Car still idles rough/runs rich after 2 months work - I'm at a wall

(David Jauch) #224

Radiators are not grounded.
To my knowledge. That is, if you mean the radiator and not bolts next to it.

ECU grounding sounds good, try cleaning everything… hope that’s it!

(Philip Lochner) #225

There is NO grounding for the injector harness. There is only 12V feed for the injectors via a relay in the trunk. Injectors are grounded BY the ECU, THROUGH transistors in the ECU to a ground NEAR the ECU in the trunk, near the battery.

(Greg) #226

Then why does the book have Michael Neal pointing out 'a ground for the fuel injection harness" ? Se my above post.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #227

No, it wouldn’t. All it indicates is the engine isn’t running right.

(Philip Lochner) #228

Its not clear to me which car/model he was referring to. I can only presume that he was referring to the model which had the “injector amplifier” mounted on top of the radiator - which WOULD need a good earth.

Otherwise he was referring to a ground, but certainly not a ground for the injector harness.

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #229

I think by this stage we have checked all the possibilities from A to Z.
The most likely reason for the rich running, which is also suspected for causing the low vacuum, is failure of the vacuum sensor in the ECU.
That is one thing cannot be checked directly by an XJ-S owner, it must be checked by substitution of a correctly working ECU.

From experience retarding the ignition by 60deg or so gives the same excessively rich running even if everything else is 100%, although not sure if it affects vacuum in the same way. Right now the coupe is out of action while I change some wiring. Otherwise I could plug my laptop into the Motec EFI system, set 60deg retard and see if vacuum drops from normal 15ins to reported 10ins.
The other thing I could do is pull the manifold vacuum tube off the external MAP sensor, which tells the Motec EFI there is no vacuum, we are at WOT and we need 5 times the idle fuel.
I have to plug the vacuum tube of course. Then if the engine continues to run I can see what actual manifold vacuum the Motec reports.
Perhaps the wiring fix will be done by end of week.

(Greg) #230

Thanks, I agree.

I contacted AJ6 engineering, and they said 16CU’s are very reliable. Before sending in my unit, he told me to check O2 sensors which is common to running rich. I have tried disconnecting them, and I thought idling in Park disabled their input anyways? And I thought they were just trim.

I’m pretty sure several weeks ago I moved all the wires over one and it wouldn’t even start.
I could easily try again. I’d move them CLOCKWISE by one to make it run more advanced, correct?

Not sure if I’m wasting my time, but would a bad pickup coil cause strange issues? I checked resistance while it was off and it was fine. Air gap is good. Was thinking of checking resistance while cranking engine. Or does it simply time the injectors on/off? I can definitely hear them clicking on/off, and they increase with rpm increase. Could injector timing be retarded compared to ignition timing? I’m 99% sure I hooked up my injector harness correctly, perhaps I should use an ohmmeter to determine what is going where.

Thanks, Richard, if you can check that, I appreciate your time. Any feedback is helpful. I can’t wait to get this V12 humming along. Even running as rough as she does, I love the sound of it.

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #231

The dizzy rotor rotates anticlockwise.
The idea is to retard it 60deg to duplicate my original mistake.
You would move each HT wire by one post anticlockwise.
If !A wire was at 12 o’clock, move it to 11 o’clock.

Before doing that I would check the timing you have now.
We only need to know timing around 700RPM or whatever your idle speed.
I know it is a nuisance, you have to jack up the front of the car to see the timing marks clearly.
Maybe you can use a mirror without jacking, never tried that.

Make no assumptions.
Check and double check.

Thinking back I had my HT wires one post out probably because the “1” on the dizzy cap was not distinct. I had timed it statically with cap removed, so had no reason to doubt the timing was in the ball park, which it really was.
That dizzy cap developed a crack not many years later and was replaced.

Roger Bywater might have missed the point that your problem was super rich and at idle. Not likely to be O2 sensors, if the ECU is working the O2 can add/subtract 20% at most.

(Philip Lochner) #232

I’m not too sure of this but I believe the O2 sensor are not used while idling, nor while at full throttle.

Moving the HT wires is most unwise - unless they really are in the wrong location. They must be where they ought to be. The ONLY way to change timing ought to be by rotating the upper half of the dissy, but this only applies if the lower half is more or less in the correct location.

Bad pickup coil is highly unlikely. The pickup coil ONLY triggers the ignition module. The Ignition module (other than generating the spark) sends a signal to the ECU by which the ECU knows what RPM the engine is running at and thus how frequently to inject - not how MUCH. Checking resistance while cranking is not a sensible test at all.

Your problem is overfueling and low vacuum, possibly interlinked.

I still think you should borrow a known good ECU just to eliminate a faulty ECU (even though I still think faulty ECU is unlikely).

(phillip keeter) #233

An idea proposed by several of us way back and throughout this thread but the idea has been met with resistance. Not sure why?

(Greg) #234

I don’t know anybody locally who has a good ECU. And I don’t want to spend $300 on a used one that may or may not be good. Not sure how to go about this. I guess I could send my ECU to someone and see how it runs in their XJS?

(Paul M. Novak) #235

You could try contacting the Jaguar Clubs of North America affiliated club in Seattle and see if they have any nearby members with an XJ-S with the same ECU that might be willing to help. That may be a long shot but certainly one worth pursuing.


(mike90) #236


I have a spare ECU laying around, from a 1989 with Marelli. AFAIK, it works; I keep it around to sanity check suspicions when diagnosing engine behavior (‘uh, must be the ECU…’ so I swap it and confirm or deny). I can send this to you if you want to check out your setup. PM me off line and we can get it done.


(Greg) #237

Really appreciate that Mike :slight_smile: But I don’t think it will work in mine, as it’s Lucas Ignition, 1988. DAC4118 16CU.

Don’t worry guys, I don’t mind sending my ECU to AJ6 Engineering to test. About the only place I would trust. He said if it does need fixing, for about the same cost I could get an enhanced ECU. :slight_smile:

Jaguar Clubs of NA, joining fee would be about the same to test out my ECU.

I appreciate everything, I will spend this weekend exhausting and triple checking everything, before probably sending ECU out to AJ6 for testing, and then I’ll be quiet without an ECU.

If anybody lives in the Seattle area, I’ll gladly buy you a case of beer for your ECU loan :slight_smile:


It sounds like you have a solid plan. Here’s hoping your ECU is bad, I guess? It would be a simple fix if it is I suppose. I’m very interested in the conclusion of your thread. I’ve an ‘86 that had idle stability issues and rich running despite a litany of parts offerings and cash sacrifices.

While you’re doing your triple-checking, maybe give those cylinders a quarter shot of oil each and see what happens.

(Philip Lochner) #239

What did you problem turn out to be?


I wish that I could provide an adequate answer to “what was wrong?”

Likely a combination of components that were slightly off spec, high fuel pressure, leaky vacuum lines, motor had sat for 15 years. All of the above?

Recently however, she is much better! Things that I have done would be too long a list. Things that made a significant difference and which I credit with the improvements…

Adjustable FPR. Set at 30 psi, rising to 37.
New set of injectors. Liberally seafoamed regularly.
ECU idle adj 1/3 from full lean.
Timing lowered, vac adv total add upped.
Went to the ported vac adv scheme Grant ran.
And the biggest improvement to idle, Engine Restore ( oil treatment ) in cyl 1A, 3A every morning for two weeks. Rings may be worn or have just been sticky.

Poor running on a MAP sensored v12 seems an unescapable downward spiral with poor running = more fuel = poorer running = more fuel.

I still have an occasional miss. So much more steady though. And power is up significantly across the board.

(Michael Garcia) #241

…the whole
poor running = more gas addage…
this is why l think the exhaust is packed with carbon,
cant breathe
and when you hit the accelerator,
with good compression,
it forces the exhaust out.
It gains momentum.

Honestly think the p.o. drove it at low r.p.m. with low octane gas for a fair amount of time.

(Michael Garcia) #242

it can not exhale…

(David Jauch) #243

Like broken or plugged cats, but should leave enough passage at idle and build up pressure with rpm and load - sounds good, but I don’t think it can be that bad?