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How Can I Tell If I Have A Blown Head Gasket?

It’s pretty rare in the Honda game but it does happen. It’s usually caused by spark knock... AKA “too much timing bro!” And I’ve been on the dyno a few times where a blown head gasket is a possibility. Problem is, sometimes it can be tricky to diagnose, especially when you don’t want to admit it to yourself! 

In fifteen years of tuning Hondas (holy shit, I really did tune my first car in 2004!) I’ve only had this happen a handful of times so I’ll recap a few of those times, how we diagnosed the problem and how to avoid it. 

The first one that comes to mind is also the most epic, and unfortunate. It was a big money build, Motec ECU, 67mm turbo, yada yada and I was flown in to tune it. After getting the timing sync’d I started tuning but it was acting strange and didn’t respond to positive or negative changes to ignition timing at WOT. After a few pulls, we had a noticeable misfire at idle so we pulled the plugs and were surprised to see broken ceramic on a few plugs. A sure sign of “too much timing bro!” That made very little sense however, since the ignition value in the calibration was very conservative. Determined to figure it out, I kept on trying to find the problem and on the very next pull there was an eruption of coolant from the coolant overflow vent that blasted coolant all the way to the ceiling! That was a pretty obvious sign of a blown head gasket!

I thought there must be a difference between what I’m commanding and what’s actually happening at the engine. Turns out that the M&W Pro14 CDI (the absolute best 4-channel ignition box that I’ve ever used BTW) has two terminals that can be jumpered together to select rising edge or falling edge trigger and that selection must match your setting in the calibration. Well no other harness maker had ever jumpered those terminals together so it wasn’t on my check list and, long story short it causes the ignition timing to continually advance beyond what you are commanding as RPM increases. The fix is to always sync your ignition timing at two RPM points, and if I had done that I would have noticed a discrepancy and been able to fix it before I cost myself a lot of money! 

The second occurrence that comes to mind happened on a turbo SOHC D16. It had no timing covers so the timing couldn’t be sync’d, so against my better judgement I proceeded with the idea that I would err on the side of caution and start with a severely retarded starting point and slowly ramp the timing in until it stopped making power. The problem, as we found out later was that the cam timing was off (advanced a whole tooth) which means the ignition timing was around 17 degrees farther advanced than I expected it to be! The power was lower than expected due to the cam timing and after a handful of dyno pulls it was overheating. But then after letting it idle for a while it would cool itself back off. Turns out that too much ignition timing caused spark knock which eventually took out the head gasket. 

A sure sign of a blown head gasket is pressurized cooling system but it can be difficult to diagnose because when a head gasket first starts to let go it may only happen at high boost. And it’s not always possible to hold your hand on the radiator hose while your doing a high boost dyno pull. The alternative is to free rev the engine but that may not give you enough pressure to bulge the hose so you may think you’re in the clear when you’re not. 

Sometimes an overheating engine may have you thinking that you’ve got a blown head gasket when it’s just a stuck thermostat. This one’s easy though, just feel the lower radiator hose and if it’s cool to the touch while the upper hose is too hot to handle then you can be sure that you have a stuck thermostat and not a blown head gasket! 

I hope that helps shed some light on this sometimes frustrating problem! 

-Reid Lunde

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