Help needed, high revs at idle until wheel stop - JeepForum.com
 
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post #1 of 9 Old 08-27-2021, 09:12 AM Thread Starter
abunea
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Help needed, high revs at idle until wheel stop

How it started: took the car to the shop for a metallic sound which I suspected was from a worn out pulley bearing. Turns out it was the connection between the exhaust manifold and the exhaust pipe, the mechanic tightened it. He also loosened up a bit the accessory belt as it was a bit too tight.



Since I picked it up from the shop I noticed it was revving much more than usual and then I noticed than when coasting out of gear, the rpms will not drop below 1500 until wheel stop. Even if I used the engine brake and brought it below 1500 and then took it out of gear, it would rev back up to 1500rpms and stay there until the wheels stopped. It also idles higher, at around 1100rpms.



Done so far: cleaned the TPS sensor connections, cleaned the IAC, the mechanic checked all the vacuum hoses (removed and checked each one), not of them seem to leak.



After the IAC cleaning and vacuum hoses check up, it's slightly better. Now the revs drop to 1200rpms instead of 1500rpms. Idle rpms also went down to 900-1000rpms. But now it also has a slight rev variation at idle (around 100-150rpms) and I noticed that if I push and release the brake pedal at idle, that rpm variation goes a bit higher up.



Hooked up an OBD2 dongle and Torque app.



O2 sensors seem ok, on average Sensor 1 is at 0.5V while Sensor 2 at 0.47V

Throttle position sensor at idle is at 13.3%, seems ok.

Vacuum at idle is around 26 in-HG, with 900+ rpms, kind of high from what I googled so far. Under deceleration it goes to around 27, on acceleration goes down to around 20-21. When I press the brake at idle and the variation of RPM is increased a bit there is also a slight change in the vacuum readings.



From what I read so far the power brake booster uses the vacuum from the engine intake manifold. No noticeable loss of braking power but I am going to look into how the booster works.



Googling some more, higher than usual vacuum points to overly advanced ignition timing, but that is not adjustable for this type of engine. And my readings are from 6 at idle till 41 under acceleration at around 2500rpms. So at idle, the timing seems fine, which is even stranger then.



It's not throwing up any error codes, any suggestions are welcome.

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post #2 of 9 Old 08-27-2021, 10:26 AM
superj
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26 is very good vacuum, dang. your changes in vacuum readings are correct for what you are doing. it should read higher on decel if in gear because the engine is trying to suck harder. it will read lower on accel because the tb is open allowing air in.

26 is great at idle though. i haven't checked either of the jeeps in the driveway but my old yj only read 17 or 20 because it was a stroker with an Offenhauser intake and holley four barrel on it.

the brake booster does indeed use the vacuum to assist braking. sometimes the idle will raise a very slight amount when pushing the brake and sometimes boosters make a swooshing sound. if you hit the brake and the idle goes up noticeably, then you might have a bad o-ring or tear in the diaphragm.

check for leaks at the head where the manifolds bolt up and where the throttle body sits. also, you can take the line form the intake manifold to the brake booster loose and put your thumb over it and see if your idle drops to normal. that would tell you its the booster, or the booster hose or check valve

the xj is the only jeep left in the driveway
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post #3 of 9 Old 08-27-2021, 10:30 PM Thread Starter
abunea
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Good to hear vacuum numbers are good, 18-21 was generic info from google since the FSM doesn't specify any numbers.

Will follow your recommendation and revert back.

Thanks!
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post #4 of 9 Old 08-27-2021, 11:43 PM
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Quote:
Originally Posted by abunea View Post
How it started: took the car to the shop.
Since I picked it up from the shop I noticed it was revving much more than usual and then I noticed than when coasting out of gear, the rpms will not drop below 1500 until wheel stop.
any suggestions are welcome.
Take it back to the shop and make them fix it on their dime.

Quote:
Originally Posted by jeepinmichguy_ View Post
TheBoogieman is a jerk.
2003' Rubicon/Zone 4.25" combo lift with Rancho RS5000X shocks/ Zone hydro shocks/5.13 gears/35" Mickey Thompson MTZ P3 tires/Black Magic brakes. Jeep #17 & 19.
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post #5 of 9 Old 08-27-2021, 11:49 PM
CDNAltitude
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High idle

You could have an exhaust manifold leak now which will cause a high idle. Since you said he loosened and tightened something up is it possible you may now have a leak. Common problem.
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post #6 of 9 Old 08-29-2021, 02:11 AM Thread Starter
abunea
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I disconnected the battery on Friday and left it off till today. Reconnected it and it started a bit rough, did not light the CEL but on the OBD2 got a P0505 - Idle Control System Malfunction. The IAC has already been cleaned, will clean the throttle body and also take it to the electrician to check the wiring on the IAC.

Unfortunately in Europe it's harder with TJs, they are rare, not a lot of mechanics have worked on one and scan tools for this are pretty rare.

The exhaust manifold was changed last year, will check for leaks on that.

LE: According to the FSM this code means - A shorted or open condition detected in one or more circuits of the idle air control motor circuits.

Last edited by abunea; 08-29-2021 at 02:36 AM.
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post #7 of 9 Old 09-25-2021, 03:55 AM Thread Starter
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Status update, no luck so far. Checked all the electrical cables, could not find any with issues. Tried replacing the IAC with an old one which I changed because I was chasing another issue. The P0505 code is now gone, but absolutely no change in behavior. They did not drive a lot so it might be that it did not get a chance to properly adjust readings and will throw it again. But at least the IAC can be ruled out as the cause. No exhaust manifold leak found. No vacuum leaks found.

It's still in the shop and they are scratching their collective heads. I am starting to think this might be a case of several issues at once, rather only one cause. I think there are 3 main possible suspects:

1. Cabling issue - a cable that has a very small issue that is really hard to find and they were not able to locate it
2. Issue with the pressure sensor for the power steering - they did mess with the steering pump and that's when the behavior started; also the PCM uses the input from this pressure sensor to calculate RPM. Not sure how this would relate to wheel speed though, as RPM stays up until wheel stop. Although the manual doesn't state it, I am suspecting the PCM could take into account the wheel speed as well as less effort is needed to turn wheels when moving vs stationary so it would be logical it starts to increase the RPM when wheels are stopped.
3. Vacuum issue - when pumping the brake while stationary the RPM goes up, which could point to a bad one-way valve or punctured diaphragm in the brake booster

Next step: they will try to improvise a valve from another make, to see if there is any change in behavior and they will take a second look at the electrics, maybe this time they will find something.
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post #8 of 9 Old 09-25-2021, 08:39 AM
CJ7-Tim
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The P0505 suggests a faulty Idle Air Controller (IAC) or issues with the IAC wiring. If the Jeep is functional with the symptoms you should order the correct part rather than attempt to modify other parts.


Quote:
Originally Posted by abunea View Post

....... it also has a slight rev variation at idle (around 100-150 rpms) and I noticed that if I push and release the brake pedal at idle, that rpm variation goes a bit higher up.
The symptoms onset after work at the shop suggests a man made cause, such as physical damage to vacuum hoses or wring, and as suggested already loose wire plug connections. Suspect an intake manifold (not exhaust manifold) air leak, a brake booster problem, damaged wires/wire plugs, or a vacuum hose problem. Vacuum hoses are easily split open by mishandling, and the rubber fittings and elbows are subject to deterioration from age and normal underhood temperatures.

A smoke test may help located any vacuum hose or hose fitting issues. Confirm that the hoses are all correctly connected in the proper order and place.




P0505 JEEP : Idle Control System Malfunction

Possible causes :
  • Faulty Idle Air Control motor
  • Idle Air Control System motor harness is open circuit or short circuit
  • Idle Air Control System motor circuit poor electrical connection
  • Vacuum leaks or damaged hoses or improper hose routing

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not to overthrow the Constitution but to overthrow the men who would pervert the Constitution” Abraham Lincoln, 1859.
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post #9 of 9 Old 10-07-2021, 09:10 PM
0bleepdajeep
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I had a vacuum leak at the throttle body shaft. I replaced the TB with a low milage unit and cured my year long vacuum leak search. Here's a video for reference.
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