Today I did a Compression Test

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Today I did a Compression Test

Postby Snowmark » 05 Jul 2018, 20:35

....... for no other reason than I borrowed a Compression Tester and I’m trying to learn stuff.

So, it’s a 1.9DG 1986 (not original engine). It’s been running fine, I think - difficult to know when I have nothing to compare it to. Anyway the test returned the values below, question is, having enjoyed a huge amount of weekends away already this year and plans for most of the school hols living in the van.......should I upgrade my RAC membership?
Cylinder 1: 105 psi, 2: 105, 3: 90, 4:105

Obviously, way lower than stated in Haynes/Wiki but is this to be expected?

Thanks for any input
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Re: Today I did a Compression Test

Postby itchyfeet » 05 Jul 2018, 20:48

consistancy is more important that values.
how did you do the test?
I do 10 turns with a hot engine plugs out throttle open.
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Re: Today I did a Compression Test

Postby Snowmark » 05 Jul 2018, 20:55

Yep, hot engine , all plugs out, missus with her foot to the floor and turning the key.

Probably only 6 turns but it didn’t seem to be rising any further.

The guy I borrowed the tester off (pretty new, Sealey set) has his doubts that it is working correctly but It seems fine. He was concerned that it gave him exactly the same reading on all 4 cylinders on his track day car..... I’d have been overjoyed with that result.

Should Cylinder 3 be a concern?
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Re: Today I did a Compression Test

Postby 937carrera » 05 Jul 2018, 21:39

In my experience, 6 turns is normally enough to get the peak, so that seems fine to me.

Wear limit is stated to be 8 bar / 116 psi so they're all a bit low. Is it running OK maybe using a bit of oil ?

You could repeat the test putting a small amount of oil into the cylinders to do a "wet" test you should get higher readings if the rings are worn. Don't put more than a teaspoon of oil in in otherwise you could bugger up the tester. This doesn't work as well on boxer engines compared to more traditional engine layouts.

After that, if you can access a compressor, then a leak tester can help identify where the compression loss is coming from, however, if the van's running reliably use it and maybe plan on having a further look when the season is over ?

Were the readings on your mates car in line with what they should be - just a bit of validation on the test results ?
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Re: Today I did a Compression Test

Postby itchyfeet » 06 Jul 2018, 07:12

i'd buy another tester, worth having one of your own and it helps verify results.

I have a gunson
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Re: Today I did a Compression Test

Postby Snowmark » 06 Jul 2018, 15:54

testing
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Re: Today I did a Compression Test

Postby Snowmark » 06 Jul 2018, 16:02

The guy I borrowed the set off said he expected values of 120/130psi but got 90psi all round so he didn’t have huge faith in the results.
Probably best I buy a set and try again.
Not using oil particularly but there is an annoying knocking noise which I fear could be linked to cylinder 3
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Re: Today I did a Compression Test

Postby 937carrera » 06 Jul 2018, 16:15

Ah, so on his car it was about 35 psi low and on yours you are below what you would normally expect.

I think that's your answer, you need another gauge..........

When you have another one take the back of the one you have and tweak the mechanism around the bourdon tube inside so that it reads accurately again.

A leak test will better identify where compression is being lost, but I don't like the sound of your knocking noise, even if I haven't heard it.

Does it make the noise when turning over by hand, on the starter, or only when running ?
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Re: Today I did a Compression Test

Postby itchyfeet » 06 Jul 2018, 16:24

sure knocking isn't tappets? they can sound terrible,
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Re: Today I did a Compression Test

Postby Snowmark » 06 Jul 2018, 19:37

Pretty sure it’s not tappets. Only noticeable when running (but then haven’t paid attention when turning by hand etc). Thought it could be worn engine mounts but they look pretty good.

Just set the timing - it appeared to be set at TDC not BTDC so I’ll give it a run and see if anything has changed.
As I said, it’s starting on the button, running fine and happy to cruise at 70mph (or there abouts). I’ll try a different compression gauge then worry some more....

I’m resigned to the fact that I may need a new engine at some point as both water jacket seals are weeping and I’m not convinced that it has been properly maintained in the past. I’d better start reading all your rebuild threads!!
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Re: Today I did a Compression Test

Postby bigbadbob76 » 06 Jul 2018, 20:41

Here's a comparison for you Snowmark-
I checked mine when I got it,
wet pressures were 15-20 psi above dry ones and cyl 2 was 15psi lower than the rest.
When I took it apart, the piston ring gaps were just out of spec apart from cyl 2 which had a broken ring.
it still did 70mph but smoked a bit on startup.
It ran fine like that for 5k miles until a totally unrelated water seal failed and forced a re-build.

I also had low oil pressure which made it tappet rattle.
Worth checking your exhaust/silencer isn't hitting the chassis, mine did and it made a nasty knocking noise that made me panic about the engine. :lol:
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Re: Today I did a Compression Test

Postby Snowmark » 07 Jul 2018, 06:22

Thanks BBB - that made me feel much better early this morning! I have just fitted a new SS exhaust so could possibly be linked to this. But, as you say, it’s running fine, not using excessive fluids, not throwing out smoke or cutting out so I’ll just get out and enjoy driving it........until it breaks, and then I will actually have something concrete to fret about
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Re: Today I did a Compression Test

Postby bigbadbob76 » 07 Jul 2018, 12:21

Your post inspired me to do a post-rebuild compression test.
All cylinders 120-125psi hot, wide open throttle.
This is less than the gauge showed before the re-build but they're all close to each other and I know it's got new rings etc so i suspect the gauge or my method rather than the compression being low. all above minimum anyway :ok
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Re: Today I did a Compression Test

Postby Sir Brixalot » 09 Jul 2018, 12:38

I've just got a compression gauge. Apart from spark plug and main coil lead it there anything else I need to do, for example to stop fuel supply and lastly is it better to swap back to petrol for the test?.
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Re: Today I did a Compression Test

Postby bigbadbob76 » 09 Jul 2018, 15:09

I just pull the lt lead off the coil, remove all spark plugs, that way there's no engine vacuum drawing fuel through so you don't have to block it off.
Sorry, can't/shouldn't comment on LPG as I don't know but my instinct says if it comes through under pressure probably best to switch to petrol.
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