Lotus Elan

Compression test

PostPost by: Bigbaldybloke » Tue Apr 28, 2020 2:48 pm

My Plus 2 has been off the road since 1986. The engine was rebuilt about a year before it was taken off the road due to a corroded chassis. The engine has had a squirt of oil down it’s plug holes a few times and hand cranked over in the years it’s been off the road. I jury rigged the engine to briefly start it for a few minutes a couple of years ago when I started the total rebuild of the car.
I’ve just done a compression test using a new tester I’ve just bought and never used before.
The car has a new charged battery and new standard starter motor fitted but incomplete fuel system so I can’t run it at the moment.
I took all the plugs out, held the throttle wide open and got figured of between 130 and 140 psi. Does this sound reasonable for a cold engine that has been stood this long?
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PostPost by: nmauduit » Tue Apr 28, 2020 3:36 pm

that does not seem like a lot (follows long disclaimer list... ;) ):
if the compressiometer is good (incl. o-ring or other suitable seal at the plug), the battery strong enough to crank the engine till it reaches a plateau (you see the needle going up in 1 or 2 strokes), have you had identical results with a teaspoon of engine oil in the plug hole just before measuring?
if yes, and all 4 compressions near identical and it can be trusted that the engine has had only light use since rebuilt, then are you sure the camshaft timing is correct (and not a super race profile)?

if all the above checks I would still go ahead and try the engine on the road before pulling it out.
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PostPost by: Donels » Tue Apr 28, 2020 5:14 pm

Sounds like you have protected it from corrosion but the values seems a little low, but.....they're all the same. So if the engine has done little running since rebuild it may not be fully run-in, so leakage past rings may be high.

Any problems are likely to be corrosion based, so visually check for corrosion, if it all looks good try a start. You can get a borescope attachment for your phone off EBay for around £12 so you can cheaply examine bores, camshafts, valve seats etc to give you a good idea without stripping it down.

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PostPost by: benymazz » Tue Apr 28, 2020 6:35 pm

Echo what was said above - I think that anything causing low compression is either caused by corrosion or a lack of running in.

I would definitely say run the engine on the road and do another compression test after that before deciding to yank it out.

Since you said you put oil down the plug holes and turned it over periodically I doubt that the cylinder walls or rings are corroded. My engine that I just rebuilt had sat in a garage since since at least 1980 without being turned over once and there was only slight surface rust at the tops of the bores. Minor surface rust on the valves and valve seats comes to mind but I do not know enough to say how serious of a problem that could be.

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PostPost by: sprintsoft » Tue Apr 28, 2020 7:02 pm

I think doing it cold is not giving you the proper result...

Workshop Manual says:

1. Warm engine to normal operating temperature
2. Throttles wide open
3. Starter turning engine at 200rpm

a. Should give in excess of: 160psi (at sea level)
b. Each cylinder within 20psi of each other

(The same figures are given for both 9.5:1 and the 10.3:1 compression ratio engines.)

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PostPost by: TeeJay » Wed Apr 29, 2020 10:58 am

Bigbaldybloke wrote:I took all the plugs out, held the throttle wide open and got figured of between 130 and 140 psi. Does this sound reasonable for a cold engine that has been stood this long?


My engine after standing for a similar amount of years, from cold with full throttle:-
130, 109, 156, 159 psi.

As my engine was out on a stand and with previous engine history, I removed the head to clean, inspect and examine it.

A complete rebuild followed. The resulting HOT full throttle:-
180, 180, 180, 180 psi

IMO
If your engine is fully assembled in the car and you only have to complete the fuel supply.
Check static timing, plug gaps and spark to plug with them all removed, easy if you have timing light.

I would then turn the engine over with the plugs out in stages until you see an oil pressure reading.

If all fluids are OK, oil, water and fuel to carbs with no major leaks, I would try to start the engine.

All being well run it at fast tick over and if that’s ok increase throttle slowly upwards to 3000 rpm.

Again if all’s well, run engine until warm and then recheck the compression at full throttle.

Hope all goes well for you. :D
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PostPost by: Bigbaldybloke » Wed Apr 29, 2020 3:06 pm

Thanks all, most helpful.
I think based on your replies I’m going to leave it be until I can get it running in the car and get it up to temperature then try it again.
As the pressures are pretty even I don’t think there is anything major wrong, but I’ll carefully check all the valve clearances before I run it. The engine is already +0.030 on the bore so not much scope there, but it’s only done around 3000 miles since it was rebored. Hopefully it’s the valves if the pressures don’t come up. Might borrow a different pressure tester to see how the readings compare with mine.
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PostPost by: nmauduit » Wed Apr 29, 2020 5:46 pm

Bigbaldybloke wrote:Thanks all, most helpful.
I think based on your replies I’m going to leave it be until I can get it running in the car and get it up to temperature then try it again.
As the pressures are pretty even I don’t think there is anything major wrong, but I’ll carefully check all the valve clearances before I run it. The engine is already +0.030 on the bore so not much scope there, but it’s only done around 3000 miles since it was rebored. Hopefully it’s the valves if the pressures don’t come up. Might borrow a different pressure tester to see how the readings compare with mine.


valve clearance is one thing, but just to be sure I would also get a glimpse at valve timing (skipping a tooth or two can alter the compression, too )... you can then assess lift and duration and get a feel for the profile you have.

on an engine that has sit for so long and not always under your watch (always with plugs? ), it is possible that old mineral oil has gummed up a valve guide or stuck some dust on a seat and it would not seal well at first : a heat cycle should restore that, too.
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