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Old 05-18-2024, 05:13 PM   #1
Unc
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Onan 7500QD temperature problem

I am in the process of doing long overdue maintenance items on my (relatively new to me) 7500 generator. I started the process with wanting to replace all the rubber hoses for the coolant system. When I took the outer covers off the generator it appeared to have been leaking coolant around the water pump and possibly the thermostat housing where it bolts onto the block or head (can't remember which it bolts to). That gasket will be impossible to change without major disassembly as one of the bolts is behind the gear cover. That will have to be a winter project for next year. I was planning on running it and seeing if it kept loosing coolant.

Prior to doing this the generator would shut off and gave me a code for overheating. When I discovered the corrosion at the back of the engine I figured it was low on coolant however after more investigating the temperature sensor hole was full of corrasion and material (I guess from old coolant). I cleaned this out with a tap and it looks wide open now.

Fast forward to things I have replaced at this point.
1. All rubber coolant hoses
2. Temperature sensor (as well as cleaning the hole it goes in with a tap. It is as open now as when it was new it appears). The temperature sensor gets tight with a couple threads showing. Looks like a poor design as the sensor is still recessed in a hole and not in the flow of coolant.
3. Radiator (old one appeared it had a pin hole that had been leaking)
4. thermostat
5. water pump

Several weeks ago I ran it with the a/c running and it ran fine for about 30 min and never shut off. I figured it was fixed at this point. In the process of revamping it I wanted to add a set of gauges so I could see what the temperature and oil pressure was doing inside the RV so I ordered the cummins gauge set that plugs into the wire harness coming out the side of the generator. I did not have this plugged in when I ran it several weeks ago.

Today I got the gauges put inside the RV and ran it for about 45 min to an hour with the a/c on. The temperature on the gauge went up to ~210-220 and then proceeded to continue climbing. Gauge maxs out at 250 and it went all the way up. I know these generators will turn off when they sense an overheat and it never shut down. 250 seems way too hot to me.

Now things I have checked:
1. Large amount of air coming through radiator while it is running so the fan does not seem to the problem.
2. Top of the radiator showed to be ~195-205 F when I shot a laser temp gun on it. Hose coming out of bottom of radiator was about 165-170. Outside air temperature was ~78 F

Has plenty of new Final charge coolant in it.

Does anybody have any ideas on what these generators run at? Most don't have a gauge but I have read that someone thought they ran ~220 F or so.

Any ideas on what to check next? Next on my list is to loosen the top cover and check the temp at the thermostat hosing with the laser temperature gun but I ran out of time today. Will be next week before I get back to that.

I'm frustrated with it and am tempted to run it until it shuts down and see what happens.
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Old 05-18-2024, 05:32 PM   #2
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Did the replacement T'stat or the original have a temp value stamped on it ?

I have the original manual from my 7500 ; I'll look through for any temp info.
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Old 05-18-2024, 06:02 PM   #3
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I think it was 180. I ordered it based on parts manual part number. All the parts were from Cummins with the exception of the water pump. It was aftermarket due to shipping time. It appeared to be identical to what I took out.

Oh I also put a new belt on it that seemed tight as a tick and scraped paint off the pulley as I put it over the water pump shaft.
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Old 05-19-2024, 04:02 PM   #4
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I replaced my 7500 water pump belt last summer ; and even thought the belt number matched , it was tight as a guitar string.
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Old 08-16-2024, 06:01 PM   #5
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Same problem

My QD7500 is doing similar things. I replaced the temp sensor and thermostat (180*) and installed a water temp guage in the block. Now it will idle all day no load at 180*. Put the ACs on and it will slowly climb to about 218* and shut off. Takes about 20 minutes. It will run longer with the oil filter door open but still shuts off. I am going to run flushing agent in it tomorrow but anyone have other suggestions? It only has 592 hours on it but it is 18 years old.
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Old 08-16-2024, 11:54 PM   #6
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Onan QD

On the several I have owned - ‘03 & ‘04 models, both with low hrs - corrosion in the cooling system causing poor circulation through the radiator and around the coolant temp sensor were the issues. They would handle a light load, but not a full load. At 250 degrees coolant should be passing through the pressure cap back into the reservoir. Both of mine would shut down before losing any coolant. After temp sensor, thermostat, radiator and belt replacement both worked flawlessly. All cabinet panels must be in place to draw air through the radiator for proper cooling. I suggest you also confirm all interior foam seals and panels are in place to force all the air through the radiator. Did you replace radiator with a new unit?
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Old 08-18-2024, 09:15 AM   #7
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Have not replaced radiator, running flush agent under full load now to see if it helps
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Old 08-18-2024, 04:21 PM   #8
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Seems to be fixed after flushing. Ran 3 hours under full load temp peaked @ 198*.
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Old 08-18-2024, 10:29 PM   #9
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Boy does this thread bring back memories!
I went through ALL of what many of you are and have. We've owned our '04 Itasca Horizon 36GD with the CAT C-7 330HP now for 13+ years and it presently sports 116,000 miles. Our QD7.5 also threw fits a long time ago on a hot summer day in Bishop CA and popped the code 33. Well, long story short, I too yanked that genny all apart to learn about it and find things out. And, after a very serious and long and hard battle of using every tool in the arsenal to extract that little temp sensor, the scenario was just like what the OP reported.

Lots and lots of corrosion all around that temp sensor in it's recessed port. It was a ton of work getting that little sensor out but I won. I installed a new T-stat, sensor and head gasket at that time. And some new coolant. It's been operating just fine for years but, it occasionally throws the code 33 from time to time.

I had one more "little" problem when I yanked the skin off that genny. I found lots of small grey magnetic particles/clumps/pieces etc. all over the inside of that gen. What the heck was/is all this???? Well, I found out it had thrown a magnet off the rotor and basically disintegrated it in the process. What a mess. I think it's about time for a new temp sensor, a new water pump belt and a new thermostat. And I'll also flush things out. May I ask anyone, what kind of "flush agent" are any of you using?
Scott
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