Monday, September 20, 2021
Monday, September 20, 2021

Here’s why diesel pusher radiators overheat… Don’t let this happen!

By Kate Doherty
Last summer, our ’08 diesel pusher with a rear-facing radiator kept overheating, especially when climbing small hills. Initially, we rationalized that the two-speed fan wasn’t shifting from low to high, so we changed the fan relay – to no avail.

We didn’t want to climb the Rockies with this problem, so we queried a local radiator shop and were instructed to spray the radiator at the back with low-pressure water from a garden hose. But first, we were told that we should spray the face of the radiator with a spray cleaner like Simple Green, let it soak in, but not let it dry before washing off. Then, we were to rinse with the low-pressure hose again. Hoping this solved the overheating problem, we began our trek from the East Coast to the Rockies. The problem of overheating remained.

Next, we checked the fan clutch again by climbing under while the engine was running. The fan was turning correctly but the engine was still running hot. We contacted a fleet diesel shop in Pueblo, Colorado, and were luckily squeezed in the next morning.

We were worried about serious engine issues but, fortunately, the shop mechanic diagnosed the problem as extremely dirty fan blades. The mechanic used a low-pressure setting on a pressure washer, climbed under the RV and scrubbed the blades clean for at least one hour. The culprit … oil from the engine breather pipe had deposited on the fan blades – collecting dirt, sand and other road debris as we traveled across the U.S. Over time, this changed the blade’s airfoil shape.

This is the same problem airplanes experience when moisture meets cold temperatures forming ice on the aircraft’s wings. The ice adheres to the leading edge and coats the top and bottom surfaces of the wing (airfoil), changing its shape and lifting properties. When air no longer passes over and under the wing supplying lift, the airplane stops flying, i.e., the airplane becomes a flying brick! 

When radiator fan blades become soil-laden, the fan turns but does not push air any longer, or at least not enough to cool the engine. Hence, diesel pusher radiators need to be inspected before long trips, regardless of the season. Don’t forget to inspect the input side as well as the exhaust. Inexpensive radiator spray cleaners are available at auto parts stores, Walmart, and online.

Save yourself a lot of angst and time and add this inspection to your pre-drive checklist! Your engine will appreciate breathing better and you’ll feel safer going up those hills. 



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Gonzo Motors
7 months ago

As a Phoenix native I know my aluminum engines are just waiting to warp- a- head. In between scheduled coolant system maintenance I make a habit of washing my radiator/ engine/trans bottom with any available garden hose directing stream away from sensitive electronics. When at the self-serve car wash I do a moderate-pressure back-flush after a presoak of dish soap. A soft tooth brush or bottle brush on a long stick helps get those in-between areas. I do mid-cycle drain of the radiator coolant via that handy radiator drain plug. My newish Civic is too hot under the hood for my liking so on hot days I pop the hood leaving the safety latch engaged. That’ll save on all that expensive neoprene under there and give the engine a rest. With certain cars I will modify the cares air foils and or add some small hand fabricated air deflectors to direct air through the smallish radiator openings. Controversy aside, in super hot weather I up’ the engine oil weight.

8 months ago

In my 12 years in HVAC (commercial), I always checked fan blades. The poster above is correct, accumulated dirt on fan blades can & will degrade their ability to move air. Often overlooked. The effect is more pronounced on radial (“squirrel-cage”) fans, but even axial (“propeller”) fans can need cleaning every so often. I do this every year to the HRV (heat recovery ventilator, AKA “air exchanger”) in my house. Unit is still going strong after 38 years.

8 months ago

Our overheating problem was caused by the thermostat (regulator) that fell apart. CAT recommends changing it every 3 years. Dawn is also a safe degreaser. Also the nylon reserve antifreeze tank can crack and release pressure. On ours I redirected the vent tube to the side of the coach away from the radiator.

Al Hubbard
8 months ago

Seems to me that any normal build-up of crud on fan blades should not be able to cause the conditions you describe in the article. Fans pick up dirt, hair and grease, and radiators get plugged, but designers know that going in. They probably designed the system to accommodate the problem to a good degree, then got overruled by the cost cutters.

7 months ago
Reply to  Al Hubbard

Agree to a point. Remember the OP had previously cleaned the radiator. My guess is oily dirt had built up on both and the overall issue was reduced air flow. For those who want to see this same issue at home, check your ceiling fan blades after the spring wet season. Surprising how much fuzz builds up on it.

You system needs to be inspected and cleaned regularly. Safe travels

Brad Teubner
8 months ago

I have been told that Simple Green Extreme is to be used on radiators, as Simple Green is corrosive to aluminum.

David Lastoria
8 months ago
Reply to  Brad Teubner

Never use a corrosive on any radiator. Please read my lengthy comment.

Phil Atterbery
8 months ago

My first & only major break down with our 38N Bounder was related to a lack of proper airflow thru a pusher radiator. The previous owner failed to clean the air passages in 35000 miles. The dealer failed to correct the problem. Halfway thru a trip to CA the engine overheats, a #1 exhaust valve got injested ruining a cylinder. $27000 later I have an engine replaced. The radiator was 40% occluded. I don’t remember the condition of the blades. All of this due to the lack of product knowledge that was contained in the owners manual. I have also added a slobber tube catch can just in case.
Oh the places we go & the thing we see.

Tommy Molnar
8 months ago

I’ve always thought diesel pushers were at a disadvantage when it comes to cooling, especially the ones with only one opening at the rear of the coach. Many (perhaps most) have a second opening on one side (usually the left side) to help in cooling. The Prevost that was reviewed today had only one opening (at the rear) though if you go to Prevost’s website and view their top-of-the-line unit, it has two. I wonder what makes the determination for one or two openings. I would prefer THREE myself, but right now I’m a bit short on cash for new RV’s so it really doesn’t matter 🙂 🙂

8 months ago
Reply to  Tommy Molnar

The number of openings depends on the engine/radiator configuration. Many DPs have a rear radiator, so basically one opening. A far better arrangement is the side-radiator, which draws cooling air from the side, blowing it out the back. Not only cools better, but access to the engine from the rear is excellent. Good luck servicing an engine by reaching around/over it’s rear radiator…

David Lastoria
8 months ago
Reply to  Tommy Molnar

Sometimes the sideways vent is where they mount the a/c condenser, with the electric fan mounted behind it. I think that is a much better design, actually.

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