You will need to upload the video to YouTube or so, and then insert the link.
Would the ECU show any codes or give some cry for help (i.e. a startup check) if it was shut down due to a bad relay, or other sort of power source? Just a split second of disconnection per each misfire.
What happens when less relevant sensors are disconnected one at a time, I don’t see what swapping the lambda sensors would do? It seems as if the machine doesn’t register that it drops out on occasion, so it can’t detect the misfire because…
The OBD2 system isn’t very generous with doling out codes, however, in my experience, its been very consistent with spitting out a code when there is a misfire, at least due to coils or spark plugs. I cant say I’ve ever had anything else ever cause a misfire, but I’ve never experienced a misfire that didn’t flag a code.
The warning lights (seat belt, washer fluid, brake fluid and chime) activate when the relevant switch connects to earth allowing current to flow. An unintentional earth, perhaps caused by damaged insulation could cause that and the random effect could be explained by engine vibration making and breaking the path to earth. A similar unintentional earth on a wire controlling engine management could explain the misfire; a misfire caused by electrical earthing might not be seen by the diagnostic system as a fault depending on what part of the system is affected. Unintended earthing may also explain the voltage fluctuations at the battery terminals.
So - I am looking for a location where warning light and engine management wires are physically close together - unless the car beats me to it and earthing eventually causes a fuse to blow.
I have found a curious thing. The two wires serving the brake fluid reservoir level switch have been cut off at the multiplug and the ends coming from the loom have been soldered together taped over and left hanging in the engine bay. This was done before my ownership so is unlikely to be the cause of the present problems. But if my understanding of the wiring is correct, soldering the feed and earth together should mean that the fluid warning light is permanently “on” - but it isn’t.
Check that the bulb in the dash is present and working properly. As has been reported on many of the Jag-Lovers lists, Prior Owner and/or their shops have occassionally “fixed” Warning Light issues by cutting wires or removing bulbs.
Dash bulb is working OK - lights up as required at ignition on. Puzzled though as to why soldering the two wires together should cause the bulb to extinguish - thought it would cause permanent illumination. Will have an other look at that later but doubt it is the cause of my misfire as the fluid sensor mod is several years old.
Had another look for the misfire today. Removed ECM and checked multiplug connections again - all fine; took covers off ECM - no signs of internal corrosion or physical damage. Found a bunch of multiplugs in the body cavity at the bottom of the A pillar behind the DECM position and got my hopes up when I found corrosion on one (photos below). Cleaned it up and reconnected - no difference.
I have found a way to reproduce the warning light illumination with an almost 100% certainty - until now it was completely random. With a warm engine on tick-over if I jab the throttle pedal quickly the warning lights come on and within a second or two go out. First thought poor connection or earthing in wiring between engine/gearbox and body but no amount of wiggling looms, earth straps and cables can reproduce the effect. Interesting because the misfire is worst when flooring the throttle at low revs when driving - as revs rise/box changes down the misfire is less pronounced/disappears altogether - so there seems to be a correlation.
I’m trying to tell you the TPS may be causing trouble without being able to be bench tested. I’ve never heard of a TPS setting off a code on these cars. It is the cause of many unique problems, it seems, if you read through the forums.
As for the knock sensor, I assume bad fuel mileage and a knocking engine? The detonation would be off, so you might have jerky acceleration?
The TPS will add extra fuel if it is rotated rapidly, that is the main reason to have it.
It is also used when down at 0.5V to tell the ECU the throttle is closed, but you could do that with a $1 micro switch. If you take, say, 5 seconds from idle to foot flat on the floor and no misfires that suggests basically the fuelling and sparking are working well. Do it on the road so as to load the engine.
The TPS may look O.K. when tested with a DMM while being rotated, but maybe when you floor the pedal it has a few gaps in its response. They are expensive, and the OEM ones are a resistive track that wears out. Could try drilling a small hole in the plastic body and squirting in WD40 or similar. I fixed the '79 ignition switch that way 20 years ago.
Thanks for that clear and helpful description. I tried to clean out the inside of the sensor with brake cleaner and an airline - put it back and at first the engine ran smoothly, then misfired worse than before - throughout the acceleration range now.
On the basis that what I did resulted in cause and effect I have ordered a new TPS (probably messed up the old one anyway now) and I will report back when fitted. I’m not holding my breath though.
Another angle I am considering further is connector block corrosion. Looking at the wiring diagrams, several systems may connect via the same block; could explain why there seems to be a link between engine management (misfire) and instrument cluster warnings.
Not sure what brake fluid solvent is, but likely to be quite aggressive and not good for sensors.
Even if the TPS is part or all of the misfire problem, does not ( yet ) seem likely to cause instrument lights to malfunction.
I did suggest before running the ignition system direct from battery for a trial. On the V12s there are two firewall posts connected direct to battery. Easy to get at, and it isolates the ignition circuits from all the relays and wiring in the cabin. It may or may not cure the misfire, but does it stop the instrument light problem ? You need more experiments and more results to nail this.
A faulty TPS is a reasonable suspect IMO because the misfiring started after work on the car including compression tests with WOT. A series of full throttle openings may have precipitated a fault in a worn resistance strip. Will replace the TPS next week and see…
To recap. The engine was running fine until earlier this year when cranking before firing became longer and longer. To trace that problem I inspected/tested compressions, plugs, plug coils, O2 sensors (replaced one), fuel pump, fuel pressure, injectors (for electrical continuity) fuel pressure relief valve, CPS, and probably some other components I have forgotten about. I finally cured the cranking problem by replacing the camshaft/engine position sensor. The engine now starts straight away but the misfire and warning light issues began straight after.
My focus has therefore been that the above work disturbed something and that the two issues (warning lights and misfire) are somehow linked. The only way I can see a link is via common/shared electrical circuitry, connectors, earth straps, relays or the like. The intermittent nature of the present problems suggests a making/breaking electrical contact under vibration or electrical load/heat breaking down insulation.
I will take it one step at a time so I can be certain as to the effect of further work. So, new TPS first, then I’ll look at bypassing the ignition system.
New TPS makes no difference. Disappointing but not entirely unexpected. Have by-passed the ignition switch and makes no difference.
Can you expand on your suggestion of running the ignition system direct from the battery - I have the two firewall positive terminals but what do I need to connect them to?
Not sue what you mean by bypassing ignition switch.
My suggestion was to run the ignition system off the battery and bypass the ignition switch.
Is that what you did ?
This is a 6 cyl engine and I guess coil on plug. It will still have an amplifier to drive the low voltage side of the coils. It must have a 12V battery supply and ground connection. Also signal wires from the ECU or a separate ignition control unit. Usually there will be a relay to supply +12V to this system.
Try using the 12V from the firewall terminals to drive the system.
Somebody with a wiring diagram might chime in and give more detail.
The idea is to isolate ignition from all the strange things and events you found in the instrument cluster area. Does it help ? If it does then the problem is not the ignition system itself. It narrows the search area.
I simply connected together the live battery power input to the ignition switch and the ignition switch output terminals thus taking the ignition switch out of the circuit.
Yes - these are coils on plugs. At ignition “on” a 12v feed is supplied to the EMS power relay which switches battery power to the ignition coils, O2 sensors, the ECM and the fuel pump relay. The coils are “fired” by the ECM grounding the output wires of the coils (each connected to a different ECM terminal)
If I connect a separate 12v supply to the switched side of the EMS relay that will tell me whether the relay or anything “upstream” of it is faulty. Is that your thinking here?
