Compression readings on 78 350ci

[Boothby1952]

Couldn't find any info on compression test results and lacking the experience of many of you, what should I be looking at for possible problems with these numbers: 1 = 121...2 = 130...3 = 135...4= 131...5 = 150...6 = 135...7 = 136...8 = 134. any opinions will be greatly appreciated.

 

[Bextreme04]

Couldn't find any info on compression test results and lacking the experience of many of you, what should I be looking at for possible problems with these numbers: 1 = 121...2 = 130...3 = 135...4= 131...5 = 150...6 = 135...7 = 136...8 = 134. any opinions will be greatly appreciated.

She's tired, but still should be running with numbers like that. Probably worn rings all the way around and a leaky valve on at least #1. You can put a little bit of oil in each cylinder and test again, if the compression number goes up you have worn rings. The generally acceptable compression test number is that the highest and lowest values should be within 10% of each other and over about 120 psi for good combustion. If you also do a leakdown test you can find exactly where the issue is.

Did you run the compression test with all plugs removed and throttle wide open? Sometimes you can get erroneous readings on one cylinder(especially the ones with the crossover) with throttle closed and not enough cranks. I usually have throttle open and crank through at least 3-5 compression strokes on the cylinder I'm testing(you'll hear the engine load up on the compression stroke of the cylinder you are testing since all other plugs are removed.

 

[cstew47]

At what altitude were these readings taken? That will affect the results. I would be concerned about #1. hopefully it's just a valve issue.

 

[75gmck25]

As already mentioned, for consistent compression test readings you need to make sure the engine is getting enough air and its turning over fast enough.
- pull all the plugs
- remove the air cleaner and wire or hold the throttle wide open
- ensure the battery is fully charged
- clamp the fuel line closed with some vice grips (if possible) so you won't keep dumping gas into the cylinders
- put the tester into #1 and turn over until you get the highest reading (usually takes at least 4-6 strokes)
- test every other cylinder using the same number of strokes as number one
- results for each cylinder should be within about 10% of each other
- if you get low readings on some cylinders, put a squirt of oil into each cylinder as you test them again. If it make the compression go up significantly, the rings are not sealing in that cylinder.

 

[Boothby1952]

Thanks for all the info. Really appreciate it. Need to run another test as I did everything but get the engine to normal running temps...then do the wet test. Altitude is about 900’.

 

[dvdswan]

Average should be around 160 psi. IIRC, normal range is 150-170.

 

[Matt69olds]

I’d check out the slop in the timing chain. Those numbers are pretty consistent, but consistently low. A stretched timing chain will retard the cam timing and cause lower compression.

Rolled the engine backward until the timing mark is at TDC. Remove the distributor cap, slowly roll the engine in the normal direction. Stop the moment you see the rotor start to move. Look at the timing marks, if you have more than 10* of free play it’s time for a chain.