So yesterday while diagnosing my car missfire on CYL 4 in my 2000 forester 251 with 270k, i ran a compression test to see if the valves may be causing the miss fire. Turned up negative on the valves, but it shot all over 200 PSI on all cyls. #4 was around 240, #2 220, and so on. 

 

i found out the problem was a mix of spark plug wire, plug and coil pack. 

 

but GD was kinda at ahhh, of how much psi the cyls were pushing.

 

Is it possible i have a high compression build in my forester? It almost dont make sence.. We know the pressure gauge is good because we used it the other day on another car. i know very little about the engine it self other than the stuff ive done to it since i bought it at 220k, which is nothing major. I do know that the person who had it before me had the HGs done at superior import, did they put thinner sti Hgs in it?. Is it possible for a 257 head gasket to give that much more compression? 

 

if this is a higher compression build wouldn't i have killed it, since ive ran regular gas in it for 50k? haha....

 

prwa

The STi gasket doesn't raise compression by a whole lot, going off memory it puts it up to 10.4:1 which with the way the ecu is tuned on the 251 is still ok on regular. I'm going to be repairing my mom's outback the same way this summer. You're results could be a combination of things, like a thinner hg and carbon build up on the pistons and chambers.

Not uncommon to see numbers that high. I had an Ej22 with 200-220 on all 4 cylinders. Nothing special about it. Just says it has good clean rings IMO.

I think spec is 185-215 or somewhere around there anyway.

what about the other 2 cylinders, 1 & 3?

Yeah im assuming that its a mixture of a couple different things that's causing it. but it runs great now, so i cant complain

 

Num 3 was 220ish psi and i cant remember num 1 but it was up above 200 aswell