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.) #143

The important ground is underneath, a Y-shaped cable that connects the sandwich plate to the front subframe to the chassis. It is commonly reinstalled incorrectly by people working in the area.

Testing grounds with a VOM is not reliable. The engine may be grounded via the throttle cable or the shifter cable, so a VOM will show good connectivity. But running starter current through those cables melts the plastic lining and the cable jams – not good on the throttle cable.

(Greg) #144

No, i’m sure I got the right one, and I tested the resistance, it is correct. And sticking paper clip in connect plug made no difference either.

in the mail for 1 week, then sat for one day. I am confident my injectors are good. SD Faircloth rebuilt and tested them, and they weren’t too bad to begin with.

I am going to investigate my injector resistor box over by the right headlight. I think it may have a bad ground, removing ground wire made no difference to engine idle, and it looks a bit corroded on the outside.

oh, and no more tests until the weekend. Now I’m paranoid about fuel wash, so I’m going to change the oil again Saturday and crank it a bit with no fuel (to coat the rings) before running the engine anymore.

(Michael Garcia) #145

…couldn’t have picked a better one to restore Greg,
it was manufactured
at the corner
style and performance…

(Michael Garcia) #146

…once upon a time
you did mention
going through a tank full of gas in a short amount of time…

(Robert King) #147

Probably time to step back and think logically. You have a relatively low mile engine, so not likely timing chain issues (unless there is evidence it has been apart). You measured good compression. If unplugging the CTS causes no change, could it be running on a fixed fuel map because the WOT switch is stuck/ hooked up wrong? I would doubt exhaust restriction unless it has been run a while in this condition, but easy to check.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #148

If this is true, I’m voting that your ECU is NFG.

(Aristides Balanos) #149

Yes indeed.
Low vacuum is an effect, not a cause.

Unless your camshafts are totally out of sync all symptoms point to a bad ECU.

I concur, open the box and check if the vacuum sensor holds vacuum,
Also adjust the Base Fuel Map to full lean and see if it makes a difference.
If not, only checking with a known working ECU would confirm if your ECU is good or bad.

(Greg) #150

I’ve got a few more things to check before I write off ECU, which I can’t get to until this weekend.

The important one is sending a fake vacuum of 15-18 inHg to the ECU. Of course, I’ll first inspect the vacuum hose inside the ECU.

I also want to investigate the intake manifold for gross leaks, and the injector resistor pack for corrosion and bad/no ground.

If indeed the issue is a very rich fuel mixture issue all the time: is the ECU not sending the correct signals, or is the ECU sending the correct signals but it’s not getting through correctly or at all. Do the injectors default to full rich if they receive no data about fuel mixture?

(Greg) #151

Interesting, I just found this in ‘da book’

EFI GROUNDING PROBLEMS: Michael Neal sends this experience: “…the radiator had been replaced and a ground for the fuel injection harness at the right front of the engine compartment had not been re-secured. It had blown two ECUs before I found this. I don’t know why but it took the ECU a week or two to blow. After I replaced the ground and ECU there were no further problems.” Reportedly, if the engine clearly is running very rich or possibly won’t even start because it’s too rich, it’s time to start looking for grounding problems in the EFI wiring.

I checked my ground there, and it was quite corroded. Upon checking it with my ohhmeter, the wire was not grounding to the frame, nor was the bolt, even though it was screwed in. The threads in the radiator brace (where the ground bolt screws into) are corroded too. Perhaps a no ground for the injection harness is the problem, and it has also blown my ECU?


Well the internal vacuum line certainly looks good! I hope it tests bad as that is such an easy fix, but I suspect it’s just fine.

If you’re concerned about rings, cyl walls and compression, you can give each cylinder a shot of oil. If you have cats in place I don’t know how bad for them this is, but I can attest to the safety and efficacy of a small amount in non-catted systems.

A different option would be fogging oil. just safety-wire the plastic straw before blasting it into the spark plug holes. If it blows off, as they are won’t to do, then it’ll wind up where you do not want it.

You can disconnect the resistor pack completely from an idling v12 and it won’t change a tick. Don’t run that way though.

Gross leaks at intake would just add air. At a single cyl would cause lean misfire there only. Wouldn’t affect all 12 like you have.

There ARE two CTS versions. One has a resistance range dang close to the ATS the other is much lower. Don’t know the vin ranges for the two. Paper clip jumper means it’s not this issue in either case.

You can buy a $50 adjustable fuel pressure regulator from Holly in the right range. This would let you manually drop fuel pressure to counter the ECUs insistence for more fuel. This would ( should ) get you idling and allow you to set timing properly at 3K rpm. Not a fix at all, but a temporary crutch.

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #153

Fired up the 1979 coupe with HE engine today, and plugged my laptop into the Motec EFI.
The Motec uses an external MAP sensor ( manifold air pressure ) and at 750 RPM idle with engine around 50deg C it registers 40 to 50% atmospheric which equates to 15 to 18 inches of vacuum.
Nothing special about this engine.

Idling at 750 RPM the injector pulse width should be about 1 to 1.5mS. I have ignition set to 6.5deg BTDC. The ignition map for the engine was based on a table I got from a forum member 20 years ago, and is in the ball park but may not track the usual Lucas dizzy characteristic exactly.

I once had a problem very similar to Greg’s. I rebuilt the 2nd hand HE engine and put a 5 speed box behind it. At the time the coupe had the original pre HE engine with the Motec EFI system. I changed over the engine, and had to change a few sensors and modify a bit of wiring. I rejigged the Motec fuel and ignition maps based on information from a member and then came the day to start it up.

I did not have the air cleaners on to give more access while getting it all sorted. First mistake was forgetting about those air cleaner bolt holes that let air into the manifold. No trouble getting the engine going first time with a bang at 3000RPM idle. One minor heart attack later plugged the holes. At one stage in the procedure of getting everything sorted I had badly fouled plugs, obviously a very rich mixture. Much time was spent figuring out if I had any Motec variables set incorrectly, any sensors malfunctioning, any wiring errors. The 12 plugs were in and out 3 or 4 times to clean and refit.

I could make no sense of the problem. Then I just started from square one as if the engine had been given to me and nothing could be assumed to be correct. Within minutes I found the plug leads were out by one, I had put the lead for cylinder 1A into the next dizzy post at 60deg delay, and all the others followed in sequence. No idea why that happened.

The odd thing is the Motec does all the ignition advance adjustment, meaning the rotor is locked to the shaft and there is no mechanical or vacuum adjustment. On a V12 dizzy with posts spaced at only 30deg ( and a consequent narrow rotor ) it is critical to get the rotor alignment exactly right so as the Motec adjusts advance you do not have the rotor missing the post at any stage.
The conventional dizzy adjusts the rotor with the timing so self compensates.
I had the old Crane Cams LED pickup which makes it easy to do static timing and a fair bit of time was spent getting the rotor adjusted to be exactly positioned when the LED signal triggered. I must have spent a lot of time on this before firing up the engine and never noticed the plug leads were wrong.

In the 35 years of owning an XJ-S I must have refitted plug leads at least half a dozen times.

Back to Greg’s situation. As mentioned twice before pulling the CTS on my stock convertible makes no difference at idle, although have not tried that while revving the engine. Looks like the CTS could be a blind alley. Don’t forget the idle RPM is really dependent on how much air is getting into the engine, and the A/F ratio must go beyond a critical limit before RPM will be affected.

The suggestion of a $40 adjustable regulator is reasonable, hoping it is easy to plumb into existing fuel rail and fittings. Small money compared with what has been spent so far. If the A/F can be fixed to give a good mixture and steady idle then ignition and timing can be tackled.

On this forum it has been quite rare for anybody to report a definitely dud ECU in comparison to so many finding other things giving grief when engines not behaving.

(Greg) #154

All good feedback, thanks, I guess for the heck of it I can move all the wires over one and see what happens, but I am quite sure 1A plug wire goes to cap port 1. And my idle was steady enough to hook up a timing light and get a reading.

I am trying to exhaust all other avenues before blaming the ECU, for sure.

Remember, when cold, car revs sorta ok, and idles rough, but idles. When it warms up in 20 minutes, it’s hard to rev and idles so terrible, it wants to stall.

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #155

Something I should have mentioned. My Motec fuel map, which must be close to a regular Jag ECU map, shows if you have no vacuum at idle the injector pulse width goes from 1 to 1.5mS up to 5mS which would be an A/F ratio of about 4/1 instead of 15/1. It would be a serious challenge to fire that mixture, and even if you could there would be LOTS of raw fuel in the exhaust.

That said, I am not sure about the tolerance on A/F ratio for reliable firing. Google hits do not give a specific answer.
My guess is +/- 30% on a warm engine. Anybody know ?

Here is something I don’t think has been mentioned. Because my convertible has given no engine trouble I have had no reason to make myself familiar with all the details. I think the injector cycle is initiated by the CPS getting a signal from the 3 tangs on the damper assembly, which is 6 pulses per engine cycle.
I guess the ECU divides this signal down to get the 1 pulse it needs to start an injector cycle every 720deg of crank rotation.

So far so good ? If you have a noisy signal that gives more than 6 pulses every 720deg you might get more injection events than you bargained for. Lots of CPSs fail and most times they give symptoms easily recognised and hence easy to fix. The CPS relies on having 3 good tangs go past it with just the right clearance to give 3 good pulses.

(Greg) #156

Lol, but I don’t have a CPS. :frowning:

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #157

Oops ! Another grand theory shot down in flames. The CPS must be for Marelli and later. I think you have a 1984 car which must be same as my 88 Lucas. I just looked up my set of “XJ-S 77 to 88 5.3” Cathouse electrical files which are an entertainment to sort out 3.6L from 5.3L data. Well, the ECU uses a signal from the dizzy via the CEI unit to initiate the injector sequence. It uses the infamous shielded wire but you already know about that. A noisy signal would be a problem, but less likely than a CPS type setup.

With a thousand feedback responses to your problem one or two of them should hit the target.
We just have to keep trying.

(Greg) #158

Mine is an 88. Shielded wire? Is that the one from ignition amp to ECU? I thought that was rpm reading for ecu. be That one I haven’t checked. Is that a simple ohmmeter reading between that plug and a corresponding ECU harness pin?

I think my injectors are opening at the right time, maybe just staying open too long.

(Philip Lochner) #159

There is no doubt that pulling the CTS enriches the mixture but we should consider HOW MUCH.

Like Richard, I have also implemented EFI (using megasquirt-2) on 3 HE engines and 1 pre-HE and I use batch injection much like the OEM ECU does. My cars idle at around 2ms. This time includes the time the injector needs to physically open which is around 1ms, so at idle the injector is actually open for only 1ms.

Warm up enrichment at around 10degC is only about 30%. That is 30% more (than operating temp = 85degC) for the OPEN time, meaning the OPEN time becomes 1.3ms (so the pulsewidth becomes 2.3ms).

However, at full throttle, the pulsewidth becomes (as Richard confirms) in the region of 6-7ms, meaning the open time is now 5-6ms i.e. MUCH MUCH more than the 1.3ms when cold.

Kirby, the V12s are A LOT more tolerant of rich mixtures when cold than when hot when they DON’t tolerate too rich mixtures . So, your observations merely confirm that WHEN COLD, the mixture comes richer still (hence the grey smoke) but when hot that 30% or so enrichment is sufficient to make the engine stall due to being too rich.

My point is (as Richard confirmed) that an ECU fuelling the engine as if at wide open throttle supplies A LOT more fuel than pulling a CTS.

(Philip Lochner) #160

Richard, can you confirm if the 1ms @ idle is open time only or total injector pulsewidth? That time is so short, that my standard low impedance injectors wont even be open yet.

Also, what injection regime are you using? Batch or Sequential, how many injections per engine cycle?

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #161

No, it’s for injector triggering. We haven’t suggested that as a cause here because it typically stops the engine dead. It cannot cause rich running.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #162

What’s the enrichment at -150degC? Because a failed CTS doesn’t simulate 10degC, it goes completely open circuit.