Hello guys;

Please help me figure this one out. Car has been perfect as of late...until yesterday.

When coming to a full stop...and only when actually stopped...the engine RPM will drop to about 500 for just a second, then the engine will hesitate a bit and then EITHER get the RPM back up to 650 (normal idle) and remain there, OR the engine will stall. Never happened until yesterday, and now happens just about every time I come to a complete stop. Stalls about 50% of the time.

To answer a few of your certain questions:

1. RPM drop and possible stall only happen when in Drive or Reverse...NOT when I put the car in Neutral. So in other words, only under a bit of load from the torque converter when stopped.

2. Same behavior with the Air Conditioning on or off.

3. Re-start after stall is normal.

4. Acceleration, RPM, etc is normal in all other driving conditions.

5. Throttle body and connections, MAFS and connections, crankcase breathers were all cleaned today. No change in behavior (I keep all these bits pretty clean anyway).

6. Fuel pressure at the rail is about 42 psi with ignition on and engine off, and drops to about 38 when engine is running.

7. Short and Long-term Fuel Trims are all good...less than 5%...so there don't appear to be any significant vacuum leaks. (I have the Torque app for Android and can monitor this on the fly)

8. No codes.

Please bring me some wisdom!



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I had the same problem then I had the tube coming out of the passenger side valve cover replaced with a tougher unit (breather). Mine had a hole in it and it got freaky at the stop lights with wanting to stall after the engine warmed up.

Problem gone.

 

Thanks Jeff...I'll put that in the mental file...but my breather tube is in good shape, with no cracks.

How 'bout it, fellas?


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Bump!

I need some help on this one, guys. I'm stumped. Please see post #1 below...


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bump - someone able to advise?

 

Have you ever inspected the ECU for leaking electrolytic capacitors?

There were some crappy capacitors used in the early years and they have been known to cause problems....fortunately it is usually visible upon inspection.

While you are there clean the ECU connector contacts also.

 

Steve...I'll check that out. I've never cracked open the ECM...I see instructions on JTIS for removing the ECM, but not for cracking it open or getting to the circuit boards. Could you tell me a bit about that and give some detail on what I'm looking for?

Greatly appreciated!


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Just remove the ECU. There are a few screws on the case to remove the cover and housing.

Handle the printed circuit board gently by the edges. What you are looking for is any of the following:
-Corrosion
-Dampness
-Burn marks

Also with a magnifier and bright light look for:
-Cracked components
-Solder joints that do not flow smoothing around the components leads...especially on the connectors and the larger, heavier components

If anything is questionable post a pic.

 

Here is a picture of some severe leaking capacitor induced damage: https://www.jaguarforums.com/forum/a...or-ecm-004-jpg. It would not necessarily be that dramatic.

 

Check your throttle body. Perhaps it needs cleaning.

 

Thanks...but check item number 5 in the original post.


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OK Steve-

I've got the ECM out. I could only access the circuit boards from the side shown below...I've got a problem screw on the other side of the case.

Everything that I can see is bright, clean and shiny. No burn marks, leaks, etc...at least not to my untrained eye. I can't see everything from this view...I'm hesitant to remove the screws holding the second, face down circuit board...but I'll attempt it if you ask me to!

But what I can see looks good, as you'll see below.

BTW, this is not the car's original ECM. Manufacture date is June 1996, car was sold in October 1996, and this ECM is dated November 1996.

 

I have been fighting with a rough idle for about a month and what I found on my car was a combination of things. I had performed several tests the first was to identify for vacuum leaks and the second to identify that the MAFS was working properly and a smoke test twice to find all the leaks. I found a cracked manifold and a vacuum hose that was leaking under the throttle body (TB) and after a complete inspection I found that the TB position sensor was cracked so my TB is back to ASI for repairs. I will not know that the problem was resolves until I reinstall the TB but I have high hopes that it is. I will attach a link so you can check the readings on the MAFS if they are not on the mark check for vacuum leaks and when you think you go it check it again. If you perform this test do it in a parking lot with nothing around or in front of you….. You will understand when you do it.

Link to MAFS test JagRepair.com - Jaguar Repair Information Resource

 

Holy ECU Batman...I think I see a smoking gun!

 

yep - steve take a look at his second pic - you get a better view of that area

 

How the @#$%^& could I have missed that?!

No way to tell what problems this is or isn't causing, but clearly something that needs to be addressed. What's my best option here, Steve?

 

It is hard to tell positively from the picture, but I think you have some moderately severe PCB track corrosion damage. Though it is possible, that is difficult to repair well for something as critical as an ECU. In addition all the electrolytic caps should be replaced. Then there could additional be non-visible damage from short circuits.

Given all of that, I say a rebuilt replacement ECU is your best bet. If you get a used one, you run the risk of going through this same scenario before long.

 

You know that I defer to you on all matters electrical. Do you know who the preferred vendor(s) would be for a rebuilt replacement?


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Gus has more experience with the rebuilders than I do, and he always recommends ASI: Automotive Scientific Inc. Your #1 Source for ECU, TCU, ABS, EBCM, Climate Control and Speedometer Repairs

 

Wow I did not see that! Yes ASI so far has a good reputation....