Compression - how much?

[nigelhudson]

I have a Volvo 2002 which works reasonably well but not as well as it should. It starts OK unless it's been unused for a few weeks. But it's slow to pick up especially when it is cold and particularly when going astern. As it's raw water cooled it never really gets very warm unless it's been running a few hours.

I'm stripping it down for a comprehensive service and my dilemma is knowing how far to go. Clearly the injectors have been out at some point in its history - there's no service history on the boat so I've no idea how long ago.

My options seem to be just pull the injectors and get them cleaned and reset or lift the head and do a full top-end job. I realise that, if I move one of the copper sleeves round the injectors, then I'm going to have to lift the head anyway. However I work on the basis that "if it ain't broke don't poke" so lifting the cylinder head is not a job to be taken on lightly unless have to.

So I have cranked the engine over by hand and there is compression in both cylinders - but is it enough? My experience is with petrol engines so I'm not sure how much resistance I should get from hand cranking a small diesel. Given that the crank drives the camshaft rather than the crankshaft pulley it seems suspiciously easy to turn over. As it turn it over at the top of the compression stroke there is resistance but I can hear the leakage past the valves as the resistance dies away. There is clearance on all the tappets so the valves are closing but I wouldn't expect a perfect seal on an engine of this age.

Is there anyone out there with experience of hand cranking these small Volvos who could tell me how much resistance I should be experiencing? This might help me in deciding whether to take the plunge and take the head off!

 

[TheBoatman]

You didn't say what colour the exhaust smoke is? But if it's starting OK then compression maybe at the bottom of the list of possible problems.

 

[agc1]

Oh heck. I have only posted a few times and now I am going to post something liable to provoke the intelligensia that roam these parts....

Diesels work best when worked hard, ie their temperature is at the upper end rather than the lower end. Before you start stripping your engine down, have you revved the knackers off it for a good hour or so? You may see alarming clouds of black smoke coming out of the exhaust. Don't panic! This is just all the unburnt cr@p coming out of the engine, so leaving it much cleaner internally, and possibly allowing the valves to close up properley, rather than partially seal and give you compression problems.

Don't expect a diesel to perform to its best unless it is warm, and don't "play" with the injectors unless you know what you are doing. If your fuel filtration is good, your problem is unlikely to be with your injectors. Injectors are not like spark plugs that you can fettle with a file/hammer/emery cloth. If you have fuel and filtration problems, you have probably knackered the injectors. If your fuel filters are clean, you probably have not got a fuel/ fuel filtration problem, and your injectors are fine.

Do I get torn to shreds now?

 

[savageseadog]

Raw water cooled engines are slow to warm up so you will need to wait a few minutes, anything up to 5 or 10 before you try to use it. Would also suggest leaving well alone if it starts and runs. I've known diesel engines with what appears to be low compression turned by hand run and start quite well.

 

[Juggler7823]

These engines are prone to the exhaust valve getting pitted and worn and causing poor starting. I suggest you take off the head and get the valves reground. If you intend to remove the injectors use the correct tool available at kepart etc for around £30. You have to remove the stud through the clamp to get the injector out. Then, obtain set of new rings (on sale or return). Put a ring in each cylinder about 10mm from top. measure gap in ring with feeler gauges. If its significantly more than 10 thou then it needs major overhaul.

 

[capt_courageous]

Hi
If you want to check the compression hire/borrow a pressure guage from a motor engineer. There is no way you can really judge it by simply turning over by hand. Unless you use the valve lifters it should be very difficult to turn it over by hand. The Volvo manual says the compression at starter moter speed should be about 280 to 340 psi.

 

[andy_wilson]

Head

I have mine off at the moment and like the anecdotes, it needs a bit of a clean up and the valves lapping. Reason for diassembly was exhaust blowing past injector. Feared it may be a sleeve, but fortunately a stripped stud-nut caused by someone previously over tightening the injector clamp was the problem.

The exhaust valves are worst, no doubt part due to temperature and part to do with the near proximity of sea water in the exhaust mixer just after the manifold. They have lapped in OK.

Taking the head off is pretty easy, just the electrics, fuel and alternator to remove.

De-coke gasket set about £60.00 + VAT and postage from KEYPART, includes gaskets, valve stem oil seals and a few o-rings. Paste and a lapping tool a couple of quid from auto-factors, borrowed valve spring compressor and dial gauge (valve stem play - OK).

Did an oil sample before the end of season that showed no adverse wear or contamination so not interested in cylinder or bottom end wear.

One of the easier jobs I'd wager.

 

[Appleyard]

Nigel..can I add my tuppworth to all the good advice so far.
Charlie is correct in what he says re giving the beast a good work out. Assuming it is still on the boat and in running state,add a tin (500 ml) of Forte engine flush to the oil and run it on tickover for about 20 mins. Drain the oil and refill,new filter etc. This stuff cleans the rings and oilways in the engine . Now add 500 ml of Forte diesel fuel additive into the fuel filter and run this through.. this will clean the injectors and valves(give her a good hard run ) .You should find an improvement all round providing the engine is in reasonable order. This stuff is not "snake oil" most diesel mechies use it ..get it from a motor factor . I have no connection with the company .Try this before you start stripping down.

 

[nigelhudson]

Thanks for all the opinions. I'm getting more leave it alone messages than whip the head off ones! For information the smoke tends to be white but it's difficult to determine whether this is steam or unburned diesel. I have been using injector cleaner in the fuel. I wouldn't mess with the injectors myself - just extract them and give them to specialist.

I'm inclined to try the non-invasive stuff such as flushing the engine and giving it a good thrash. I did run it for an hour at 1500 rpm before starting to drain and strip it.

Happy to hear any further thoughts you have.

 

[Stemar]

A cold engine will never be able to give its best. If it's not getting warm, is the thermostat OK? If it's stuck open, that could be part of the problem and one that's easy to fix. I'd check this before the various declagging tricks mentioned above.

BTW, if you do have to pull the injectors, a tip given to me by a guy who knows what he's doing is to soak the injector with penetrating oil - the proper old fashioned stuff, not WD40, 'cos it evaporates too quickly - for 24 hours, then put a spanner on and jiggle the injector with a twisting motion. Don't force it, but persuade it. Keep going until it's turning 1/2 a circle then lift it out.

If you disturb the copper sleeves, it's a head off job and you risk getting seawater into the bores. I think I'll be draining the engine down before I start!

 

[oldharry]

Low compresion will 'always' = poor cold starting. Yours starts, so the compression cannot be too much of a problem. As others have said, if you have not run it hard, and it always tends to run cool (which it will if it is raw water cooled anyway), then maybe all it needs is a good hard thrash for a minimum half an hour, preferably longer.

I use my diesel car for commuting. A run up the motorway after a couple of weeks of stop start never properly warmed up motoring makes a quite surprising difference to it, and I know at least one garage which includes a 20 mile motorway thrash as part of its diesel 'tune up' when servicing diesels which are used mainly for shopping trips.

 

[TheBoatman]

[ QUOTE ]
I did run it for an hour at 1500 rpm before starting to drain and strip it

[/ QUOTE ]
Gezzs man 1500 rpm is nothing especially if its not under load - take the boat out and get 2500-3000 rpm on it for an hour+. Then tell us what the results are?

If you had a 300+ hp engine then yes, lets talk, but a small hp engine needs to be run at a minimum of 2500 revs.

Sorry if this post seems aggresive - it's not meant to be!

Peter.

 

[Rick]

First suggest you use cold start procedure if you're not using it - it is to set throttle to required setting (about 1/2) and then pull engine stop lever fully out, then push it back home, then turn key to start. This procedure "rolls' the fuel rail over into an overfuel situation, which ends as soon as governor starts to work.

Second suggest that the full bore technique is the go, but do it under load. I did it to mine, and it started at 2,900 revs with lots of brown / grey smoke that progressively cleared as max revs rose to 3,400 over about 20 minutes. It ran like a dream after that.

 

[oldharry]

Quite agree, boatman. Off load at 1500 rpm is really no different to tickover, and can actually damage the engine if it is done frequently. Unless a diesel is made to work, carbon accumulates in the cylinders and exhaust, reducing efficiency and power. Extended light load running does not allow the engine to warm up properly, causing uneven warming of the block and head, which can cause head gasket failure, and cracking of both block and head castings. Inadequate warming up means the oil does not come up to temrpature and volatile fractions from the fuel do not get burnt off. Excessive carbon in the bores and valves gets in to the lube causing increased wear. In the meantime the head is not properly warmed through, and particularly in the marine environment condensation may occur in the rocker box. If the valave springs are affected, they will corrode and become pitted leading to premature valve spring failure.

A good hard thrash at 3/4 plus throttle on load for at least half an hour deals with all these problems, cleans everything out, and leaves the engine running much more sweetly.