#3 cylinder exhaust running 100F hotter
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Race Director
Joined: Jan 2003
Posts: 17,723
Likes: 3,708
From: Central Arkansas
#3 cylinder exhaust running 100F hotter
Interesting note yesterday after I spot checked my water temp with an IR gun, which I do occasionally to make sure my gauge and temps are still in sync. I decided to read the exhaust ports just before the manifolds and noticed all were running somewhere in the 300+/- F range, except #3 which was reading about 415-420 F. This was at idle but holding the throttle at around 2,000 rpm’s for a bit didn’t vary the overall difference much. Overall I was surprised they were actually this low but maybe the aluminum heads contribute to that. I pulled the NKG plug and it was dark gray but dry. I hit it with carb cleaner and the gray washed off and it looked almost brand new. Sticking a camera inside didn’t show anything to me. Pistons were pretty clean, head surface and valves have some carbon in some areas. Measured the wire resistance and it was 772 ohms over roughly 3’ so that’s good. The #3 intake runs close to the crossover which I don’t have completely blocked off. But if it were affecting #3 it would probably be affecting #5 and #4 and #6. So, a bit of a mystery but I can’t see digging into it any further. I don’t think 100 F variance is regarded as greatly unusual. Possibly the L72 intake just runs a bit lean on that cylinder?
Racer
Joined: Dec 2025
Posts: 329
Likes: 124
From: Pachuta, MS
You might pass a propane torch near the intake ports to see if there is any change in idle. You might have a small vacuum leak leaning out that cylinder or it could be nothing more than a slightly loose exhaust manifold that doesn't transfer heat quite the same in that area. The worst case would be a partially blocked coolant passage ... if any of this is significant.
