CA18ET - Missfire on boost

[alv]

Hey everyone, wondering if anyone has any clues about this. My car has been super difficult to start, cranking for ages before it would fire. When it eventually started it would run fine, but then would occasionally cut out on idle.

I did a few things to try and remedy this, new plugs, lead, ignition, fuel filter, fuel pump water temp sensor, cleaned idle air control valve, check and adjust timing, dizzy cap, etc - eventually it transpired that the fuel pump control circuit on the ecu was on it's way out (and eventually burnt out). With thanks to the guys on the scimitar forum I have now solved this issue and have wired in a relay instead to control the fuel pump.

However - now this is solved i'm getting a missfire on boost. I'm going to switch back some of the original bits to see if that does anything, but I'm running out of ideas, and i want to get the car ready for classics and cake in 4 days!

Any ideas?

[incarsolutions]

try closing the plug gap a little and see if its any better

[alv]

Plugs are gapped correctly to 0.9mm

[goldnrust]

It wouldn't hurt to gap them down to 0.7mm, even if it's just a test. If the misfire improves then it's likely a weak spark issue, if it doesn't change then it seems likely that the spark strength isn't the issue.

[alv]

Ah I understand, thank you

ok great - I'll try that. Would a weak spark be more of an issue when the engine is under load / boost? As it seems to run fine when idling and revving with no load

Thanks

[ChasR]

Yup, I've had that before.

Where did you get your spark plugs from? The last time I bought some cheap NGKs they would never fire right. A set through someone known and they were much better .

[alv]

Hmmm ngk BCPR7ES from a local motor factor about 5 years ago. They sat in the glove box in their boxes while the car was laid up - do spark plugs have a sell by date?! I'm gonna clean my old ones and stick em back in tomorrow just to test.

[scimjim]

www.scimitarweb.co.uk/sgwrs/viewtopic.php?t=11828

Any help?

[alv]

yes - thanks scimjim

the coil is new, but is indeed an ebay one. It wasn't doing this before so i'm gonna try swapping back to the original coil and see if it improves

[steveg]

My 940 drove Ok but had a really bad missfire as soon as it started to boost at all. I fitted new plugs but also changed the distributor cap and arm as both were pretty old. It was fine afterwards so I'm not sure if it was one or both of them together.
I'd be suspicious of any of the new bits as the quality of a lot of stuff seems to be going downhill, even reasonable well know brands.

[alv]

it looks like I have a bad injector - and my plugs are so sooty after just 20 miles. Think I've got a few issues here

Pulled the codes off the ECU and it's reading fuel pump error (which I expected because I bypassed the fuel pump circuit in the ecu) and air con. Which i guess is good, so I can rule out the CAS...

[steveg]

Another Volvo problem is the coolant temp sensor for the ECU. When they fail or start failing they make the engine run too rich. If all of the plugs are the same it could be something like this but it's only a guess.
Have you looked to see how much oil is making it's way back into the engine from the turbo ? I think they all do a bit but if it's excessive it won't be helping things.

[alv]

I've changed the coolant temp sensor for a new one, but didn't test it... I'll give it a check

[ChasR]

I'd put the old coil back on to eliminate things ; I've had a few new coils introduce problems into the mix.

[alv]

I'll give that a whirl - thanks

[slipngripross]

Having owned a few of these and normal Nissan issues for misfire are Coilpacks, MAF Sensors and plugs

[alv]

So I guess considering how rich it's running maf is a good bet. Checking my fuel pressure tomorrow

[alv]

lol - fuel pressure 100 psi instead of 36. better get that sorted

[scimjim]

measured before or after the fuel pressure regulator?

[alv]

Before... between pump and regulator.