Genesis’ 30 Second compression check.

Genesis’ 30 Second compression check.

Genesis,

This question came to me while reading your reply in another thread.

When you first crank them up cold, what symptoms are you looking for? Please elaborate…

Thanks,

Greg

Ok, here 'ya go...

First, note that this ONLY works for Detroits - engines with glow plugs or other starting assists will tell you little or nothing.

Second, block heaters must be OFF. The engines MUST be at room temperature. Batteries must be sufficiently charged to roll the engines over normally - use the parallel switch if in doubt.

You must calibrate expectations on the outside air temperature. Realize that Detroits ignite ONLY on compression heat - therefore, how they behave when lighting off tells you a LOT about their compression.

So, what you want it a completely cold engine and good power to roll them over.

Assume temperatures are in the 70s or thereabouts, and the overnight temps didn't go below mid 50s to 60ish. Basically, you need to know what temperature the BLOCKS are - shoot them with an IR thermometer if you need to. The following assumes the BLOCKS are around 70F.

Throttles are NEVER advanced on a start, so we have them at idle.

When the button is hit, the engines should roll over a few revolutions before catching. They should light off and settle down to a clean idle in less than 10 seconds - 5 is better. You WILL get some white smoke on the intial start - if you don't, then either (1) its the middle of summer and the blocks are 90 degrees, or (2) someone cheated and the engines are warm. Note that even a BRIEF start before you get there will queer these results - if you see NO smoke on a truly cold start, you've either got Jesus engines, its the middle of summer and the blocks are near 100 degrees, or someone cheated. If its not the middle of July or August, bet on the cheating. The reason for this is that Detroits run the rack to full fuel immediately when the starter begins turning the engine - so you are ALWAYS overfueled on a start. On a warm restart they come up before fuel pressure builds, so there's no white smoke. On a cold start the fuel pressure comes up first, you get overfueled injection into a cold cylinder, and the result is white smoke as it can't atomize and burn it all.

If compression is low on one or more holes, they won't all light off at once. You will get white smoke until they are all firing. If compression is significantly low you will get a LOT of white smoke and no light-off for a significant amount of time - 20 to 30 seconds isn't unheard of. The reason is that the fuel basically didn't light at all; you're seeing unburned, atomized fuel coming out the pipe.

You're looking for (1) the speed the engines fire up, (2) the amount of white smoke you get on start, and (3) how quickly it clears. If you get a good, clean start without hunting or misbehavior, a small to moderate amount of smoke on the start and it clears within 10 seconds or so, the compression is probably ok on all cylinders. Suspicions mount the more these metrics are "off" for the block temperature.

You should also get NO fuel on the water. If you get a tiny bit on the initial start but it disappears immediately that's not horrible. But if you get white smoke and visible fuel on the water that doesnt clear in short order then either you have a leaking injector or a cylinder that isn't firing.

The problem with this "quick test" is that misadjusted racks and bad injectors can produce these symptoms - as can low compression. HOWEVER, the odds of there being low compression and you getting a CLEAN startup when the engines are truly cold are pretty low.

BTW, check the engines for vibration once they're started up. A common "mechanic trick" if you've got an unscrupulous seller with one bad hole is to back off the injector for that hole so it doesn't fire at idle. This unbalances the engine and can be extremely damaging (since its unbalanced at ALL power levels!) but it will "hide" what would otherwise be a mosquito-control event on a cold start. The engine shaking on you from being out of balance will give it away - if you look.

With engine temps in the mid 80s to 90+ you should see almost no or even no smoke on start and immediate clearing. As temperatures drop from 70F towards freezing more time to light off and more smoke is NORMAL and its even a good idea in the cooler times to hold the STOP switch and crank for 5-10 seconds first to warm the combustion chamber with pure compression heat (no fuel going in) and also get the oil (which will be very thick) started towards the journals and turbos.

As an example a couple of years ago I looked at a 6-71 powered boat that was in an environment where the OATs went into the 50s overnight. We did a cold start test like this. It required upwards of 30 seconds, with white smoke rolling out of the mains - mosquito control heaven - before they fired and stayed lit. Even after 4-5 minutes, there was STILL significant smoke.

A full compression test came back with all cylinders just under minimums, and the deal blew up as the seller wouldn't make a proper adjustment for the trashed engines.

The key to cold starts is NOT pre-heating, although that's certainly good (from a standpoint of engine longevity and such) - its being able to roll the engines over at a sufficient RPM to get enough compression heat for them to fire. That means batteries in good shape and clean electrical bus connections to the starters.

compression test

Genesis,

As a detroit owner I will have to question the validity of your test during coooler temperatures and in cooler climates. I purchased my boat with 8V71TIs, and went to the expense of having all 16 cylinders compression tested. For our readers who don't know detroits, this means that the fuel lines must be disconnected at each cylinder, and because bending them during removal is easy, replacing them is necessary after the cylinder is tested.

The left engine tested well above factory minimum compression values, while the right engine test showed 4 of eight cylinders slightly below minimum. Both engines smoke significantly after start in temperatures below about 70, and while the right engine does smoke more than the left, both can take up to 20 minutes at 1200 rpm or more until they produce minimum smoke.

For those individuals contemplating a boat with detroits, I would argue that a compression test is an important part of the prepurchase engine survey. This is especially true if you live in cooler climates, or are having the survey done during cooler weather. The fact is that this more complete survey is expensive; about $3000 for two engines, but the potential for an overhaul of two engines is at least $28.000, excluding turbo chargers, gears, and accessories.

Up here in the great lakes, the smoke test you've described is inadequate and can be misleading, and does not necessarily indicate an engine at the end of it's useful life, particularly when temperatures get into the 60s and even 50s quite regularly. Bear in mind too that many recreational boat owners will use their boats only 75 to 100 hours a year, and there can be a few years left in the useful life of the engine even if compression is near the minimum recommended. Most trucking companies wouldn't dream of opening an engine up solely on the basis of compression that is at or even slightly below recommended minimums. They will wait for other minor symptoms to begin (such as oil consumption) to reach a certain point before overhauling.

I will admit however, that producing smoke in the marina is undesirable, particularly in this day in age, and at some point an owner will want to improve or correct the situation.

I don't deny that what you've described may be useful in climates with temperatures above 75, but let's not construe that to be universally true under cooler cold conditions.

Thanks,

A satisfied detroit owner.

forgot to add...

Sorry, in the previous post I forgot to add the following two points;

my boat does not yet have block heaters, and, great lakes water temperatures start very cold at the eginning of the season, as low as 36 degrees or so, and even at the height of summer may only be in the 60s, so the raw water side of the cooling system contributes to the time it takes to bring the engines to normal operating temperatures, and for the smoking to subside.

Satisfied detroit owner

Disagree

First, unless you bend the fuel pipes, there is no need to replace them. If you use the correct tools (there is a split socket for this purpose) to loosen and tighten the fuel line nuts, torque them properly, keep the pipes in the same place (mark them with a felt-tip pen so you know which end is which and keep them in order!) and don't damage them there is no reason to replace them. I've never replaced mine, they do not leak, and I've replaced my entire injector set once (long story - some idiot had the wrong injectors in my engines when I got the boat!) and pulled a couple of "bad actors" another couple of times. I have no fuel dilution in my oil come change time according to the lab and nice oil puddles on top of my fuel line nuts when I pull the valve covers. 'Nuff said.

Someone was yanking your chain (at your significant expense) if they told you that fuel lines had to be replaced whenever disturbed. If I recall correctly those things are roughly $25 each! 32 of 'em is $800 - for nothing. It simply is not true that they need to be replaced unless you ape them, and if you do, then you may well ape the new ones too. Overtorquing the flare-style pipes will crack the flares and then you will get fuel leaks into the oil - bad news. If you have the new style pipes with O-ring seals, then close inspection of the O-rings is called for (any crap on the seal area will cause it to leak) and the O-rings should be changed if questionable, but replacing the pipes themselves as a routine measure is uncalled for and calls into question the mechanic's ability to properly wield his or her tools and not damage things.

Now you know why I don't let so-called "mechanics" on my boat any more, and do my own work. If some jack tried to tell me that I had to spend $600 for new fuel pipes just to change out injectors (all 12 of 'em in my engines) I'd tell them to go stuff their overcharging bill-stuffing head where the sun doesn't shine, and get the he|| off my boat - if they can't turn a wrench without breaking things they sure ain't taking off any of my valve covers!

Second, what were the ACTUAL results of your compression tests? You are aware that there is a tolerance between worst and best cylinder, along with a minimum, right? If you violate the former you not only end up with an unbalanced engine under power (different compression results in different power output in each hole) but you ALSO get light-offs where one or more holes are not firing - and that makes a shizload of smoke, as I noted. If you're below minimums on ANY cylinders you will get smoke like crazy on starts even in reasonably warm weather.

I live in the Florida Panhandle.

In the winter, we get overnight freezes. In the summer, we get temperatures in the 80s overnight

I've owned my Hatt now for close to 4 years. I've seen her behavior on cold starts (with no block heaters) in every set of conditions from freezing cold (literally - going over to run for a while to add heat to the engine room so the fresh water system doesn't freeze overnight!) to blistering (100+ days) hot.

My engines have NEVER caused me that kind of trouble. If the weather is particularly cold, I will pre-heat by holding the STOP switch (or the governor stop lever if I start in the engine room) and cranking for 5 seconds or so - sometimes twice - before attempting a start. I do this for startup smoke control - its not necessary, but it helps. I do not have block heaters, as there's no good place to install them on my engine configuration.

If its taking 10 minutes for your engines to clear up at fast idle after a cold start in weather that you can run around in without a winter parka, then something's not right.

Its not necesasrily a compression problem - improper injection timing can do this, as I noted. But in your case you do know you have a compression problem.

Any colder than that, and the water isn't liquid anymore.

In very cold weather (under 40-50F) I DO use the parallel switch as the higher rotational speed at the starter contributes to a faster and better light-off.

We must qualify "smoke" here too. Light wisps on an engine with a 30 degree F block are not "smoke". Doubling as the mosquito control truck IS smoke. Such is expected in 30 degree weather ON STARTUP, but definitely NOT for the next 10 minutes!

As I said, you have to adjust the criteria based on your location and the temperature of the engine and (less so) surrounding air.

Its all about how cleanly you can get that fuel to ignite.

I have roughly 800 hours SMOH on my engines, and roughly 1800 hours TT since new. I bought the boat with "new" majors (that were half-done - at least the kits were done right) and roughly 1000 hours, and run about 250-300 hours a year. The last year has been much lighter due to the hurricane problems down here this summer. I consume about one gallon of oil per engine in 100 hours running time, and I get the kind
of light-off behavior I've described. Warm oil pressure is roughly 20 psi at idle, 55-60psi at 1500 RPM and above. There is no haze on the horizon behind my boat when I'm 100nm offshore and there is nothing to "color" my view behind me.

When I had the wrong injectors in my engine and the "mechanics" had them set up wrong I had quite a bit of smoke and a good bit of haze behind me. Now, with them set up correctly and with the correct injectors I have just a puff of smoke in reasonably warm conditions, and little (comparatively) even when its cold outside. I do get a nice "bolus" of smoke on a cold start in cold weather and water, but it sure doesn't hang around pumping that out for very long.

Water temps here run from 50 to over 90, but unless your cooling system isn't working right only the upper side of this is a major factor; Detroits have bypass thermostats and if your engines won't warm up correctly in cold water the bypass seals in the thermostat housings are probably shot. They're easily replaced, and should be if they're not working right.

If you are below minimums I bet your cross-hatch is gone and the wear indicator on the compression ring is probably gone too. So long as you don't have any broken rings or other really bad behavior you can keep running the engine - one of the "virtues" is that Detroits will run even if totally screwed up - but that doesn't mean you meet specifications - you don't - your power is probably down and EGTs are likely up.

I don't have a problem with complete engine surveys. They are expensive, but worth it in many if not most cases. The point is that if the engines have problems then why spend the $3,000 on engine surveys unless you know the seller will make an appropriate reduction in the selling price IF the survey comes back with unacceptable results - before you spend the money?

MANY sellers will behave in exactly the wrong way under such circumstances - "there ain't nothing wrong with those motors!" - even though you could double as the Mosquito truck even in reasonably warm weather (50-60F OATs) If I'm buying, I want to know that BEFORE I waste my time on the surveys.

My 30-second "compression test" does not prove compression is bad if the engines fail. If they pass, however, it is damn good evidence that all is well in the compression department, and we can proceed with the rest. On Detroits, if there is no reason to question compression you can do a boroscope and save the compression, saving you $50 a hole or so, yet still detect incipient problems that could cost you big bucks.

My "30 second compression test" a screening tool, not a diagnostic one.

Smoke

I agree w/ Genesis. My 6v92's are on the Great Lakes. They have 3300 original hrs and they still fire quickly, even down to mid 60 degree temps. Even at colder temps, the smoke clears within 30 seconds. If you have a bad injector then the test will obviously not tell you anything about compression, but this is just a quick test that is supposed to tell you if you need to dig deeper or not.

smoke test...

this has to be the BEST post on Detroit that I've ever seen , either here,, boat diesel or anywhere else. granted, I've only been interested in diesels in 15 months

thank you Genesis!

I bought my 53 with 8V71Ns in summer time (south florida) and they didn't smoke on a cold start. traces of smoke was slightly visible for a few seconds on the stbd engine but you'd have to be looking for it real close.

Engine survey revealed worn cross hatching, more severe on stbd engine, which clearly indicated something wasn't right about the 300SMOH claim / paperwork which showed litle details about the work. The rest of the boat was very good so with a 5% adjutment, I closed. I decided to skip the compression test after discussing this with the surveyor and considering the overall condition of the boat and engine operation. although it wasn't the case in this deal, I also wonder how sellers feel about an unklnown mechanic tearing down their injectors to perform the test. I know i'd be a little worried...

As temps started going down in the fall, below 70 in the morning, I noticed a little more smoke from the stbd engine. same this year, in the past couple of weeks, a little bit of smoke is visible for a few seconds, mostly stbd engine.

They both fire right away and seem to settle to a smooth idle almost right away, certainly within 5 seconds, even when temps dips in the 40s or 50s in the morning after a cold front. Port engine barely uses any oil but the stbd does, about 1 gallon every 10 hours... something I can live with for now.

Only start up issue I have is that right after the port engine fires up, unless I advance the throttle a little and idle around 800rpm, it will stop after a 5 or 10 seconds. I'm guessing I might have some air in the fuel line but any suggestion is welcome.

On the other hands, there are a few heavy smokers down the dock... not sure if they're DD but suspect they are judging by their rumbling noise... a couple of years ago, I was at a marina in the keys around Xmas time and the guy next slip asked me to close up everything and not stay outside on the boat because he was about to fire up his "GMs and they all smoke". He wasn't joking... the cloud of smoke lasted a good 5 minutes and from 100' out, I couldn't see the boats... I thought I was going to turn into an evironmentalist that day

good to know the trick about short cranking with the stop button pressed in cold weather... and I really the idea that these beasts are so forgiving especailly since I alwasy cruise around 1500rpm...

again thank you for a very informative post.

pascal
miami, fl
1970 53 MY

compression test

Genesis,

I should have mentioned that the starboard engine inspection showed some cross hatch wear, but they were not worn away either. The ring wear indicator did not show a problem.

The port engine had all compression values near new, and it still smoked in cold conditions, say around 50 or 55 degrees for a daytime high. Under these conditions, when either engine reaches normal operating temperatures, the smoke dissipates, typically within 2 or 3 minutes. But in very cold weather it can take several minutes at fast idle (again we're talking about blocks that have sat in ambient temperatures of 35-40 degrees for weeks.) Both engines start fairly quickly, and I also use the stop button for the first 5 seconds or so.

My point was only that I wouldn't use the smoke test during cold conditions as a deal killer, particularly if I liked the rest of the boat. Even if the engine didn't smoke, I would still request the engine survey from a reputable detroit shop.

I agree with your comments about replacing the fuel pipes; when I questioned the tech about this he said that they've had leaks occur in the past after such inspections, which as you can imagine create hard feelings and problems between owners and potential buyers, and that their "insurance policy" was to simply to replace them after the compression check because they were relatively inexpensive. I didn't know how "inexpensive" they were until I got the bill.

In my case, I wonder if the injectors or timing are contributing to the smoke, but I wouldn't think so if the smoke dissipates when the engines reach normal operating temperatures. I plan to have a tune up in the spring, we'll see what that turns up.

This was very interesting to me as I have two boats around me that have DD 6v71s with 2500 plus hours that will clear the docks when fired up. I have wondered why this was and now have a better understanding. I do have a question for Genesis about the cold start. I have DD 671Ns with just over 1000 honest hours in fresh water only. Fortunately for me mine do not smoke on a cold start and fire immediately as well, as long as the ambient temperature is above 50 or so. However, on a cold start at most any temperature my engines will want to hunt for several seconds until a smooth idel is reached, say 30 to 60 seconds. Both engines are set at about 600 RPM at idel with throttles pulled back in nutreal. I thought this might cause the hunting because if I advance the throttles slightly on a cold start the engines will light as usual but will idel smoothly with little hunting. This start up has the idel at about 800 RPM but for sure under 1000. Genesis, would you elaborate on this hunting situation--I never thought this to be a problem.

Here's the deal....

... on timing and injectors.

The height of the injector controls when the fuel goes into the cylinder.

For the most power, you want the fuel to go in so that it IGNITES just as the piston starts down. Note that fuel doesn't instantly ignite. Therefore, exactly when you must inject the fuel varies with engine RPM. Note that it does not vary with load.

If you inject early, then the fuel ignites while the piston is still going upward. This is VERY bad, because the pressures in the chamber will get extremely high and the engine is trying to be stopped by the fuel burning. This can in extreme cases snap conrods and in no instance is it good - it doesn't contribute to your engine's longevity or life.

If you inject late, then the fuel ignites while the piston is on the downward stroke rather than just as it starts down. The compression ratio is much lower in this case, and thus the fuel burns slower and with less pressure developed. More of the heat in the fuel ends up remaining in the exhaust rather than turned into rotation of the crankshaft. You get more heat and smoke, and less power.

Injector height is critical - it simply has to be right or the engines do not run correctly and develop all the power they are supposed to.

Exhaust valve clearance is also critical. If they're too loose, then the valves open late, close early, and do not open all the way. The result is poor breathing, excessive smoke, low power, excessive EGTs and in extreme cases broken valve keepers and dropped valves - a catastrophic failure.

If they're too tight, then the valves may not close completely (!) during compression, and they both open and close early. This results in compression leaks, burned valves, excessive EGTs and low power. In extreme cases it can result in the piston HITTING the valves - a catastrophic failure.

Valve clearances MUST be accurate within 0.001. Beware that when checking hot clearances if the engines start to cool down (and they will and do!) you get a false reading - if you adjust on this basis when hot you'll set them too tight and that's very, very bad. I'd rather be off by 0.001 loose than 0.001 tight, if I've got to err, because there's more wiggle room there. But too loose is bad too - so don't go overboard.

An injector that has a bad spray pattern will appear to run fine when completely warm, but will smoke like hell when cold. Poor atomization results in poor combustion. The problem with leaving this alone is that eventually you'll do damage with fuel washdown and carbon build-up from this - so you want to address it.

What were the actual compression NUMBERS on your test Detroit? Note that there are two sets of "minimum" numbers in the Detroit service manuals (!) in different sections - so whether you were really below minimums or not is open to question. But - if you're getting that kind of smoke - I bet you are.

I've never seen a Detroit that was below minimums and still had a good wear indicator on the top compression ring unless the problem was in the cylinder head. If that's where you are leaking you need to get that head off and fix it, because ultimately that will burn up the valves and now you're replacing the head, plus you run the risk of a catastrophic valve failure which will total the cylinders in which the valve(s) drop.

Note that compression on Detroits has to be tested with the engine fully warmed up and with the engine RUNNING at 600 RPM. The usual method is to do half the cylinders at a time, literally running the engine on half its cylinders for the purpose of the test once they've been warmed up. A compression test on a Detroit done on a cold engine, or with the starter is invalid.

6-71 DD

Genesis! I would like to get yours and others opinion on starting my motors. I am going to start my motors for the first time in 2 1/2 years. They are 6-71 natural DDs. About 2200 hours or less. How do you think I should I proceed. Air Temp will be in the 40s, water temp in the low 30s. I've been instructed in many differant ways and procedures. I need to narrow it down with a lot more insight as not to cause any damage. This will take place in about 1 to 2 weeks as soon as the docks are in and I want to be ready. Bill