I was looking at testing the compression in my 89 L98 350. When you run a compression test. Do you run it on a cold engine? Or do you run the engine up to temp. Then pull all the plugs and do the compression test?

I know some do it both way. But doing it on a cold engine would be easier. As on a hot engine, would be harder to get the plugs out quickly and not burn yourself. And I'd guess you'd have to work fast to keep the engine from cooling down too much by the time you get to the last cylinder.

And can you get buy doing the test on one battery? As I believe you have to crank it 4 times per cylinder. Which means 32 cranks.

 

I did mine with the engine cold. You don't necessarily have to crank it 4 times per cylinder. Just unplug the coil wire from the distributor cap and crank as needed. The gauge will not go down until you hit the bleeder button/valve.

Hope this helps.

 

Yeah, I've read to unplug all the connections at the distributor cap as mine is HEI coil in cap. And also unplug the INJ 1 and INJ2 fuses to keep fuel from spraying.

I think the reason they say to crank it 4 times is to build up as much pressure as possible and to go with the 4th crank pressure.

How many times did you crank yours over?

 

Here's from my FSM:

Compression test:

1. Disconnect the BAT terminal and the four terminal connector on the HEI distributor. Remove ECM fuse.

2. Throttle open, all plugs removed, and battery at full charge. The lowest reading cylinder should not be less than 70% of the highest, and no cylinder should be less than 100psi.

This should be done with four compression strokes per cylinder.


NORMAL - Pressure builds up quickly and evenly to specificed compression on each cylinder.

PISTON RINGS LEAKING - Compression low on first stroke. Tends to build up on following strokes but does not reach normal. Improves considerably with oil.

VALVES LEAKING - Low on first stroke. Does not tend to build up on following strokes. Does not improve much with addition of oil. Use 3 squirts from a plunger-type oiler.

 

Right. I just meant that you watch the gauge until it peaks, which may be 4 cranks or you could just turn the key forward for a few seconds. Either way, the gauge isn't going to go down until you release the pressure.

 

I'm going to search around with regards to doing it cold or warm/hot. It would be tricky doing it warm/hot since you have to move fast as the engine will start cooling down. If you did it warm/hot. It may be best to do 1,3,5,7. Put the plugs back in, warm the engine back up. Then with a fresh battery do 2,4,6,8.

 

Do you need to remove the "four terminal connector on the HEI distributor?"

Looks hard to get to at the back of the engine.