Compression readings - 903cc

D

DSpieg

True Classic
I'm trying to decide (from a near-overheating episode a month ago) whether the cause was the start of a blown head gasket (may have been getting some combustion gas in the coolant, not sure though) . I was advised to check compression, finally did it yesterday.

The readings (cold engine, all plugs removed, unfortunately forgot to hold the throttle plate wide open) were: 120#, 125#, 131#, 130#. Are these reasonable numbers? Should I retest under different conditions?

'71 Fiat 850 Spider, stock 903cc. Head bolts recently retorqued to 39.8 ft.-lbs., and no overheating since then over several drives.
 
DSpieg said:
I'm trying to decide (from a near-overheating episode a month ago) whether the cause was the start of a blown head gasket (may have been getting some combustion gas in the coolant, not sure though) . I was advised to check compression, finally did it yesterday.

The readings (cold engine, all plugs removed, unfortunately forgot to hold the throttle plate wide open) were: 120#, 125#, 131#, 130#. Are these reasonable numbers? Should I retest under different conditions?

'71 Fiat 850 Spider, stock 903cc. Head bolts recently retorqued to 39.8 ft.-lbs., and no overheating since then over several drives.
Click to expand...
If the coolant is not leaking from the heater tubes or the radiator, you are good to go. The 850 was designed to manage air flow to the radiator with various shrouds around the engine and a fan/shroud boot that (I forget) sucks or blows air thru the radiator. Does your car have the complete shroud set intact? So if the radiator has been tested for flow and is good and it still overheats when facing a severe test like mountain pass on a 90 degree day, you might want to check the engine compartment shrouds.
 
Jeff Stich said:
Do the test over with it at WOT. Otherwise you're just speculating.
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Agree if you’re concerned with performance but for leaking head gasket you’re looking for one cylinder to be drastically different from others. As long as parameters for each cylinder tested are the same you’re good to go.
 
Thanks for all the great advice! I'll re-do the compression test when my Official Car Cranker a.k.a. Daughter Unit returns home to lend me a hand. Car is idling and running real well BTW, but running 10-15 degrees F hotter than usual - granted the recent drives were in ambient temperatures of 80-95 degrees F.
 
DSpieg said:
I'm trying to decide (from a near-overheating episode a month ago) whether the cause was the start of a blown head gasket (may have been getting some combustion gas in the coolant, not sure though) . I was advised to check compression, finally did it yesterday.

The readings (cold engine, all plugs removed, unfortunately forgot to hold the throttle plate wide open) were: 120#, 125#, 131#, 130#. Are these reasonable numbers? Should I retest under different conditions?

'71 Fiat 850 Spider, stock 903cc. Head bolts recently retorqued to 39.8 ft.-lbs., and no overheating since then over several drives.
Click to expand...

Shouldn't the engine be brought up to temp before the compression test?
 
Measured temps with, yes, my spanking new laser thermometer (but not from HF). Saw nothing that didn't make sense, other than one exhaust pipe temp running about 40-50F less than the others. Shot the thermostat, radiator inlet and outlet hoses, all seemed reasonable.
 
DSpieg said:
Measured temps with, yes, my spanking new laser thermometer (but not from HF). Saw nothing that didn't make sense, other than one exhaust pipe temp running about 40-50F less than the others.
Click to expand...

Was that cool pipe for the same cylinder that had a 120# reading?
 
Nope, #4 had the "cool pipe" and #1 had the lowest compression. I simply have to re-do the compression test a few times and correctly (engine warmed up, throttle full open).
 
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