Back-firing/misfiring when hot...help
 

My car has developed a mis-firing problem, but only when the engine is very hot (ie after fairly hard driving or on motorway journeys of more than 20 minutes.

I have ruled out MAF sensor and coils.

Could it be the lambda sensor?

The spark plugs look fine, no deposits, not looking burn't either but I am worried about that the fact that they are recently new and gapped at only 0.7mm.

Just an update incase someone has the same problem. I put a new lambda sensor in today and it seems to be running well.

What Scooby is it??

It's a WRX MY94 wagon. I took it for a long drive today and....back-firing again. Had to limp home, stopping every 15 minutes to let it cool down a bit.

When I got home I deactivated a VOS chip that I had installed 3 weeks ago, and then took it for a long drive again, straight away. No backfiring!

In all I have bought new coils, new spark plugs, a new MAF sensor and a new lambda sensor and ALL I HAD TO DO WAS REMOVE A DAMN CHIP!

Lots of money down the drain, not to mention the cost of the chip (150quid).

Although at least my car has loads of new parts now, and is probably running better as a result.

In fairness to the chip, I will get help from the guy producing them, and perhaps there is a loose connection or something.

Boost may be set too high for an early car ?
MAP problems ?

Mark A

It was doing it as I was going along on the motorway off-boost! So I guess it's not boost related.

Some sort of mapping error with the chip, or the chip overheating, or a loose connection that got worse when heated.

...(I guess, open for advice/suggestions).

Sounds like the TD04 with a TD05 map problem perhaps.. your car will have a smaller TD04 turbo and the guy won't realise..

UK 4 and 5 door or same age have TD05 and so does the WRX 4door..

Get your money back and get a www.ScoobyECU.co.uk

Simon