Head Swap compression testing

[ewolfe57]

My head swapped Eta is running, but there's some smoke and coolant leaks and I'm trying to make sure the headgasket is fine before proceeding. I know it's definitely leaking coolant around some hoses but I want to make absolute sure it's also not leaking from the head.

I did a compression test today and got pretty consistently 120psi across all cylinders. This is definitely lower than stock but I know the head swap with E pistons will lower compression. Is 120ish normal/not a bad sign? Any other simple tests I can do?

[TobyB]

One of the easy tests that tells you a lot-
on a stone- cold engine, take off the expansion tank cap
to relieve all pressure, then put it back on, tightly.

Start the engine, 'goose' it by opening the throttle quickly a few times, then shut down.
Then check the cap- if it has full pressure, there's a pretty good chance that combustion
gasses are getting into the coolant passages.

But all sorts of little things look odd after a head pull- I have learned to just
assume it's ok, pay close attention to gauges, fix the leaks and go drive it.
It seems like big surgery, but the patient recovers far more quickly than you or I do.
If you stitched him all back together right.

t

[e30davie]

If it were me i would be fixing all of the known problems and leaks first.

Then getting a coolant system pressure test, my mate has a kit, you attach it to where the radiator goes and just pump it up by hand. if it holds pressure that means your head gasket isn't leaking coolant.

You definitely got the timing belt on properly with the marks lined up? could explain your low compression results across the board if you are a tooth or two out.

[ewolfe57]

Thanks for the input, I appreciate it. I went ahead and ordered new hoses for the cooling system because they're 30 years old so it can't hurt either way. It's possible the low numbers is due to how I was doing the test; I had to do it solo so I couldn't watch the gauge go up, so it's possible I could have gotten more out of it, but I did it the same way across the cylinders, so at least it's consistent.

It's definitely possible the line that goes out of the back of the cylinder head is loose so I need to check that out; it's also the one gasket I didn't replace. Is the hose pipe on the back of the head a place that ever leaks?

[wilholl]

Did you do the compression test with the throttle wide open?

[ewolfe57]

Yeah, it was wide open. Not adjusted since the head swap so maybe a little out of spec but I would be more inclined to think I hadn't turned it over enough to get full compression. At least it's consistent numbers across all cylinders though. I should be able to tackle the leaks this week and can find out if it's just the cooling system or a head issue.

[e30davie]

Dont worry about wide open throttle, will work regardless. My engine got 190 across all 6 with the throttle closed. You had all of the other spark plugs out when comp testing? and the battery was charged up? ie the engine is spinning nice and fast?

An engine in running condition you should see full compression on the gauge after 3 or 4 rotations of engine. Shouldn't take very long at all.

[ForcedFirebird]

I also have done thousands of compression tests and don't buy into the throttle open scenario, they will still come to the same conclusion.

Check your belt timing. It's easy for it to be a tooth off, and can cause low compression.

[digger]

one reason for doing WOT is you usually dont get any fuel injecting because it activates "flood clear" without needing to pull relays etc. not sure of motronic has flood clear though

[ForcedFirebird]

I believe it does. When a poor running m20 comes in with a hard start from flooding, keeping the foot on the throttle clears it out. I always pull the ECU main relay, this makes no spark and no fuel conditions.