Lotus Elan

Compression tests a bit odd....

PostPost by: reb53 » Mon Dec 17, 2018 5:17 am

Several months ago I did a compression test and got the following results,
200, ( rearmost), 175, 200, 200.

The 175 was a little worrying as that was the cylinder that died many years ago, punching bits of ring into the head.
( After the re-build it was ok, the same as the rest. 70K miles since then.)
I didn't try any oil down the bore, just thought I'd keep a wary eye on it.

Today I ran the test again and got,
200, 200, 200, and 150.
Some oil down the "150" brought it up to 200 immediately.

And the previous 175 was now ok....( ? ).
So I'm a little confused as to what could be going on.
I know if an engine sits the rings can stick and need freeing up, but my car is used regularly.
All tests were run with the engine hot and the throttle fully open.

My current plan is to keep a very close eye on things but wondered what others think.
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PostPost by: alan.barker » Mon Dec 17, 2018 5:34 am

I'm surprised the oil made a difference which points to Rings.
The usual cause for low Compression on a Cylinder is a burnt Ex Valve.
I lmagine it's No 1 on your Sprint due to the Servo and Headlamp Vacuum take offs which can make it run weak. So Exhaust Valve gets too hot and burns.
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PostPost by: reb53 » Mon Dec 17, 2018 5:41 am

Hi Alan.
I thought, how does he know I have a Sprint, as I forgot to mention it ?
Then saw the poster's name... :)

Still odd that the previous low reading has come up though.
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PostPost by: alan.barker » Mon Dec 17, 2018 8:48 am

Hi Ralph,
do you think it could be an Exhaust Valve sticking a bit. Maybe a squirt of Redex/Uppercylinder Lubricant or whatever you use in NZ could do the trick. I think it's worth a try and quick to do :wink:
Alan
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PostPost by: nmauduit » Mon Dec 17, 2018 10:01 am

what you can do to get an other piece of information is a leak down test (putting compressed air into the chamber and trying to find how fast and where it goes). The setup is not expensive but needs an air compressor to operate.

Regarding a burnt valve, I would think once it starts leaking from being burnt it goes down quite fast, 150 compression is not that bad and the fact that it's restored to specs with oil points to a minute compression leak.
Then after 70k miles a valve guide may be worn out and lead to a valve seating poorly, possibly springs a bit on the low side (could be assessed with the head on), a small debris may be stuck on to the valve sealing area, oil burnt on a valve stem causing it to stick... Examination of plugs (richness and combustion quality) and careful follow up for evolution (was the compression measured engine hot or cold?) may help you to determine if (when) a overhaul is to be scheduled.
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PostPost by: david.g.chapman » Mon Dec 17, 2018 11:22 am

You may have done this of course, but did you repeat the compression test again without oil? I have had an instance where I got a low reading and then all was OK when repeated.

If you get a consistent low reading like that with the oil test pointing to rings, then I am afraid my next step would be to take off the head, drop the sump, undo the big end and raise a piston up to look at the rings. The top one may have broken. If you leave it the bore could end up scratched as the ring continues to break up. :(

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PostPost by: alan.barker » Mon Dec 17, 2018 12:34 pm

When i bought my Elan Sprint it had 69,000 miles on the clock. I am the 4th owner and it has known history.
For me and my experience of Twin cams it was the right moment to refresh the Engine.
I removed the Engine and stripped it down.
When i looked at the Pistons there were a few broken Rings and all the Rings worn thin + Ring Grooves worn wide. Cylinder standard and never been rebored. No real Lip at the top of the Cylinders + no scores.
I bought a new old stoke set of pistons very cheap, deglazed then rebuilt Engine. Using new Bearing Shells + new Oil Pump.
The Engine now runs perfect :mrgreen:
Alan
ps. the set of 4 old stock new Pistons only cost £100
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PostPost by: reb53 » Mon Dec 17, 2018 8:00 pm

Thanks for the suggestions to date.
And, yes, the engine was hot when tested.

I intend to retest on a regular basis until a clear pattern emerges and I can then decide what to do.
If it comes apart for a rebore then all valves would be replaced as I remember what happened the last time the engine was apart and I replaced the valves, ( as a precaution.)

The old ones were sitting on the bench and as I idly tapped one against the other, to see how "attached" the carbon was, the head fell off one, only 2 tiny little bits of shiny metal holding it on..........

I might have to get you to chuck a set of pistons into your luggage next time you head for NZ Alan ! :)
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PostPost by: Hawksfield » Mon Dec 17, 2018 11:34 pm

Hi reb53
Are saying that your two different test results show No 4 piston was low first 175
Then the next test No1 piston was low at 150, and both recovered on retest??
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PostPost by: reb53 » Mon Dec 17, 2018 11:59 pm

Hi John,

Several months ago no. 3 was 175, but now is ok, ( 200 ). The others were all 200.

No. 1 just tested 150, and came up to 200 with some oil. The others are all 200, ( including no. 3 which was 175 several months ago).

Imagine if you could just leave all things on your Elan for a few months and they came right on their own.... :)
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PostPost by: Panda » Tue Dec 18, 2018 10:14 am

I think that when you loosened and removed the spark plugs some particles of carbon lodged on the valve and prevented the valve from seating fully. Whenever I do a compression or leakage test, I loosen the spark plugs and then run the engine for a few seconds, and only then, perform a compression test. Fifty years in the industry has proved to me the necessity to do this on Lotus and Alfa twin cam engines.
Alan P.
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PostPost by: reb53 » Tue Dec 18, 2018 10:22 am

Thanks Alan, something I hadn’t thought of.
I’ll retest a few times and if I’m lucky I can postpone the rebuild for several years!
I’ll let you know the results.
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PostPost by: Hawksfield » Tue Dec 18, 2018 12:38 pm

Hi reb53

If your engine is running fine I would take regular compression test's to suit your mileage, once a month if you use it regular and record figures to analyse
Otherwise enjoy using the car if you had a real problem you would know by poor running :)
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PostPost by: alan.barker » Tue Dec 18, 2018 1:16 pm

With 70,000 miles the condition of the Rings and Pistons need to be checked. To do that they need to be removed.
This is normal for a Twin Cam imho.
Alan
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PostPost by: reb53 » Wed Dec 19, 2018 2:18 am

Christmas has arrived early !!

I loosened the plugs a few turns and briefly ran the engine to ensure any bits of carbon weren't floating around, ( though probably not as the plugs were out a few days ago anyway).
Then with my trusty assistant keeping the throttle fully open I re-ran the tests and got...., wait for it......

200 200 200 200 :D :D

So I suspect the odd readings were caused by either some carbon crud, poor technique, or both.

Thanks for the replies, and enjoy your Xmas !

Cheers,
Ralph.
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