V12 Compression Test

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Nick V12 e type
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#1 V12 Compression Test

Post by Nick V12 e type » Thu Jun 07, 2018 11:29 pm

Hello, I’m starting to make progress with my series 3 v12 e type. I need to remove the engine to carry out some rust repairs and thought before doing so it would be sensible to check out the condition of the engine. I’m hoping it does not need a rebuild, we will see.
I want to avoid if possible removing the engine, repairing the body, putting the engine back in and them finding the engine needs work and having to remove it again.

The oil look fresh and clean and is at the tight level (a good sign) and I started the engine a few weeks ago using a petrol can in the boot. However without an exhaust pipe fitted it was very loud and scary. I was pleased it cut out after about 30 seconds.

My plans are to carry out a compression test, use an endoscope to have a look at the cylinder walls and then if all looks good see if I can get it running. I have now fitted an exhaust pipe so if it runs the noise will be manageable.

Compression test: I’ve removed all the plugs and will disconnect the power to the fuel pump. As this is the first time I’ve attempted a compression test on any engine I would be greatful for any advice or guidance on the best process to follow.

Thanks

Nick
e type series 3 V12 2+2 1972, Tesla Model S 2016

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cactusman
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#2 Re: V12 Compression Test

Post by cactusman » Fri Jun 08, 2018 5:59 am

Best to disable the ignition by removing one of the low tension wires to the coil. Make sure it cannot short to ground by taping up just in case. Remove all twelve plugs. Make sure the loose plug leads cannot fall into any drive belts etc. Fit the compression tester to cylinder one. Crank for say 10 secs. Take the reading down. Repeat for the other eleven. Make sure your battery is fully charged as you have a lot to check :bigrin: :bigrin: Not sure what the pressures should be for a V12....others will know but at least 100 psi I would guess....but look for even pressures across all twelve...plus or minus 10 or so psi. Given your engine has not run these will be cold pressures. They may be different if the engine is hot so if things look ok and you get the engine running you can repeat later when the engine is warm.
Julian the E-type man
1962 FHC
1966 MGB....fab little car too

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angelw
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#3 Re: V12 Compression Test

Post by angelw » Fri Jun 08, 2018 8:54 am

Hello Nick,
The "proper" method is to have the throttle wide open to ensure that the intake gets a good supply of air into the cylinder. It seems not to make a whole lot of difference to the result of the test, but it would if the duration of the cranking was short. As it costs no more to do it right, conduct the test with the throttle open.

A compression test will only tell you what the general compression is and not whats causing it to be low, should it prove to be that way. A Leak-back Test will determine for you if compression is being lost past the Piston Rings, Inlet, or Exhaust Valves.

Regards,

Bill

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#4 Re: V12 Compression Test

Post by Nick V12 e type » Fri Jun 08, 2018 7:03 pm

Thank you the guidance. I will follow the advice and report back following the test. Fingers crossed, cheers Nick
e type series 3 V12 2+2 1972, Tesla Model S 2016

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#5 Re: V12 Compression Test

Post by Nick V12 e type » Tue Jun 12, 2018 9:37 pm

Hello,
compression test done, the most difficult part was cleaning out all the muck that had accumulated in the wells within which the spark plugs sit. Lots of poking about and copious use of my workshop vac prior to removing each plug.

I followed the advice, all plugs out, coil disconnected, fuel pump disconnected and battery fully charged.

Here are my results, cold engine, no oil added to the cylinders and throttle wide open.

Bank A 1. 155 2. 155 3. 155 4. 140 5. 160 6. 155

Bank B 1. 135 2. 150 3. 150 4. 155 5. 155 6. 155

I’m thinking this is a good set of numbers 2 cylinders a little lower than the other 10 but I’m hoping the experts will agree with my view that the engine is sound based on these results.

I welcome your experienced feedback.

Nick
e type series 3 V12 2+2 1972, Tesla Model S 2016

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cactusman
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#6 Re: V12 Compression Test

Post by cactusman » Wed Jun 13, 2018 8:38 pm

Not having a V12 myself other may know better but they all look pretty close. I'd say compression wise you are ok. My xk six showed around 160 +/- 10 or so. So yours are very similar.
Next thing I'd do is crank the engine with no plugs etc and just check that you do get some oil pressure. Ideally use a mechanical gauge as the pressure transducer is notoriously unreliable and inaccurate on the electrical version.
If you get some pressure showing the pump is working I'd get it all together and start her up and let her idle a bit to get the oil right around everywhere. You can check for major leaks in the oil, water and fuel systems while the engine warns. If there is no plume of smoke and oil pressure is at least 40 psi at idle and, I think, 60 at 3000 rpm all is well.
Julian the E-type man
1962 FHC
1966 MGB....fab little car too

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42south
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#7 Re: V12 Compression Test

Post by 42south » Thu Jun 14, 2018 1:31 am

Hi
Don’t panic if the hot idle oil pressure is 15-20psi, or even lower.
These engines are renowned for low oil pressure at idle.
It’s unlikely to be the oil pump, as they are also very reliable.
Cheers
Mark
Mark Brown
1971 S3 Etype
Son: when I grow up, I want to be a pilot
Dad: You’ll have to choose.

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#8 Re: V12 Compression Test

Post by Nick V12 e type » Thu Jun 14, 2018 11:07 am

Julian and Mark thank you for your comments, very welcome. Oil pressure gauge ordered I will report back on my progress.

Thanks again

Nick
e type series 3 V12 2+2 1972, Tesla Model S 2016

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jagwit
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#9 Re: V12 Compression Test

Post by jagwit » Thu Jun 14, 2018 3:47 pm

One reason for low compression COULD be that carbon behind and between the rings and the piston has caused the rings to become stuck in the grooves in the most inwards position, thus not causing good sealing between piston in cylinder.

I once bought a high mileage Land Rover Discovery 1 3.9 V8 for my daughter to use for her studies - long story...

Anyway, when I measured compression, cyl #5 was way down on the others. I decided to run the car with 2-stroke oil (2SO), 1 x 500ml per 80L tank fill. The theory is that the 2SO softens the carbon between the piston and the rings (and everywhere else...) over time and that eventually those rings become free to move again and thus affect better sealing.

After 3 tanks with 2SO, #5 had vastly improved and after about 6 tanks, it was on par with the others.

This recommendation can only be applied to a car WITHOUT catalytic convertor and lambda sensors.

BTW, there are a host of other benefits to using 2SO on an old engine....
Best Regards
Philip
71 E-type V12 Coupe,
80 XJS (EFI by Megasquirt & EDIS-6 + 5sp manual overdrive)
73 Jensen Interceptor
74 Interceptor (EFI by Megasquirt + overdrive 4sp auto)

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JJC
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#10 Re: V12 Compression Test

Post by JJC » Sat Jun 16, 2018 1:25 am

Note : Should not be getting 30 lbs of oil pressure at idle....should be around 8 to 10 lbs. Certainly should not be getting 60 lbs at 3000 rpm....more like 25 to 30. High oil pressure indication at 3000 rpm means electric (useless) sender is bad, or oil flow is being restricted someplace. Perhaps restricted at the bearings. Be carefull.

John

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#11 Re: V12 Compression Test

Post by Nick V12 e type » Sat Jun 16, 2018 3:45 am

Hi John,

Thanks for your comments and guidance, I’m waiting on my mechanical gauge arriving and then will need to get the car running prior to any oil pressure assessment so may be a short delay prior to reporting findings.

Many thanks for you contribution

Nick
e type series 3 V12 2+2 1972, Tesla Model S 2016

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