Hey guys,

I purchased a 2002 AP1 S2000 roughly two months ago with only 36,000 miles on it. From day one I could feel quite a bit of vibration coming from the engine/transmission. However, I didn't see the check engine light on, so just assumed the engine mount/trans mount bushings were a bit worn so didn't really pay too much attention to it.

Since purchasing the car, I've autocrossed it 3-4 times. While I did bounce off the rev limiter a few times, I made sure I didn't over-rev the engine on downshifts, since the valve retainers are known to crack on the AP1s.

The first time the CEL came on, it was for the fuel cap not being secured/tightened down. The old cap kept loosening by itself every so often, so I purchased a new cap, cleared the code, and just forgot about it.
About 2 weeks after the initial fuel cap issue, 6 more codes came up: P0300, P0301, P0302, P0303, P0304, and P1456.
I immediately got a compression test tool and checked the cylinder compression (in psi): 135, 210, 210, 180. I squirted some engine oil down the cylinder one spark plug hole, and the pressure increased slightly from 135 to 150 psi. I then took it over to a shop to do a leak down test, the results were: cylinder 1 & 4 at 80%, and cylinder 2&3 at 50%. I was not allowed in the shop but the mechanics reported that they could hear air escaping out of the throttle body. The shop suggested the head gasket was bad, and also the block could be damaged. However, I wanted to get a second opinion, after the leak down test I took the valve cover off and checked the valve clearances, they were a bit on the tighter end of the tolerance but I don't think they are significant enough to cause such a huge problem. The valve retainers also seemed to be intact.

Any suggestions as to what I should check next? I was thinking about pulling the head and replace the head gasket, and also check for bent valves.

Thanks in advance guys.

 

Could get a bore scope down the plug holes and inspect for cylinder wall damage.

 

you didn't test compression before you purchased the car?

 

I recently had the same codes... after going through just about all of the potential easy fixes. It ended up being valves were out of spec . Too tight on the exhaust , only 1 of the 4 cylinders was way off. But was throwing all the same codes you have listed and no more CEL after the adjustments

 

my 2001 AP1 developed a misfire a couple of years ago -

quick fixes:
replace coils DENSO 6732301 - a lot cheaper than OEM
​​​​​​​Injectors professionally cleaned
check and fix valve clearances - this tool (and a set of feeler guages with angled ends) is your friend:

ended up being a mix of 2 and 3 to finally resolve

EDIT- oops you already checked the clearances sending healing thoughts to your block from across the pond

 

I wonder if the motor overheated at some point?
What does air out of the throttle body suggest?