mazdaverx7

i'm looking at an 84 944 that may possible have a bad head gasket and i'm wanting to perform a compression test before i buy the car. i am aware of how to perform a proper compression test and i have a nice compression tester. however, i would like to know what needs to be done to properly perform a compression test. for example, what do i need to do to shut down fuel and or spark? i do not have a service manual and i apologize for asking, but what's the minmum psi that's acceptable per the fsm and what spread between the cylinders is acceptable. thank you in advance.

joejoe

Pull fuel pump relay, I believe the starter will still spin. Pull all plugs and disconnect coil wire. If possible run the car up to (or near ) operating temp. as a cold engine will give different readings. When doing test open acellerator all way to allow total air flow. If barn door on afm is closed it will definetly give low reading. I believe 150 per cylinder is good, but main thing is difference between cylinders (less than 5% is good). I am sure if this is incorrect others will chime in.

max_boost951

I like to perform cylinder lead-down test as well...it will let you know if any valves are hung up and head gaskets leaks

xsboost90

if you pull the DME relay it kills the fuel pump and the spark.

mazdaverx7

very good, thank you. please excuse my ignorance, as i'm still learning the 944, where is the DME relay?

max_boost951

UBEC (underhood bussed electrical center) a.k.a. fuse block. I believe it's G5...correct me if I'm wrong.

Van

On an 84, isn't the relay block under the dash in the driver's footwell?

PorscheDoc

Yes, on an 84 the fuse panel is under the dash, above hood release area. IIRC the DME/fuel relay is the 2nd or 3rd from the left on the top row.

Get the car to operating temp, pull the DME relay, start it and let it die. Remove one plug at at a time and check the compression with the gas pedal on the floor (throttle body open). Biggest mistake is doing it with the throttle body closed.

mazdaverx7

thank you guys for the responses! i really appreciate the insight. very similar to the RX-7's i'm used to working on. for those we pull the EGI relay (fuel pump relay) and open the throttle 100% when cranking at normal operating temp.

944Ross

Mark the orientation of the DME relay before you pull it; there's no way in hell you can see into the socket to figure it out!