Bad earth - opinions?
 

Hi folks, I have an '95 WRX that had an engine transplant from another identical car around five years ago.
Since then (or around that time) it's had a tiny missfire at around 2000rpm when the weather's damp, where the engine seems to hesitate until you apply more throttle, despite changing plugs, coil packs, and checking all obvious electrical connections. My thought are that the problem is electrical, but I'm running out of ideas what to look at next....
Should the block have an earth strap to the car's chassis? If so, I don't have one, only the main loom connector under the intercooler... Another thought was the TPS sensor on the throttle body? Could the immobiliser cause similar issues (the car always takes 2-3 seconds of cranking to start - which seems longer than it should be)? How about the knock sensor retarding ignition to the point of a missfire?
Sorry about the long explanation, I'm hoping that someone else has experienced similar niggles and found a simple solution...!

You got any error codes showing?

You could instal another earth direct to battery -ve from the block and one from the bolt holding the ecu down, can't hurt. Use thick flexible cable such as is sold for ICE feeds, plus decent ring connectors and SOLDER the ring connectors.

Then if it's still doing it, that's ruled out.

After that, I'd approach a decent mapper.

Any mods to the car? Have they been mapped?

Thanks. Only mods are a cat back exhaust and aftermarket filter (foam type), but the missfire is definitely weather/ temperature related (disappears when engine is warm).
I'll check for error codes and see if that show's anything...

Is this an open foam filter? Its pretty possible and not uncommon for them to soak in a little water in wet conditions leading to the type of misfire you drscrbr

No, it's an HKS panel filter. I wonder if it's getting a little condensation when it's damp?

worth checking. Also worth checking that the cover under the o/s grill on the bonnet is there. These are often removed to try to aid airflow. What happens is water runs through it onto the maf or air box.

Thanks, already checked that... It's pretty much back to standard while I try and find this fault.

Have you tried changing the plug leads themselves?

The reluctant damp start problem on my classsic went away when I fitted a new set of plug leads (from importcarparts, not the eBay ones).

You have got the earth from battery to front panel haven't you?

Mine's an early classic, so coilpacks are in the cylinderhead. They seem to be in good condition, with no cracks behind the rubber ends...
Battery is earthed correctly.

Update: Travelling along at 60 the other day, the engine died (rpm reading 0 on the revcounter even though car was still in gear at while slowing). Turned the ignition off, then on again and the motor sprang back into life... I'm thinking that the ignition switch could be on it's way out?

Well, I found the problem!
Bad earth to the alarm/ immobiliser under the dash. Tightened up the bolt holding the earth straps and it's behaved fine ever since...
Thanks for all your help, I guess no-one could have guessed that would be the problem!

That's great news.

Thank you for coming back and confirming the cause and fix. :thumb:

This is one to add to the list of misfire, poor starting electrical solutions.

Moderator, is there any way we could have a ScoobyNet 'troubleshooting' section where we could list faults, symptons and quick checks by category so that other owners could benefit from these types of 'quick fix' solutions?

Sam