Evening everyone,

Bit of a weird one, just had my engine rebuilt, and did a compression test and getting 90 across all cylinders (engine cold, throttle closed)
This seems a lot lower than expected, would this happen if the engine wasnt timed correctly?
thanks in advanced

 

Compression tests should be done with engine up to temperature and throttle wide open.

 

What Mufc said.
Throttle closed makes it so no air can enter the engine, therefore it's working against a lot of vacuum. Plus, warm is how engines operate, therefore test them warm.

 

Check out page 780 in the repair manual. It specifies how to do the compression test and how to interpret the results, and even what to do with low pressures.

Note there is no 'ideal' pressure reading. The important part is to make sure the lowest reading cylinder is within 75% of the highest reading one. Since you have the same readings on all 8, your compression is either good on all, or bad on all. Re-test per the manual and advice above.

Why is there no 'ideal' pressure? Because of the testing equipment. For an ideal gas, pressure and volume vary inversely. Assuming temperature and hose diameter are constant, using a hose 10 ft long will give you pressures 10 times lower, compared to using a hose 1 ft long. There is more space for the air to compress in the longer hose, therefore the pressure will be lower.

 

Yeah I thought I did it wrong
When I get the car back I'll do it properly. But as they're all near enough exactly the same is that a good thing?
only reason I ask is that its seems like it's misfiring, and I can't find the fault, had injectors tested, they're fine (however some seals were a bit damaged so could be that), so trying to figure out this issue is annoying me!

 

Are there any DTCs stored in the system due to the misfire(s) situation?

 

Only thing i saw was under the pending codes (no EML) was a knock, but i didn't get a chance to look at it with the SDD.

 

Keeping the throttle open won't make a difference. All 8 spark plugs are removed for the test, therefore there is only vacuum being created in the cylinder being tested.

But definitely get it up to temp before testing again.