sensible pre-purchase tests for a diesel engine

[stefan_r]

I am looking at a boat and wanted to get the engine and outdrive surveyed.

I have asked the local Volvo agent for Poole (who should know what they're on about) but they say that they don't need to do compression tests...

Their logic is that without new batteries and tip-top starter motors (boat is 14 years old with 650 hours) that compression tests don't really show much.

this sounds like rubbish to me because I thought the test proved the cylinder got up to ..x.. PSI which showed the rings were okay, no scorng on the cylinder etc. As long as the piston completes a cycle in a cylinder then the PSI should be attained and make me happy??

I can understand a cold engine may show slightly low results until everything expands...but this is the first time I have heard that compression tests are irrelevant!!!

BUT!!! very srange that Volvo are turning down a few hundred quid to do some work.

Instead they say their engineer will come down for a few hours and have a look and listen for any problems....

What do the forum suggest as the right tests for a KAD42 and outdrive???

Shall I insist on compression tests anyway???

 

[Clyde_Wanderer]

Volvo Have a point to a certain degree.
A diesel or petrol enging will have slightly higher compression readings with new or near new starter motor for the very reason thet the engine spins faster when cranking, giving less time for the gasses, which are been compressed, to escape down past the rings, and a combination of new starter and battery can make a marked difference to the starting of an engine, especially an older one with some wear on it.
But there can be lots of other factors that can effect the starting of an older engine.

 

[capt_courageous]

Hi
The engine specification will certainly show the expected pressure at 'starter motor' revs. while cold. I guess they just don't want to take the injectors out.

 

[Talbot]

Have you considered having the oil analysed, as that will show you more than just a compression test anyway!

 

[vyv_cox]

I don't understand their argument re compression tests. The absolute value is relatively unimportant, within a certain range for the engine. The real value of the test is to compare the values for each cylinder, which will highlight any faults. I can only think that with such a low number of hours they assume everything will be in order.

It may be that a good mechanic can give you sufficient information after running it for a while. Putting it into gear and running it hard against warps will give some information by ear and by looking at exhaust. Condition of the bilges re leaks of oil and water should be considered, plus gas leaks during running, of course. General appearance may also be a clue, original belts should still be in good condition, no burning or rust around the head, mounts not softened, etc.

 

[oldharry]

[ QUOTE ]

I have asked the local Volvo agent for Poole (who should know what they're on about) but they say that they don't need to do compression tests...
?
BUT!!! very srange that Volvo are turning down a few hundred quid to do some work.

Instead they say their engineer will come down for a few hours and have a look and listen for any problems....


[/ QUOTE ]

Quite right too! Diesel compression is usually more than twice that of a petrol engine, and at those sort of pressures, cranking speed compressions reveal little more than the pistons are going up and down, and the valves are opening and shutting. This can be proved much more simply by starting the engine!

An experienced diesel engineer can tell you much more by using his eyes and ears on an engine that is running. He will know the softer exhaust note of compression loss, and can tell the condition of valves, injectors and pistons simply by listening to the way the engine is running - the level and sound of diesel 'knock' for example will tell him a great deal about the condition of the engine, and coupled with an hour or so looking around and checking, he will very soon be able to tell you whether the engine is in good shape.

Most diesel faults show up as a change in the noise it makes when running, either at tickover or at speed on and off load. Compression blowby from worn or damaged bores produces crankcase pressure, which he will spot instantly once the engine has warmed up.

Also diesels do not like being disturbed and dismantled unnecessarily - so do not insist on tests that will cost you hundreds, tell you nothing that cannot be checked more simply by other means, and may disturb the smooth running of an otherwise good engine!

/forums/images/graemlins/ooo.gif

 

[Oen]

It's certainly best not to disturb the engine(s) unnecessarily.

But it's even more certainly vital to have oil analysis done. Swansea Tribology Services:

web page here

Have saved me about £30K so far.

 

[reeac]

The bottom line is whether the engine starts promptly from cold. If it does then compression and associated items [starter motor etc.] are OK .

 

[oldharry]

[ QUOTE ]
The bottom line is whether the engine starts promptly from cold. If it does then compression and associated items [starter motor etc.] are OK .

[/ QUOTE ]
Yes, but..... if it doesnt it may still be perfectly OK. And most direct inject engines will still start perfectly with badly worn rings /bores. I once had a Perkins that started perfectly, though the bores wore so worn it had to go up 2 sizes on rebore! The only symptom was oil loss through the breather. Equally a BMC 1.5 I had would take 2 minutes or more cranking cold start, although recently recond, because of a duff connection in the heater circuit.

 

[stefan_r]

[ QUOTE ]

Have saved me about £30K so far.

[/ QUOTE ]

WOW! Out of interest what did they find that was so costly ?

 

[vyv_cox]

Used oil analysis

I know what the suppliers of this service will claim and I have a good deal of experience in the technologies used for it. I would be very wary of making decisions based upon a single sample. Spectrographic oil analysis should only be used as a trending tool and it is easy to be misled by some high values (or some low ones) for certain elements. The values obtained from a sample taken immediately before an oil change and immediately after will show absolutely no similarities. So the most you can expect is to find out whether the oil needs changing.

A few years ago I did a job in which a fairly large gas engine had failed in a big way, despite regular sampling for oil analysis, with good results. It turned out that sampling and oil changes were totally uncoordinated and the last few samples happened to have been taken on almost new oil. Bearing failure was not picked up and the engine was wrecked.

 

[stefan_r]

Re: Used oil analysis

so how many hours (apart from as many as possible!!) would be a minimum for a reasonable result??

 

[vyv_cox]

Re: Used oil analysis

The test counts the metal particles of a specific size range in a sample, plus looks at viscosity, acid value, water content. Assuming that the oil is changed every 150 hours in an boat's internal combustion engine it makes sense to sample when the oil is about to be changed. Trending the results over, say three oil changes at minimum would perhaps give some meaningful purpose. Samples need to be taken either during or immediately after operation when the oil is hot. Nothing magic about it, it's just known as taking a representative sample.

In industrial equipment sampling frequency is selected according to the equipment. So for gas or diesel engines running continuously sampling might be carried out at 1000 or 1500 hour intervals. For rotating equipment like gas turbines we normally sample every six months or maybe less frequently. Helicopter gearboxes have continuous onboard monitoring plus very frequent laboratory testing! It's known as Risk assessment.

 

[Oen]

I had surveys done on two mobos. Both had significant engine problems, only picked up through spectography. Without going into nauseating detail, the engines concerned were, though relatively new and low-houred, shot. Had I purchased, I'd have been faced with engine replacement.

 

[Andrew_Fanner]

1. Does it start?
2. If it starts, does it stop?
3. If it starts, does it do so without clouds of mucky smoke?
4. If it starts, does it run with a clean(sih) exhgaust when run for a while?

ect.

That will goive you a good idea whether or not it is basically sound.

 

[andy_wilson]

Sensible pre-purchase tests for a diesel engine

1. Inspect all service records. Their presence or lack of is the first main test, as is that of scheduled oil analysis results. (edit) Ask to see on-board spares department.

2. Familiarise yourself with the specified performance parameters of this installation in this vessel, the loading and fouling situation, visually inspect the installation, check the oil (and anything else pre-start) and go for a test run. See if it accellerates faultlessly up to designed speed.

3. After at least 20 minutes satisfactory running include a long run at full load to assure yourself of the fuel delivery and cooling capabilities.

4. When back in dock, note the hours since last service and take an oil sample for analysis. Taken in isolation this is of limited use, as outlined below, but given a briefing, the lab will advise you if there is any cause for further investigation.

5. Then decide if a compression test or detailed engine survey is required.