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

(Greg) #123

One last thing I’ll throw out there, piston rings stuck on their grooves? This car had been sitting many years, and I see a lot of carbon build up everywhere. I also think gasoline or soot is getting into the oil. And I’m getting a lot of blow by to pcv valve at idle. Can stuck rings cause low vacuum?
When I do compression test, I’ll squirt oil down plug holes, I didn’t first time.

I WILL do exhaust test too.

I know this thread is getting L o n g, sorry!

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #124

Agreed, it sounds wrong.
Yet again, I pulled the CTS on my convertible at idle with engine fully warm, and again at about 20deg C, and nothing changed.
The convertible ia an 1989 with Lucas ignition. Actually, looking up the PO’s info it was first sold Dec 1988, so really a 1988 model. The car starts and runs perfectly.

We have a mystery.

(Philip Lochner) #125

Greg, please do just one experiment for me?

Pull the tube that connects to the center of the cross pipe near the bulkhead off and block the vacuum leak with your finger but not the rubber tube you pulled off.

Please then report your observations.

(Michael Garcia) #126

mine idles at 1000 rpm
in park
750 in drive…
when l got this unit,
l looked for a gas station
that had a high turnover ratio
for premium unleaded.
one that had tanks in the ground
that were less than five years old

…do all of
the spark plugs
have about the same
amount of deposits on them…

have all injectors been cleaned…
maybe add 104 octane boost to less than 1/4 tank
see how it behaves…
keep over a half tank of gas,
especially in winter months,
to avoid condensation build up…


I feel like you’re really making progress here. Good damper, timing is controlable and predictable, new amp takes a variable out of the equation, strong spark is more than enough for idle, tripping throttle full load is not stuck on, so many possibilities eliminated!

Personally, I suspect your pickup is just wire-color coded backwards. Can you take a pic of where the red wire is soldered to?

I believe that you may need to confirm the wiring for your CTS again. Not that I don’t think that you didn’t do a proper job, but because the CTS, ATS, and TPS are all interconnected in the harness. And you appear to have a failure of both the TPS and the CTS.

ECU pinouts…

5 CTS signal
7 TPS signal
21 ATS signal

A compromised circuit feeding 19 would take out your TPS and CTS.

(Greg) #128

I am busy with work this week, so won’t be able to test her until Saturday. I probably need a break :slight_smile:

I’ve only pulled 2 spark plugs so far, but they were both covered with soft soot. I cleaned one off, put it back in, one day later totally covered again. Injectors have been cleaned and overhauled. I also verified injector harness is good.

A bit of history - this car has 48,000 miles, first owner 1988-2012. So that’s only 2,000 miles/year. And I know they lived on an island. Looks like they sold it after it failed smog twice (running rich). I don’t have any records on why. Second owner let the car sit for 6 years. And now I have her at 48,000 miles.

Regarding circuit to 19 on ECU, how can I further test it? I was getting the correct read out at the harness pin. Are you saying check it again? Or is there another test? Or do I need to check ECU?

I am still confused:

  • Is my super low vacuum (under 10 inHg) causing the fuel mixture problems???
  • Or are fuel mixture problems causing my super low vacuum?

What leads me to believe I have more of a vacuum issue and not a fuel mixture issue, idling at 10 inHg, when I rev it I can only achieve 10 inHg also. When letting go, it jumps up to 14 inHg and falls back to 10. So it all looks good, it’s just about 6-9 off. And although smoother at 1500 RPM, it still sounds like cylinders are randomly missing. To me, that’s valves/compression issues. And remember, something like gas or soot is getting into my oil. It looks like diesel oil within 1 week! But I’m no expert, can a very rich fuel mixture cause low vacuum???

So next weekend I am going to test compression and exhaust. That should answer my questions. For all I know, it’s a bit of EVERYTHING.

Oh, I assume the timing chains on these cars are pretty solid? They don’t jump chain links easily, do they? (my valve timing could be off?)

One last clue, which makes sense to running rich: Car runs bad when cold, about 7-10 inHg, but I can rev it. Once car gets to operating temp, like 180-190F, car runs terrible, can barely hold an idle, vacuum is 5-7 inHg, and I can’t rev it anymore.

First time restoring a car, and I picked a Jaguar V12 :):rofl:

(Philip Lochner) #129

Greg, the MAIN cause for this is overfuelling. A faulty CTS or CTS related issues won’t cause sufficient overfuelling to foul plugs that quickly.

I believe the vacuum signal from the engine is somehow not getting to the ECU causing it to think the engine is at full throttle at all times. This COULD cause sufficient overfuelling to foul plugs that quickly,

I once bought an XJS to use its engine for a Cobra replica I was building and that car’s vacuum line was disconnected at the ECU. It was not happy at idle, but ran, gobbled fuel but ran surprisingly smooth, albeit underpowered. That experience sounds so similar to what you are having.

(Greg) #130

I’ve tested that, I connected a long vacuum line directly from cross pipe straight to ECU. No change. There is also no change if I disconnect that vacuum line to ECU.

I double checked CTS: at 58F, the CTS is reading 3010 ohms. At ECU pins 5,19 it reads 3020 ohms. When disconnected, the ECU pins read infinite. When I put a paper clip in plug, the ECU pins read 0.0.

I am adding bad ECU at the bottom of my list for likely culprits, since TPS, CTS, and ATS all seem to be delivering correct signals, but disconnecting them makes no difference. ONLY difference to idle was when I triggered throttle switch and it bogged down.

(Paul Wigton) #131

Grasping at straws here: are the injectors stuck WFO???

(Greg) #132

I listened them, and heard them clicking properly, increased as rpms went up. Just have them rebuilt and tested too.

(Greg) #133

Before compression tests, two more things on my list: Intake manifold and Grounds.

  • I’m still not convinced the intake manifold isn’t adding to this issue. I’ve re0-torqued all the bolts. At idle, I sprayed a bit of carb cleaner on the port FAR AWAY from the distributor, and ‘think’ I noticed a wee bit of a drop in idle. With that big electrical dizzy right there in the middle, what’s a safe way to test the intake manifold gasket? Water spray?

  • The engine tested good for ground. But there is a ground coming up to the radiator frame at far right corner of engine, above right headlight. I tested, and it was not grounding at all. Not sure what it’s for, it’s wires ran up along radiator area. I tried grounding it with alligator clipped wire, and no difference. I am not sure where all the grounds are for this car, but since there’s a bit of corrosion on everything, could bad grounds cause ECU feedback problems? I made sure the grounds in the trunk were good when I put in the tank, but it’s worth double checking.

(Philip Lochner) #134

Greg, there is another little test you can do:
Connect a vacuum pipe to the ecu, then, with engine idling, suck on it with your mouth (creating artificial vacuum that will BS the ECU in thinking its getting vacuum). If the ECU is good, it will reduce the fueling and this may cause the engine to smooth out - at least at idle. Sucking more or less should find a point where the engine is “happiest”. Then you have proven that the ECU is ok and that lack of vacuum is the root cause.

Well, that quoted statement confirms that the ECU is indeed feeding the engine as if the engine is at wide open throttle.

So, either the ECU is faulty (which I doubt) or the engine is in such a bad way that its not pulling a vacuum enough for the ECU to pull back on fueling. It seems we are dealing with the latter.

With sufficiently little vacuum, the “full load enrichment switch” (shown by red arrow) will also activate enriching the mixture even more. However, I don’t know by how much. The point is this also won’t help matters.

I have heard that if an engine runs this rich for long enough, you get cylinder wall “washing” where the oil on the cylinder sleeves are washed away by the fuel which leads to accelerated ring and wall wear which would result in reduced compression (and less vacuum).

At this point Greg, my money is on finding out why the engine produces so little vacuum. Other than vacuum leaks, incorrect cam timing (or very “hot” cams) can also be a major culprit.

(Greg) #135

[quote=I have heard that if an engine runs this rich for long enough, you get cylinder wall “washing” where the oil on the cylinder sleeves are washed away by the fuel which leads to accelerated ring and wall wear which would result in reduced compression (and less vacuum).

This could explain why my oil looks like diesel oil, and i’ve gone down from 12 inHG at idle to 7 inHG at idle only a few weeks later. I only changed oil 1 month ago. And I have a lot of blow by at idle. I’m not driving it (can’t like this), and I only idle and rev it now and then for testing purposes, so hopefully the rings aren’t being damaged?

Oh, and when I did compression test back in Oct, and got good readings, I had JUST changed the oil and ran it a couple minutes first.

(phillip keeter) #136

Why would you doubt it?

(Philip Lochner) #137


Of course its not impossible for the ECU to go bust, like any other man made machine, I just think its unlikely that the ECU is faulty.

The test I suggested by sucking on the vacuum input to the ECU (unscientific as that may be) would pretty much confirm that the ECU is ok (on a good engine). Alternatively, Greg could try another ECU to confirm that the symptoms remain the same.


Open that ECU up. It’s just screwed shut. I’ll put a Beer on your interior vacuum tube being split wide open. This is an easy check. 15 minutes.

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #139

Washing oil off the bores with excess fuel is not good.
Two years ago I bought 10 litres of Lucas upper cylinder lube, a good price on Ebay.
My two XJ-Ss do not get a lot of driving, and in the winter driven once a month on a fine day. I reasoned the Lucas stuff was cheap compared to fuel and I like the idea it adds lubricity to fuel for protecting the fuel pump in all 3 of my Jags.
It also condenses ( I hope ) on cylinder walls and valves in the 4 weeks between XJ-S drives.

The other claims Lucas makes are probably fairy tales, but I think the lubricity claim is good. I use about 3 times the recommended dose.

(Greg) #140

Here’s my ECU. Looks shiny and clean, and had never been opened. I will check vacuum hose when I get a chance.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #141

Disagree. When the CTS failed on my '83, it belched thick gray smoke when cold and wouldn’t run at all when warm. It would easily foul plugs if run for any length of time.

I’m wondering why Dowling has completely different experience with his convertible. Could it be that the CTS was changed at some point? And if that’s the case, is it possible that Greg has the wrong CTS?

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #142

Not too long ago we had some reports of freshly-serviced injectors being slap jammed if not installed and run very soon after returning from the shop. Apparently the test fluid they run through them is NOT good for storing them. So, how long did yours sit before installation?