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Hot Start Cylinder Misfire

high_point

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Hey Folks,

Seems i'm encountering an issue with my Vz (6 cylinder 175 kw), i'm getting a cylinder misfiring/locking out under very specific circumstances.

On a cold start, car runs fine and will run fine indefinitely.
However, do a trip and leave the car to rest for 10 minutes, turning on the car results in an instant error code indicating that cylinder 3 is misfiring and the car starts vibrating/chugging. From the feel of it, the cylinder misfires every rotation. it isn't a sporadic misfire.

Now, I can make the fault disappear if i get a decent speed on the car and travel for a bit, then stop and immediately restart the car. On restarting, the fault disappears and misfires stop.

So from what i can guess, there's a heating/cooling issue on some part in the car and getting up to speed must cool it down.

Further info on the car:
it's been converted to liquid injection Gas, issue occurs on both gas and petrol which should rule out the possibility of it being the injectors.
The fact that it runs fine on the initial cold start makes me think that it wouldn't be the spark plugs or coil packs.

any ideas on what part might be causing the issue? Have done some reading and a common faulty part that seems to be popping up seems to be the ECU, not sure if i would have any way of testing that its the case.
 

exanVZ

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maybe you could try swapping a coil pack around and see if the problem remains on the same cylinder, or the one you moved it to, that should help you narrow it down, coils seem to be a common issue on VZ V6s.
 

rambunctious

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maybe you could try swapping a coil pack around and see if the problem remains on the same cylinder, or the one you moved it to, that should help you narrow it down, coils seem to be a common issue on VZ V6s.
Second that.
I had misfire and let it go for a while until it got worse, so changed 2 coils, only to find it was 4 coils, so changed the lot along with plugs.
No problem since.
 

high_point

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Well finally got around to servicing the car, changed all the spark plugs and swapped a coil from one of the other cylinders.

Issue is still there and on the same cylinder. There's definitely something heating up and causing the issue, whether its the ECU or the wiring or something else is the question.

Would there be any sensors that could affect a single cylinder? if it was the O2 sensors i would think that it would cause issues with more than one cylinder.
 

99notout

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have a look under the plenum as to how they attached the lpg injector wires onto the petrol injector wiring. I have seen them just twisted on, and also some left with no insulation at the join. It's a fail point as moisture gets in and corrosion sets in as well. It's worth checking before starting to blame the ecu. To test the ecu, you will need to check the pin outs, and back probe into cyl 3 output wirs and see if it has a good wave form, or just clip in a noid light for a no signal test.
 

high_point

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So heres an update after a couple of years.

was getting ready to swap out the ecu but was trying to diagnose another issue related to the gas injectors.

started poking around with a multimeter and some safety pins to try and identify the wires to the injectors, after doing all my tests and putting the harness back onto the ecu, the misfires on cylinder 3 after a warm start have stopped.

the only thing i can think of is that there must have been a slighty bent or corroded pin in the connector and me poking it with a safety pin has either moved the pin slightly or removed the corrosion. Maybe the heat was causing the pin to lift a bit and make a bad connection.

so if someone comes across a similar issue, a clean of the connectors on your ecu might be worthwhile.

not sure if this is permanent but hasn’t happened on my last 5 trips. Will post back here if it pops up again.
 

wannaeatyourbrains

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Yeah, it is good info. A guy here pointed out the same thing to me, but said it is on the connector. He showed me how easy it is to pop off the face of the connector to get at the connector pins and check them out, deal with them if necessary.
 

high_point

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Well seems like the issue is back in a way.

However, might be a new issue. No longer getting a misfire on cylinder 3, but cylinder 1 and 5 seem to be having problems.

Cold start, works fine as per usual.

Hot start, it struggles to turnover now, will take a good 5 seconds of cranking to get it going, and sometimes it just plain fails to.
When it does start, misfires are reported on cylindesr 5 and 1. Sometimes only 5.

Seems to have a bit more luck starting if i reset the ecu, but its now becoming a real pain the ass.

@wannaeatyourbrains do you happen to have a link to the post you mentioned with the connector being dismantled? looks like there's a clip or two that might pop off but might like to see a bit more info on it before i go screwing around.

Problem i've got now is i don't know if this is a completely new issue or a remanifestation of the last. Almost about ready to give up on this car.
 

wannaeatyourbrains

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Sorry, no link mate. This guy from Jordan was helping me troubleshoot a VZ, and he was explaining to me how the pins in the connectors get pushed out of place or broken often by rough hands locking them onto the computer. He pushed a few of them back into place with a little spike to square everything away, and it worked! It was a really good tip.

There is nothing to it, but. The connector has a flat black plastic face plate with all the holes in it where the pins go in. Just go to the first crack you see around the edge of it, and pry it there. It just pops off. It comes off easy as, so you don't need to break anything. Pops on again when you are done.

That's from memory. I haven't had a go myself yet cause the situation has not arisen since. But I was paying close attention.
 

wannaeatyourbrains

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Have you checked you got sparks coming from your coil packs, and your injectors are firing? I reckon I would be checking that first. You can see if the coil packs are firing if you get someone to turn your motor over for you. I don't know about LPG injectors but it is easy to see if regular ones are working. You can see if you got power to the injector plugs with a test light or LED from a kid's toy. That would narrow things down for you.

The computer is robust, but you can get them tested and repaired, cause misfire is one common issue. I had the one in my car tested by Kevin Stigant in Perth, who is a good guy.
 
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