Misfire 945 1997 230FT

[jlawler]

My beastie has developed a misfire and I can't work out what's causing it.
It started being a bit rough about a week ago, and it would last about a minute and then run fine. I put it down to damp and to check out the leads. It really felt electrical as the feeling when driving was sudden/jerky not a fuel like power drop. Not really dependant on speed but a bit more noticeable at constant speed rather than when accelerating.

Then this morning it failed to start. It flooded but I got it started once I blew a couple of plugs out. Now the misfire won't go away.

So I have replaced or sapped (from the lower milage donor car)
Plugs (new)
Distributor cap (new)
Rotor arm (new)
HT leads swapped with an older set but still good, no change
Coil swapped, no change
MAF swapped, no change

I found the air inlet hose at the turbo had perished and was probably leaking I thought "Ahaaa, this must be it", so cleaned up and remade the joint (must replace). But this had no effect either. I also disconnected the battery after this in case the mapping had deviated a lot (I had that with an inlet manifold leak previously).

OBD came back with 1-1-1 for Injection and Fuel (so nothing obvious there).

Swapped the Fuel pump relay (just in case) but that's not it.

I don't think it's the Hall sensor as I have had one of these go on a previous car and I could see it on the Tacho (jumps down to zero) and this is steady with this misfire.

So now I'm stumped.
What would be the next thing to look at? Fuel Injectors (I have a spare set)?

Thanks

Justin

[Selby]

Plugs gapped correctly?

[jlawler]

Replaced with new plugs, Volvo originals and all looked Ok.

[360beast]

What colour is the spark?

When was the fuel filter last changed?

What condition is the rotor arm in?

[jlawler]

-Spark looks orange on 1,3 and 4 a bit better on 2. Also tends towards the outside of the gap but I was resting them on the heat shield above the exhust.

-probably 60 or 70k on the filter I changed it when I got the car

-replaced the rotor arm with a new one (no change) old one only had about 30k on it. I replace them every 2 years,

[jlawler]

One thing I did notice when I was checking the fuel pump.
I disconnected the rubber hose (the one that comes from the metal pipe)
When I turned the engine over fuel came from the rail side i.e. not from the rubber hose I'd disconnected.
Is this coming from the return line?
Started and stopped with the ignition so I assume the pump is Ok (it was replaced about 4 years ago).

[360beast]

That's a weak spark, it should be blue or white not orange.

If you disconnected the fuel hose that goes straight on to a metal pipe coming from the fuel rail then that is the return line so yes it will come from the rail.

If you disconnected it from the metal spaceship looking thing (a non return valve of some sort I believe) then that is the feed line. It is nigh on impossible to remove the line without cutting/heating so to remove it you would have had to use a 14mm spanner to hold the fuel rail and a 17mm spanner to undo the line on to the fuel rail. I'm guessing you removed the return line then, so yes that is fine for fuel to come out of the rail.

[jlawler]

Thanks for the information.

What would cause a weak spark?

[360beast]

I'd make sure the spark plugs are gapped correctly to 0.7mm first.

Try the ignition module from the donor and see if that makes a difference. If not then clean the back of it and the heatsink it is sat in then apply some new heatsink compound as a bit of maintenance.

[jlawler]

Ok I'll give it a try.
I assume by the ignition control module you mean the one mounted near the passengers side wheel arch above the battery:
Volvo Ignition Control Module - BOSCH - 0227100124

I swapped it but didn't notice any heat sink compound on it. I'll put some on tomorrow.

The only thing that is hot there is the fuel injector ballast resistor.