Track day misfire .. what could it be?

[johnv]

Trackday yesterday, the car started to miss at the the very start of each session, not badly, maybe just one cylinder .. just for half a lap or so then it would clear up up and be fine for the rest of the session, no smoke as far as I know, temp and OP fine. Any ideas as to where to start looking? Thanks

[atm92484]

Have you done a leakdown test?

I once had the same thing occur - turned out to be a head gasket that was on its way out. The extra heat from running under load would get it to reseal. Eventually it went but not after it lead me on a wild goose chase through everything else.

I've never had a bad coil start out bad then improve (always started ok then degraded as it got hot) but it is possible that it is the coil. Maybe airflow from the car moving at speed cools the coil enough that it starts working again?

[GraemeC]

We’re you sat ticking over for a while before the sessions? Could be a plug fouling up and then burning itself clear after a bit of time on track

[johnv]

We’re you sat ticking over for a while before the sessions? Could be a plug fouling up and then burning itself clear after a bit of time on track

Yes .. but its done a couple before this one .. starting off with Blyton . . and not had any issues

[johnv]

Have you done a leakdown test?

Haven't taken it off the trailer yet! What you say does make sense but I probably did 3 ir 4 sessions flat out after it started happening so I would have thought something bad would have happened if that was the case?

[iain1967s]

Plug lead insulation breaking down and arcing to the nearest metal part that it routes close to - grille, dipstick, alternator bracket etc. ?

[Joel Welsh 4769VU]

Distributor cap cracked, loose?
Little moisture in it, then dried out.

[atm92484]

Haven't taken it off the trailer yet! What you say does make sense but I probably did 3 ir 4 sessions flat out after it started happening so I would have thought something bad would have happened if that was the case?

Mine went on for a few sessions before going. Sometimes it was only first session of day and wouldn't do it until first session next morning. It definitely wasn't the usual head gasket failure.

[Peter Laidler]

I'm with Graeme C here. Start at the easiest and cheapest and correct upwards - or in money terms, downwards! Unless you have the high load facilities to test the plugs it's difficult to diagnose the actual 'problem' beyond the fact that it's broken down. But it's easy and cheap to replace the set and from then onwards it's a process of elimination knowing that at least it wasn't the plugs. Plug leads usually fail under load

[johnv]

Mine went on for a few sessions before going. Sometimes it was only first session of day and wouldn't do it until first session next morning. It definitely wasn't the usual head gasket failure.

I'll try all the above then change the head gasket .. do you know why yours failed? I can't see that mine has any excuse to .. the car's never overheated, the head and block are freshy skimmed, BK450 hg, ARP studs etc , I'll be disappointed! Car starts and runs sweetly atm ..

[Dan Tweed grey]

Had this problem, put fresh spark plugs in and it sorted it.

[trevorhp]

Had the same problem on a trackday and it was a poor connection on the coil.
Simple to check and cheap to fix