Misfire on the race car...

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alfaquad
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Misfire on the race car...

Post by alfaquad »

Hey guys! Just a quick one - running a 2.0 8v on Dicktator, single coil in the race car. What would make the car have an intermittent misfire at high revs only it seems. Car idles fine and drives fine on warm-up lap then as the race starts it starts misfiring and becomes progressively worse as the race progresses. I turned into the pits after 5 laps as the miss got so bad I didn't want to risk breaking engine mounts! Car started with this at my MPC race at Zwartkops, but we sorted it by replacing a dead spark plug. Then at Midvaal it was fine in qualifying but in both race (even though we fiddled with things and replaced a the rotor) it was misfiring badly.

When checking for spark, all four plugs have proper spark - but this is only tested at idle. Think my first option is to get a new distributor cap. Cleaned mine, but think it may be cracked and I can't see it. Seems like plug 4 is wetter than the rest, leading me to believe that it's the cause of the miss. Don't think it's the coil. Will go through all the wiring (which is a bit of a mess) but my first instinct is that dizzy cap/rotor is shot.

Give me your thoughts please!
2007 Volkswagen Touareg 3.0 V6 TDI
1966 Austin Mini 1000 / 1958 Porsche 718 RSK Replica
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panic-mechanic
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Re: Misfire on the race car...

Post by panic-mechanic »

To check for spark jumping start it at night with no lights on and you'll see if there is a spark loss issue. Also check in the management what the dwell time is, maybe somebody accidentally changed it.
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alfaquad
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Re: Misfire on the race car...

Post by alfaquad »

panic-mechanic wrote:To check for spark jumping start it at night with no lights on and you'll see if there is a spark loss issue. Also check in the management what the dwell time is, maybe somebody accidentally changed it.
The weird thing is it ran 100s when we changed the rotor and cleaned the dizzy cap between race 1 and 2. Took it out the gate by Midvaal and gunned it to almost redline in 4th down the road - not one hint of a miss and it pulled damn strong. On temp and everything.

I'll check the spark tonight quick.

Think I may have a spare dizzy cap here, but can't really test the thing in the street here at revs. Plus I need to be taking my gearbox out to finally fit an LSD!
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Re: Misfire on the race car...

Post by PapaJo »

Are you using copper wire plugleads or the carbon type?
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Re: Misfire on the race car...

Post by Donavan »

It might be fouling the plugs on cold start.

Might be a good idea just to change the temp sensor and all 4 plugs as an option of narrowing it down.
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Re: Misfire on the race car...

Post by panic-mechanic »

Do not discount somebody having messed with your dickator settings. We have seen it more than once....
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Re: Misfire on the race car...

Post by alfaquad »

panic-mechanic wrote:Do not discount somebody having messed with your dickator settings. We have seen it more than once....
Didn't get round to testing the dizzy cap yet.

You say the dwell timing might be off? Hmm. Why would the issue be intermittent though? Fine in quali and when testing on the road, but then acts up in the races?
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Re: Misfire on the race car...

Post by panic-mechanic »

More heat. Electrical components have more resistance the hotter they get. This the coil could be breaking up. Too little dwell will mean it can't fully charge and too much dwell will see the coil overheat.
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Re: Misfire on the race car...

Post by alfaquad »

panic-mechanic wrote:More heat. Electrical components have more resistance the hotter they get. This the coil could be breaking up. Too little dwell will mean it can't fully charge and too much dwell will see the coil overheat.
It turned out that one of the three wires going into the distributor plug had worked itself out. Luckily I had a spare plug so after wiring that up the car ran without issue again!

Ran the full 1-Hour Endurance at Kyalami this last weekend, besides the clutch cable stretching to the point where the clutch still worked, but not 100% about 4 laps into the race and my headlights switch burning out causing me to lose a minute cause of a quick pit repair, the car ran without skipping a beat the whole race. Gavin Ross' Challenge Class A car pipped me on the last lap of the race beating me to 1st in class by 1.8s :oops:
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Re: Misfire on the race car...

Post by panic-mechanic »

good stuff !
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