Compression lost in my engine

[Ricci]

The engine codes must be fixed first. A 95 will not be really noticeable. The speed sensor will not allow the engine to rev up while driving. I thomk That is the problem.

But the shop is saying

In regards to the Lotus we first scanned the engine for codes. Found 2 codes 1. #24 for Vehicle Speed Sensor and 2. #44 for Oxygen Sensor. We then proceeded with a compression test. The results were as follows. 1. 95 2.

1. – We first check for any engine codes, 2 were found

A. #24 – Vehicle Speed Sensor

B. #44 - O2 Sensor

2. We then completed a compression test.

1. 95

2. 150

3. 130

4. 140

3. Completed Oil and Filter Service with an engine flush to see if we could bring the compression up. Also completed an intake decarbonisation service. No luck at this point.

4. Completed Leak Down Test, found compression to come back up, leading to believe there is a problem with the bottom end.

5. I spoke with the engine rebuilder and it can be rebuilt but no estimate can be given at this time.

[molemot]

Low compression will be either: a hole in the piston(!)

or leakage past the piston rings

or valves leaking.

The normal diagnosis procedure is to add a few drops of oil to the low cylinder through the plug hole, then repeat the compression test. If the compression figure comes back up, then the rings are leaking; if not, then it's the valves. Since the next steps are the same in both cases, you have to get the engine out and remove the head...then you can check the valves fully. Easiest way is to remove the camshafts so that all the valves are shut, then put the head with the combustion chambers uppermost and plugs fitted and fill the combustion chambers with paraffin. Then wait...and inspect the inlet and exhaust passages for signs of paraffin, indicating the valves are leaking. If they are OK, then proceed to removing the pistons and checking the rings and bores. As for an estimate....valves can be replaced and ground in fairly cheaply, but pistons and liners and rings would be much more expensive, £1,000 +. Just for parts..... good luck!!

[Sir Paolo]

Rick, Agree with John, but I note you state compression 'came up'. Was the test done compression test done warm?

Either way no.1 is more than 10% lower than the adjacent cylinder, which usually indicates a problem.

Also check that there's no issues with carbs - this can affect compression readings. If there's a choke problem etc. Worth checking before you start stripping the engine.

Also and I'm speaking from experience here, you may find that if/when the engine is in bits, other parts are approaching the end of their useful life, and need replacing 'while you're at it'.

Lotus ownership was never meant to be cheap!

[alan]

I would not rebuild an engine based on this compression test alone. Firstly few people know how to do a proper compression test (eg ensure there is no air restriction, throttle open, air filter off, battery fully charged, engine warm). If for example the throttle was not open then small differences in the throttle opening could cause these sort of results.

Secondly I would want to do a running compression test as well as cranking to help identify weak valve springs etc. Thirdly I would want a leak down test to find out where the air is getting out.

95 is not that low and will probably not be noticable. I would sort out the other issues first - even on three cylinders the engine will be able to rev up when not under load.

[molemot]

The original poster says that a leak down test was done...and gave a higher compression reading? As for the significance of a compression test, that lies mainly in the comparison between cylinders. This engine has 95, 150, 130 and 140. So I would say that the test is being done properly, otherwise the higher figures wouldn't have been there. Also the 95 reading is indicative of a problem; if it's the rings then the engine will either be burning oil or will shortly start to...and if it's the valves, then the defective seal will lead to at least one burnt valve if the engine continues in use. None of this should stop the engine from revving...but will limit the power available and ought to be rectified.

[Ricci]

Thank you for all of your advice it Gives me something to go on And I have a better understanding on when the mechanic is talkin to me...

My mechanic said there was another 1990 Lotus that have similar problem and it was a fuel injector!!! we have to start there first and eliminate all the problems Before we start looking at an engine job also change spark plugs

Low compression will be either: a hole in the piston(!)

or leakage past the piston rings

or valves leaking.

The normal diagnosis procedure is to add a few drops of oil to the low cylinder through the plug hole, then repeat the compression test. If the compression figure comes back up, then the rings are leaking; if not, then it's the valves. Since the next steps are the same in both cases, you have to get the engine out and remove the head...then you can check the valves fully. Easiest way is to remove the camshafts so that all the valves are shut, then put the head with the combustion chambers uppermost and plugs fitted and fill the combustion chambers with paraffin. Then wait...and inspect the inlet and exhaust passages for signs of paraffin, indicating the valves are leaking. If they are OK, then proceed to removing the pistons and checking the rings and bores. As for an estimate....valves can be replaced and ground in fairly cheaply, but pistons and liners and rings would be much more expensive, £1,000 +. Just for parts..... good luck!!

Thank you for all of your advice it Gives me something to go on And I have a better understanding on when the mechanic is talkin to me...

My mechanic said there was another 1990 Lotus that have similar problem and it was a fuel injector!!! we have to start there first and eliminate all the problems Before we start looking at an engine job also change spark plugs