the Jeep in a month or so knowing it had an issue and we are in the process of moving so life is crazy. I can't find any air leaks so I decided to pull the throttle body off and clean it really well just in case that was it. Put it back on and same thing was happening. Here is where the solved part comes in:

I have the throttle assist/hand throttle on here that the PO installed. The screw on the side comes loose every once in awhile so I end up having to tighten it up by hand while driving. I was at a stop and the RPM's were high and I randomly turned the screw for the hand throttle out and all of a sudden the RPM's dropped to normal. Hmmmmmm. As I kept driving there was no more high idle anymore. Wait what?!? ( That was my thought while driving) So when I got home I took a look and this is what had happened:

IMG_1102.PNG



So basically the brass fitting stayed in the throttle and the cable sleeve came out and with the bend in it it would hold the throttle on. I took the cable off from the throttle body end and have put 30 miles or so and it's good.

Lessen learned....start with the easy stuff! lol At least I know my throttle body is super clean now.
 
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