XJ XJ8 / XJR ( X308 ) 1997 - 2003

Erratic and elevated idle rpm

 
  #1  
Old 04-11-2019, 08:06 PM
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Default Erratic and elevated idle rpm

My Jag is randomly showing elevated idle rpm in drive, neutral, park (in any gear). The downshifts also seem to occur at higher rpms as well and I need to apply more pedal to the brake to stop the car.

It happens irregularly, where I'll start it and it drives fine, but then do a hard acceleration and initiates the erratic idle problem. Sometimes it happens at start up, and sometimes it goes away by shutting the car off and turning it on again. Sport mode doesn't seem to affect the issue.

When operating normal, my idle rpms are: ~600 RPM in Drive, ~660 RPM in Neutral/Park
When operating normal, my downshifts occur at ~750 RPM or less if it downshifts to 1st gear when in sport mode.

When the erratic idle occurs, my idle rpms are: ~750 RPM and maybe a little high when in Neutral/Park

When the erratic idle occurs, my downshifts occur at around 900 RPM.


I've never messed with the small screw that manually adjust the resting position of the pedal link, so I don't think that is the issue. I've recently changed my alternator, but that's about it. No CEL's or anything. All the throttle body connections seem like they're connected tightly, not loose. I'll try cleaning the connections this weekend and see if that helps but I'm not sure why it's doing this.

Any suggestions?
 
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Old 04-11-2019, 08:15 PM
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Sounds like a vacuum leak...but possibly could be a failing TPS?
 
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Old 04-12-2019, 11:48 AM
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I would think that a vacuum leak would cause a lumpy, unstable idle all the time, but my issue only happens once every couple days. This makes me believe it to be an electrical issue. From reading other threads, it seems possible that it's attempting to complete the idle calibration cycle, which it shouldn't be doing. So I would assume, like you mentioned, throttle body failure, or a bad electrical connection.
 
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Old 04-12-2019, 12:25 PM
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I had an erratic vacuum leak, which I didn't know it was a vacuum one, until I was replacing my knock sensors and while reinstalling I noticed the lines seemed loose. I trimmed them back so they snugly affixed to the fittings and that cleaned up a hesitation issue I had. Being those lines were by the SC and not under load to leak, I didn't find them myself until then. In any case, it wouldn't hurt as a preventative measure.

While there, have you checked the adjustment on your throttle cable? Be sure to check the pinning also on the TPS.
 
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Old 04-12-2019, 07:51 PM
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Hi Erasmo,

If your scan tool has the ability to view Live Data, try watching your Short Term Fuel Trims (STFTs) for both banks. If, when the idle rises, the STFTs go lean (positive, indicating the ECM is adding fuel to the Air-Fuel Ratio due to excess air), suspect an intermittent vacuum leak or issue that mimics a vacuum leak, such as the EGR valve or Idle Air Control Valve sticking open.

If the STFTs go rich (negative, indicating the ECM is reducing fuel), suspects could be something like the carbon canister purge valve sticking open, allowing excess fuel vapors into the intake (which causes rich running at certain rpm, or mimics a vacuum leak at others), or the fuel pressure regulator not releasing properly (possibly due to internal corrosion), causing elevated fuel pressure.

Please keep us informed.

Cheers,

Don
 

Last edited by Don B; 04-15-2019 at 11:05 PM.
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Old 04-15-2019, 01:15 PM
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I made some observations about fuel and ignition parameters while my problem occurs. The car started driving fine, and then again, once I gave it some throttle, it started doing the erratic idle thing.

I noticed the STFT to be negative, and the ignition timing was set to -10 degrees while I was in park, but then went to positive timing while I put it in drive, yet the STFT continued to be negative. Could it be cutting out on fuel because of the ignition timing, or could it be retarding the timing because of the negative fuel trim?

Below, I have some screenshots (with comments) of my monitoring display from my bluetooth OBD scanner. What do you guys think?


Notice the STFT of -7.0 while coasting.

Idling at around 1400 rpm, with not throttle input.

Closed loop, using a feedback to control fuel mixture.

Notice the ignition timing of -10 degrees while in park and idling.
 
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Old 04-15-2019, 01:25 PM
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Also, I did replace both knock sensors about 9 months ago, and have changed some of the PCV hoses. I also Cleaned the EGR valve around 9 months ago aswell.
 
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Old 05-11-2019, 12:01 AM
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Just an update:

I got a failsafe/limp home mode come up after some driving today. I pulled over because of the malfunction, and turned the car off. I used my OBD II scanner to check the code, and upon restarting it, the CEL was gone, but a P1612 code came up as a Pending (unverified) fault code. I have searched online for the code and only came up with a Throttle Offset Malfunction. I also found this description to the P1612 code (which seems to be triggered conjugately with P1229):


This seems to be the explanation to the erratic idle i mentioned before.

Any ideas where to go from here?
I have yet to clean the throttle body connectors though, so....
 

Last edited by AZjag1999; 05-11-2019 at 12:14 AM.
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Old 05-11-2019, 12:23 AM
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1612 is not a Jaguar specific code. 1229 is and it means the following:

 
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Old 05-11-2019, 12:30 PM
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Here's the snip from a different version of the DTC Summaries manual - I'm attaching the full manual.

M., would you mind attaching the full manual you referenced - I'd like to be sure I have it in my collection.




Cheers,

Don
 
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Last edited by Don B; 05-11-2019 at 01:00 PM.
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Old 05-11-2019, 03:50 PM
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Hi Erasmo, whenever doing a P-code search, be sure to put Jaguar before the P-code to get a more vehicle specific relevant response in searches.

Possible causes:

Faulty Throttle Motor Faulty
Throttle Motor Relay Throttle Motor harness is open or shorted
Throttle Motor circuit poor electrical connection

P1229 Jaguar Description The Engine Control Module (ECM) monitors the Throttle Motor Control Circuit. The ECM sets the OBDII code when the Throttle Motor is not to factory specifications.

You can also use the term DTC, thus Jaguar OBDII DTC... https://www.obd-codes.com/jaguar
 
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Old 05-11-2019, 07:06 PM
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Originally Posted by Don B View Post
M., would you mind attaching the full manual you referenced - I'd like to be sure I have it in my collection.
The DTC manual is attached. Covers engine and transmission P-codes.
 
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0 - Jag Codes - Complete.pdf (197.8 KB, 2 views)
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Old 05-11-2019, 08:01 PM
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Originally Posted by M. Stojanovic View Post
The DTC manual is attached. Covers engine and transmission P-codes.
Thanks, M.!

I did have that one - it's for the Denso 32-bit EMS but is probably also mostly applicable to the earlier 16-bit EMS used on the AJ26/27, with minor differences here and there.

Thanks again,

Don
 

Last edited by Don B; 05-11-2019 at 08:03 PM.
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Old 05-12-2019, 12:33 PM
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I see it's JTP 10371. There's also JTP 10381 in case you don't have it. Pages 1-83 appear the same. The later pages about trans codes are somewhat different.
 
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Old 05-18-2019, 12:29 AM
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Any luck Erasmo? IIRC, the early years had problems with the terminals on the TB going bad. It was fixed by using gold colored pins.
 
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Old 05-18-2019, 03:27 PM
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I was able to get all the connections sprayed with some electrical connector cleaner this morning, then went for a drive and it idled pretty high at around 800-900 rpm, but then settled back to the regular ~600rpm after a few minutes. I'll see how it behaves from now on.

What is interesting is that records show that one of the previous owners had the same problem, where it would idle at 900rpm sometimes, so the dealer checked the air filter and pcv hoses, then cleaned the throttle plate and claimed the fault was not reproduced after that. This happened at 47,000 miles, then at 52,000 miles, the whole throttle body was replaced because the car was going into limp home mode. The part # for the throttle body is C2A1470EK, so I'm not sure if that is an updated part. The throttle body I have on now is only around 65,000 miles/9 yrs old.
 
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Old 05-18-2019, 10:49 PM
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If you saw the gold pins on the PCM side of the TB connections, you should be good there. I'll check my 02 sport and 03 r1 to see what part numbers their TB's have.
 
 
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