Compression Test Confused!?

pos · Dec 5, 2011

Hello,

I have just run a compression test on my 200tdi (to try and find the cause of the lumpy idle / white / blue smoke on startup) and I am not happy or convinced with the results. The gauge showed bang on 220PSI on each bore. This is massively down on compression and I find it very strange that all four cylinders are exactly the same. Does anyone think I might have had a leak with the gauge / testing equipment or is this actually possible? I do not have any pressurisation of the crank case, so the rings / pistons / head gasket are all in good shape.

Should I try anything else or would this immediately point to terrible valve seats?

Thanks,
-Tom

jamesmartin · Dec 5, 2011

was that the highest pressure reached or did you just give it a couple of turns

pos · Dec 5, 2011

jamesmartin said:
was that the highest pressure reached or did you just give it a couple of turns

Hi James. That was the highest pressure shown on the gauge over a period of about ten seconds per bore without oil. It stayed at that pressure for about five seconds out of the ten but as soon as the engine stopped turning, the gauge would take about 3.5 seconds to return to nil. Is this right?

thanks,

-Tom

pos · Dec 5, 2011

I'm thinking a leakage test next to see whether or not air is escaping down the intake / exhaust manifold. There is very little blow-by.

jamesmartin · Dec 5, 2011

pos said:
Hi James. That was the highest pressure shown on the gauge over a period of about ten seconds per bore without oil. It stayed at that pressure for about five seconds out of the ten but as soon as the engine stopped turning, the gauge would take about 3.5 seconds to return to nil. Is this right?

thanks,

-Tom[/quo normally guage holds pressure till you release it .so is guage accurate,the fact there all the same is good sign especially if took the same turns/time to get there

pos · Dec 5, 2011

jamesmartin said:
pos said:

Hi James. That was the highest pressure shown on the gauge over a period of about ten seconds per bore without oil. It stayed at that pressure for about five seconds out of the ten but as soon as the engine stopped turning, the gauge would take about 3.5 seconds to return to nil. Is this right?

thanks,

-Tom[/quo normally guage holds pressure till you release it .so is guage accurate,the fact there all the same is good sign especially if took the same turns/time to get there

Click to expand...

The gauge certainly did not hold the pressure until I released it, so i think it is safe to suspect that I have some faulty equipment. I have been informed that it hasn't been used for a long time. I am going to leak test it at some point over the week. Obviously if I can feel air being blown down the inlet manifold or out of the exhaust then I know that the valve seats need some attention. If there is some air blown out of the oil filler cap then there is obvious bore wear / potential blown head gasket. Another thing to consider would be the timing. If I am a tooth or more out, then the exhaust valve will be open or the inlet valve might not have fully closed as the piston reaches TDC. I don't know whether or not it'd run a tooth or two out though.

-Tom

jamesmartin · Dec 5, 2011

best thing is 2 tests, at tdc injection stroke both valve would be shut even if a tooth or 2 out ,and both open on exhaust stroke ,you would notice though in running

tombardier · Dec 6, 2011

I know on a petrol I would have the throttle fully open when cranking (because you'd have much lower readings with the throttle closed), however, not sure how that'd work in a diesel with the fuel cut off to be honest :/

Karlos28 · Dec 6, 2011

the land rover engines are WOT all the time effectively.. the intake is never restricted. on a petrol you have a butterfly valve that stops the air flow..

tombardier · Dec 6, 2011

they're WOT what?

tombardier · Dec 6, 2011

wide-open throttle I take it?

Karlos28 · Dec 6, 2011

haha sorry! Wide Open Throttle

The Mad Hat Man · Dec 6, 2011

Leak down test is betterer.

tombardier · Dec 6, 2011

have you done a wet test yet though?

tombardier · Dec 6, 2011

although having said that, I'm thinking of petrols again. Don't know if you can just dump oil down the injector holes :/

pos · Dec 6, 2011

I will have a go at leak testing it this weekend all being well. As for trying a wet compression test, I didn't do that because I determined that the gauge was fooked on a dry test. It's the same principle with a diesel as it is a petrol engine when doing a wet test except you would usually just drip the oil into the cylinder via the injector or the glow plug hole before you screw the adapter in. Using the glow-plug hole is the obvious option, much less to remove and no chance of getting dirt in your fuel lines.

-Tom

tombardier · Dec 6, 2011

You can't know that the air isn't leaking past piston rings until doing a wet test though. The gauge might be ok!

pos · Dec 6, 2011

tombardier said:
You can't know that the air isn't leaking past piston rings until doing a wet test though. The gauge might be ok!

There's no blow-by / pressurisation of the crank case when the engine is running hot or cold, so there can't be much in the way of compressed air + fuel getting past the rings I wouldn't have thought?

tombardier · Dec 7, 2011

Wet test is the only way to prove that. Also, if you have got dodgy valves then that could mean that your compression gauge is fine. Nothing so far is proving that the gauge is knackered surely?

Shifty1962 · Dec 7, 2011

tombardier said:
wide-open throttle I take it?

Diesels dont have a throttle plate to open so it makes no odds what you do with the go pedal.

Compression Test Confused!?

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