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Old 08-10-2019, 05:19 AM   #1
Mini-Skoolie
 
Join Date: May 2018
Location: Toronto, Ontario
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Year: 2009
Coachwork: Thomas Built
Chassis: HDX
Engine: Cummins 8.3L Turbo Diesel
Rated Cap: 48
My Thomas Built HDX overheats above 55 mph

My new 2009 Thomas Built HDX Conversion with 8.3 Cummins RE has 285,000 km on the clock. It runs smoothly and quietly on flat road but arrived with 2 annoying problems. (And I am just learning about diesel engines so any assistance is greatly appreciated).

The first is that it overheats as soon as it exceeded 90kph or 55mpg. Before that, rock solid at 200F. Above that, a steady rise to 225F where the overheat alarm sounds. Thermostat, rad cap and coolant have been replaced as first effort to no avail.

Second issue is loss of speed when climbing hills. Downshiftng the Allison 5 speed manually helps maintain speed but on really steep grades of say, 7%, we can slow to a crawl. I want to tow a runabout car however I am fearful with both of these issues.

Any suggestions on where to look for remedies much appreciated.

SuperFreddie
In Buster the Cat House
2009 Thomas Built HDX SafTLiner
converted to French Country Cabin on wheels
8.3L Cummins inline 6
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Old 08-10-2019, 06:18 AM   #2
Skoolie
 
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Any coolant loss? If so have you replaced any and with what type?
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Old 08-10-2019, 06:52 AM   #3
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Quote:
Originally Posted by superFreddie View Post
My new 2009 Thomas Built HDX Conversion with 8.3 Cummins RE has 285,000 km on the clock. It runs smoothly and quietly on flat road but arrived with 2 annoying problems. (And I am just learning about diesel engines so any assistance is greatly appreciated).

The first is that it overheats as soon as it exceeded 90kph or 55mpg. Before that, rock solid at 200F. Above that, a steady rise to 225F where the overheat alarm sounds. Thermostat, rad cap and coolant have been replaced as first effort to no avail.

Second issue is loss of speed when climbing hills. Downshiftng the Allison 5 speed manually helps maintain speed but on really steep grades of say, 7%, we can slow to a crawl. I want to tow a runabout car however I am fearful with both of these issues.

Any suggestions on where to look for remedies much appreciated.

SuperFreddie
In Buster the Cat House
2009 Thomas Built HDX SafTLiner
converted to French Country Cabin on wheels
8.3L Cummins inline 6
Since you already replaced the thermostat, coolant, and cap, the other important items in the cooling system to check are radiator, fan, and water pump.

The point of a radiator is to create an large surface over which heat is exchanged from the coolant to the outside air. That surface needs to be clean on the air side as well as the coolant side to do the job. On the outside look for bugs, dirt, and grime built up between the fins. You can address that with a general purpose degreaser or a special radiator fin cleaner solution and gentle water pressure. (Do not hit the delicate fins with a pressure washer).

To assess the scale buildup on the inside surface of the radiator you need to drain the coolant and take one of the hoses off the radiator. It should not look like a coral reef inside. If it does, you can flush with a scale remover but I would just replace the radiator.

If you have an electric radiator fan, verify that it kicks in. On an engine driven fan you will have a viscous clutch that I would just replace at the mileage of your vehicle.

While you are troubleshooting the cooling system, check the turbo charging system for leaks. Low boost pressure leads to over-fueling and overheating. On the exhaust side, look for tell-tale soot and on the intake side check hoses for cracks and clamps for tightness.

What about the water pump? They typically leak well before the impeller is so eroded that it will affect cooling. I would only take the pump off if everything else mentioned failed to rectify the problem.
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Old 08-10-2019, 09:04 AM   #4
Mini-Skoolie
 
Join Date: May 2018
Location: Toronto, Ontario
Posts: 12
Year: 2009
Coachwork: Thomas Built
Chassis: HDX
Engine: Cummins 8.3L Turbo Diesel
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Originally Posted by Sehnsucht View Post
Any coolant loss? If so have you replaced any and with what type?

No coolant loss. Coolant replaced with tstat.
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Old 08-10-2019, 09:06 AM   #5
Mini-Skoolie
 
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Coachwork: Thomas Built
Chassis: HDX
Engine: Cummins 8.3L Turbo Diesel
Rated Cap: 48
Thank you alpine44 for very comprehensive response. I've clearly got to dig in for the rad and clutch replacement.
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Old 08-10-2019, 09:15 AM   #6
Bus Nut
 
Join Date: Jun 2015
Location: Asheville, NC
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Year: 1999
Coachwork: American Cargo 14'L x 7'8"W x 7'H Box
Chassis: Ford E350 Cutaway
Engine: 7.3L Powerstroke
Rated Cap: 11500 lbs
Quote:
Originally Posted by superFreddie View Post
Thank you alpine44 for very comprehensive response. I've clearly got to dig in for the rad and clutch replacement.
I hope that this will help you to get the overheat problem resolved. Don't forget to check the turbo systems as low boost pressure would also explain your problems getting up a hill.

The Cummins C-series (8.3L) in your bus is a great engine that is tuned very conservatively from the factory in comparison to the B-series (5.9L) for example. Any imperfection in fuel or boost system will make them feel dog-ish. The good news is that these maintenance related imperfections will not lead quickly to expensive failures like with more high-strung engines. But they should be addressed nevertheless.

If a 8.3L can move a heavy military truck up a hill it should get your bus up there as well.
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Old 08-10-2019, 12:13 PM   #7
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thats a Rear engine bus.. your fan is either Hydraulic or belt driven with electric clutch.. that fan should run on high speed a lot with a rear engine bus when its hot outside.. if its hydraulic, the valve usually has low and high speed... sometimes they use the engine computer for the fan, other times it is a separate sensor.. sounds like since it stays cool driving around town that it is stuck in Low speed mode..

-Christopher
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Old 08-10-2019, 03:06 PM   #8
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IF the water pump is original to the engine the impellers are most likely eaten up. Dollars-to-doughnuts that's most likely the problem unless the pump has been replaced.

I had a 1993 Oldsmobile 98 that had the same issue, original pump and overheating. Pulled the pump and the impellers were almost non-existent. new pump and presto! The cooling issue magically disappeared.

Good luck!

M
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Old 08-10-2019, 05:56 PM   #9
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Quote:
Originally Posted by M1031A1 View Post
IF the water pump is original to the engine the impellers are most likely eaten up. Dollars-to-doughnuts that's most likely the problem unless the pump has been replaced.

I had a 1993 Oldsmobile 98 that had the same issue, original pump and overheating. Pulled the pump and the impellers were almost non-existent. new pump and presto! The cooling issue magically disappeared.

Good luck!

M
I've seen that happen in a vehicle running without anti freeze, just water - the block rusts, and the friction of the rust particles takes the blades off the water pump
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Old 08-10-2019, 10:06 PM   #10
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Possible not enough anti-freeze in the system, as another member said... (Anti-freeze vs water increases boiling point)

But something a lot of people still overlook when adding coolant is that most Extended Life Coolant is not to be mixed with standard ethylene glycol. When these are mixed in the same system, it is known to clog up the system with solid deposits that can restrict coolant flow and damage the engine. This has been a known issue almost from day one when Extended Life Coolant hit the market (well over 10 years) and people still manage to screw this up. I don't care if it does say it's compatible, stick with the plan, man... Short-change your vehicle, and it will let you down every time... ESPECIALLY something big and heavy. Lots more zeros when these need repair... Just saying, you can pay a little more now, or a lot more later.

That being said, I have seen a number of oddball cooling problems over the years...

Sticking thermostat
Weak / bad water pump / gasket (only leaked at boiling point)
Leaking heater control valve
Leaking heater core
Leaking intake gasket
Leaking head gasket
Pinholes in radiator / heater core / hoses
Cracks in radiator tanks
Debris blocking airflow through radiator (leaves, dust, etc.)
Air trapped in the system

Check for all these. Rear-engine buses do indeed have their own set of challenges and idiosyncracies when it comes to cooling.
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