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Old 06-08-2022, 09:05 AM   #1
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Join Date: Jun 2022
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2003 Itasca Horizon 39qd Cat Engine Overheating

Hi All,

I've been experiencing an issue with my 2003 Itasca Horizon 39QD motorhome. It has the Cat 330 engine and 6spd Allison transmission.

I drove it about 1500 miles down the I-35 without any issues but as I started to get into the warmer weather of Texas, I noticed my temp gauge increasing past the center line "normal" level. I slowed down a bit and it didn't seem to be an issue for the remainder of that drive to corpus Christi.

Since then, I have driven another 800ish miles northwest to new mexico and have been experiencing much more regular/serious overheating issues. Any slight grade and especially any significant hill, is causing the temp gauge to fly up pretty quick into the danger zone, with the warning light coming on a few times and I've had to pull over. Temp goes down quickly and I can continue. Lots of coolant in the tank when I check once I'm parked and engine is cool.

I've read about needing to keep the RPMs in a higher level to keep the cooling sufficient on these Diesel engines and I've been downshifting from 6th to 5th for even slight inclines, and that's helped keep it in the normal range.

I also just ended up driving much slower and kept it no more than 55-57mph. When driving that speed in 6th gear, even on flat surfaces, I found the temp still creeping up past where I'm comfortable so I have been keeping the transmission in 5th gear for the entire drive, aside from when downshifting into 4th for steeper inclines. This kept the temp where it should be, even over a 5-6 hour drive, with only minimal temp gauge increases when going up bigger hills, then temp would come back down. I've been driving in South Texas and new mexico and it's been pretty hot outside so I'm assuming things might improve again once I get to a cooler climate.

I believe I need a radiator replacement as some of the fins at the top of the rad have rotted away and although it's not leaking as far as I can tell, the Rad doesn't seem to be working at full capacity.

I'm planning to drive north another 1800 miles to get back to Canada later this month and would like to avoid buying a new radiator just for the drive because I'm ditching the RV once I get home.

My question is, how bad is it for the Allison transmission if I drive it pretty much the whole way home in 5th gear and don't use 6th? The RPMs aren't too excessively high in 5th, staying around 1700-1800rpms at 55-57mph. I don't care about fuel mileage at all and I can handle the slowness. I just want to know if my solution of driving in 5th gear primarily, and not using 6th to keep the temps down, will put too much strain on the transmission over that long of a distance, and I'll be ruining the transmission.

I'm trying to find a place to get the rad cleaned if possible before I take off but I'm in a small town with limited options.

Any insight on the long term use of 5th gear in this situation would be very much appreciated.

Thank you!
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Old 06-08-2022, 09:09 AM   #2
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Join Date: Oct 2014
Posts: 6,570
When was the last time the FRONT of the CAC was cleaned? Needs to be done from the bedroom/closet access.

Have you verified that the small/separate V belt that drives the waterpump is properly tensioned?

When was the last time the thermostat(s) were changed?

You should not need a "work around" except on long, steep grades.

You will not hurt the Allison by driving in 5th, but fuel consumption will be up.

Better to address the root cause of the overheating rather than concentrating on a "work around".
Brett Wolfe
Ex: 2003 Alpine 38FDDS. Ex: 1997 Safari Sahara. Ex: 1993 Foretravel U240
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Old 06-08-2022, 09:39 AM   #3
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Thanks very much Wolfe!

I'm not sure if it's ever been cleaned. I'm doubtful it has.

I've only had the RV for about 2 months and realized that the lifestyle isn't for me so I'm returning it to the owner. I'm too young to be living in RV Parks haha. I put some money into getting it road worthy and that bought me a years worth of using the RV from the owner, so it's not a huge financial hit thankfully.

Not sure about the thermostat, and I haven't verified the belt.

I definitely agree, best to solve root problem. That said, I already put over $10k into the RV to make it road worthy and I'd like to avoid putting another $4-5k into it for a rad, when I'm just driving it back home to return it anyway.

If the work around I've come up with won't cause any permanent damage, I'm happy to eat the additional fuel cost and time on the road, just to get it back safely without any more overheating and without spending a bunch of cash on a rad.

Thank you again!
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Old 06-09-2022, 05:53 PM   #4
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I am assuming you have a rear radiator. There are several posts on here about the necessity of cleaning the the CAC and the radiator. If you have not yet extended the breather tube, there are posts on that topic as well. I get in there and spray it down with simple green and hit it hard with hot water at lease two times per year.

As mentioned above you may need to replace the thermostats and radiator at some point but i would clean it well first. The search function on this topic will be your friend.
Frank, Columbus, Ohio
2001 Itasca Horizon Cat 3126B 330hp, 2018 Jeep GC Limited, EPS harness or 24' Enclosed Car Hauler
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Old 06-11-2022, 10:10 PM   #5
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Location: Powell River, B.C.
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You won't hurt the transmission running in fifth , the converter locks up and the oil circulates just fine ; actually converter locks up in all gears but first , if memory serves.

But depending on tire size you can allow the coach to shift into sixth , by selecting Econo mode , my shift completes at 58 and 1700 rpm in sixth is 63 mph .

Brett , has mentioned the water pump drive belt tension , be aware that some Cat 3126E and C-7 engines don't have a separate belt for the water pump ; it's driven by the serpentine belt.
Those engines with the V belt , accessing the tensioner mount bolts for an adjustment , on a rear radiator coach can be problematic AND , a number of members have broken the bolts while trying to adjust/replace the belt .

Then the rad and CAC have to come out to get at the broken bolt to remove it.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.
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cat, engine, heat, heating, horizon, itasca, overheating

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