VQ25DET, starts fine then starts missing after 20-50 secs

[TurboHicas]

First post in here but I've read quite a few. Had 2 Skylines, 4 x C34 Stageas and just current M35 last 2 years. Missus still has a C34.

This one, VQ25DET, started missing about a year ago and a new set of plugs fixed it, completely.

Recently it has started missing when I get right up it - like passing with my foot down when the revs get over 5 grand.

That made me think about the fuel filter, if it was restricting flow when everything is sucking full thirsty.

Did another run that I do, bout 900 km round trip, and just a few km from home it suddenly started missing like a bastard on idle. Then it started missing under throttle and was ugly but didn't get any worse. I nursed it home in 3rd, spinning her over with light throttle to minimise the uneven-ness. Anyway, situation now is, it starts and runs fine - from cold - for about as long as it takes warm up the exhaust.

In about as long as it takes for the exhaust to cool off, it will start and run even again, but for less time.

Ran it for about 10 minutes as I parked it up on a ramp to get under it. After running longer, it would start and miss straight away 20 minutes later. Put my hand up its skirt - turbo was still nice and warm.

So it seems heat related and I don't know if it could be the oxygen sensor, a coil breaking down from running with huge plug gaps, the fuel filter - mmmm nahhh. Gave the air mass sensor a few whacks - that didn't fix it.

Any ideas or "try this to see if"s ?

[Leroy Peterson]

In just about every M35 random engine issue thread:

 

- replace or test a different MAF

- replace or test different coil packs

 

 

[GTSBoy]

It will be coils.  They get hot, they suck balls.  They cool down, they work again.  Rinse and repeat.

[TurboHicas]

Ok, thanks, I'll get new coils. ?

[Sack87]

Coils

[TurboHicas]

Nahh, isn't coils. Put in a new set. Ran nice and smooth until I booted it and now it's missing again.

[Sack87]

Download Hobdrive so you can get codes and see if there are any. Providing you have an OBD reader fitted. 

[TurboHicas]

I've got a code reader thing you plug in under the dash - there are no fault codes.

I'm thinking it's the oxygen sensor because it plays up after getting warm. Could be the MAF too I guess.

[TurboHicas]

It's the MAF. Here's some info that might be handy.

While searching for info I read about a guy who had a MAF problem where the engine would run reasonably well without the MAF plugged in but ran like shit when he did plug it in.

The guy advising him said that although the engine will die if you unplug it when it's running, the system defaults to an "acceptable" mixture setting when started without the MAF plugged in. The purpose being to make it driveable to get home or to repair.

So I unplugged my MAF, started it and the engine idled smoothly on all pots.

Plugged it back in, started it and it ran like it had a rag in the intake again.

I know not all MAF faults affect the idle but it's a simple and sure fire test to see if it is the MAF when a rough idle is part of the symptoms.

[KiwiRS4T]

Yep Aussie M35 owners were importing wholesale quantities of MAFs. Don't know if they ever found a supply of acceptable alternatives to the OEM one.

[TurboHicas]

Well it wasn't the MAF sensor - put in a new one. And it isn't the oxygen sensor - got a new one from Kudos and put it in.

If I unplug the MAF sensor and start it, it runs on all pots but is in limp mode - limited to 2K revs.

Plug the MAF in and it misses.

Any ideas?

[GTSBoy]

If it is not the AFM, because 2 AFMs run the same, but plugging it in makes it run shit, then clearly it is the wiring/plug.

[TurboHicas]

Checked continuity from mouth of plug back about 20cm on each wire individually, no loss of contact on any of the wires while flexing the wires around where they are bent into the plug.

I'm looking at the ECU unless I can find a big air leak has suddenly opened up somewhere.

[GTSBoy]

Yuh, and the wire goes aaaaaall the way back to the ECU, where there is another big plug full of termination pins.  These have been known to be pushed back out of the plug by clumsy enough handling.

[TurboHicas]

My Mrs sez I'm plenty clumsy. I better check them. Where are they?

Eta. Found it. Nothing going on there. Will try replacing it.

Edited March 20, 2019 by TurboHicas

[Sturdy Sleeper]

DId this ever get resolved? lol