# How to identify a problem diesel?



## davidpm (Oct 22, 2007)

I have the opportunity to spend a lot of time aboad a boat with a Universal M25 that I belive is on its way out. We had a compression test and it is really low. It also stopped once due to overheating. So I got to thinking what would give this engine away if I didn't know it so well?

It starts easy. Glow plug for 30 seconds, starts immediately.
White smoke for 45 seconds
No venting from the crankcase overflow tube.

The above would lead one to belive that something is amis but maybe not too bad. I know better.

There is a bad smell below but that could be attributed to a simple exhaust problem. 

So what would I look for to confirm the low compression diagnosis?
I see this as an opportunity to improve my surveyor skills.
I have some inside knowledge, how could I deduce the state of the engine from more subtle clues?


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## DrB (Mar 29, 2007)

*Do You Need to Use the Glow Plugs...*

everytime from a cold start? If it's warm, like 60 deg F or above, do you need the GP? You should be able to start the engine without GP most of the time IF you have good compression in a diesel. GP are intended, I believe, to assist in cold weather starts by helping to warm the pistons without extended cranking of the engine.

DrB


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## bwindrope (Feb 25, 2007)

Well this might be a sucker question but I'll step forward. Sounds to me a lot like part of the problem is leaking valve guides that drip oil, causing the white smoke on start up. Valve guides won't cause low compression, but are symptomatic of metal wear and could relate to the other culprit. Valve adjustment could also be bad or the valves or seats could be bad, causing low compression. Exhaust valves could be bad venting exhaust into the engine compartment through one of the vents. She starts well, which is good, and this doesn't correlate with having bad valves, but hey, I'd be checking valve adjustment anyway because it's so darn easy and so often the problem. 

There's a stab for someone to laugh at.


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## jrd22 (Nov 14, 2000)

From what you have said it would be hard to tell the engine is in bad shape. Usually, an engine with low compression will be very hard starting and it will blow blue smoke for quite awhile when cold, or under a load. There will also be blow by which will drip into the bilge on older engines that are not vented to the air cleaner for reburning. I test a diesel by cold starting it, without glow plugs or just minimal depending on air temp, and by how much smoke there is on initial startup (blue/oil, white/unburned diesel) and for how long (some engines typically smoke but will then clear within a few minutes). If it starts well cold with no excessive smoke then I check oil pressure against the manufacturers specs. when warmed up to normal operating temp. I'll then run it under load at full throttle for 5 minutes, there should not be excessive smoke and it should not over heat (temp will increase but not significantly). After running it hard I check oil pressure at idle and compare to manuf. specs. Then I shut it off and it should shut down quickly, only one revolution or so, not slowly wind down after shutting off the fuel. If it passes these tests then it's time for an oil analysis and check the antifreeze to see if it is clean on FWC engines. External examination can tell you a lot before you even start it, look for leaks (oil or water) especially around the front of the crank and at the back of the oil pan (main seals). Look for corrosion and rust and if found determine what's causing it. Any discoloration of the oil or antifreeze needs to be checked out. Generally, if you are not extremely familiar with engines you should have an engine survey done, and it's not a bad idea even if you are an "expert".


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## djodenda (Mar 4, 2006)

Just a data point, but my M25, which has 2500 hours on it and runs well, takes a lot of cranking to start, unless I use the glow plugs for at least 30 seconds. Starts right up after that.

This isn't necessary if the engine is warm, but if it' been shut down for more than an hour or so, even in the summer, I use the glow plugs. It's easier on the starter.

I think this is a normal characteristic of the engine.

Oh... and it's really hard to get a reliable compression test with one of those cheapo gauges that have a rubber tip that seals up against the injector/spark plug hole. The only way to have a reasonable shot a getting somewhat accurate readings, is with the type that has a hose that screws into the injector/spark plug hole. 

David


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## captbillc (Jul 31, 2008)

white smoke on start up is caused by fuel not burning completely in a cold engine. low compression will cause this. is the engine using oil? if not it could be valve leakage causing low compression. how many hours on the engine?


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## HoffaLives (Feb 19, 2007)

davidpm said:


> No venting from the crankcase overflow tube.


something wrong here if i understand you correctly - if the engine has bad compression due to cylinder wear you will get blowby showing up in the crankcase vent. given the symptoms you describe-low compression and overheating - are you sure you don't have a bad head gasket? (although they will usually show one or two bad cylinders). 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. the big factor determining if it's time for a rebuild are the hours and years on it.


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## timebandit (Sep 18, 2002)

OK, I'll guess!!!

Partial plugged exhaust.

No air out, no air in, no compression.

Usually the engine will start ok and idle but when you apply power it grunts and gets hot but no power.

If there is not a lot of blow by then the rings are sealing pretty good.

You can check the over all condition of the engine by placing the suspect cylinder on tdc and pressurerising the cyl with co2 and listen to see if the air is escaping from the intake, exhaust, engine vent.

If the leak is at a valve pull the cover and check the adjustment.


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## davidpm (Oct 22, 2007)

djodenda said:


> Just a data point, but my M25, which has 2500 hours on it and runs well, takes a lot of cranking to start, unless I use the glow plugs for at least 30 seconds. Starts right up after that.


This is the exact behavior we have. We took it to a supposedly reliable marine yard that specializes in this engine. Old Lyme Marina, Clinton CT. They treated us well but the project manager told me the mechanic would not do a wet test due to fear the engine would kick over and damage his gage. When I asked him how that was even remotely possible with all injectors removed he could not give me an answer.

I suspect that not every mechanic has the same experience and has a good day even at the best yards.


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## davidpm (Oct 22, 2007)

HoffaLives said:


> something wrong here if i understand you correctly - if the engine has bad compression due to cylinder wear you will get blowby showing up in the crankcase vent. given the symptoms you describe-low compression and overheating - are you sure you don't have a bad head gasket? (although they will usually show one or two bad cylinders). 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. the big factor determining if it's time for a rebuild are the hours and years on it.


That's exactly what I'm thinking. It starts easy if glowed for 30 seconds.


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