Detroit 6V92 - Hard to Start

Any idea why one of my Detroit diesel engine is hard to start after one month of rest?

My Port engine starts first time (no wait). Now my SB engine will start after several long attempt, then with some "puff" will slowly get to throttle RPM. Once done, all is fine and they both operate the same (Oil & Temp).

It sounds like a fuel problem. Air in the line from drain back. I assume it starts ok after an hour, day or week but not after a month?? Both engines are fed from same manifold and tank?? If you go to boat diesel with same question they will suggest to you the steps needed to trace the problem.

Francois said:

Any idea why one of my Detroit diesel engine is hard to start after one month of rest? My Port engine starts first time (no wait). Now my SB engine will start after several long attempt, then with some "puff" will slowly get to throttle RPM. Once done, all is fine and they both operate the same (Oil & Temp).

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You haven't provided enough info for a proper evaluation. It could be anything from a weak battery bad fuel corroded injector tip or even a stuck ring. Boat diesel is a good Detroit resource you may want to check out.

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It's loosing it's prime, have you checked for fuel dilution in your lube oil? Could be a external leak between supply pump & fuel galley or a crossover line or bad injector which would show up in a oil analysis. Good luck

Thanks for all for your answers

Sunchaser, your assumption is correct. My engine starts just fine the next day or couple of days. It is after one month that the difficulty occur (for a non mechanic, that is bizarre !!). After it started, I can run these engines with 10+ hours non strop with no issues.
Both of my engine run on different tanks and I think manifold as well. Do you think I can "bleed" the return fuel line?

Cafesport, if I had bad fuel, would this engine operate with "hiccup" while running? (which is not the case). For the corroded injector tip (which they may be rusted after 28 years of operation...

River Cruiser, I think I will try the oil analysis and see... (good idea)

Francois said:

Thanks for all for your answers Sunchaser, your assumption is correct. My engine starts just fine the next day or couple of days. It is after one month that the difficulty occur (for a non mechanic, that is bizarre !!). After it started, I can run these engines with 10+ hours non strop with no issues. Both of my engine run on different tanks and I think manifold as well. Do you think I can "bleed" the return fuel line? Cafesport, if I had bad fuel, would this engine operate with "hiccup" while running? (which is not the case). For the corroded injector tip (which they may be rusted after 28 years of operation... River Cruiser, I think I will try the oil analysis and see... (good idea)

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Your engines are kind of unique in that they are 2 cycle and there is no need to bleed them once the 2 fuel filters are full. You can just roll them over and they should care of themselves.
Healthy 6v92s emit a steady growl and should have no hiccups. If your engines harmonic beat sounds like a bass a nova you have a problem. You mentioned smoke no? What color and for how long? 28 calendar years equals how many engine hours for these engines? Oil analysis might also offer some clues.

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I would look for a tiny air leak, or the fuel supply check valve is gunked after 28 years .

DD sells a small hand pump that can pressurize the system , so air leaks become fuel leaks , lots easier to locate.

About $70.

Cafesport, smoke is white for about 8 minutes after I start them. The hours is about 2800 each.
I'll look into oil analysis and replace my fuel filters.
Thanks

FF,
Thanks for the tip. I may try that in the spring for my fueling problem that I think is still not solved. Would it work w a regular Bosch system?

Francois said:

Cafesport, smoke is white for about 8 minutes after I start them. The hours is about 2800 each. I'll look into oil analysis and replace my fuel filters. Thanks

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What is the temperature where you leave the boat ( maybe try a heater in the er next time).White smoke could be a number of things. Any loss of coolant, can you smell coolant or fuel in the exhaust, any sheen on the water? 2800 hours depending on the hp rating could mean you are past due for some replacement parts. Probably best to draw some oil and get it analyzed before you begin any disassembly. Do the engines sound the same once they get up to temp? Do they both pull max rpms. Should have probably asked this first but has anyone done anything before you had this problem?

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"2800 hours depending on the hp rating could mean you are past due for some replacement parts"

That is what I was thinking, when they get tired they take a bit to light off and they smoke. Check fuel system out completely, but also might want a DD mechanic to do a full review.

Cafesport, no coolant lost and smoke does not small coolant nor fuel. Before I bought the boat a couple of months ago, the engines were professionally maintained and the survey came out good.
I'll just see what the oil analysis says.

Thanks for your guidance !

2800 hours is nothing or time for an inframe , mostly depending on how hard they were run.

At 1800rpm its nothing , but if the cruise was 2100 full tilt with turbo and after coolerto the fish killing spot , ideling for hours trolling then on the pin home, an in-frame kit is about $1200 or so.Install $5K +

The engine will need to be lifted but not removed for the overhaul.

To rebuild it it will need to be removed .

Time smoking is not the thing to observe , coolant temperature is.

If you start up , isle for a min or so then run 800RPM till the water is 120F , 1200rpm till 140F , then 1600, when (at what temp) does the smoke clear?

IF it takes 160F to clear , its time for those new cylinders!

Not to hijack the thread but I will, I worked for several years for a contractor that had a spread of Terex scrapers & dozers all Detroit powered. The death knell for them was overheating, the scrapers were almost always running against the governor when in the cut loading, on the haul road & at the fill area. They were always on the edge of overheating depending on the ambient temp, because of this someone in the office got the idea of installing smaller injectors in the 12v71 in one of the scrapers. What also made this seem like a good idea I guess was that fuel had taken a big jump in price. The way it worked out was 5 scrapers that still had the N70 injectors were running slower until they could pass the slower one with the N65 injectors. There was no appreciable difference in engine temps but there was a decrease in loads per day & none of the operators wanted to run the slower one. A Detroit mechanic told me that they "run the best just before they blow up" of all of them I worked on I never saw one have a catastrophic failure, most just got hard to start & low on power. They are sensitive to the quality of air filters because being a 2 stroke they use twice as much air for combustion as a 4 stroke. If we got 2000 hours on the 12v71 this was considered pretty good. Some of the other smaller Detroits did much better almost as good as the Cats;-)

Good info river cruiser, I will change my air filters and discuss the injector idea with a Detroit mec.
Thanks

I will change my air filters and discuss the injector idea with a Detroit mec.
Thanks

The DD dealer will have a bunch of charts showing HP/fuel flow at various RPM with a number of different injectors.

LOTS to chose from .

For any 2 stroke DD the goal is to operate at 60% power, and up.

White smoke for several minutes, and slow to start is almost always caused by air in the fuel intake line. It takes awhile to get the fuel back to the injectors and then awhile longer to clear out the air in the injectors.

Check for a fuel line check valve. Some D.D. had them and when they fail to close properly will allow slow bleed down of the fuel in the lines and the return line which allows air in. Then the fuel pump must reprime the system and clear the air which makes for a lot of cranking and bumbling.

Although a totally different engine mine has a check valve in the return line and once I found and changed the old valve, the restarting after a long sit time was totally different.

The reason DDs have a blower/compressor/supercharger is not to make them go fast but because 2 stroke engines require mechanical forced air induction to start (larger ships do it with fans which are stopped after engine is running). A look at the blower and coupling might be a good idea as well.

A customer had a similar problem with his Hatteras. They found a leak in the fuel system from the tank manifold. Some of the fuel valve stems were leaking. If your tanks are located lower than the engines it could be the same for you.

Blowers on these engines have a shear coupling. Its a shaft that is replaceable by removing the cover behind the blower drive. But, that is not the problem here, if the blower spins its ok, if not the engine wont run. Fuel bleed back seems to be more likely.

Wow

I've got a 73 Buick that I need to prime to start quickly. Leave it for a day or two and it starts fine but after a week it needs to be primed. Had the carburetor rebuilt thinking that would solve the problem but it didn't. I'm thinking a hairline crack in the carburetor leaks the fuel either into the intake manifold or out on the outside of the manifold. I ruled that out in the past because I couldn't see the fuel but now think it leaks so slow it evaporates and is not visible.

I mention this as I've had this car for 14 years or so and still have the leaking problem. Don't drive the car often so priming it is less trouble than fixing it.

But w the DD in question here I think it's most likely a leak of fuel or air most likely fuel. Mechanics usually have "leak down" testers or pressure testers that may identify whatever seal is not sealing completely. Perhaps very small differences between the two engines could compared to localize the difference and thus the problem. I'm practically convinced from the text so far that it's a fuel or air leak problem. If a good mechanic can find it on your boat then maybe a good mechanic can find a similar problem on my boat. Looking fwd to good news.

Your Buick probably has a Quadrajet carb. These carbs have small plugs, kinda like little freeze plugs, in the fuel wells. They are notorious for tiny leaks. It leeaks into the intake manifold. Kinda ruff on the cylinder walls and rings. Rebuilders dont pull and replace them. It's any easy fix. On the DD, put a 12 volt fuel pump inline and plug the return at the tank. Pressurize everything and the leak will show up, if its an air leak. To determine If its a leakdown problem unplug the return.

if you have a lift muffler and it is almost full of water that might be your problem too, happened to me on a delivery, putting fuel in the port tank and the boat leaned way over and the exuast was under water to much and filled up the lift muffler and the engine would not start, to much back pressure,took the plug out of the muff and drained the water and oit started right up.A LESSON LEARNED one of many I incountered over the tears by the way the starboard tank was dry 500 gals on one side is heavey?

I've not seen any DDs fitted with water lift mufflers, but if so and the outlet gets far enough underwater to fill the muffler I question the use of the water lift to begin with, poor design, but possible. Detroits dont do well with to much exhaust back pressure and trying to push water uphill would be plenty.

kulas44 said:

Detroits dont do well with to much exhaust back pressure and trying to push water uphill would be plenty.

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Considering the 6V92 natural is good for over 54 inH2O backpressure it would take quite a waterlift to create problems.

GM Cleveland 2 strokes started quite well with up to 15 psi backpressure on submarines. They had to be able to build up that much to blow out the snorkel exhaust mast.

My 6v92 service manuals call for .6 in of mercury at no load and 1200 rpm as a max. for exhaust backpressure. 1 inch of merc equals 13.6 inches of water. so about 7 or 8 inches water max at 1200 rpm. But, that is at 1200 rpm. That blower may not move 8 inches of water up the exhaust outlet, especially if its a little worn. I still dont think this is the reason for the slow starting.

it wasn't a poor design, the boat was tilted way over and the ex was way to far under, I have seen many DDs with lift muff,on many yachts, and I have been delv. them for 35 years

also the reason they use them is a boat with a aft cabin they save a lot of space and can make it wider without a big muff behind the walls so they put it in the engine room. makes sence???