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rpm at idle?

C

ckndrob

NAXJA Forum User
Location
maine
I have an 00' with a 4.0 and 129,000. It's been running at 10 psi when at a stop.... Last week I had a rough idle error code which the mechanic adjusted the idle to get rid of. Now the idle at a stop in drive is 500 rpms and twice today the check gauge light came on and the pressure gauge dropped to zero.

what should the rpm be at idle? I think it needs to be raised to increase the low psi I have to avoid the check gauge light. I already changed the sensor and had it checked with a gauge so she is low.
 
1996 and newer OBD-II, the engine idle is non-adjustable. The ECU controls the idle speed (and timing) and will overcome any manual adjustments to the idle (or timing). Idle should be about 750 rpms and warm idle oil pressure should be above 13 psi as noted in the Service Manual.
 
What weight oil are you running?
 
Uh oh, another 0331 with low oil pressure? Somebody slapped a head on and called it good...

To the Original Poster-How long have you owned it? Know any history? More info = more answers.
 
we did already replace the cylinder head and I am using 15w40 oil. Not sure if the two are related but my mechanic brings the rpm back to 750 could my p1294 rough idle code come back on me?
 
owned for about 8 months. PO did replace cylinder head. I was told it may need a new oil pump... In the meantime I changed from w30 to w40, not much difference in psi but since the idle was adjusted to 500 to check gauge light now comes on so I will bring it back to the shop to have adjusted to 750 to keep the pressure up.
 
P1294 (M) Target Idle Not Reached. Target RPM not achieved during drive idle condition.

Possible vacuum leak or Idle Air Controller (IAC) malfunction. Try cleaning the IAC.
 
First, the idle is controlled by the ECU. I don't know what this mechanic claimed to have done, but all he did was rip you off.

All Xj's from 1991 on have a ECU the electronically controls the idle. I'm not sure about the earlier RENIX XJ's.

If you have a rough idle, chances are there is an issue with the fuel/intake system. You either have a small vacuum leak somewhere or your fuel pressure is too low at idle speed, basically starving the engine for fuel. There is a remote possibility that your TPS is sending a faulty signal to the ECU when the throttle body is in the idle position. Also causing a rough idle.

Your idle should be anywhere from 750 - 950 rpm. 750 for a manual trans, 950 for auto trans.

Now you have another problem. When you do find the cause of your low idle, you now have to cosider what that mechanic did tou your engine to "adjust" your idle to begin with. That could make matters worse and cause a fast or high idle.

Good luck my friend.
 
Changing the head will fix a cracked or warped head, but will not fix bearings that may have been damaged by coolant that leaked into the sump because of a cracked head. There have been numerous threads addressing this problem. Bad oil pump, maybe, but wiped out bearings may also be the reason for poor oil pressure. And, what oil filter? No FRAMs, please, although I don't want to open that can again...
 
after the cylinder head was replaced I began to get the P1294 - rough idle error. The jeep would run at 750 rpm and rpm would fluctuate slightly. I changed the IAC, cleaned the throttle, and ran some sea foam though the throttle, checked for vacuum leaks (could not find any) and still got the P1294 code.

last week the mechanic said he adjusted the TPS to clear the P1294 but the idle is now running only at 500 rpm and I now get the check gauge light with 0 psi.

I stopped by the shop and the mechanic said to drop the Jeep off so he can re-adjust it. Said he needs to use a meter to make the adjustment. I am worry that if he brings the rpm back to 750 that the engine light and P1294 code returns.
 
If the P1294 code comes up and your mechanic can't diagnose and repair it, maybe you need a better mechanic.

1996 and newer OBD-II, the TPS is non-adjustable.
 
So today I got the Jeep back from the shop. A week ago I dropped it off for a P1294 roguh throttle code - it was reving at 750 rpm with fluctuation. When I picked it up last week they made some adjustment and the idle when warm was down to 500 rpm followed by a Check Gauge light and 0 psi (after the Jeep is warm). Previous post says RPM cannot be adjusted so I need to check with them on how the reduced it.

Today the mechanic said they played with the meter and tried to make some adjustments but I see no change. The idle is still around 500 when it's warm and the check gauge light will come on with 0 psi. They mentioned that if they got the rpm up (not sure how base on comments here that the rpm is not adjustable) the P1294 code would come up.

So... any thoughts? How can I get the RPM close to 750 rpm to avoid 0 psi and a check gauge light and avoid the P1294. I highly suspect the engine may have either a bad oil pump or is worn but she's runs great and no noise or knock.
 
Oh - would unplugging the battery to clear the computer and then run the Jeep for a few minutes reset anything?
 
So I did disconnect the battery thinking it would maybe reset the idle rpm but it did not. My idle RPM is still at 500.

Would a bad TPS cause a low idle issue? Maybe I should also remove the throttle body to clean it better. I just want to get my rpm at warm idle back to around 750 to avoid the check gauge light and 0 psi I am getting at 500 rpm. I do recognize I may have the need to replace my oil pump next but if I can jsut get the RPM back to 750 it would keep me going through the winter.
 
Well removing and cleaning the throttle body didn't do anything to improve/increase the RPM. What my mechanic told me is they loosened the screws on the TPS and with the little play moved it slightly and retightened - I guess this little adjustment lowered the RPM from 750 to 500 to clear/workaround the rough idle code issue.

I spent some time looking for a vacuum leak but could not find any. I might try to loosen the screws on the TPS to see if the slight movement makes the rpm change back to 750 and then tackle the P1294 rough idle code some more.
 
Find a new mechanic. You can't adjust the idle and the little bit they moved the TPS will have no real effect. The TPS may temporarily change the idle for a few minutes, but it will go back to what the PCM wants. When was the last time you had a tune up? Have you tried a seafoam treatment? Idle should be between 500-900 rpm, per the FSM
 
A failing battery can cause a low idle issue. Yeah, sounds dumb, but there it is. It's a Jeep thing. :D I had a low idle issue for over 6 months before the battery got bad enough where it would noticeably crank slow. New battery, problem solved.
 
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