Why doesn't my engine act its age?

[skipjack27]

I have a major puzzle that I'd love someone to throw some light on. The puzzle was actually broached in an earlier thread on a crankcase breather problem, but I'm reluctant to pursue it further there. I received much invaluable advice in that thread and I'm reluctant to impose further on those very kind and knowledgeable respondents. By starting a new thread, they'll have the opportunity to pretend they haven't seen it!

Briefly: my Volvo AQAD41A (1450 hours; 23 years old; no previous rebuild as far as I know) is venting oil through the crankcase breather at a quite alarming rate. The problem only appears at revs above about 2700, and gets progressively worse as rpm increases until there is a continuous trickle of oil into the bilge in the 3100-3400rpm range. Mechanics ran a compression test and borescope, with the following results:

1: 16 (232)
2: 17 (247)
3: 19.5 (283)
4: 19.5 (283)
5: 16 (232)
6: 19 (276)

Above results were provided to me in bars, and I have converted to psi in brackets. Volvo says the service limit for this engine is 320psi. The borescope revealed "vertical scoring evident and cylinders polished (no cross-hatching)".

So, a pretty clear diagnosis of the breather problem, apparently: this is a very tired engine that is in dire need of a rebuild. Except...

- starts instantly in even the most freezing conditions (not literally freezing here in Perth, W.A., but still extremely cold in midwinter). Don't even have to use glow plugs. Just advance the throttle a little above idle, and she instantly bursts into life every time. In 18 months, I have NEVER experienced the slightest difficulty in starting her up. Combing other fora for a clue to what is going on, I noted the following reply to someone having similar problems: "how confident are you regarding the compression test? an easy start for a low compression diesel engine is an oxymoron. a worn-out engine will generally let you know by smoke and hard to start."

- blows NO smoke at all of any colour (white smoke on startup, which disappears after a few minutes - this is a known problem with this model, that was corrected on later models). I've hung over the transom at WOT and am utterly unable to detect any smoke at all.

- as far as I can determine, the engine is using essentially no detectable quantities of oil. This has been a little confounded by the breather problem, but when that has been avoided there is no evidence at all that she is burning oil (which is, of course, entirely consistent with the complete absence of any black smoke)

- runs extremely reliably: never conks under any circumstance, and appears to have all the acceleration and speed I could hope for in a boat of this sort. With a clean bottom, she reaches her rated 21 knot max speed at WOT.

What is going on here?? Mechanics have remarked to me that this model was possibly "the best marine diesel Volvo ever made", but it can't be that good. According to the compression test, this is an engine that is dead on its feet: very low compression in some cylinders, and substantial variation across cylinders. And yet, if it were not for the breather problem, I would never have suspected that the engine was not in prime condition.

I'm a relative diesel newbie, but I've owned gas engines for quite a long time - and I know all about tired gas engines. Impossible to start in cold weather (and not that easy in warm weather), conk every time they are put into gear, burn almost as much oil as fuel, poor out the kind of exhaust that originally gave rise to the "stinkboat" sobriquet, etc, etc.

So, can anyone educate me about diesels? How can such a tired engine behave so impeccably (with the sole exception of the breather problem)? Is this what diesels do? Could the compression test be seriously awry (the firm I deal with tends to put its youngest and/or least competent mechanics onto relatively small boats like mine)? But if it was inaccurate, why do I have such a bad breather problem?

Any perspectives on this apparent paradox would be greatly appreciated.

Jeff

 

[Don S]

Re: Why doesn't my engine act its age?

I must admit, I have never seen a Volvo 41 with bad compression that would start easily. I like many others, can only do so much troubleshooting from thousands of miles away without actually seeing, hearing, smelling and feeling the engine running.

Get a new compression test. This needs to be done per Volvo specs: This is from the manual.

A compression test indicating the cylinders and valve
sealing is carried out as a easy and reliable way of
judging engine condition.
? Run engine to operating temperature and shut it off.
? Dismantle all injectors and test cylinders in turn
and order. (Note: that means remove the injectors, not take them apart Don S)
IMPORTANT! Cleanliness is extremely important,
no dirt must get into the fuel system. Plug
terminals on disconnected injectors and fuel
lines.
Compression should be read off at normal starter
motor speed. A deviation of up to 10% below the figure
given in the Workshop manual ?Technical data?
is acceptable.

From the Tech Data manual, the compression should be as follows: PS: PSI X 145 = MPa

Cylinder pressure at starter motor speed 4 r/s (240 rpm) ... 2.5 MPa (25 kp/cm 2 )

2.5 MPa = 362.5

Obviously you are well below that with your readings.

After retesting the compression in the normal method with a known accurate compression gauge, I would follow up with a leakdown test to determine where your compression is going. Rings, valves, etc.
Some may suggest you put a couple of tablespoons full of oil in each cylinder and redo the compression test to see if the compression comes up, but if you do, it could also affect the leakdown test results.

As I have said before, faulty injectors can wash oil off the cylinder walls and cause scoring.

Has anyone looked at the turbo? If it is leaking oil at WOT, it could be pushing oil into the manifold. You would also notice oil leaking out of the inter-cooler weep hole next to the water drain (If it is not plugged up). Oil would be all over the starter too.

Is your WOT rpm within specs?

 

[dsiekman]

Re: Why doesn't my engine act its age?

Let me begin by saying this is a total theory and i am in no way a mechanic...but...

Are you familiar with the concept of water injection in diesel engines? It has been around since the 30's and is used to increase power and cool combustion temps. The problem with water in an internal combustion engine is that water cannot be compressed. As such, something has to give. However, diesel engines run much higher compression than gas engines. Is is possible that you have a very minor leak in one of your gaskets, water in the fuel, or were getting condensation inside the engine allowing water into the oil? This could displace oil as described and also boost compression at startup until the engine was warm enough to burn it off. Granted, you'd have to be pretty darn lucky for it to work out to be the perfect ratio...

Again, this is total speculation and may not even be possible. I hesitate even putting it out there, but in my line of work, all ideas are worth considering.

Good luck and keep us posted.

 

[Howard Sterndrive]

Re: Why doesn't my engine act its age?

Some diesels start pretty well with lowered compression.
As engine compression decreases, the starter cranking speed increases and creates the heat of compression with piston velocity. Put your thumb over the end of a bicycle tire pump - pump slow..... then fast and see which burns your thumb.
An extra 100 rpm on the starter also spins the injection pump 50 rpm faster.

 

[Don S]

Re: Why doesn't my engine act its age?

Some diesels start pretty well with lowered compression.

The Volvo 41's were not in that group, trust me.

 

[Joshua Nichols]

Re: Why doesn't my engine act its age?

Worked on tons of diesel engines...She is ready for a overhaul.. Are you ready to spend the cash? If not, go ahead and run her.. I had a little 3 cylinder perkins diesel that would start up and run with very low compression.. Lots lower than the numbers you posted.. It was amazing.. The guy had used starting fluid on it trying to start it after running out of fuel and trying to bleed of the air. Broke one piston and shattered the rings on the others.
That oil you see is blow by.. A sign of overhaul time.. Should you be flipping out? Not really.. The thing will likely run another 5 years or more..
Seems kinda strange that with that little operating time that its worn.. But it happens.. Dirt ingestion is the killer along with using starting fluid

 

[GLENN M]

Re: Why doesn't my engine act its age?

only thing i can think of to cause pressurised crankcase,is blowbye.no mystery here its for whatever reason down on compression and getting past the rings.why who knows yours still starts like new when majority wont,one of the great mysterys i suppose.you can donate it to science,rebuild it, run it the way it is,or see if volvo wants to take apart and study it.

 

[proshadetree]

Re: Why doesn't my engine act its age?

The borescope revealed "vertical scoring evident and cylinders polished (no cross-hatching)". That statement alone is worth a thousand words. Its build time. The easy start and mo smoke is just a mystery. Like stated before lots of diesel engines have bad compression and start like champs. Never worked on a Volvo, but many cat and Perkins are worn out and run like champs, no smoke and start great. You must have a freak because Don said it shouldn't start good. The compression tests and borescope will not lie.

 

[dodgeram2500]

Re: Why doesn't my engine act its age?

Just a thought... Will a compression tester thread right into the injector holes or do you have to use an adapter? If you need to use an adapter it increases the volume of the cylinder thus you get lower than actual compression numbers. I ran into this once on a motorcycle with funny spark plug threads once, had to use a 3 or 4" brass adapter with o rings and all my numbers were much lower than spec, but all fairly even.

 

[proshadetree]

Re: Why doesn't my engine act its age?

Must be a lot of adaptors to lose 100 psi. He stated his walls were worn out.

 

[skipjack27]

Re: Why doesn't my engine act its age?

Thanks for the replies. A great deal of food for thought.

But, I'm still no nearer understanding the puzzle. In Glenn M's words: "one of the great mysterys i suppose.you can donate it to science,rebuild it, run it the way it is,or see if volvo wants to take apart and study it."

Don S is quite clear that 41's shouldn't act like this while having such low compression, and he suggests getting another test done. I'm strongly inclined to take his advice. As someone else remarked, these compression figures are surprising for an engine that's only done 1450 hours. I'm quite prepared to rebuild the engine this coming winter, but would be irritated if it wasn't really necessary and the oil venting problem has some other explanation. So, I guess the first thing to do is to confirm the original compression test and just accept that I've got a freaky engine if the figures are confirmed. If the compression results are confirmed: rebuild. If not, perhaps explore alternative solutions to the venting problem.

Thanks, everyone, for clarifying the problem for me.

Jeff