F155 Low Compression - how to proceed? (1 Viewer)

[stoshzack]

Hi All -

I finally confirmed what I was thinking all along, my stock (and project) '70 FJ40 F155 engine has low power. Ran a dry compression test and got 90/80/80/80/85/90. I know normal is 140-150. My question is how to proceed? I heard people having success removing the head for milling flat, new gasket installs, and reinstalling the head (something I have never done). Or is this a complete engine rebuild?

Thanks for any help

 

[thebigredrocker]

Was the engine good and warm when you checked and did you have your accelerator pedal to the floor?

 

[SteveH]

Do a wet compression test by adding a teaspoon of oil (or a measured amount, like a full pump from a pump oil can) to each cylinder (use ATF or 10wt. oil), and report back. If compression jumps significantly, then the rings are shot and you pretty much need a rebuild. If it doesn't, then the valves are implicated and perhaps a valve job can bring the compression back.

As the previous reply said, test it hot with the throttle held open. Don't be scared of pulling the head, having it re-done, and reinstalling - it's easy, but requires quality parts and close adherence to the FSM. Don't wing it.

 

[stoshzack]

Thanks all. These were cold numbers using a remote starter. I'll run it again with a hot engine.

"With throttle open" - does this mean run the engine normally with throttle open and just disconnect and replace each spark plug with the compression tester during operation?

 

[Rock40]

No, not exactly. When you do the compression test, all the spark plugs should be removed from the cylinder head. You want the throttle wide open so the engine is able to draw in plenty of air, he doesn't mean the engine is actually running. Get it hot, then remove the plugs and test it. Make sense?

 

[stoshzack]

Thanks @Rock40 - that makes a lot more sense than what I was thinking

 

[thebigredrocker]

I would also unhook the fuel line when doing your test. Just put a bucket under the line. I may be wrong, but I think washing your pistons with fuel will results in less lubrication and lower than actual compression readings.

 

[LAMBCRUSHER]

disconnect the dizzy and fuel line. remove all 6 plugs. install comp tester to first hole and crank motor over 5 revs, maybe 6. record results and test SAME HOLE AGAIN. repeat for a total of 3 readings per hole; 5 or 6 revs each- always allow the same number of revs across all tests. block the throttle plate to WOT for max air flow capacity. if your numbers are still low, add a couple ccs of oil to each cylinder just before hooking up the comp tester and run a "wet test". as explained, this will give insight as to where the air is escaping from. regardless of the results, ADJUST YOUR VALVES. and test it all over again. Valves get tight as the engine wears, which can allow for a low compression reading...the very first thing a person should do following the panic sensation after a bad compression test result, is to adjust all the valves to .008 intake and .014 exhaust and test it again...HTH