Motor Mystery...

Harry_p
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Joined: Thu Oct 22, 2020 10:34 pm

Re: Motor Mystery...

Post by Harry_p »

I’ve always read that most home testers aren’t that accurate and not to focus too much on the numbers just to ensure they’re all even. I don’t know how much truth there is in that.
Italian: ‘69 fiat 850 saloon, ‘80 alfasud super, 2012 Mito multiair 135
German: ‘72 VW dormobile camper, ‘87 Mk2 GTi 8v, ‘89 e30 m3, ‘96 e36 m3 evo saloon, ‘98 e36 318ti compact sport, ‘06 merc clk350
British: ‘94 metro gti 1.8, ‘91 metro 1.1c

justsuds
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Location: Chorley Lancs

Re: Motor Mystery...

Post by justsuds »

Atmospheric pressure is 14.8 psi ish, therefore you have 10:1 compression - ish, which is pretty good !
Cheers, John.

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Spacenut
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Re: Motor Mystery...

Post by Spacenut »

alfadave wrote:
Tue Sep 07, 2021 8:48 am
Good news and bad news really?

Four even compressions is good.
Under 150 psi , not so good?

Have you tried another compression tester, to confirm yours is ok?
Indeed - compression tester is brand new, but you are right, the absolute pressure reading could be off. That said, I was more interested in getting a consistent set of closely spaced readings.

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Spacenut
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Re: Motor Mystery...

Post by Spacenut »

justsuds wrote:
Wed Sep 08, 2021 10:00 am
Atmospheric pressure is 14.8 psi ish, therefore you have 10:1 compression - ish, which is pretty good !
Cheers, John.
Thanks John - that's good enough for me :D

alfadave
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Re: Motor Mystery...

Post by alfadave »

Haynes p.25 says minimum satisfactory pressure is 150psi

My cheapo Gunson Comp tester has the green zone between 150-200 psi

I would beg/borrow another tester to independently check.

But don't panic, just keep driving it!

kammatic
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Location: Bedfordshire

Re: Motor Mystery...

Post by kammatic »

Compression ratio is swept volume against clearance volume isn't it?
I've never thought of it as 15 psi atmospheric/ 150 psi compression means we have 10:1. Does it work like that?

For a cheeky quick way of doing it, an it's actually how alfa/fiat did it as early as the 90's with the examiner, you put an amp clamp to the starter main feed, disconnect fuel supply to engine so it won't start, and crank it. Measure the current draw to the starter on each compression event and you get a relative compression test. I can knock them out in 10 mins these days.

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Spacenut
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Re: Motor Mystery...

Post by Spacenut »

kammatic wrote:
Thu Sep 09, 2021 11:30 am
Compression ratio is swept volume against clearance volume isn't it?
I've never thought of it as 15 psi atmospheric/ 150 psi compression means we have 10:1. Does it work like that?

For a cheeky quick way of doing it, an it's actually how alfa/fiat did it as early as the 90's with the examiner, you put an amp clamp to the starter main feed, disconnect fuel supply to engine so it won't start, and crank it. Measure the current draw to the starter on each compression event and you get a relative compression test. I can knock them out in 10 mins these days.
I think you are probably right about the compression pressure Kamm, but I am now close enough across all cylinders to know that there is not a specific problem with the LH bank. I like your current sensor idea, we remotely monitor microwave amplifier tubes in orbit in much the same way - input current is proportional to helix current. Once the relationship is quantified the technique is quite accurate. Do you have a calibration for cranking current vs. compression ratio? I guess it will vary depending on starter torque, ring gear diameter etc.

I will probably borrow another compression tester to make sure.

Lauren

kammatic
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Location: Bedfordshire

Re: Motor Mystery...

Post by kammatic »

I'm sure there is some way to calculate compression pressures against current draw, once all the variables are taken into account ,, but its not done like that in service / workshops..
The idea behind the amp clamp method is the achieve a quick relative compression test.
So, image customer brings car / truck Into workshop complaining of a miss fire/ rough running, you could argue its either fuel, ignition, or mechanical issue in the cylinder . So we use the current draw test to look at relative compression, if one or even two are a lot lower, you found the issue. On engines with cam sensor, I scope that and overlay with the current draw scope pattern, so you can see which cylinder is which .
It's honestly the best trick I've learned in years to quickly ascertain the cause of problems

chriswilliams
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Re: Motor Mystery...

Post by chriswilliams »

How to detect a cylinder head gasket leak:

Buy the "cylinder head test" kit on ebay. Mine cost £8.50

Its a brewers trap, a rubber bung and a blue test fluid.

Warm the engine, remove the radiator cap and push in the bung.

Add a little blue fluid to the trap and start the engine again.

Once it all settles down if you see the colour change from blue to yellow, you have a head gasket leak! The leak produces CO2, which forms carbolic acid with the fluid in the trap and the blue turns yellow when the water becomes acid. Very simple really!

That's what I found, the subject of another post.

regards,

Chris.

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Spacenut
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Re: Motor Mystery...

Post by Spacenut »

Thanks Chris.

It's been a while since I posted the above, after doing a few thousand more miles I am increasingly convinced that I have one or more badly seated valves, possibly as a result of running for a long time with tight valve clearances.

Poorly seating valves would explain the poor compression and also the incessant popping and banging through the exhaust on the overrun - sounds like a turbo wastegate! I used to think it was the short exhaust, or even the not-quite-gastight joints, but that would not explain the poor compression.

Not sure whether to repair of replace at the moment. The engine needs work as the crank oil seal is leaking too :cry:

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