Compression test tresults

[ElanByName]

Hi All this is my first post since I bought a +2/130S 4speed Elan last August, engine/head rebuilt several years ago with low annual mileage since (~300). The car has a previous history of misfiring (discussed on this forum) when all sorts of fixes were considered. Indeed the day I viewed the car there was a little difficulty starting it.

Despite an ongoing misfire, sometimes one, sometimes two cylinders, initial investigations around the ignition system revealed nothing untoward so I moved on to a dry compression test, results as follows : 140-100-100-140. This indicates a leak between cylinders 2 and 3, potentially causing a misfire on one or two cylinders? I have had contact with the previous owner who acknowledges that the head was never retorqued after the rebuild, but he wont confirm how many miles have been covered. This doesnt sound good on the basis that the car has potentially completed 3k-4k miles since using the MOT certificates as a guide.

All other engine parameters look ok - no overheating etc, no water in oil

Please can you advise on the following

1) After how many miles should the cylinder head be retorqued
2) Is it worth just retorquing the head now to see if the problem is cured?
3) Do I replace the gasket, or get the head checked etc?
3) Looking at other threads here , compression on typical good cyclinders appears to be iro 175lbs compared with the 140 I am getting. Is 140 on the low side and do I need to delve deeper?
4) Any other thoughts?

Please let me know if further information would help.

Many thanks

David

[alan]

imho with 2 and 3 at 100 psi you need to take the head off. Also check for flatness

[Chancer]

Did you do the compression test on a wide open throttle and wait for each cylinder to "pump up" the reading to its maximum?

If not that may explain your lowish readings of 140psi but it could also be the guage, I only ever rely on them as a comparator and on that basis you have 2 cylinders that warrant investigation.

[oldchieft]

Hi David
One thing that is missed is the type of Schrader valve fitted to the compression tester.

Has the valve been changed?

It should be a low differential type valve or you can get low readings.

I found this out when I begged some old tyre valves from a tyre shop to replace the leaking valve in my tester and found it was reading low even when compared with a tyre inflater.

I sent mails to Schrader to get the right type but got no answer, so I did my own fix.

If you get a long type valve with a spring on the outside then just pull off the coils and cut them till it is just a light pressure to keep it closed.

Having said that you still have a problem with 2 and 3 cylinder.

If you can get an air connection to one of the plug holes (an old spark plug and a bit of welding) and turn the engine so both valve on that cylinder are closed stick it in gear with the brake on. Put the air on then you might find air come out the other plug hole.

But you might not wish to waste time on this and just lift the head, it will need to come off.

Jon the Chief

[ElanByName]

Hi David
One thing that is missed is the type of Schrader valve fitted to the compression tester.

Has the valve been changed?

It should be a low differential type valve or you can get low readings.

I found this out when I begged some old tyre valves from a tyre shop to replace the leaking valve in my tester and found it was reading low even when compared with a tyre inflater.

I sent mails to Schrader to get the right type but got no answer, so I did my own fix.

If you get a long type valve with a spring on the outside then just pull off the coils and cut them till it is just a light pressure to keep it closed.

Having said that you still have a problem with 2 and 3 cylinder.

If you can get an air connection to one of the plug holes (an old spark plug and a bit of welding) and turn the engine so both valve on that cylinder are closed stick it in gear with the brake on. Put the air on then you might find air come out the other plug hole.

But you might not wish to waste time on this and just lift the head, it will need to come off.

Jon the Chief

Jon
Many thanks for all the info. The tester I used is a very old (but almost unused Skes Pyckacant unit. I have also used it recently on my TR5 which gave consistent readings of 175psi on each cylinder, which is about right I believe. Therefore my assumption was that the Lotus readings were at least indicative if not accurate. Is my logic sound?

[ElanByName]

imho with 2 and 3 at 100 psi you need to take the head off. Also check for flatness

Alan,
Thanks, I guess you are confirming what I thought. Could you give me an idea of the max mileage recommended before retorquing after a head replacement please?
David

[ElanByName]

Did you do the compression test on a wide open throttle and wait for each cylinder to "pump up" the reading to its maximum?

If not that may explain your lowish readings of 140psi but it could also be the guage, I only ever rely on them as a comparator and on that basis you have 2 cylinders that warrant investigation.

Hi and thanks for the reply.

The test was performed with the engine turning over on the key for about 10-12 revolutions until the maximum readings were achieved. Why do you recommend a wide open throttle?

Thanks

David

[Chris]

I would suggest that you buy, make, or borrow a differential pressure tester. Or you local garage man may have one. This will eliminate any variations caused by throttle openings, or engine turn over speed etc. you will however require a compressor.
Chris

[rgh0]

Hi David

You need to test with the throttle plates open otherwise the pressure achieved is reduced by the vacuum you pull in the inlet manifold.

cheers
Rohan

[AHM]

Hi David,

Did you do your test with a hot or cold engine?

Adding to what Rohan has said, it also eliminates the other induction variables, particularly vacuum take-offs on no1 and no4 that could give those cylinders higher readings.

To go back to your questions.
1. About 500 miles depending on who you ask - you should also check valve clearances
2. It costs you nothing to try
3. neither until you have ruled out lots of other things. Is the miss high revs, tick over, or all the time? have you checked HT lead resistance, timing, valve clearance? Carb set-up? there are vac take-offs that upset the carbs at low revs as do leaking throttle spindles....
3a. It is a subjective measure with many variables, what you are looking for is consistency
4. There are various ways of doing the compression test, but I think most people do it on a hot engine, with the plugs out, and the throttle open. Also look-up " leak down tester " on youtube

[ElanByName]

I would suggest that you buy, make, or borrow a differential pressure tester. Or you local garage man may have one. This will eliminate any variations caused by throttle openings, or engine turn over speed etc. you will however require a compressor.
Chris

Thanks Chris, good suggestion. Ive had a look on ebay and there a a few reasonably priced kits, so will probably buy rather than make. A compressor was on the shopping list anyway, just need to bring it to the top of the list!
Thanks again David

[ElanByName]

Hi David

You need to test with the throttle plates open otherwise the pressure achieved is reduced by the vacuum you pull in the inlet manifold.

cheers
Rohan

Hi Rohan
THanks for highlighting this, goodpoint.
David

[ElanByName]

Hi David,

Did you do your test with a hot or cold engine?

Adding to what Rohan has said, it also eliminates the other induction variables, particularly vacuum take-offs on no1 and no4 that could give those cylinders higher readings.

To go back to your questions.
1. About 500 miles depending on who you ask - you should also check valve clearances
2. It costs you nothing to try
3. neither until you have ruled out lots of other things. Is the miss high revs, tick over, or all the time? have you checked HT lead resistance, timing, valve clearance? Carb set-up? there are vac take-offs that upset the carbs at low revs as do leaking throttle spindles....
3a. It is a subjective measure with many variables, what you are looking for is consistency
4. There are various ways of doing the compression test, but I think most people do it on a hot engine, with the plugs out, and the throttle open. Also look-up " leak down tester " on youtube

Hi,
Thanks for the comprehensive reply.

What you seem to be suggesting (with Rohan) is that the readings for 1 and 4 may be artiifically high, and therefore the real readings may be nearer those for cylinders 2 and 3?

Thanks for the other comments against my specific questions. Here are is a bit more info/thoughts

1) According to receipts and MoT paperwork, the rebuilt block and cylinder head has completed on average 100 miles per year since the rebuild in June 2009. The previous owner has confirmed that cylinder head bolts had not bee retorqued at the time of sale and I assume valve clearances have not yet been checked as the mileage is still within 500 since the rebuild (almost 5 years ago).

2) True!

3) It is missing all the time on at least two cylinders. HT lead resistance and ignition timing has been checked although the fuel supply side hasnt. Are you suggesting I may be jumping to the wrong conclusions looking at compression test results etc?

3a &4) Agreed. The results are consistent on repeated checks, however I am only able to complete the test on a cold engine.

How do I perform meaningful leakdown/compression tests on an engine that fails to run because it is too lumpy (2 cylinders at best)? Maybe I dont

Thanks again

David

[AHM]

What we are suggesting is that you have a consistent method and eliminate things that are nothing to do with what you are testing - So doing the test with the throttle open eliminates everything to do with carbs and vac take-offs. I don't know if you have vac take-offs on 1 and 4 or if you have strombergs or webers so can't draw any conclusions. Open the throttle and eliminate the question.

3. I recently bought a new distributor cap, changed the leads over but was looking at the inside of the cap.... 1342 anticlockwise looking at the outside of the cap is 1243 from the inside. Every time I took the cap off to check I looked at the inside and checked 1342 then put it back - It took me a while to work it out.

If you have the correct mixture and a good spark at the right time anything will run - Having the right compression is desirable but not essential. So I would suggest that not running is a separate issue.

You can't really do a meaningful compression test without the running clearances of a fully warm engine... so yes, I think you are jumping to a conclusion..... partial test results and a likely unrelated issue are your symptoms.

How do I perform meaningful leakdown/compression tests on an engine that fails to run because it is too lumpy (2 cylinders at best)? Maybe I dont

You can't - you need to get it to run first.

[kstrutt11]

It should run fine on all 4 cylinders with those compressions, is the miisfire at all rpm or just idle?

If it is washing loads of fuel down 2 cylinders it could even explain the differences in compression.

I would go back to basics
Check the valve clearances
Check there is a spark at each plug
Check the firing order
Check the carb to head o rings
Finally strip and check the carbs

Kevin