944 Mystery Misfire

[Guest]

My H reg 944 S 2 occasionally misfires under power. It runs lumpily for a second or two, then revs freely again. Sometimes the problem does not recur for weeks. Problem is, it's not what you want when overtaking on a an "A" road. All attempts to diagnose by specialists have failed. Any similar sufferers?

 

[Diver944]

Intermittent problems are the worst because they never do it when the mechanic is looking at it [] The first thing to do with any 944 electrical glitch is to replace the DME relay because these can cause all sorts of problems and even breakdowns. Everyone should carry a spare in the glove box because they only seem to last about 5 years. Under £20 from OPC and parts suppliers and plugs into the fusebox.

My S2 had a chronic misfire when under power a few years ago but it happened suddenly and constantly and was accompanied by lots of black smoke (unburnt fuel) out the exhaust. This turned out to be the engine temperature sensor thinking the engine was cold all the time so overfuelled (as if the choke was on all the time). Does yours kick out black smoke when misfiring?

This sensor is the blue one at the front of the engine almost directly in line with the bonnet catch. It may be worth cleaning or 'wiggling' the connection to see if you can induce the fault????

 

[Guest]

Thank you very much. In the past when it stalled and was difficult to start I removed the relay, put it back - and all was well. I will now replace.

Regards,

GM