generator help needed

[gcbist]

I have a 1979 43'DC with an Onan 7.5 MDJE generator. It has been performing perfectly until.....One of our forum members was kind enough to send me the repair manual...now I am hoping to get some help diagnosing the problem so that I can have a chance to try the repairs myself. Here goes...

I was on anchor the other day running the genset for about 30 minutes when it shut down...probably a high temp auto shutdown. I found the closed fresh water system hot so I waited a couple hours and checked again. There was no coolant in the system at the radiator cap. I found water under the genset in the oil drip pan..about one cup. I checked the engine oil and found it clean with no water. I checked all the hoses I could get to and found no apparent leaks.

I have started the genset since then for a period of 4 to 5 minutes for diagnostic purposes. It starts as always and I find more water, but now the fresh water closed system maintains its water level but has not been heated. I then check to see if I can see a water leak and find no obvious sign of leaking, and find an additional 1/2 to l cup of water in the drip pan. I had refilled the closed fresh water system with distilled water for diagnostic/trouble shooting purposes and ran the genset for a few minutes only to find the system remains cold. The water line from the thermostat to the exhaust manifold feels very warm, other fresh water lines are still cool.

When I first checked the genset the day it shut down, I started it for a few minutes and observed what appeared to be air bubbles in the exhaust discharge. When I did another diagnostic start just yesterday, there was little or no water discharged although air was being blown out, and I immediately shut it down.

I am not a mechanic and as you are all aware the genset is hard to get to. I am hoping to diagnose the cause of the problem prior to starting to dismantle hoses and pumps if needed. I am not sure I will be able to determine if anything I take off is broken or causing the problem. I'm having a diver clean the bottom and check the seawater intake for blockage. I have gotten into the bilge and can see the thruhull and raw water filter/cartridge/seastrainer, but if that is what is causing the problem, I have got big issues. A prior owner was nice enough to install a 2500 watt inverter and 4 Trojan 105's for anchor power although he installed them on a platform directly above the thru hull making it next to impossible to see, let alone try to work on.

I've thought about the raw water or closed fresh water pumps could have quit but don't know if that would cause a leak and the high temp. Using mirrors and bending my arm where there shouldn't be any joints, I can't see any water leaking from the pumps.

I hope someone out there could offer some suggestions that would lead me to a diagnosis prior to starting to dismantel hoses and pumps to determine the cause of my problem.

 

[captddis]

First look for the air bleed petcock on top of the manifold for the t stat. Start the gen and crack the bleeded , coolant should come out. If no coolant it is air locked. Some of them are a real PITA to bleed. You might have to force some coolant through the system to get the air out.

 

[Nonchalant1]

I have a 1979 43'DC with an Onan 7.5 MDJE generator. I then check to see if I can see a water leak and find no obvious sign of leaking, and find an additional 1/2 to l cup of water in the drip pan. I had refilled the closed fresh water system with distilled water for diagnostic/trouble shooting purposes and ran the genset for a few minutes only to find the system remains cold. The water line from the thermostat to the exhaust manifold feels very warm, other fresh water lines are still cool.

It sounds like you're not getting raw water pumped around correctly. No water in the exhaust particularly sounds like that. The 1/2 cup in the pan can be overflow from the freshwater system or raw water. If you put color in the fresh system water, or green antifreeze, you can tell if the 1/2 cup you're getting is internal water or flotation water. You should be able to find that leak, except if there's no water in the exhaust you can't run the genny much at all.

I have an MDJF so different beast, but there is no hose from the thermostat to the exhaust manifold. Raw water runs from the heat exchanger to the exhaust.

I'd start by checking the raw water pump. First, is the belt that runs it tight and no breaks or chips out of it? I had one with a piece chipped out of the inside and when that point got to the pulley, it'd reduce the tension and stop running the pump. Next: how long has it been since you changed the impeller and is the impeller cover tight?

If those are OK, it is possible to disconnect the output side of the raw water pump, run a hose to a 5 gal bucket and start the genny. You'll see right away if you're getting good raw water flow. The genny won't overheat (or even heat up as you noted) for a while, but no raw water in the exhaust is bad, so only do it for 10 seconds or so. If you get no flow, your sea strainer or zinc can may be plugged up or the pump impeller is shot. If you get good flow, your problem is after the pump (like a plug in the exhaust injector hole or old impeller vanes stuck in the passages from a prior owner).

Doug Shuman

 

[Trojan]

I have the same unit. The first thing I would do is remove the 3 screws on the front of the fresh water pump and check it. Most likely the impeller is bad. When the f/water impeller starts to go bad it does not cool the closed water system as well. Then you start to over heat and there goes the antifreeze. I had the same trouble last year. The fresh water pump is gear driven off the cam gear. The closed system is belt driven. That would be my starting point as long as the strainer is clear.

 

[Nonchalant1]

BTW - If you do find that the impeller is bad........if it has any missing pieces or missing vanes, you have to go find them. Do not leave rubber pieces in the fresh water side and hope they'll stay out of trouble. Look in the first bend of the cast channel in the output side of the FW pump, and any bends or restrictions in the hoses or fittings, in the inspection plate for the heat exchanger, etc. until you find them. Otherwise, they'll come loose and move to block up something further down the line at the most inopportune moment possible. The worst case scenario is that everything is running fine until a small piece makes it through the heat exchanger and gets stuck in the exhaust water injector, plugging that hole. The result can be no water in the "wet exhaust" and an exhaust hose fire , which is really hard to put out.

Doug