Lotus Esprit with low compression

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Re: Lotus Esprit with low compression

Postby cs_ » Tue Mar 22, 2011 1:48 pm

Hi Alan, I believe the compression loss can be seen in the cranking waveforms you provided. The area of interest is the amount of pressure before EVO on the compression stroke. Notice that on cylinder 4 there is -8.8psi and cylinder #3 is -5.5psi. Both cylinders are showing some leakage but cyl #4 is the worst. I would suspect that the other cylinders would read similar to #3.

If there is a leak in a cylinder air will escape while compressed which will cause low pressure at the end of the power stroke - the closer to atmospheric the better the seal. This is due to there being a lower volume of air in the cylinder at the end of the power stroke compared to at the beginning of the compression stroke.

Now would be a good time to do a leakage test. You can also do a cranking test with the wps in the intake, crankcase and exhaust.
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Re: Lotus Esprit with low compression

Postby mrblonde » Tue Mar 22, 2011 9:51 pm

hi folks, Cs thanks for that explanation helps me a lot! i had mentioned earlier about rise and fall towers uneven and noticed the larger dip if that is the right word? and thought i was on the right track to see compression escaping you mentioned both cylinders show leaks?
could you share what you see for us others? many thanks!
ps this is what i mentioned earlier i saw
Attachments
pico cranking low with overlay.gif
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Re: Lotus Esprit with low compression

Postby cs_ » Thu Mar 24, 2011 4:13 pm

Hello MrBlonde, thanks for the picture it does a good job showing the loss.

you mentioned both cylinders show leaks?
could you share what you see for us others?


Both cylinders are showing a loss but since we have three cylinders that are balanced I would be willing to assume the amount on cyl #3 to be "normal for this engine" for now and focus on cyl #4.
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Re: Lotus Esprit with low compression

Postby Alan » Thu Mar 24, 2011 6:21 pm

Both posts above are correct. The peak cranking pressures and minimum pressures just before EVO are as follows:

pressures.gif
(5.46 KiB) Not downloaded yet


Another possible cause of low compression is restricted air intake (less air sucked in so less to compress) but we can rule this out as the intake part of the waveform looks similar across all cylinders. If we all agree that cylinder 4 is leaking then we can move on and try and find the cause.

Lets look at another cranking waveform. This time we have added #1 ignition as a reference and measured exhaust pulses using one WPS500 pressure sensor and inlet manifold with another.

lotus inlet and exhaust.gif


The top trace (blue) is the exhaust waveform, the green trace is inlet and the bottom trace is #1 ignition.

We can add more waveforms if anyone wants, but in our opinion this one shows the problem rather well. Now its over to you!

** Updated 28th March **

To clarify / expand on the above, this is our understanding of the waveform.

If you look at the above waveform the position of the vertical ruler shows a missing intake pulse (green trace). The ignition trace shows #1 firing, so the position of the vertical ruler is in the middle of #1 intake. Remember that there is nothing wrong with cylinder #1, its cylinder #4 that has a problem so this seems odd at first.

The explanation is that we have a leak from the intake valve on #4. When #4 comes up on the compression stroke, this leaks air into the intake just as #1 opens its intake valve. Rather than sucking air in through the air filter (which causes the intake vacuum) some of the air has instead come from our leaking cylinder.

This leaves the question of why the engine produces full power and seems to drive OK, indeed the running compression pressure readings at WOT are identical from each cylinder. Its possible that at higher RPMs the air has less time to leak from cylinder 4 so the effect is reduced, its also possible that at lower speeds the intake valve is "hanging down" and not being shut properly by the spring. At high RPMs the higher air pressure may be snapping the valve shut.

Either way its time to give the top end of this engine some TLC. It might be a few weeks before we have the time to do this but will update this thread when we have any more news.
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Re: Lotus Esprit with low compression

Postby mrblonde » Thu Mar 24, 2011 11:12 pm

hi again Alan ,CS and thanks! well i see big difference if all things are supposed to be even, not perfect but similar! i see a big loss on intake side,slight on exhaust, as we know four is the offending cylinder we could break down the cylinder events and offsets but you have proved very quickly the old lotus need some tlc in my opinion?
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Re: Lotus Esprit with low compression

Postby Dr. Jake » Sat Mar 26, 2011 1:19 pm

I think you should check the valve adjustments.
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Re: Lotus Esprit with low compression

Postby Alan » Mon Mar 28, 2011 2:54 pm

I have updated the thread 3 posts above to explain what we understand the problem to be.
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Re: Lotus Esprit with low compression

Postby mrblonde » Mon Mar 28, 2011 9:33 pm

hi Alan apologies should have measured those events :oops:
this caught my eye because i recently had one with similar symptoms at ours
low cranking compression and runs fine otherwise i was looking for an expo? it came in for non start i got very similar readings to yours i had low compression on one cylinder and two weak coils replaced the coils and runs fine no obvious power loss to owner?
i thought Dr Jake was going right way maybe worn tappets as you say maybe springs?
thanks again look forward to what you find!
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Re: Lotus Esprit with low compression

Postby mdf12mm » Sun Apr 03, 2011 8:25 pm

With a fault like this we would always check valve/tappet clearances and cam lobes
lack of valve clearance and or worn cam lobe(s) will cause erratic results on dynamic compression tests etc
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Re: Lotus Esprit with low compression

Postby Kev » Mon Mar 12, 2012 9:48 pm

Is this screen still live for the Lotus? Or has it been resolved!

Kev :D
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