compression test

[ampwhu]

i think i know the answer, but i thought i'd ask anyway..

if i do a compression test on each cylinder (remove plug and insert tester as well as disconnect the coil etc), would that show up if i have burnt a valve out? i'll tell you why, the car sounds as though it's running on 3 cylinders. if it isn't the useless condensors on sale now, then i'm hedging my bets on it being a valve.

any thoughts?

[bmcecosse]

First - set all the valve gaps correctly - then do the comp test. Engine should be hot after a run - take ALL the plugs out - prop the throttle open (brick on pedal is fine) and then crank it over with the gauge in each plug hole in turn. Few revs each cylinder till the needle settles....

[ampwhu]

but will it show if a valve is burnt out? i remember the last time i did a comp test...... each cylinder was about 160psi

[bmcecosse]

Well in that case - the valves are fine !

[ampwhu]

so it must be these silly condensors on sale these days.....

[bmcecosse]

Not necessarily........it may be the coil!! most likely is rotor arm/dizzy cap/HT leads - but possibly the 'low tension' lead inside the dizzy -have you checked it/replaced it in living memory??

[ampwhu]

Not necessarily........it may be the coil!! most likely is rotor arm/dizzy cap/HT leads - but possibly the 'low tension' lead inside the dizzy -have you checked it/replaced it in living memory??

it wont be the coil. it has never been a coil in over 25 years of ownership.

low tension lead was replaced 3 months ago along with the points and condensor. dizzy cap is new along with the leads and plugs. i've just made a 400 mile trip with no bother at all in all the wet and damp weather as well.

i reckon on mechanical issue......... dropped valve......

[bmcecosse]

Well it won't be a 'dropped' valve.....that would be very obvious - but it may be the valve gap(s) have closed up in that long run. The main reason I advise 15 thou gap for the exhaust valves....

[ampwhu]

but back to the original question, would a compression test show if a valve has burnt out?

i don't want to remove the head to find it is ok.

[mike.perry]

Think logically. If the valve was burnt out there would be a hole in it so you would have a low compression on that cylinder. All your cylinders are good so no valves, pistons, head gasket problems.

[ampwhu]

thanks. when i said further up that i recorded 160psi, that was last year. i've yet to do it since i've had an issue.

i'm off to comp test all 4. i'll report back later........

[bmcecosse]

Ahh -last year - a very long time ago. But do set the valve gaps and take the car out for a few miles run before doing the test.

[ampwhu]

well it wasn't a valve, coil or rocker gaps. it was the head gasket.



on 1 and 2 i had 150psi and on 3 and 4 i had about 45psi. so i removed the head and was faced with the obvious.

i have just spent the afternoon changing the head gasket for 1 i had in stock here. over the past few weeks i have noticed oil and water in my tray i leave under the car in the garage. this must have been a telling sign that i ignored.

after i had fitted the gasket i did a comp test and got 150psi on all 4 cylinders. so i started up and the car ran fine. i also had 60psi old pressure from cold.

a satisfying days work. i am now 'celebrating' with a cold spitfire

[bmcecosse]

Don't forget to re-torque now, after that first heat cycle....and then set the gaps. And you originally reported running on 3 - it's rather more than that when the gasket blows!!

[ampwhu]

Don't forget to re-torque now, after that first heat cycle....and then set the gaps. And you originally reported running on 3 - it's rather more than that when the gasket blows!!

i didn't do that last time. i guess i should. i have torqued it to 45lb. i didn't run it for too long as i have the rad out and no coolant. do i torque it to the same after a run then? and why would the gasket blow? i haven't lost any oil, water and they haven't mixed together.

not a long shelf life. about 18 months at a guess??

[bmcecosse]

Because you didn't re-torque it! Fit the rad - warm it up, let it cool somewhat and then retorque.

[ampwhu]

went and re-torqued it this morning after i had let it get up to temp. it did need re doing, you were right. took the car rof a run and it seems fine. healthy oil pressure as well. temp gauge didn't move from just below the normal position (as before). so i guess i am happy. just the exhaust blow to sort out next weekend.