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

(Greg) #41

But why? I’m already giving the ECU 12" via a vacuum line directly to it, bypassing the one under the car.

(Doug Dwyer) #42

Oh, OK…I missed that. Never mind


(phillip keeter) #43

Have you considered having the ol’ ECU tested?

(Douglas) #44

Wow–that’s incredible. Are you certain it was fully out of gas? I don’t suppose any could have gotten away like down to a gravel/grass driveway that you didn’t see escape?

It just seems to me like even if it was running incredibly rich it wouldn’t use up that much that fast. I dunno though–maybe it did and if so then clearly whatever’s letting it run so rich is your primary problem!

Have you tried revving the car up high at all? If it was sitting maybe there is carbon buildup that an “Italian tune up” as I’ve heard it called could address. Your comment about black smoke before made me think of that but as I’ve never done it myself I don’t know if it really is effective.

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #45

At idle the inlet manifold pressure should be around 30% of atmospheric with warm engine.
My coupe has an aftermarket EFI system calibrated in bar where 1 bar = atmospheric pressure.
Hence 30% is around 10ins Hg, and usually expressed as a vacuum.
The setting of the aux air valve makes a difference to vacuum at cold idle.

However, the vacuum in itself is not a critical issue here, more pressure = less vacuum = bigger throttle opening = higher RPM.
No matter what, the ECU should adjust the injector pulse width to suit.

The ECU looks up the vacuum and RPM which then determines a particular cell in the fuelling map.
That cell has an injector pulse width which then determines how much fuel to inject.
That fuelling is modulated ( adjusted ) by any O2 feedback typically over a range of +/- 10%.
The Jaguar ECU disables feedback at idle ( Is there an RPM limit anybody knows ? ).
The CTS, and to a lesser extent the ATS, will adjust for more fuel with a cold engine.
The fuelling map is far more sensitive to change in vacuum than it is to change in RPM.

Plan of attack:

  1. Chase the reported weak spark.
    Connect a 25thou gapped plug direct to coil output, bypassing dizzy, and ground plug.
    Repeatedly ground negative side of coil ( KIrbert suggests rubbing the wire over a coarse file ).
    Blue spark ? If not, suspect the coil or much less than 12V on positive side of coil.

  2. Blue spark good. Connect ignition system again.
    Disable fuel pump.
    Rig up as many plugs and leads from dizzy as you can and ground the plugs.
    Simpler if you use old plugs with 25thou gaps and leave good plugs in engine.
    Crank engine and check for good spark on all plugs.
    This checks ignition amp, dizzy insulation, HT lead insulation and plugs.
    It checks the rotor in the dizzy connects to the HT post at low RPM, maybe not at high RPM.

  3. Ignition system proved good, but does not check timing.
    From previous posts timing claimed to be correct.
    Enable fuel pump again. Prime fuel rail.
    Start cold engine.

  4. If it runs rough could be the sooted up plugs reported earlier.
    Could be mixture too rich.
    Double check the HT lead connection from dizzy. I once had them all out by one and the engine started but did not run well. It took me a while to find the problem and I have often done work on the top end of a V12 - silly mistake.

  5. With my aftermarket EFI system anything can be changed easily with a laptop.
    Not so lucky with the Jaguar ECU.
    You can change idle mixture with the trimmer in the ECU.
    You can do it by changing fuel pressure.
    You could do it using a potentiometer in place of the CTS.

Frustrating situation Greg, but in the end they always get fixed.

(Michael Garcia) #46

on youtube
they show how to check for defects in o2 sensors…

(Michael Garcia) #47

…could it be
clogged exhaust system?

no joking here,
but like
the banana in the tailpipe claus?

(WayneC) #48

Greg said: “This HEI has me confused with all the wires going into it.”
It’s been awhile since I had mine out, but perhaps a couple of photos would help you some.

For wiring:

Look here for internal repair information:

(Michael Garcia) #49

…every engine needs the tar knocked out of it periodically, these v12s’ love to run above 3000 rpm…

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #50

That ain’t right. The difference between ign on and idle should be about 10 psi – unless the engine’s running really badly so it’s not pulling much vacuum at idle.

Do you have a sensor in the left side of the fuel rail that controls vacuum to the LH FPR? If so, you might try bypassing it, running a vacuum directly from manifold to FPR. See if it makes a difference.

(Aristides Balanos) #51

I would cut those cables and connect the correct way if I was you.



You set the timing to 18 BTDC @ 3000rpm with vacuum advance disconnected; correct?

Have you jumpered all the trim sensors yet? I recall you tested CTS wiring to ECU and it was good. Any change at all with CTS jumpered vs connected vs disconnected? If not, suspect ECU.

(Michael Garcia) #53

…may have run
lower octane,
quazi cheap
gas through it
for some time…

happy new year…
here is to another
trip around

(Michael Garcia) #54

worst case scenario,

as one were to troubleshoot
and make proper changes,

dont forget to
unplug your battery
for a little while

as to allow
the internal
to reset…

(Greg) #55

Thanks for all the great advice! I need to go to work today, but will do some more tests as soon as I can. From what Kirby and Steve said, I’m starting to wonder about the quality of vacuum to the FPR, so I am going to do several vacuum tests around the car and at the FPR. I’m beginning to wonder if I’m getting different vacuum on B bank than A bank?

Yes, I unplugged vacuum advance when checking timing.

The ECU is my last resort. Don’t have a spare to try. Plus I’ve read here they rarely go bad. I will at some point pull it and inspect at least. Can a home mechanic test one out?

on a side note, this is probably a red herring, but when I reversed the wires on the distributor pick up to HEI, the car started and idled smooth! But when I gave it gas, it pinged pretty badly. Then over the next minute, the idle became worse and worse, until it was idling even worse this way.

(Steve) #56

You can test vacuum to the FPR easily. Is the balance pipe fitted between the two banks? If so, vacuum should be fairly equal between the two banks but worth checking.

And again. With engine warmed up, 12 in Hg manifold vacuum at idle is VERY, VERY low. If this is indeed correct, I think you have to check the alignment of the cams and/or check for stretch of timing chain.

Good luck.

(Greg) #57

OK, I tested the vacuum lines, no difference between A and B bank. And I made sure the vacuum line was going to the FPR correctly, same fuel pressure at 33 psi at idle. Revved it, jumped to 36, fell back to 31, and then settled on 33.

I didn’t have time to let it fully warm up, but after 10 minutes of idling at about 750-800rpm, I could only get 7 in Hg. If I revved it, it jumped towards 0, and then hit 14 inHg, and then fell back to 7.

I am going to double check unplugging CTS once warmed up later today, but I remember yesterday I did that, and the engine idle didn’t change at all. Same with putting the paper clip in the wire plug.
But could a bad ECU cause low vacuum? I wouldn’t think so.

When I replace the plugs this weekend again, I’ll double check compression. In the last 2 months of testing, my coolant level hasn’t budged. And there is no clicking or valve noise listening to both banks.

I still wonder about reversing the pickup lines and getting it to idle smoothly for a very short time. I did note that even though the new pickup has the red/black wires reversed, the correct wires from the plug go to the top and the bottom of the magnetic pickup based on the original one. I may do this test again, and check vacuum at the same time.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #58

Yeah, that’s really bad. Should be 15-20. However, it’s likely a symptom rather than a cause. The subject line says the car is running poorly, and running poorly means low vacuum at idle.

It does sound like your FPR’s are working as they should, given the poor vacuum situation.

Did you ever resolve the ignition pickup situation? Those two wires reversed will screw things up.

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #59

I thought there was a report of orange instead of blue spark a couple of dozen posts ago.
If so the ignition system needs verifying.

Disconnecting the CTS with cold engine should make a difference.
That should cause less fuel and can stall an engine.
If the fuelling was too rich to start with maybe little effect.

Reversing the red/black wires dizzy to ignition amp may be negated if the engine is timed correctly afterwards. The dizzy will just be rotated a bit more ( or less ) than normal to compensate.

(Greg) #60

Got it to operating temperature (about 190F), can only get 10 in HG now at idle! I’m losing ground here.

If I rev it to 2-3K, so the vacuum goes to 0, the engine actually sounds pretty good. Not 100% smooth, but very close. It just runs rich and sounds awful at idle.

At 190F, I disconnected the CTS that I verified is working along with the wires to the ECU, not any change whatsover to the idle. Could I perhaps be dealing with a bad ECU? Could running way too rich at idle cause vacuum to go down if it runs poorly?

Regarding spark, I pulled one of the spark plug wires at coil while engine was idling, the spark was 90% blue. Perhaps just weaker spark when trying to start the car, which could be ignition.

Also, I totally messed with the timing now. I loosened the three mounting bolts for the distributor just a bit and put cap back on. With engine idling, I can get it from 7 inHG to 10inHG by rotating distributor CLOCKWISE as much as possible. Is that more advanced or retarded? Perhaps when I put the distributor back in last month, I was one tooth off? But my timing light using cylinder #1 was accurate. I also remember Kirby saying the timing marks below could be off. Ugh, ignition timing is so hard, I haven’t done it in like 30 years. Maybe I should have bought a Marelli :wink:

oh, one thing that nobody’s mentioned (or if you did, sorry I missed it), can the TPS cause issues like I’m having?