QUESTION
I have been having starting and idle problems so I took my Lotus Elan S2 to a local garage that has an electronic engine analyzer and ran the engine.
The oscilloscope indicated that there was some leakage between the HT leads and when checked they were dirty.
Other parts of the ignition system were checked and proved to be serviceable.
We then played about with the leads, disconnecting each of the spark-plugs in turn.
Disconnecting cylinders 1 and 2 made a big difference to the running of the engine, 3 and 4 less noticeably.
We then used a compression tester on each of the cylinders in turn. 1 and 2 gave readings of 110 psi, while 3 and 4 were less than 100psi.
Removing all the spark-plugs and retesting saw little change in 1 and 2 but slightly worst in 3 and 4.
A squirt of oil into 3 and 4 showed a slight improvement in compression.
We used two different compression testers but I'm not sure how reliable the readings were.
I have now changed the spark-plugs and the engine is now behaving for the time being.
What else can be done to investigate this problem?
R.W. - Melbourne, Australia
ANSWER
As your Elan has electronic ignition with dirty HT leads, then this can cause leakage and cross-fire if the leads are not spaced apart.
The HT leads should be checked and cleaned on a regular basis to make sure you avoid mis-fire problems.
With this in mind and the readings you achieved, I have some questions for you -
Where the tappet clearances checked to ensure that all the valves closed fully prior to the compression test?
Where the throttle butterflies held fully open whilst the compression test was carried out as this influences the readings?
How many miles/klms has your Lotus twin cam engine covered?
How much oil does the engine consume?
Are you experiencing problems with the rear most spark-plugs fouling?
During your recent visit to Lotus Marques, as you started the Elan engine to return home it was running on only 3 cylinders
As soon as you moved off, the engine cleared and and ran on all 4 cylinders
These symptoms point to a cylinder with low compression but once warm all 4 cylinders will operate.
A low compression ratio is usually the result of a worn engine that will also use oil.
The Lotus twin cam big valve engine with a 10.35 compression ratio should have in excess of 160psi.
If you saw compression figures below 100 psi then this is approaching the lower limit where it is difficult to achieve combustion.
This will lead to starting difficulties and poor idle when the engine is cold.
To investigate this problem further, I would recommend a cylinder leak-down test.
This will check engine internal wear and tell you exactly how much the piston rings and bores are worn.
Once you have about 25% leakage, then generally speaking an engine requires rebuilding.
Engines that have high cylinder leakage (blow-by) have excessive crank case pressures that can cause venting problems.
Lotus twin cam engines that have high crank case pressures will vent a mixture of gasses and oil vapour which are usually fed back into the rear carburettor (feeding cylinders 3 and 4).
This cocktail is then ingested back into the engine and can cause spark-plug fouling problems which results in poor combustion.
An other test to have a look at how your engine is running, is to install a colortune tune spark-plug in each cylinder.
The colortune has a glass insulator top that allows you to view the combustion process and the quality of the spark.
Copyright © 2012 Lotus Marques
I have been having starting and idle problems so I took my Lotus Elan S2 to a local garage that has an electronic engine analyzer and ran the engine.
The oscilloscope indicated that there was some leakage between the HT leads and when checked they were dirty.
Other parts of the ignition system were checked and proved to be serviceable.
We then played about with the leads, disconnecting each of the spark-plugs in turn.
Disconnecting cylinders 1 and 2 made a big difference to the running of the engine, 3 and 4 less noticeably.
We then used a compression tester on each of the cylinders in turn. 1 and 2 gave readings of 110 psi, while 3 and 4 were less than 100psi.
Removing all the spark-plugs and retesting saw little change in 1 and 2 but slightly worst in 3 and 4.
A squirt of oil into 3 and 4 showed a slight improvement in compression.
We used two different compression testers but I'm not sure how reliable the readings were.
I have now changed the spark-plugs and the engine is now behaving for the time being.
What else can be done to investigate this problem?
R.W. - Melbourne, Australia
ANSWER
As your Elan has electronic ignition with dirty HT leads, then this can cause leakage and cross-fire if the leads are not spaced apart.
The HT leads should be checked and cleaned on a regular basis to make sure you avoid mis-fire problems.
With this in mind and the readings you achieved, I have some questions for you -
Where the tappet clearances checked to ensure that all the valves closed fully prior to the compression test?
Where the throttle butterflies held fully open whilst the compression test was carried out as this influences the readings?
How many miles/klms has your Lotus twin cam engine covered?
How much oil does the engine consume?
Are you experiencing problems with the rear most spark-plugs fouling?
During your recent visit to Lotus Marques, as you started the Elan engine to return home it was running on only 3 cylinders
As soon as you moved off, the engine cleared and and ran on all 4 cylinders
These symptoms point to a cylinder with low compression but once warm all 4 cylinders will operate.
A low compression ratio is usually the result of a worn engine that will also use oil.
The Lotus twin cam big valve engine with a 10.35 compression ratio should have in excess of 160psi.
If you saw compression figures below 100 psi then this is approaching the lower limit where it is difficult to achieve combustion.
This will lead to starting difficulties and poor idle when the engine is cold.
To investigate this problem further, I would recommend a cylinder leak-down test.
This will check engine internal wear and tell you exactly how much the piston rings and bores are worn.
Once you have about 25% leakage, then generally speaking an engine requires rebuilding.
Engines that have high cylinder leakage (blow-by) have excessive crank case pressures that can cause venting problems.
Lotus twin cam engines that have high crank case pressures will vent a mixture of gasses and oil vapour which are usually fed back into the rear carburettor (feeding cylinders 3 and 4).
This cocktail is then ingested back into the engine and can cause spark-plug fouling problems which results in poor combustion.
An other test to have a look at how your engine is running, is to install a colortune tune spark-plug in each cylinder.
The colortune has a glass insulator top that allows you to view the combustion process and the quality of the spark.
Copyright © 2012 Lotus Marques
