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MG TD TF 1500 - Cyl compression
|I am going to do a tune up on my mgtf 1250.|
What should the compression be in PSI s ?
Does the eng have to be warmed up ?
Throttle wide open or closed ?
All plugs out or only the one you are checking ?
There seems to be some confusion on these three things
|the instructions with my compression gauge state the following; warm up engine, remove all plugs, place compression tester in spark plug hole, place throttle in full open position, engage starter and spin motor. read compression value on gauge. (i'll add: car in neutral and parking/emergency brake set.) |
the value you read is not as significant as the relationship between all cylinder values. what you would like is for them to be within 10 per cent of each other.
if you have an air compressor, have you considered using a differential compression tester? it would tell you rings or which valves and give you numbers to use to compare the cylinders.
if you don't have an air compressor you can put a little oil on top of the pistons and repeat the compression test with your compression gauge. if the rings are the cause of the low compression the reading on the gauge will rise when you test after the oil application.
It is very important that the engine be warm, not just idling for a few minutes. Take it out for a drive first. I used to not think that the WOT (throttle full open as Tom says) was important, but I've seen a few installations since where the engine could not get the air it needed to show a good compression, so now I open the throttle completely. My procedure follows Tom's quite closely, but I'll add I usually go for six cranks, but I know mechanics I respect who go 10.
You can then immediately proceed to setting the valves and you will be done with the 'breathing' part of a tune-up.
This thread was discussed between 26/07/2009 and 27/07/2009
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