Vectra 3.2 intermittent misfire.

[paulvectra3.2]

2003 Vectra GSI, 187k miles.# Old, battered, much loved.

So, I'm engine braking down a hill on the motorway, 5th gear, 1500rpm. Check engine light starts flashing...

Curious. That's a misfire warning then. Application of right foot to accelerator shows a dead miss on one cylinder. Ten seconds later it clears, and all is well.

I drive 100 miles a day to and from work. Mostly M25.

A few days later, half way to work it shudders, and a few seconds later the misfire's back. Doesn't feel like an ignition miss (not sensitive to throttle position). Pulled onto the shoulder, turned it off, turned it on again - all good. Drove to and from work.

A few days later it returns - over the course of ten seconds I lose pretty much all power, violent shaking. Rolled onto the shoulder. Sat there for a few minutes, looking at my scan tool which was plugged in for the whole drive. Nothing really to see - fuel trims and O2 looked fine until the moment I put the clutch down and pulled in.

The only fault codes stored are misfire - sometimes it's cyl 3, sometimes 1 or 5, on one occasion 6. Got a P0300 random misfire code as well.

Gave it a brand new Bosch MAF and fuel filter halfway through this - no change to the fault.

I'm really stuck. I've got no direction to go in. It had coilpacks and plugs a year / 20k miles ago (genuine Bosch) following a coil failure.

My bit of the M25 has no hard shoulder for a lot of it, and losing an engine there would lead to a very stressful time.

I'm nervous about driving it on the motorway.

The fault never occurs other than on a prolonged motorway drive (50 - 60mph). Won't do it on the back roads - driven gently, or driving it like I stole it.

Any ideas?

Doesn't feel like an ignition miss.

First time it did it, I think was in an RPM range with closed throttle that indicates the injectors weren't being called upon to fire (injector shut-off seems to happen above 1000rpm in 5th with foot off the accelerator when operating normally). I think this points away from it being an injector or fuel pump issue.

Not the MAF (replaced).

Is this a failing crank sensor? I can't picture what else would trigger the misfire monitoring whilst coasting downhill.

When not playing up, I can drive it gently, or try to break the sound barrier and it's fine.

Any ideas?

[Propeller]

My guess:

* camshaft sensor / crankshaft sensor
* ground wire

Greets Andre

[northpole]

crank or cam-sensor(s)

[Jon1796]

Oh don’t forget ecu fault !!! and when was cam belt/tensioner last changed. a live data run recorded can show up things that can show what is the problem. o.c/v. is a good software to use on these better than some others.

[paulvectra3.2]

I was wondering about it being a cam / crank sensor.

The fact that it did it whilst coasting down a hill makes me think it can't be injector or spark related.

It hasn't thrown any cam / crank correlation codes.

When it did it the other day, I turned the car off and then immediately re-started it and the fault cleared. I have it in my head that when the crank sensors start to fail, typically you get a no-hot-start as part of the problem - which I don't seem to be experiencing, but I could be wrong about the no hot-start thing being common.

It got a timing belt kit fitted in the summer (with the correct tools to set the cam timing).

I'll have a poke around the wiring tomorrow, and see if I can get my hands on a couple of sensors over the weekend.

I've got some data I recorded with a scan tool. It shows nothing odd on the lambda readings for either bank until I'd actually pulled over after one of the incidents.

I've been driving to and from work on the back roads all week (two hours each way - it's killing me) and it won't break like that, so I've not got any useful scan tool data.

I've got a Snap-On Verus - open to suggestions about what PIDs to record / graph on a test drive. I'm struggling - O2 sensors, fuel trims, MAF... can't think of what else might be insightful.

Does anyone know the symptoms of a failing ECU? There's one for sale on Ebay at the moment, and I think I can probably do the coding / programming if I buy it (but £240 if it's not the fault would hurt a bit).

[paulvectra3.2]

Genuine GM cam and crank sensors on order.

Is there only one cam sensor? Or is it one per bank? Car's parked out on the road and it's dark outside at the moment.