Onan 7.5 MDJe White smoke

Hi everyone, I need a little help on figuring this out.

My onan generator blows white smoke when it's on a load but not when there is no load on it. Can someone tell me what it might be?

I have checked the the raw water pump and thats running good.

It's winter and it's cold out - condensation?

Actually I just noticed you are in California, but I bet the water is colder than in the summer?

You will get more help if you post this under Electrical Tab.

fred2015 said:

Hi everyone, I need a little help on figuring this out.

My onan generator blows white smoke when it's on a load but not when there is no load on it. Can someone tell me what it might be?

I have checked the the raw water pump and thats running good.

Click to expand...

Hi Fred,
Welcome to the forum.

What do you mean by the water pump is running good? Do you see water coming out the exhaust or just white smoke / steam? Loss of water flow either from reduced flow of the pump or a partially blocked heat exchanger may generate steam as the water all gets vaporized going through the exhaust elbow when the exhaust gas gets hotter under load.

Ted

Hi OC,

When the water pump is on, a flow of water is coming out, but when I put the load on after the batteries are low, I get water and white smoke. Maybe steam? I need to recheck it, and this just started to happen.

Fred,


I would be looking at the injectors, probably have one dribbling (not atomising fuel properly) under load. Can be caused from carbon build up on the injector tips.


Cheers
Kev

Huge white smoke can easily be a blown head gasket , if the unit is direct sea water cooled.

Are you talking a light vapor like after an auto cold start , or Big white smoke, like a bug foger??

Check temps of exhaust pipes leading from muffler to overboard. There water and exhaust will be thoroughly mixed. Normal is for temp to be about 40F or less above sea temp. Above 40F above, and there may be a sea water restriction, somewhere.

Sometimes atmospheric conditions make for steam formation without anything wrong.

Try the simple stuff first.

When was your last air filter change?


Via iPhone.

Hot water vapor (steam) or raw diesel fuel.
Borrow a laser/IR temperature gun and take some readings at full load on the water cooled exhaust tubing. Above 140F shows a raw water problem.
If the exhaust wreeks with a strong smell of fuel that makes your eyes water, check the injectors.

That engine does not have an air filter. Also being a two cylinder, it will not run or barely run if injection/compression/etc is bad on one cylinder.

fred2015 said:

Hi OC,

When the water pump is on, a flow of water is coming out, but when I put the load on after the batteries are low, I get water and white smoke. Maybe steam? I need to recheck it, and this just started to happen.

Click to expand...

I have the same unit and I've noticed that the "white smoke" under load is usually steam and caused by not enough raw water flow. With the generator running, check the temperature of the exhaust elbow. If it's getting too hot to touch, then you have a water flow problem. Check these things:

Open up the sea strainer check for junk, but also open the water valve and note the water flow. The inlet is small and easily blocked or restricted by kelp or junk which may restrict the flow into the strainer.

The factory raw water pump on the MDJE, pumps just enough water to keep the unit cool. But if your pump is missing a couple of vanes or the pump is worn out, the water flow drops off quickly. Not enough water injected into the elbow and you get steam.

The raw water is injected into the top of the exhaust elbow from the heat exchanger. If the generator is run a lot with a light load, it tends to build up carbon right at the water injection port which will eventually restrict the water flow into the exhaust. Reduced water flow into hot exhaust equals steam.


If your raw water pump is bad and you have the Onan gear driven pump, it's up to about $1400 now. Let me know and I've got information on a work around option to replace that pump with a belt driven pump.

Ski in NC said:

That engine does not have an air filter. Also being a two cylinder, it will not run or barely run if injection/compression/etc is bad on one cylinder.

Click to expand...

Wow. No air filter, must be a noisy little beast especially on a boat.
Follow ski's lead on this, he'll get the smoke cleared. Thanks for the info.


Via iPhone.

Your nose can tell you a lot too. If you have noticeable white smoke, smelling it will quickly tell you if it's unburned fuel vs steam/vapor of some kind.

not enough raw water flow or poor mixing due to clogged mixer head.
start at the through hull and inspect the innards of everything til you find the blockage