Misfire on both banks V12 HE

Hello all,

I’ve written a few times on the forum relating to issues on my 1985 XJ12, but once an issue gets resolved a new one pops up (see e.g. thread relating to galloping after distributor service.

My current issue is as follows:

  • Misfire on both banks.
  • Somewhat difficult to start, needs throttle input first time starting to get to an idle, but retains idle once started.
  • Serious shaking due to the misfires.

During the past winter I serviced all injectors with the Mr. Injector overhaul kit, i.e. new filters and hoses. During the service I also tested the injectors, all hold under pressure, give a decently uniform flow rate, and work as expected. All injectors also seem to fire when mounted in the car.

Distributor cap and rotor changed. Old rotor had a few mm. wiggle when mounted, and old cap was over a decade old. Spark plug leads have also been changed. Coil was changed for single style coil from SNG, although it seemed to struggle to produce a nice spark. Old dual coil system back in the car, with nice spark.

Spark plugs were changed a few thousand km. ago, but were removed to check and re-gap. Given the issue with the coil and the difficulty in starting, they reeked of fuel and were noticeably wet. Spark plugs were cleaned with brake cleaner and a hard toothbrush as well as re-gapped to correct spec.

Compression was checked a while back, with all cylinders seeming to perform well, all in the 200psi range, with one (B2) down at approx. 170 psi. Will most likely do a recheck of compression in the upcoming days.

Timing was checked and set to approx. 20* BTDC @ 3000rpm, with vacuum advance disconnected. I’ll recheck the timing given that 20* is a bit too advanced, however, I don’t really feel comfortable doing it before the misfire issue has been, at least somewhat, resolved.

Once running the engine responds well to throttle, rising freely and rapidly in RPM with throttle input; however misfire persists in all rpm’s. No known vacuum leaks, and can’t really hear hissing around the engine bay. Idle is at approx. 750rpm when warm and slightly above 1000 cold, AAV has been removed and appears to function correctly. Butterfly valves correctly set and cleaned.

TPS was recently changed to the newer version, but measures slightly below 0.3v, rises correctly up to about 5v with WOT. When giving throttle, and exceeding 3500rpm (although briefly), A bank gives off black smoke, unclear if it’s due to sitting without driving, or if it’s unburnt fuel.

A few more observations:

  • CTS measures correctly with engine heating up.
  • Intake temp. sensor seems to be a CTS, measures 2.4kohm. Engine appears to run more rough when disconnected; but there appears to be no difference between plugging it in to the sensor or shorting it directly.
  • No difference when removing vacuum from FPR’s (except for increased idle, which goes away when covering the hose).
  • TPS and CTS wires to ECU appear to be intact when measuring resistance and continuity at the ECU connector.

To further diagnose the issue I’d need some help from you, namely:

  • How can I track down which cylinders are misfiring?

I’d like to avoid just pulling plugs and seeing if anything changes, since last time I did that the HEI module started acting up (difficult to find a replacement which wouldn’t overheat) and my rev. counter died and I had to buy a new one.

  • Where can I measure fuel pressure on the V12 HE?

I’d like to measure it post-injectors, since It’d be easier to depressurise the system, and since, in theory, it’s the fuel pressure post-injectors which would indicate if any of them are leaking, and if they are actually getting good pressure. I can’t seem to find a good place to measure the pressure, since all hoses on the right hand side seem to have specialised fittings.

Furthermore, do you know of any issues which would/could cause similar problems? I’ve had trouble finding any similar posts on this forum or other forums.

Best regards!

Are you sure your firing order is correct?

The second coil maters only above 5.000 rpm.
Engine should work fine at idle and low rpm even with just one coil.

Yes, checked a few times.

When i disconnect air temp sensor which means cold air, idle rpms go up with the richer fuel mixture. If yours gets worse, i would think you’re running too rich? Is your B bank fuel pressure regulator working correctly?

When my girl experienced persistent multiple misfires in both banks following a lengthy and complex restoration it was suggested the perhaps she needed an Italian tune up. It was explained to me that with all my tinkering and adjustments, followed by that short but obligatory “start her up and see how she goes now” I was carbonating up the plugs by not getting her up to temp.
Attributed to Enzo Ferrari, the Italian tune up is where you simply put you motor through an up to temp high revving session to clean her out. I.e.Take her for a hard drive.
I didn’t want to risk damaging something so I just let her (lumpy) tick over idling in the shed for about an hour and revving her every 5 minutes. The misses slowly dropped away and she smoothed out.
Just a thought.

I’m not sure, since I don’t really know where to measure the fuel pressure, it doesn’t change with/without vacuum. I have a spare lying around so I think it might be wise to try to replace it and see if there is any difference. The richness might also explain the fouled plugs and black smoke when revving hard, but hard to tell if it’s just old goo being blown out.

It hasn’t run for more than approx. 10 minutes at a time, so I think that might be worth trying, thanks!

So you can’t measure fuel pressure, unless you jerry rig a tee in the rubber hose coming away from fuel rail.

Vacuum to the FPR should lower the fuel pressure a bit. If you remove the vacuum (plugging up the vacuum line) the car runs much richer. This would assume full throttle.

If you notice no change, then perhaps your regulator is not regulating and so you’re running way too rich.

Without reading any further, these two bits of information say that you aren’t admitting enough air. What you have assumed are misfires is actually the engine barely stammering to run at ~500rpm and that is why it is shaking. I’d clean, check and adjust the throttle openings and make sure enough air is being admitted both through the throttle and via the air valves. The AAV is temperature controlled plus there is also an additional electrical air valve on the Bbank when it cranks. There is guidance on the Roger Bywater website for how to adjust the throttle stops and the throttle linkages correctly.

The coolant and air sensors are Bosch sensors and have the same calibration curve which is published in many places. The primary one for fueling is the coolant sensor and the air sensor will have smaller adjustment input to the fueling. Pulling either or both will result in rich running. Greg is correct in what he says about fuel pressure:- it’d run rich with fuel pressure ~50kPa higher at WOT than at idle if its manifold reference were removed. Any of these being out of spec would lead to richer running at idle.

Pulling plugs, though inconvenient, does give you useful diagnostic information.

kind regards

Hi again!

I’ve gone ahead and filmed a short clip which show the current status of the car. It’s idiling at just above 1000rpm’s, which is expected since it’s briefly after starting. You can see the engine revving freely, and barely make out the shaking (it’s considerably more noticeable IRL).

IMO the most important part of the clip is when I hold my hand behind the exhaust. As you can hear, the engine misfires, but not in any regular order, it seems sporadic and like the misfiring cylinders are shifting between one another. I have an IR thermometer, which I tried to use to measure the temperatures of the heads/block next to the spark plugs. The temperature seems uniform, some cylinders are max 10c off, but catch up after one or two minutes.

Since my last post, the following has been done:

  • B-bank FPR replaced, no noticeable difference.
  • TPS has been readjusted to fall within spec (was previously at aprox. 0.28V changed of 0.34v (within spec and moving freely through the range)), no noticeable difference.

The RPM gague seems to be correct, and idling at aprox. 750 when warm. Misfire continues through rev range, which would indicate it not being an issue related to low rpm?

I’ve also gone ahead and rechecked the gap of the butterfly valves, which is correct, and installed the new nylon bushings which have been recommended on the forum!

Attached is the video, about 50 seconds long. The voltage increased just after stopping, and is basically in the middle of the voltage gauge. On the electrical side I’ve also tried adding a second ground wire from the battery to the body, but with no noticeable change.

Best regards!