Misfire I can't find...
Turbo Mini Forums

Hi,

I've got my new engine running, bedded in the cam etc, but when I drive it it's got a misfire which seems to come and go randomly. It's a brand new engine, new gt17 turbo, on megajolt, with intercooler etc. Running no boost at the moment. It requires a rolling road setup obviously, but the rolling road operator told me to put a few miles on it before I bring it, and I want to make sure there are no teething issues when I arrive there.

So light throttle cruising in mainly 3rd and 4th, but it does do it in second, it seems to drop down onto 3 cylinders.... If I put the clutch down it misfires on idle for around 5 to 10 seconds, then returns back to 4 cylinders. AFR is around 12.5 when it starts to misfire and it doesn't seem to change. Pulling the choke out richens the mix on the gauge but doesn't alter the misfire. Fuel pressure doesn't drop as have tried with the fuel pressure kit on while driving.

It does it when the engine is warm and cold. I've checked and adjusted the valve clearances, backed them off to .016 throughout incase they were a bit tight, but still the same.

Fast running out of ideas so thought I'd ask for some advice! Thanks in advance

James

Done a compression test? Changed the plugs, leads (maybe even coil pack)?

If something is worth doing, it's worth doing half of.

Sounds like it could be sticking valves.

I think Nic had this problem, I've had it too.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

Not done a compression test, but it's a brand new engine. They are new plugs, leads look perfect and I've swapped the coil pack.

I hope not Will, guides nipping the valves killed my last engine.. I hate to say it but it feels similar to before when it happened, but happens at much lower revs and very light throttle.

And the head has been rebuilt too.

might just be worth pulling the head and running a reamer up the guides anyway.

If something is worth doing, it's worth doing half of.

Yeah true, but it's a ball ache to take off in situ. Plus it should have been done.

If everything else is in order it can't be much else.

If you have 100% eliminated everything else then it needs to be tried.

I believe it's a 9/32 reamer from memory.

If something is worth doing, it's worth doing half of.

I can't think of anything else really. Unless it's the edis or crank sensor, but can't see it really, as it feels like one cylinder going down

Just to add, when I turn the engine off, I get a loud 'pop', not sure if it's from the exhaust or back through the air filter

I had similar when I first switched to megajolt. I earthed the shield wire at both ends for the trigger wheel sensor and it went away. But before that the fault would just come and go. The car was fine one min and like a bag of spanners the next.


Also had a stumble at idle when I switched to injection which was down the the trigger wheel sensor not being perfectly aligned as they were new parts with the kit.

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

I thought earthing both ends of the sheilding just made a closed loop and made it ineffective ?

My car occasionally does that loud pop a second or so after I switch off, and after the engine has come to rest. Doesn,t seem to have any effect on running and I don,t worry about it.

Yeah, shield should only be earthed at one end. Otherwise it'll do nowt or induce more noise.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

We'll i don't understand then as it was the only way I could stop the fault. It was like that on the car for years.

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

if all alse checks out . . . it sounds like a classic guide nipping up, i think i did your engine and maybe did some work on your head or did the head from fresh? in may time if iam correct . . . .

i always ream the guides out if they are manganise one with a new 9/32" reamer which only does a few heads at a time, But sometimes this happens from time to time.

the affected cylinder guides needs to be checked with a reamer again drop me a pm and we can take it from there.