Hello Everyone!

For the past 3 months I have been having an idling issue with my 2000 VDP with only 32k miles. The car is a garage queen. I commute with my Tacoma everyday and save this car for special occasions. My Xj8 drives and runs perfect except when it idles after it reaches normal operating temperature. The engine's rpm will drop to 500 then immediately surges up to 7-800rpm as if the car is trying to save itself from stalling. It does it at exactly every 9 seconds interval in gear or not. It doesn't happen when the vehicle is in motion or just cruising without stepping on the accelerator. It only happens when the car is not in motion (in gear or not). If the AC compressor kicks on, this idle drop won't happen, but once the compressor kicks off, the problem is there again. I've already driven the car for 1000miles, and it still has no check engine light or codes displayed when I plug in my scanner in the OBD port. I've been hoping for a ck engine light so that I can figure out where the problem is. I've already replaced the throttle body and gasket, fuel pump and fuel filter, battery, all 8 ignition coils, spark plugs, and the MAF. The problem is still there. All services are done on time. One thing I do notice is that the relay for the ignition coils (in the ECM compartment) clicks on and off during this idle drop/ surge. I have switched out the relay with a known good one, the problem is still there. Is there anyone who has experienced the same issue with their XJ8 or XK8 and got it fixed? Any suggestions will be appreciated.

 

Welcome Nighwkman, please introduce yourself in the new members section. Very impressive, a 32k Jag and 17yrs old, great find.

As for this, this sounds like a MAF, but you say you've already introduced her to a new one, so that's checked off. I see you've spent a lot of money chasing this....just curious, did you get the car this way? For all you've replaced, have you checked your vacuum lines? Like the one going to the MAP on the firewall, which can become dislodged and unnoticed, if messing around the throttle body, since it comes up under to plug in. Its small and on an offset on the firewall just behind the throttle body. The pic below is upside down of it.
Do you have a Bluetooth OBDII reader so you can review your fuel trims, O2's and such?

 

Something like an elm327 is very cheap and likely needed.

 

+1 on what highhorse said about the vacuum line to the sensor and generally about vacuum leaks. See attached picture for location of the sensor (green) and the not yet connected vacuum line (red).
+1 on logging the fuel trims. How it looks from your description, the engine surge appears when warmed up - means the fuel system operating in closed loop mode. Therefore logging the fuel trims over a short period of time might help to narrow down the issue, as the idle drop would definitely affect the trims.

See also attached JTIS extract for further troubleshooting the idle drop (see symptom "Engine surge").

 

Thank You everyone for the prompt replies! To Highhorse, Yes, I did purchased the car with the idle issue. I bought the car from an uncle who could no longer drive due to his old age. It was parked in the garage for over 5 years. I had he car towed home, bought a new battery, drained the fuel tank, fogged the cylinders overnight, changed the fuel filter, coolant, engine oil and filter. The next day I tried to start the car with fresh gas, no fuel pressure, so I went ahead and replaced the fuel pump and fuel sending unit, and viola! It ran! The car runs and drives like a new car except for the idle issue. I have 2 scanners, one gave no codes but a P1000, and the other (red one) supposed to read live data but triggers the limp mode every time I plugged it in and gave me a fault code P1642 (CAN communication) and P1000. After the code is erased, everything is back to normal again. I figured it must be an incompatibility issue with Jags. I replaced all the spark plugs with Denso Iridium and a set of new ignition coils. The idle dip/ surge is still there. Replaced the MAF, still there, I hear a clicking sound coming from the ignition coil relay whenever the idle dips at exactly 9 seconds intervals, so I swapped it out with a good one, same thing. The last thing I replaced was the throttle body, again, same thing. I think the MAP sensor doesn't exist in (see picture) the 2000 XJ8 unless it's the item I am pointing at on the TB. Could this be a bad grounding issue since the relay is clicking on and off? I just ordered the elm327 suggested by JagV8 to get some fuel trims reading for you guys to analyze. I still have timing chain tensioner on my list of "things to do" for this car. BTW, my uncle told me the car was fine before he parked it.

 

That is your air assist control valve you are pointing at.
Here is a pdf for your review of Engine Management Systems...http://www.jagrepair.com/images/Trai...ual-9-5-01.pdf
What xjr2014_de has posted for a MAP is what I have also, but I do not see it on yours. Here is a screenshot from that pdf for what it says you should have. Can you review it and let us know if your assembly is the same?

 

I just remembered, that the MAPS in question is only available and used for AJ27 SC cars. Referring to the engine management training, the MAPS only comes with cars which have exhaust gas recirculation (EGR). And EGR is supposed to be fitted for AJ26 SC and AJ27 SC engines only.
That's the reason why nighthawkman can't locate the sensor in his engine bay ;-)
Since the document only mentiones the MAPS in combination with AJ27 SC engines, I don't know for sure if the MAPS is fitted to AJ26 SC engines as well.
Apparently early '97 AJ26 N/A XK8's had EGR fitted too - but it was deleted during the same model year.

 

Sorry for the late reply. That is correct, my vehicle doesn't have an EGR hence the missing MAP sensor. I am getting the bluetooth scanner this week in the mail. I will update you guys with the readings. I have a slight suspicion that my car may have a bad ECM.

 

Hi guys, I got my Elm 327 today. I plugged it into my XJ8. Like my other scanners, it can't connect or read live data. Just to make sure it's not the scanner problem, I plugged it into my Tacoma and it worked like a charm. The only code I can get out of this car is P1000 (which is a test cycle incomplete code), or P1642 when I plug in my other scanner which also triggers the limp mode. Is this an ECM issue? Is this something that I can fix on my own?

 

I suppose checking the OBD wiring etc as per workshop manual would be wise since you should be able to use live data from an OBD tool.

Doesn't sound likely at all to be the PCM (aka ECM). Go for the likely suspects...