Missfire, backfire, fast idle, cutting out running like shit all of a sudden

CantSitStill

Specialist
Jul 22, 2022
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Hi Guys,

I have a really strange issues with my car after it sat for a week. I have been fitting a new android head unit which all went well.

Now when I went to take it out tonight it started fine but within 5 mins of running at idle it started to missfire or at least drop revs once or twice. It then started to splutter like mad and cut out a few times. Tried to drive it down the street and it was hesitating like crazy and then a huge backfire. It's not driveable at all. I get no codes what so ever even after 30 mins of crappy running.

I did notice a few things though which might not be related:

1. When I opened the tank after running it for about 30 mins it hissed like a lot. There was either lots of pressure or vaccume in the tank. Is that normal after it's been running for a while?

2. The idle after a while increased to 800rpm instead 650rpm

3. I heard a weird squeak/squawking noise coming from the rear for about 10 secs in 2 secs interval when the car was idling. Very strange noise. Can only think it was coming from the tank. I have heard it before normally after start up when warm for example and always goes away after 1-2 mins.

4. Low pressure fuel is good but targeting 82 psi, I knows that's high but assume its just not adjusted yet after resetting the adaptations. The rail pressure looks normal at 600-800 psi. These pressures hold even when it's stumbling so assume fuel side is OK.

It's FBO 2007 335i with hybrids/meth injection/lpfp stage 1 upgrade. Just got a new map for the hybrids from wedge and the last time I had it out (7 days ago) was running like a champ. I was very happy until now :-(

Its got 100k on the clock and I have done loads of maintenance in the last 10k miles:

New NGK step cooler plugs
New coils
Normal oil leaks fixed
Oil changed a few times
PCV valve Rob Beck
Walnut blast
Rear o2s new

Vanos are the original ones but did an ISTA test recently are they were perfect.

Main o2s are original as well.

Any suggestions before I start replacing stuff at random? I read something about the evap being faulty? It ticks like crazy on the off but I read thats normal.

Thanks!
 

CantSitStill

Specialist
Jul 22, 2022
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25
0
I'm thinking since I can see the afr's happening on both banks at the same time it would suggest its nothing to do with o2 or injectors since they operate on both independent banks so if it was one of them the afr's would only be affected on one bank. Basically it leaves the intake system or the vanos....evap valve jammed open maybe, vanos packed in, throttle plate erratic, some other intake leak but can't see it being any hoses since it was running perfectly when I pulled up 7 days ago
 
Last edited:

CantSitStill

Specialist
Jul 22, 2022
79
25
0
Have you checked for boost leaks?
Not yet but this is so random. Even with a full boost hose off I have seen it run smoother. It can run fine for say 1 min, then go back to running crazy. It's like something is malfunctioning. I'm going to try removing the evap hose off the throttle body today and blocking the port to see if it smooths out. Then I will try ISTA and see what the vanos is doing when it happens.

Will check for hoses off as well. Got the kit to boost leak check it.
 

CantSitStill

Specialist
Jul 22, 2022
79
25
0
Looking at my video again I can see the Hpfp drops to below 400 psi and the revs are above 650rpm, pretty sure the DME would never target that low a rail pressure.its suppose to be about 700psi at idle so unless the rev fluctuations are causing the rail pressure fluctuate...maybe the Hpfp is on its way out
 

CantSitStill

Specialist
Jul 22, 2022
79
25
0
Sorted. It was the evap system full of fuel and was dumping fuel in to the throttle body. Ffs. It was caused by a vent hose on inside of the tank that had popped off above the fuel bucket. It cuased the tank to only fill on one side (yet to work out why) and fuel to get sucked in to the charcoal canister. Took me 3 hours to blast all the fuel out but back to normal... For now!
 
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