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

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #61

First off, I think the dizzy would have to be adjusted quite a ways, like 15 degrees. Second, I suspect the timing might wander a bit. The spark is triggered when the voltage reverses, and it reverses very crisply when the point of the star passes but who knows when it reverses between star points.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #62

Have you checked for leaks in the vacuum line to the ECU in the trunk?

(Greg) #63

Yes, I’m currently running a vacuum line directly from the tee in engine to ECU. Next I guess is pull ECU and see if I internal vacuum line is ok.

(Dzia) #64

This may be of some interest. On my Triumph, I do not use a timing light. I hook my vacum gage to intake and rotate dizzy until I get about 18 or 19 hhg for hen back off one or two inches. Perhaps a bit of timing chain stretch, damper partially off or wear on cam dizzy gearing? I can make the engine drop to about 10 or 11 if I want to. Try timing the car without the light using only the vacumn gage, got nothing to loose.


(Robin O'Connor) #65

[quote=“gregma, post:60, topic:371213”]
missed it), can the TPS cause issues like I’m
[/quote] no the TPS generally is a go/no go unit. I ad one on my ‘40 that the shield wire was exposed and caused a stall but it started straight up again and ran fine until the next occurrence.

(Greg) #66

I was thinking of timing it that way, but I can only turn the distributor so much without popping it out and moving it a tooth. May try that over the weekend. I can get vacuum to go up 3 in Hg by turning it clockwise, is that advancing?

(Greg) #67

So one new possibility - I tried swapping back my old GE module, and no diff. So as I was putting in my new GM module, I thought I’d re-splice some of the wires because they looked worn. Got it back in car, and now I’ve got a no start! Weak to no spark from coil. Perhaps a whole new ignition amp box? I’ve read archives where replacing the whole box helped. I am 100% sure I hooked up everything correctly inside, and checked my splices.

(Greg) #68

Thanks for all the advice, I’m going to take a good 10 days off and not touch it. Bit too frustrating for me, i don’t want to ‘go off’ the car.

(Robert King) #69

One easy test is to check for restricted exhaust; with a vacuum gauge connected hold throttle open for a steady 2500-3000 rpm. As the throttle is initially opened, vacuum will drop then recover to a reading at least as high as the idle reading. Holding that rpm, the gauge should not drop- if it steadily drops, a good indication of restriction. You can then remove both oxygen sensors and repeat, if there is an improvement that confirms restrictions in exhaust.
While the sensors are out, you can use an inspection camera to look at the monolith of the upper catalyst; should be a honeycomb with no accumulation of debris; if it looks plugged or melted, you have your answer.

(phillip keeter) #70


(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #71

Today gave the convertible its annual wash and polish with Canuba wax.
Ah, car makers no longer think of elegance and aesthetics when styling a car.
The XJ-S is very much a 9.5 out of 10. The E-Type roadster is 11 out of 10 but a bit beyond my means for now.

Down to business. With hot engine pulled the CTS on B bank ( I hope that is CTS and not temp gauge sensor ). On the original pre HE in the coupe CTS was on A bank.
The stock standard convertible gives so little trouble, and nor does it get driven often, that consequently I am not 100% up to speed on all the features.
Pulling the CTS with hot engine makes no difference, as to be expected. I let the engine cool down for 3 hours, but it is a warm day and the coolant probably still 35deg C. Started engine, pulled CTS and no change at all. That did surprise me, but that must be normal. In the depths of winter with a stone cold engine pulling the CTS should be more significant. Anybody tried it ?

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #72

Greg, as a general rule the kind of problem you are looking at is about 50/50 an ignition issue or a fuelling issue. You have to be systematic in troubleshooting and try and eliminate possible causes one by one.

The easiest one to check is ignition because you do not need any fancy tools and you can eliminate all the components one by one. You reported a weak spark way back and that may or may not be significant.
I made a suggestion how to check this a few posts back, and other forum members can add to it.
It is vitally important to nail this spark issue.

If ignition is O.K. you could have a timing problem, but if you get the engine running you can check it even at idle to make sure it is in the ball park. There is the other mystery you reported of the red/black wires from the dizzy being possibly reversed. However it is not clear if you timed the engine before or after the reversal. I am fairly sure if reversed you can still swing the dizzy enough to come out of it with correct timing.

Then you can start on the reported rich running at idle.
There will be an answer.

(Matt Furness) #73

Let me know your email address and I’ll send some tech info I have which may assist you. Worth a read while you are in the “contemplation” stage!!!

(Aristides Balanos) #74

In my car if I disconnect the CTS the engine immediately stalls.
This make sense as the ECU thinks that the engine is infinitely cold and there is massive over-fueling.

If the module is good changing the box will make no difference.
Something is wrong though…
When you changed the module did you put thermal grease under it ? Maybe it fried ?
Maybe a bad coil ?

Yes it is advancing.
Why not move it a tooth and try ?
From what you say I think you are running with not enough advance.
When you over-advance vacuum would start to fall again.

This is how I do it also.
Not super accurate but it works.

I think you might be dealing with three different issues here:

When Spark and Timing are sorted try adjusting the ECU Fuel Base Map.
This happened to me when I fixed a bunch of air leaks and the ECU adjustment wasn’t correct anymore causing massive over-fueling, even beyond of the O² sensors trimming capacity.

As for the TPS, do you have 0.32v at idle?
Because if you don’t and at the same time the ECU sees only 7" vacuum it would think that you are cruising along and over-fuel indeed.

You will sort it out eventually I am sure.


(Philip Lochner) #75

I am posting this reply not having read beyond post #1 and herewith present what I would ckeck first FWIW:

Check that the vacuum line from the centre tap on the manifold cross-pipe in the engine is connected to the vacuum input on the ECU in the boot and not leaking on the way. This is in my view the primary cause for such severe over fuelling as to cause black smoke;

If my comment 1) applies doing so WOULD have no effect as the engine is already running super rich. I am assuming you do know the difference between the intake air temp sensor, engine coolant sensor for ECU and engine coolant sensor for the dash gauge.

I only discovered in recent years how quickly new plugs can die, being fouled by over fuelling. Therefore, I’m saying that the misfires could be due to over fuelling AND/OR fouled plugs - not completely dead yet, but not good either.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #76

On what planet? On THIS car, a disconnected CTS makes the ECU think the engine at cryogenic temps and pours on the fuel. It should make a GREAT difference when you disconnect it. When the CTS is bad, the recommended temp fix is to short the leads together with a paper clip so the engine will run well enough for you to get home.

(Nick Johannessen) #77

Reading through this thread is an interesting look at crowdsourced diagnostics. The number of theories, tips and more or less relevant anecdotes is overwhelming! Let me offer a short anecdote and a plea:

Many years ago a good friend of mine, Jaguar enthusiast and highly rated engineer, was asked by the owner of a late S3 XJ12 to take a look at it. The car had been modified in the UK and brought to Norway. Some issue or other started it on a long and weary round from specialist to specialist, each scratching their head, adjusting here and there, and passing it on. When my friend had it delivered it barely ran. He got out the service manual, started at page one and went through each point, checking and setting. When done the car ran as sweet as a V12 Jaguar should.

What I’m suggesting is that one of you make up a checklist, from the most basic to the most esoteric, that will help find the problem(s) with this poor car, and then post it on the website afterwards. Any volunteers?

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #78

Agree about the 15deg. The rise and fall of the signal should be symmetrical, so the ignition amp should trigger reliably with reversed wires.

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #79

On planet Earth with my car. Quick check on Youtube to make sure the B bank is for CTS.
For sure when I pulled the CTS connector with engine fully warm no change.
With cool engine, around 35deg C, pulling it made a very small difference, perhaps 1 or 2 % change in RPM.
The car starts and runs well. So what gives ?

(Greg) #80

I’ve been told there’s a good checklist by a Jag Guru on the old forum as a PDF for non-starting problems, but can’t find it. Anybody know where it is?

  • I have ruled out fuel supply. I am getting clean fuel now, injectors and harness are good, and I have good fuel pressure.

  • Michael had mention clogged exhaust, but I would think that would allow it to idle well, and have issues at high rpm. I’ve got opposite problem.

  • I will replace fouled spark plugs, and double check compression is still good to rule out bad compression. (180-195 cold in October)

  • I am going to one by one check through ignition and timing, time to be methodical. But first replace ignition amplifier (i believe I screwed it up by rewiring the grounding wire that I think had diodes in it, which I removed. I could have also shorted a bad wire with my fiddling on it.)

  • To deal with the reversed pickup coil mystery, one good idea I got from a forum member is to test spark from coil in both cases and choose the one with the strongest spark. (hope there’s a difference!) I at least verified the pick up coil is giving out good resistance. (3.35 kohms) I will also double check air gap. I think I had set it at 0.012, but didn’t check all points on the star, and used a metal feeler.

Thanks again for all the advice. It’s overwhelming, but at least I don’t feel alone.