ACURA MDX HONDA COLD START MISFIRE P0300 P0301 P0302 P0303 P0304 P0305 P0306 (2005 MDX)

COLD START MISFIRE DUE TO A DEFECTIVE/BLOWN HEADGASKET WITHOUT OVERHEATING. CAR RUNS GREAT AND NORMAL ONCE IT WARMS UP. MISFIRE ON CYL #5 ONLY UNDER INITIAL START UP. COOLANT FOUND IN CYLINDER. ONCE COOLANT BURNS OFF THE CAR RUNS GREAT. CAR DOES NOT OVERHEAT.

[@madmechanic3213]

This is probably what's wrong with my Accord with nearly 300,000 miles, but I've been too lazy to do anything about it. I _finally_ did a valve adjustment (only because the valve covers started leaking after 20 years!) and it ran so much smoother. It wasn't until then that I was able to notice the slight misfire when I'd pull it out the garage and let it warm up a minute. It was audible through the exhaust, but it never set a check engine light so I just rolled on. It finally got me one day when I pulled out and it was barely running with a flashing CEL. Multiple cylinder misfire ... I threw some fresh spark plugs in it and that helped, but it was still there so in went six new coils. Still did it ... When I was doing a compression check one morning I noticed one of the spark plugs was glossy. I just cleaned it up and when I was done I noticed it ran smoother with no noticeable misfire. Maybe one day I'll investigate it further, if it doesn't make me do so by force.

[@aaronpollock7237]

Ive spent almost 2,500$ doing EVERYTHING to try to get mine running right!!! Including just doing a valve job!! I think I have the same problem you discovered, did you end up fixing it? And if so what'd you end up having to do to get it running right? I'd greatly appreciate your input. Thank you. Awsome detective work finding that.

[@dmvmechanic3741]

Client decided to go ahead and fix it and it's running great. We replaced the headgasket just on the Front head and used new cyliner head bolts (8 bolts). You could reuse the original ones if you'd like but we decided to get new ones from Acura. Mahle bolts are good also. I have another case study with same issue on a different MDX with 168k miles, also Front head gasket (the one close to radiator). We did the same fix on it and it's running great. I'll post the video soon. The key to finding this issue is allowing the engine to cool down, apply pressure to coolant system at 1 bar. Use a borescope camera to check inside the suspect cylinder for coolant intrusion.

[@michaelpham4730]

How does coolant get into #5?

[@dmvmechanic3741]

Headgasket failure allowing coolant from the cooling jacket to seep into combustion chamber