VR6 start-up induction backfire

[JohnJ 0]

Hoping someone can help with a very intermittent problem:

 

Every now and again my '96 VR will backfire through the induction system, just as the starter is engaged. Has happened about 6 times now and usually destroys the ISV dampener box :mad: . Only happens when the engine is cold. The car starts and runs perfectly 99.9% of the time.

 

The HT system seems fine, no arcing, and the coil pack has already been changed. I have a K&N induction kit fitted but this was on long before this particular fault came about.

 

I am beginning to think that perhaps an injector is getting leaky, or even a really random ECU/wiring fault. :?

 

Any thoughts?

[Rpmayne 0]

I blew my ISV dampener box up when the fuel was primed for too long. My was a terminal ECU fault which kept the fuel pump priming and the injectors open constently. This was cured by a new ECU. Your problem doesn't sound that drastic though, could be a injector problem flooding I suppose.

[JohnJ 0]

It did occur to me that it might be the ECU throwing out bad data, but the problem is so intermittent I am a bit dubious.

 

Might just wait until the car is off-the-road for a few days anyway and I will get the injectors ultrasonically cleaned. Might even try some injector-cleaning fuel additive but I wont expect too much success!

[Kevin Bacon 5]

It did occur to me that it might be the ECU throwing out bad data, but the problem is so intermittent I am a bit dubious.

 

Might just wait until the car is off-the-road for a few days anyway and I will get the injectors ultrasonically cleaned. Might even try some injector-cleaning fuel additive but I wont expect too much success!

 

Try an inline fuel pressure guage and see how much the pressure drops over night. Should remain at 4bar, if it's a lot less then one or more injectors are leaking.

 

The VR never has been an instant starter like the 4 cylinder engines but they should always fire within 2 to 3 turns of the crank. The Crank sensor needs to line up with it's relevant flywheel detent and send a clear signal to the ECU. All my BMW 6s were exactly the same.

 

You can also check the injector open times with VAG-COM. Should be around 4ms at idle. If it's a lot higher, then there's a problem.

 

If you know anyone else with a good Corrado VR6, might be worth spending a weekend swapping parts over, including the ECU, until the problem is found.

 

HTH

 

K

[JohnJ 0]

A couple of good pointers, thanks! :D

[Will 0]

I blew my ISV dampener box up when the fuel was primed for too long. My was a terminal ECU fault which kept the fuel pump priming and the injectors open constently. This was cured by a new ECU. Your problem doesn't sound that drastic though, could be a injector problem flooding I suppose.

 

Is that a black, plastic container about 4" tall and 2.5/3" wide with a smaller breather pipe attachment? If so I did the same thing yesterday doing a compression test without the fuel pump fuse in :roll:

 

Where abouts does it go? Looks like it's broken the plastic on whatever it was attached to so I'm hoping it's not a ba**ard to fix........

[Rpmayne 0]

That black box sits between the throttle assembly and the bulk head. There's two air pipes going to it and a barbed pin which fits into another pipe just for support. If you look round that area you should see the remains of the fittings. I blew mine up and bought the whole thing for about £20. Its only a box with a bit of foam in it. Easy to fit.

[PhatVR6 0]

easy to bypass to, just bin it.

[JohnJ 0]

If you are going to bypass the dampener then be careful what material you use, dont pick anything that will pinch and restrict air flow (or you will screw up your idle speed) and remember the location at the back of the bay, over the manifold will get v.hot.

 

Speed fit plastic plumbing pipes and a couple of 90 degree bends are an easy temporary fix!

 

HTH