Onan Quiet Diesel shuts down

[wimberleyman]

Yesterday I changed the oil/filter and fuel filter on the Onan Quiet Diesel 7.5 generator in our 2001 Cruisemaster. I started the generator and checked for leaks and everything was fine. This morning I drained the coolant, flushed the system with distilled water and then added new coolant. It seemed to me that it did not take enough coolant. The overflow tank was full but that only amounted to 2-3 quarts. Now the generator continually shuts down after a few minutes. I believe there is an air-lock in the system causing the generator to overheat and shut down but I don't know how to bleed it. I tried bleeding from the drain on the bottom while the generator was running but that did not work. Also the overflow tank has not gone down any which really seems strange.

Prior to this service the generator was working fine. I was just trying to follow factory maintenance guidelines before taking a long trip this summer.

Any help in resolving this issue or tips on a bleeding procedure would be greatly appreciated.

WM

[RKL]

Do not fill coolant thru overflow canister . You must unscrew plate covering radiator cap and fill tube. Pull out fill tube and add coolant very slowly to prevent the two other vent tubes from cloging up with coolant. Google " coolant change on onan quiet diesel" and there is a you tube video on how to drain and refillyou may have to drain out everything that is in it now and start over.. Rkl

[wimberleyman]

Thank you so much RKL! I found the video and watched it today. Tomorrow I'll start over and do it the right way. Hope it goes better this time around.
This is why this forum is so valuable!

WM

[Gary RVRoamer]

Your Onan always gives a diagnostic code when its shuts down - the Start/Off switch blinks out a code, which is covered in your Onan manual. Verify that it is "over temp", but it likely is. The 7500/8000 is notorious for being difficult to get coolant full. Simply waiting several hours for it to cool is one solution. But open up the cover as RK11 says.

However, 3 quarts of coolant is what your generator holds.

[wimberleyman]

Today I refilled the radiator following the steps in the video recommended by RKL. After following the steps for slow fill and bleeding the air from the system the generator runs fine. I ran both AC's using the generator for about 30 minutes with no shut down or loss of power. Thanks to all who provided helpful information.

WM

[RVNeophytes2]

Originally Posted by wimberleyman

Today I refilled the radiator following the steps in the video recommended by RKL. After following the steps for slow fill and bleeding the air from the system the generator runs fine. I ran both AC's using the generator for about 30 minutes with no shut down or loss of power. Thanks to all who provided helpful information.

WM

I've been having a terrible time with my 10K Onan, ascribe much of it to years of sitting idle*, before we bought the coach in January.

Originally, we had shutdowns due to a variety of fault codes, and I changed the oil, oil filter, fuel filter, air filter. Finally, we'd settled into a single fault code, an overheat. Our SilverLeaf readings confirm this.

Sadly, flushing the system and cleaning the radiator did not work. Getting a new thermostat didn't either. I removed and cleaned the temperator sensor to no avail. Assuming I had a water pump problem, I started to pull it and once I saw what a horrific task that would be, manually rotated the pump. It is smooth-running, I can feel the impeller rotating with its vanes intact. I back-pressure flushed the pump and got some black grit, but nothing huge. A test-run just now led to a 200F temp after 15 minutes of running. It seems there is a lack of flow.

Unless someone has a better idea, I'll keep flushing and refilling with fresh water; after two or three more cycles, I watch the video you've mentioned in this thread (which I've embedded below.)

* The dealer pointed out that the generator hourmeter only said 264 hours, which is what it still indicated until I replaced it, yesterday after about 35-45 hours of use... These woes actually make me think the 264 hr number probably isn't too far off-target, eg it sat unused for long periods.

HOW TO: Service an RV Generator Cooling System - YouTube

[RVNeophytes2]

Originally Posted by RVNeophytes2

Sadly, flushing the system and cleaning the radiator did not work. Getting a new thermostat didn't either. I removed and cleaned the temperator sensor to no avail.

* The dealer pointed out that the generator hourmeter only said 264 hours, which is what it still indicated until I replaced it, yesterday after about 35-45 hours of use... These woes actually make me think the 264 hr number probably isn't too far off-target, eg it sat unused for long periods.

A perfect, upbeat ending to this process, in line with the OP, hopefully of GREAT value to future readers: It dawned, as I leaned over the rear of the unit pondering how to remove the water pump, that the generator is just like a light aircraft. Wait for it...

In the sense that it is a pressurized-cowling of sorts. Meaning, that it has to be enclosed for the fan to move air through the compartment.

So, I put on a new radiator cap for good measure, re-boxed the unit, held my breath and started... Under full load, it barely tickles the 160F thermostat opening point.

Oh. Did I mention? The hourmeter was working, all along. A "technician" had wired it backwards...

[dbs944]

Originally Posted by Gary RVRoamer

Your Onan always gives a diagnostic code when its shuts down - the Start/Off switch blinks out a code, which is covered in your Onan manual. Verify that it is "over temp", but it likely is. The 7500/8000 is notorious for being difficult to get coolant full. Simply waiting several hours for it to cool is one solution. But open up the cover as RK11 says.

However, 3 quarts of coolant is what your generator holds.

My Onan 7500 diesel starts up fine, runs for 20 seconds then quits. Starting it immediately again it will run for 5 seconds then quit. Letting it rest for several minutes, it will again run for 20 seconds.

No code, just quits. It has been 7 months since I actually ran it (my fault) and checking the fuel consumption it appears after many times trying to get it started, I probably haven't pushed more than an ounce or two of fuel through the system.

Repair guy says it is an idler thing-a-ma-gig - probably $1000 to fix. If not that, the some other componet at about $3000.

Does this sound right? Plenty of fuel, plenty of coolent. No error code. Started and ran fine in January.

[HicksRA]

I found out the same thing on my QD 7.5.
Running it with the top off, the air is not forced through the radiator and it will heat up and shut down after a few minutes.

[Coaster1]

I learned the hard way on a previous coach that the bay door where the jenny is located had to be closed or the unit would overheat and quit. Something about drawing in air from a central location. Just sayin.

[woodsbull]

You changed the coolant in your disel with distliied water? You want to make sure to use SCA coolant or additives for your coolant. I am not as familiar with little diesels, I imagine the same holds true, but for big pusher diesels the Supplemental Coolant Additives (SCA), or equivalent, are required to protect the cooling system from fouling, solder blooming, and electrolysis corroding your piston liners and from general corrosion. ... Supplemental coolant additives (or equivalent) are used to prevent liner pitting and corrosion in the engineand scale deposits in the cooling system. I probably just saved some from a 25K repair bill. Ask your diesel tech. Your welcome

[96 Wideglide]

Onan diesel generator engines aren't wet sleeved. No SCA needed.

[mchero]

How about the fanbelt? I have seen posts where the belt gets a glaze and will slip.