Another Onan Generator problem

[Crypto Weenie]

Last month while traveling near Phoenix and running the generator to power the house A/C, the gen died/stopped. The codes indicated overheating. Because we were near the Cummings repair facility at Avondale, I called and scheduled an appointment for the following morning.

I have an Onan 7500 Quiet Diesel Generator (HDKAJ), mounted in a 2003 Newmar Dutch Star motorhome. It has less than 400 hours running time. I have maintained it religiously since day one – I am the original owner.

Cummins kept the coach for almost eight hours; they shotguned the issue by replacing the thermostat and temperature sensor, all to no avail. The said that it could possibly be the radiator, or the inverter(?). The belts and hoses are all in good shape.

Now that we are home, I pulled the radiator and had it checked; opened and cleaned, and pressure checked after closing it up. The inverter – it functions correctly and I am unsure why they suggested it could be a contributing issue.

Current status: It starts right up will run for at least 20/25 minutes, then powers down. It will start right back up and run for a period of time before stopping again.

I am open to any and all suggestions you may have.
John

[TandW]

So if the fuel delivery system had been opened up to effect the other repairs perhaps now it has an air leak. Just throwing darts.

[twinboat]

After it dies out, remove the fuel filter and see if it's empty.
If it is, you have a slight air leak in one of the fuel line connections or bad fuel line.

[Forza Bill]

Originally Posted by twinboat

After it dies out, remove the fuel filter and see if it's empty.
If it is, you have a slight air leak in one of the fuel line connections or bad fuel line.

Diesels are pretty simple, Air or fuel. Is your air filter clean? Have you changed your fuel filter, and there are no fuel or air leaks in the fuel as Twin suggested? Are you sure you are using the proper coolant in your cooling system? Is there an error code? Since the Thermostat has been changed, these are the things I would look at next. What would an inverter have to do with this problem? Not sure why that was suggested! Good luck!

[djh77]

I have a similar problem.......
Our 1710 hr 7.5 QD starts quickly, runs 5-10 mins and dies.
Then will start immediately and
run for days (during power outages), never quitting.
Air filter, oil, and fuel filter changes revealed black dirt in fuel filter.
No change in problem!
Ideas?

[153stars]

Without digging through the diagrams to tell if the overheat circuit is closed or opens for overheat alarm. You may want to inspect all the grounds in the the generator or cut chafed wiring. As well as connections to the circuit boards. The inverter mentioned could be internal inverter on the generator. Some of Onan generators are variable speed inverter generators. Forgive me not running the model#.
Definitely check actual radiator cap as fill as the thermostat opens purge air. Since you had the radiator checked I assume fins are clean. Belts run the fan sound good... anything else that interfere air flow or fan blade.
The service and diagnostic manuals are available online. They have easy to follow troubleshoot flow charts. The tests are straightforward. Flight Systems Inc repairs and supplies circuit boards. They also have the troubleshooting section.

[vseasport]

I had an identical problem. It turned out to be a build up of scale in the engine block. i pulled the new thermostat I stuck my finger in the passage and could scrape crud off the passage wall with my fingernail. I drained the coolant and filled the system with a solution of “Maritime descaler”. Flushed the system, filled with coolant and it never over heated again.

[kwftlaud]

not being familiar with the diesel genset I have a question.
Does it have an electric fuel pump like the gas versions do?
If so, I'd replace it.

I had a gas onan genset that worked just like yours does.
I guess the fuel pump would stop working when it got hot.

Just a guess

Kip

[andy29847]

Check the fuses under the slide off panel near the breakers and stop/start button. Those thing live in a harsh environment and the contacts can become corroded.

[Crypto Weenie]

Thanks for all the input. The problem was and continues to be coolant over temperature, as indicated by the error code. I have checked to insure nothing restricts proper airflow. I am beginning to suspect the water pump may be at fault. Visually, I can see the pump pulley turning, but I do not hear evidence of a noisy pump. But . . . who knows. The pump is available from a number of sources. I’ll attempt to pull it off tomorrow.

[Coarsegold]

Originally Posted by kwftlaud

not being familiar with the diesel genset I have a question.
Does it have an electric fuel pump like the gas versions do?
If so, I'd replace it.

I had a gas onan genset that worked just like yours does.
I guess the fuel pump would stop working when it got hot.

Just a guess

Kip

I think that is a gas motor Onan problem. I had the same issue with my gas model and the fuel pump was the issue when hot and run time of 10 min and turn off.

[THenne1713]

I had same thought as a FUEL Issue symptoms, and YES, Diesel has a Very Similar LIFT PUMP that sucks fuel up to engine. But things do NOT have to be logical/ make sense, YT video recently where they found OIL CHANGE STICKER DOWN IN THE OIL PAN; (how well cud that clog a pump inlet screen?) I would consider pulling WATER PUMP, INSPECTING pumps & PORTS; another video on 40-50yo tractor (or old car?) they found mechanic's rag still stuffed into a water port?

[THenne1713]

Assuming no bad new parts, and parts were actually changed, that leaves WIRING, WIRE PLUGS and CIRCUIT BOARD

(years back, had car in for warranty x3, x3 said they changed HEATER VALVE, but next day= same problem); Out of warranty, found I had to change valve every Fall or no heat; I will always believe they never changed and DID Lie.

[THenne1713]

i SUSPECT IF PUMP OR PORTS ARE THE ISSUE, IT WILL BE OBVIOUS WHEN PUMP REMOVED AND ALL INSPECTED? I know I have had electric pumps that plastic impellers not attached to shaft, and have seen metal impellers damaged, or shaft/ bearing wobble issues? Good Luck.