I have a 1996 XJS with the AJ16 engine.

I am experiencing a momentary voltage and RPM stumble every 32 seconds when the engine is warm and at idle.

The car seems to be fine when driving.

I have recently:
1. replaced all four 02 sensors
2. replaced EGR valve
3. replaced all six coils with MADE IN JAPAN
4. replaced coolant temp sensor
5. replaced thermostat (to 87C)
6. replaced spark plugs with RC12YC Champions - gapped at .035
7. cleaned throttle body by removing and cleaning all over
8. checked, unsuccessfully, for vacuum leaks
9. replaced air pump
10. replaced fuel filter
11. replaced IACV

Not sure where else to look. The fact that it does this every 32 seconds, makes me think the engine, or sensor, or computer is looking to do something? As if it's trying to engage something briefly.

I do sense the engine stumbling a bit when I come to a complete stop.

According to my OBD2 reader, it idles at 544rpm. I'm wondering if that is on the low end?

Could it be:
a. The TPS? I have no problems accelerating from a stop, or from anywhere for that matter.
b. The CPS? No problems starting the car.

I mention both of these because they both use magnet sensors? Could they just be falling out of spec?

c. MAF Sensor?
d. Alternator?
e. Grounds?

Is there anything else I'm missing. The regularity of this 32 second stumble makes me want to think it's something mechanical coming on when the engine goes open loop? Cold engine does not have a stumble problem.

Thanks for any advice.

 

Can you hook up our reader just to see if any codes that haven't set a light are stored. Just yesterday I got a P0116 myself which is a CTS. strange since I just replaced it last year with a PART FROM jaguar heritage in the UK. I did notice the CTS they send me looked totally different from the ones I bought before.

Anyways, have you rules out the coil connectors?

 

No codes. Stored or active.

 

Hey according to my ObD2 reader my Aj16 is idling at exactly the same 550 so that s the factory default, should not be causing the problem

 

Do the stumble change in any way with increased RPM or increased load on the electrical system?

 

I experienced the 32 second stumble on my '95. I replaced a lot of parts with little change until I replaced the MAF. After replacing the MAF, the Ecm was able to gradually achieve a good idle.

 

Hey Vee,

I had idling and rich running issues on my car and replaced a bunch of stuff too (but nothing like as much you did!) - anyway I decided erasing the saved ECU fuel maps by doing a hard reset was worth a try.

After I did the reset, I started her up and let it idle.

The revs fluctuated between 400 and 1100 then gradually less and less and finally stabilized at the right level. It took the ECU about a minute to learn the idle setting.

I can't say if that might be the problem on your AJ16, but it worked on my AJ6.

Check out this thread. 3rd page has best explanation

https://www.jaguarforums.com/forum/x...d-reset-38112/


Larry

 

Not that I can detect. While driving, I sense no dip in RPMs nor can I see a dip in voltage. This problem is only detectable when idling, either in gear or park/neutral.

When I took to a Jag Specialist in Springfield, VA, they used a PDU to reset my adaptations. I took it there this past August for them to resolve my idle issue, it has crept back. So much for experts. I appreciate the recommendation.

 

That sounds like GREAT news.

I happen to have a spare MAF. I will install it tomorrow and post the results. What are the chances that the other MAF is bad too????

 

Don't forget to check the MAF connector too! It sits in a bend possition

 

MAF was replaced with no change.

The RPMs and Voltage just drop every time I come to a stop.

Can it have something to do with loose belts?

 

I can't say for sure, but when I had a stumbling idle issue, it stumbled every 32 seconds. I believe that may be the Ecm cycle. It may adjust the fuel map every 32 seconds in its search for one that brings the fuel air mix to defined specifications. One sensor you haven't replaced is the crank position sensor. It is one of many inputs the Ecm uses to run the engine. You may want to disconnect the battery first to force the Ecm to relearn everything before dropping $100 on a CPS. These are mysterious beasts, especially since we cannot see what the ECM is thinking.

 

There might be some helpful information here:

JagFORUM Logon

Read geoff's post about the voltage dropping as the car would warm up on one of the coil connectors.

Maybe its time to start measuring voltage?

I would not be surprised if it's a wiring or connector problem.

 

The CPS is the last part, isn't it?

Im the meantime, I will start taking readings. Shove a pin in the back end of the connectors and alligator them to a multimeter?

 

When I had this problem on my 1996 4.0, I replaced all coils and plugs, no change. I cleaned the throttle body. No change. Replaced the IAC. No change. I replaced the CPS. Fixed the problem instantly. Ran about 18 months with a perfectly steady 600 rpm. Developed a high idle this fall and I replaced the EGR valve and fixed that. She runs fine and idles smoothly at 600 rpm again.

The one thing I have purchased but not installed as yet is the TPS.
This problem seems to bug quite a few '95-'96 4.0 cars. I was about to give up as it can be very frustrating.
Remember all these devices are pretty old as electronic things go and living in a very hot engine bay.
Just keep the faith. Post if you have success. Cheers, RagJag

 

I installed a new crankshaft position sensor. No dice. Same problem.

I don't know whether this was happening before or not, but the voltmeter needle now drops when shifting into gear from neutral or park. Is that normal?

I guess the TPS is the last sensor to replace. Knock sensor worth considering?

 

Well I turned the car, not engine on, and disconnected the IACV just to see what would happen. When I started the car with the disconnected IACV, the engine runs at a higher idle, but the dip is gone. Idle is at 700rpm in gear and 900rpm out of gear. (just looking at the needle, I didn't have anything plugged into the OBD2 port)

I think that would rule out any kind of power draw or alternator problem, no?

Something is clearly directing the idle to drop, or the idle is set too low? I know Mark's car is set to 550rpm, and runs well, and mine is just a hair below that.

I still have a TPS that can be replaced... (Knock sensor too?)

 

Does your throttle body have an air bypass screw just to the right of the I a car valve?

 

Sorry. The word prediction messed up my message.
Does your throttle body have an air bypass screw just to the right of the idle air control valve?

 

I only have a 3mm allen screw on the left side of the of the throttle body. It is only accessible with a modified (cut down) allen wrench. It seems to affect where the throttle spring rests.

Does that make sense.

I definitely do not have the adjustable screw that is accessed by the top of the throttle body.