Nearly at wit's end. 1995 XJS Convertible, AJ16 4-liter 6-cylinder, automatic trans, ~200k miles, owned two years. Car has always run beautifully. A few months ago, while driving home from a Cars & Coffee, the engine briefly cut out, just a fraction of a second, then more often and longer, then over the course of 5-10 minutes barely able to keep running, then pretty much dead. Next morning, starts up and runs fine, again until it's good and warm (20-30 minutes), then symptoms return. Next morning, fine again, etc. More detail: When the engine cuts out, the tachometer needle dips to zero briefly, the ammeter dips, and if the check engine light is on, it blinks off momentarily, too. All other gauges are rock steady. Also, it's not a single-cylinder miss, the whole engine shuts off. Sometimes it would throw weird codes into the (not quite ready for prime-time, first generation) OBD-II computer, but not always. The one code it's still throwing is P1790, and that's in the transmission computer, not the engine ECU. I've spent hundreds at a local Jaguar specialty shop, but they haven't been able to find or fix it. Here's what's been done and what is known:

• It not fuel-related
• The crankshaft position sensor (CPS, CKPS) and throttle position sensor (TPS) have been replaced with known good components.
• Air intake and throttle shaft cleaned.
• The mass airflow sensor has been checked good and air leaks have been ruled out.
• Sometimes the engine will restart after sitting for 10 minutes but the idle speed hunts up and down. In the morning, it's rock steady.
• The mechanic tried blowing cold air on various components to see if overheating was causing a malfunction, but no joy. Of course, the cam position sensor is tight up to the engine block and not likely to cool much with air.

The mechanic has thrown up his hands and said if money were no object, he would just start swapping components until the problem went away, starting with the cam position sensor and the ECU. I'm hoping the collective wisdom of this forum can either help me zero in on where to look or perhaps help me find loaner components so I can do the swaps without draining the retirement fund. Grateful for any help offered.

 

1. Coolant Temp Sensor - easy and cheap to replace.

2. EGR - more expensive but straightforward to remove. Make sure you get a new gasket with it.

Does this happen when you're idling?

 

What's the DAC # of your transmission ECU.
I have one from my 94, 4.0 you are welcome to try.
Steve
Pacifica

 

Yes, the engine will die while idling. Thanks for the suggestions!

 

[We've spoken on the phone - Steve in Santa Rosa] Thanks for the reply. I'll have to check the transmission ECU and get back to you. I hope it comes up in my OBD scanner. Otherwise, where do I find it?

What's the DAC # of your transmission ECU.
I have one from my 94, 4.0 you are welcome to try.
Steve
Pacifica

 

Ok. I thought while it was driving.

I’d like to know what other codes you’ve seen. They should be stored as pending, no?

The failing transmission ECU usually begins with the car going into limp mode. I wouldn’t get my hopes up there, although it is plausible.

When idling, do the RPMs move much. Engine should be hot, and quite frankly, the more loaded up the engine is, the more helpful it will be. AC, radio, headlights, etc. Give it about two minutes and see if you can see the RPM needle tick down. During these two minutes, do not turn anything else on or off, and the engine needs to be in gear, not Park.

This is the list based on what you’ve told me:
1. Idle Air Control Valve is bad. Cheap part, be careful when replacing as the 5.5mm bolts snap easily since they’re loctited in.

2. EGR Valve is gummed up. You can remove this part and dunk it in gasoline and scrub it up and see if it makes any difference. I’ve never had luck with cleaning a bad EGR but other people have.

3. Coolant Temp Sensor. This one is so cheap and easy it would be a crime not to try this.

4. Skipping TPS, MAF and CKPS because you’ve addressed these, as well as cleaning the throttle body.

5. I would say oxygen sensors if the car was idling rough, but it isn’t, so I’m passing on that too.

6. Lastly, this might be a stretch, but your light blue relays might be dying, which is a known issue in these cars. Your car has about 11 of them. Two of them operate the ECU and the EMS. One in the trunk, I believe works with the transmission. Probably would be worth it to just move known good relays into these spots and see if they make any difference. What do you have to lose?

 

Thanks, Vee. The engine cuts out when driving and/or when idling, it doesn't seem to care. I have not experienced the idle varying, but the mechanic described the repeatable situation where they leave the car idling, it dies, they wait 10 minutes, restart it, and the idle varies, as if hunting for a place to settle. I'm afraid that's the best description I have right now. Regarding OBD engine ECU codes previously seen, there was an O2 sensor code that never appeared after the O2 sensor was replaced; and TPS sensor and camshaft sensor codes that never returned after the TPS was replaced. The car does not currently throw engine codes.

 

@95XJSteve I had a similar issue with a '95 X300 (same engine). It would run fine, then randomly shutoff. Would sometimes restart straight away and run fine, or would stumble and die. Would startup and run perfect the next morning. Never threw a clear code, but would throw codes common after the engine cuts out like a transmission code. In my case, it was a failing solder joint in the ECU. Someone mentioned that there was EU regulations implemented around that time regarding electronics/solder and cars of that vintage would have failures in computer modules, lcd screens, etc.

Whether it is a solder joint in the ECU or something else, it is heat related.

Good luck

 

Mac, that sounds very much like my car. I was thinking about the ECU, which is expensive and complicated to replace. I believe it has to be reprogrammed when moved to a new car. I agree that the problem must be heat related. My idea is to put an ice pack on the ECU while I drive it and see if the symptom disappears.

 

I also had an issue in my 96 when I first bought it in 2010. It would randomly stall out...no codes. Sometimes at a stop light, sometimes when decelerating to a stop, other times while driving on the freeway. I started to regret the purchase, but was one day rewarded with a CEL code referring me to the EGR. I replaced it and never had that (particular) problem again.

I would still replace that temp sensor as well. Any temperature related problems could easily be attributed to this strangely crucial piece to the AJ16 fueling system.

 

I'll just add a couple of suggestions after experiencing some of the same problems. Make sure your throttle valve is properly adjusted to .002 inches. Mine would stall sometimes when I took my foot off the gas. I discovered that when the idle air control was fully retracted (warm engine), my throttle valve would completely close and block off all air. When the engine would stall the transmission light would come on even though there was nothing involved with the transmission. There is a set screw for the throttle valve. Does your gas tank whoosh (?) when you remove the cap? I found on both my cars that the Rochester Valve was clogged and needed to be cleaned. Not sure if this would cause your problems though.

 

Those are two very good things to look at.

The butterfly adjustment isn't very common, but it can definitely create the problem you're experiencing because the air the engine is getting is not lining up with what the MAF and TPS is telling the engine to expect. I would have hoped that your jag-specialist would have checked it when it was cleaned, but you never know. (I would check his work and remove the hose at the intake and see what that butterfly looks like. Don't forget to open that butterfly up and see what the other side looks like. Gunked up throttle bodies are part of the design of this engine, unfortunately. If they replaced the TPS, I would hope they would have taken the 5 minutes to clean both side of that throttle body.

The Rochester Valve is a common problem. Replacement is cheap and somewhat simple. I'm not sure if it can create a fueling issue since the fuel pump would cavitate the tank before starving the engine of fuel due to the tank's inability to breathe.

 

Vee, the mechanic did clean the throttle body and the area around the TPS, as he said he knows that’s often how they fail.

 

Steve,

I don't think it can possibly be what you've referred to as the cam position sensor (the engine position sensor). That is only required to assist starting. The ecu doesn't use the signal at all once the engine is running. You could theoretically disconnect it completely once the engine has started. The ecus don't have a reputation for failing during running, they are more likely to prevent starting than to cause cutouts.

The 2 things that do seem to have been reported by US owners as causing random cutouts are EGRs and crank sensor. You mention that the crank sensor was replaced by a "known good component". I assume you mean from that it wasn't new unit? Did you replace that after the problem started or before?

The other you might want to check is potential corrosion on the pins of the ecu and the connecting plug. That's also been reported as a potential cause of cutouts.

Good luck

Paul

 

Paul, those are great observations. The CKPS was replaced with a new component after the problem surfaced, but did not make any change in the behavior. The TPS was replaced with a used “known good” component, after which some OBD codes did not return but which did not fix the problem. I’ll have a close look at the ECU connectors as you suggest. Really appreciate the thoughtful response.

 

One other suggestion, search the X300 forum for similar symptoms. Same engine, and since there were a significantly more X300's sold there will be more info to mine for gold.

Cheers

 

Eliminating the ECU is very easy. Pull it and see if the pins are corroded.

if you wash this car a lot and if you haven’t sealed the harness entry point and protected the connectors , chances are water has trickled down. I would check those ECU connectors ASAP.

Things that can fail in the way you describe, after warming, seem to be, in my opinion, either the ECU or the could, unlikely the TPS, but possible.

Also, sorry to be negative, but when you say the TPS has been replaced with a “known good part” , what does that mean? A recent new part from another car? Because if the “know good part” is an original from another 28 y.o car, it’s not a “known good part”, that being said, I don’t see your failure mode as a TPS. My thoughts are a bad ECU. When mine had corroded pins I remember the failure mode being just like yours.

 

Spikepaga, thanks for the comments. I'll have a close look at the ECU and hope to find what you describe. Regarding "known good," the CKPS was new but the TPS was, yes, a used part. I'm not really focused on that at this point, because neither the behavior nor the OBD codes suggest there's a problem there. I'll be returning to the investigation this weekend and report back when I have news.

 

Quick update: I pulled the covers and found that the ECU connectors are clean and pretty, but the unit itself was floating, not anchored to anything, Aha! Maybe it needs to be grounded to the chassis! I bolted it firmly in place, but no joy. The engine still dies after it warms up. The ECU itself was still cool to the touch, so I don't think ECU overheating is likely to be the problem. Does this image of the ECU tell anything to you vets?

The ECU

Connectors are clean and pretty.

 

My next guess would be the TPS but it’s just a guess because your failure mode seems different ……My TPS failure more was the car literally blinking of *while driving* at speed….so it would die for a moment and come back to life. The TPS is pricy though. I would start with a CTS of it hasn’t been replaced yet