Engine randomly quits completely and then catches itself at warm/hot idle (runs much better stone cold), doesn't seem to have any rhythm to it.. the more throttle you give it the more it clears up.
The odd part to me is it started out of nowhere, went for a cruise on the highway and it was running great, started missing when I went to park it.
- headphones recommended

Things I've tried since the issue started
-TPS - replaced with a hall effect sensor like Kirby mentioned.. somewhere (write-up and cad files coming soon!)
-rebuilt injector harness - needed to be done anyway
-checked pcv system - seems to be functional
-regapped plugs and cleaned seats - gap was .035" now .025" - no difference
-overrun valve springs tightened - may have made it worse?

Parts recently replaced (before issue but not immediately before)
-Plug wires
-coil (MSD blaster 2)
-ignition module (MSD circle track unit)
-distributor cap&rotor, mech advance rebuilt and greased, vac advance replaced
-throttle bushings
-all vacuum systems besides timing advance, pcv, and modulator removed
-fuel filter, regulator, and lines
-new injectors
-air rail deleted and holes epoxied off
-coolant temp sensor replaced

I still need to try hooking up a spark tester and readjusting throttle butterflies but beyond that I'm pretty stumped

edit: spark light shows consistent spark and also discovered the exhaust smells lean.. surely it's not a vacuum leak I got rid of all the vacuum crap argh!

 

If I had to bet, it would be the MSD amplifier unit in the Lucas box that is failing. A genuineGM one is probably best, but some guys have had success with the Pertronix unit, I seem to recall. Also the condenser unit inside the Lucas box can fail so worth removing that.
Also worth checking the wires that come up from the dizzy bottom to the amplifier unit, there is a molded plug halfway up and this can fail or get iffy.
Check carefully the connectors to the coil, the wires can fracture inside the insulation.
Finally check the connector and the state of the white wire that leads from the Lucas box rearwards to the loom. This gives the ECU the 'engine is running' signal.
Also check the carbon brush is still there iside the dizzy cap.

 

I have a spare amplifier so I'll probably try swapping them even though the dang msd unit is practically brand new, condenser and zener diode were removed when I overhauled the little box, wiring to distributor and pickup coil has all been replaced as well as the trigger wires for the coil.
... the white wire to ecu is in terrible shape frankly but I can't imagine it only dropping pulses at idle
I've redone the other engine harness that terminates behind the fender but couldn't find a connector for the main one, does it run straight into the body harness with no plug?

 

I watched the video.

To clarify, the condition you are trying to repair is the surging heard just before you manually open the throttle? Yes?


If the surge is what you're trying to repair then adjusting the butterflies is a great place to begin!

Cheers
DD

 

it does almost sound like a surging idle but it's not, the engine shuts off completely and comes back to life about half a second later.. it never goes above ~750rpm
also you can hear it still miss with the throttle cracked open, although much less

 

Check out the video in Post #11 of this thread:

https://www.jaguarforums.com/forum/x...-193753/page2/

Is this a match for what your experiencing? A new TPS fixed the problem in my case. Try disconnecting your hall effect sensor and see if anything changes

If not, considering all you've done already, I'd start a deep dive into wiring and connectors. Something suspicious might be uncovered. And for giggles, jiggle the wiring at the back of the ignition switch. Does the engine react to the jiggles?

I'll do more mulling

Cheers
DD

 

Replace the white wire anyway. I had it causing what felt like surging but was actually the engine cutting out. Even if that isn't the issue here, its just asking for trouble as some point if it hasn't been addressed and looks bad.

 

Doug, no the issue is different than that it would cut out completely and come back to life at ~2-300 rpm, on the verge of stalling, also not affected by putting it in gear which should rule out a vacuum leak

I rechecked the tps, it reads 0.3v at idle and 4.6 at WOT, and was getting ready to go for another test drive hoping to get better video of what was going on.. on a random hunch I unplugged the intake air temperature sensor before driving it and it seems like the issue has disappeared which makes no sense to me.

only other things I've done between earlier and now is throw the pcv valve through my ultrasonic cleaner and look at the throttle butterflies (already set correctly, though not by me)

According to the wiring diagrams the IAT does share it's voltage source with the tps, maybe a bad IAT can alter the output of the TPS?? A new IAT is on order I'll be sure to let you folks know if that does the trick..

 

Quote:

According to the wiring diagrams the IAT does share it's voltage source with the tps, maybe a bad IAT can alter the output of the TPS?

Seems plausible, at least


Cheers
DD

 

Trace the wires from the IAT, towards the TPS.
There is a splice around the AAV area, and the solder in there is woeful at best. Apprentice stuff to me.

All mine had all sorts of issues when first in the fleet, and that solder was a common problem with all 3.

That sihelded wire is important, as said, and I would not trust an original wire now its past a teenage age.

 

I've seen threads about the joint you're talking about but never realized where in the harness it actually is, would make perfect sense if that's the source of my issues! Would you recommend redoing the joint completely or just adding solder to it?

Also as mentioned before I would love to redo the entire engine harness but there doesn't seem to be a good place to disconnect it.. do I have to just cut it and add my own connectors?

 

Redo the joint completely! And if the wire to the sensor is crisp, replace that and the connector too.

 

What Greg said.

The wiring at the rear of the engine bay is waaaaay past its use by date, no matter the distance traveled, AKA, Garage Queen.

That wiring, and many other known things were ALWAYS priority #1 for all my cars. Being Daily Drivers, and the distances we traveled, just because, near enough was a death sentence for us.

Fill the beer fridge, roll up the sleeves, and get into it, repeat in 20 years.

 

The engine harnesses (two normally) are connected to the main harness by two large moulded plugs at the rear of the engine bay, somewhere about centrally between the firewall lip and the engine.

 

One of the harnesses splits into both of those molded plugs on the inside of the right fender, the other harness goes straight through and off to narnia... I've already redone the one that terminates at those plugs, it does not include the shielded wire unfortunately

 

The one that goes to narnia goes straight through to the ECU inthe boot. Not much you can do about that one, and normally, because it is not in the centre of the engine bay, it is OK.
You can trace back the shielded wire until it is not failing and renew it from there forward. A carefully soldered connection will be OK.

 

Alright, IAT replaced with the $20 unit from Moss and relevant harness rewired. Took the old girl for nice hard drive through one of the mountain passes out here and I don't think it's ever ran that well before!

Still got a pretty good miss at idle but it's an actual miss now not the engine cutting in and out.. I have a sneaking suspicion that it's compression related, sticky rings or something of the sort.. my current plan is to add marvel mystery oil to the tank/sump and drive the **** out of it

 

Always a sound plan with the V12!

 

The redline is there for a reason.

Check the spark plugs, mainly 1A and 1B, then 5A and 4B. They are SO EASILY cracked when installing, and thats why I buy a oox of 20 each time.

 

I just recently removed and re-gapped all of them, broke 3 in the process but I'm pretty confident all the ones in there now are in good shape.. they are taper seat plugs however and the surface they're supposed to seal with has definitely seen better days. How much corrosion/dirt in that area is too much?