poor engine compression

[kalindi]

I have just had a telephone conversation with the French engineer who has completed a compression test on my elderly BMC. My french is not so good but my understanding is that three cylinders are ok and one not> However he seemed to say not to worry because the problem was carbon caused by poor injection. He has recalibrated the injectors and all should be well. Does this make any sense or is it just wishful translation. I will see him on Saturday and a face to face conversation with many diagrammes will make things easier to understand but any advice before hand would be helpful.
Thanks again in anticipation

 

[Cliveshep]

The short and pragmatic answer to that one is that if the engine starts, runs fine etc on Saturday translation is irrelevant. It isn't unusual for cylinder compressions to vary a bit, any more than a bit can be more serious, causes can be sticking rings, worn rings, broken rings, worn valve guides, burnt valve seats, blown head gasket, cracked head, or a combination of any/all of the above. Do you REALLY want to worry about all that NOW? It might all be fine when you run it on Saturday as you think the mechanic intimates, and you can then heave a sigh of relief and go boating!

 

[Latestarter1]

Few engine manufacturers still list compression data for their engines. Compression checking is gasoline engine territory.

It has long been established that a blow by check or the slightly less reliable leak down test are the only reliable measures of diesel engine health.

Because of the nature of a high compression diesel engine a compression test can easily show disparity on a perfectly healthy engine a small drip of fuel in the pre combustion chamber of your old indirect injection engine will give a significantly different compression reading.

However pulling four injectors checking each cylinder refitting the merely 'pop' testing the injectors is a super way of generating revenue.

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[boatmike]

It sounds as if you do indeed have one cylinder down on compression. Contrary to other posts diesel engines can and should be compression tested and the results should be reliable enough. The reference to carbon is a bit vague but it is entirely possible that a blocked or badly calibrated injector on one cylinder could cause carbon build up. The most common way carbon can cause compression loss is a build up on the exhaust valve stems which can stop the valve seating properly. If this is the case the only proper cure is a valve out decarbonising job and re-grind the valves, but if it's not very seriously low on compression and he has the injectors now working OK the quickest cure is often to give the engine some heavy work to do at full throttle for a while. This can often get the temperature up and blow a good deal of the crap out. Don't be afraid to hammer it. Hard work never did a diesel any harm.

 

[Marsupial]

If one cylinder is down on compression there will be a reason for it and it wont be an injector, although an injector could have caused it. They should all be within 10% of one another. A good mechanic would have put some thick oil in the low cylinder to see if the reading changed (upwards) by a significant amount - this would indicate worn bores or a sticking piston ring or three are the culprit. If the reading didnt change then its valves, stuck and or burnt, seats burnt, broken, missing or head gaskets leaking,or cracks and other horrible expensive stuff. All ASSUMING THE TAPPETS ARE CORRECTLY SET!! very important and often neglected.

IF you can get the thing firing on four cylinders do as boat mike says its the cheapest option.

 

[oldharry]

"However pulling four injectors checking each cylinder refitting then merely 'pop' testing the injectors is a super way of generating revenue"

Couldnt agree more! That test and the result is virtually meaningless. Appreciate there is a language problem, but what does he mean?

Compression testing a diesel is a waste if time except under carefully controlled conditions, and is almost always a waste of time. Leak down type testing gives a much more accurate picture, and an experienced mechanic can tell from the noise and rate of leakage what the trouble is.

 

[Marsupial]

A bit harsh not a complete waste of time, the readings I am used to are 400, 375, 0, 390 or something like it! At least you know where to look for the trouble. Not sure that many mechanics would have the kit to do a leak down test (for those who dont know(no offence meant) you need a compressor and few guages and valves and a good ear) but they do know about oil.

 

[Heckler]

I agree, a compression test is the definitive test, there is a lot of good stuff on here BUT also a lot of "thumble thumb" urban myths repeated
Stu

 

[Marsupial]

yes, like many other industries "the engine repair industry" thrives on urban myths. But lets be fair on a forum with no actual evidence (like an unwilling lump of metal) its hard to give absolute advice but only general ideas and often the good stuff gets mixed with the not so good. Unfortunaltey it can take "an expert" to tell them apart and even then when did two experts actually agree? Like many others on this forum I try to give others the benefit of my many years experience in such a way that the recpient (if he/she acts on it) wont do themselves or thier equipment any harm.


Have a nice day