Intermittent misfire

[Shaun_E]

I have the nightmare problem of an intermittent misfire. It initially only occurred under load from low revs/wide throttle opening but later became more prevalent. It finally manifested itself on the the start line at Loton Park - as I raised the revs for the launch, the engine cut out. This was repeated 3 times in a row at which point I gave up on the run.

As the car was stationary the last time I think we can rule out fuel starvation through surge (which was initially suspected)

Possible culprits I have in mind are: alternator, ECU, sensors/wiring (TPS/CPS).

However I have checked the following:

Alternator is charging (14.4V showing), battery is fully charged (12.8V showing).

I connected the laptop to the Emerald and TPS shows 0-100% through full range of movement, temp sensors show expected values.

I did have problems connecting to the Emerald and have ordered a new lead to eliminate that. Perhaps the ECU could have a dodgy connection somewhere?

Another symptom is that the shift lights keep resetting themselves which suggests an electrical malady. As they get the same feed as the ECU/coil then that might be an area to investigate.

Anyway, I'm after any suggestions on where to start looking and feedback from anyone who has had a similar problem (and resolved it )

Engine is a Duratec with throttle bodies and an Emerald ECU and it runs wasted spark ignition with a Ford coil pack.

Thanks,

Shaun

[Grubbster]

I had a misfire problem on my Sigma recently which took a while to resolve. CC said to give all the relays a push to make sure they were in properly so that's easy to try. But my problem was quite unusual - the big red cable that goes to the starter motor was touching the dipstick tube - over time the insulation wore away slightly and it would sometimes arc across from cable to dipstick tube and this caused the misfire (as it is a dead short to earth!). Eventually the arcing burned a hole through the dipstick tube which lead to an oil leak - only then did we find the problem. So lucky not to have caused a fire. It's unlikely to be the cause of your problem but a very easy thing to check.

[Mark Durrant]

Shaun

 

A few items to check:

 

FIA switch which are known to fail

Fuel pump relay loose. John Powell had this and a cable tie stopped it from coming loose again

Did you have to modify the wiring loom for the Duratec conversion if so check any joints/changes that were made

 

Good luck

[Allen Payne]

My (Omex?) shift lights caused a misfire when connected to the coil for their signal.

 

Reconnected them to the tech feed they worked fine and the misfire went away

[Nick Chan]

Hi Shaun, problems I had with my sigma that were difficult to chase:

 

Earth lead on the near side that is fixed from the chassis to the engine mounting point was very slightly loose and took Steve Grenald a little while to trace on his RR. The engine would misfire under high load but be fine under 5k. Also a leak in the barometric sensor feed from the inlet manifold. Mine caused a severe misfire low down

 

Good luck

 

Nick

[John Vine]

A wild thought... does the misfire appear to come from just one cylinder? If so, have you checked the wiring to each coil-on-plug (only relevant if you haven't upgraded to the new silicone-sheathed sub-loom)? I suffered an intermittent misfire on my R400D, and eventually discovered a broken wire inside the COP connector on #1 cylinder.

 

JV

[Shaun_E]

Thanks for all the ideas. Will let you know how I get on.

New coil pack and plug leads will be ordered. I don't have coil on plugs but HT is a likely culprit.

Replacement FIA cut off switch ordered.

Will do a thorough check of the wiring, especially the new bits and the earths. Will check relays too.

New ECU lead ordered and haven't ruled out the ECU yet.

Will try disconnecting the shift lights.

Any other suggestions will be gratefully received.

[Shaun_E]

I've done the following and still no joy. More suggestions gratefully received.

Replaced FIA switch

Replaced coil pack

Replaced plug leads

Disconnected alternator (and have a spare one in case) but running on battery only still misfires.

Cleaned up engine earth and reattached with zinc plated bolt and removed the powder coat from the engine mounts at each end. Checked resistance and it is good (low).

Reloaded ECU map

 

Tomorrow I will order a new TPS and a new CPS. This is getting expensive ☹️