I have a new (to me) Zenith 750 Stol with a Viking 130 engine. I have been returning it to service after it sat in a hangar for about 2.5 years after the original builder passed away. I've solved a few issues, and had Jan upgrade the gear box with the 48 states upgrade, put in the new light weight exhaust and muffler, etc...

Everything is good, except the coolant temps get high even though I've only just taxied the airplane around (I was doing brake conditioning and brake checks). After about half an hour the temps climb very slowly, and will eventually push above 220F, or even higher if allowed. 

The first time I saw high temps, I checked to make sure coolant was flowing and found the radiator was not getting hot so...

I suspected there might be air in the system preventing the thermostat from functioning, so I checked to make sure the coolant tank had plenty of Evans coolant, marked the level when cool, and ran the engine several times to get it about 200F. Coolant is remaining exactly at the former level not being sucked in after the engine cools off, so while there may have been an air pocket initially, it's good now. 

But temps on while on the ground still eventually rise to push the 220F point. Radiator and hoses now get warm, indicating that at least some coolant flows, but not enough to keep the temps in the operating range.

So now I'm guessing at what to do. Radiator looks good, but there's no fan like in a car, and it could just be that on the ground there's not enough airflow. Or it could be that there is a problem with the thermostat, and it's not allowing enough coolant flow through the radiator (radiator never really gets hot, just warm now.)

Any advice is appreciated. I'm thinking I will just replace the thermostat to be sure it's good (that's a cheap part) and I've read in the forum about some who have increased the radiator inlet space or added a shroud to get more air flow. 

I'm in Southern California, and the temps have been cool for all these tests, as in mid to upper 60s F.

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The thermostat is good if the radiator is getting hot. That means it is opening and flowing. I think that would be unnecessary. 

Please keep in mind, considering the history and previous bad prop strike, without having it properly gone through (by the previous owner) - we don't know how the internals are on the engine itself - as far as what shape they are in or if it has been overheated before. 

As far as the cooling, you do have a gen 1 radiator, the newer style radiator is a simple and cleaner setup with far better cooling and a premade shroud and a different cooling setup. I can lead you to videos on this if interested. 

That all being said, no cooling system is designed to fully cool the engine on the ground for an unlimited time at elevated power. At idle it can. 

Some things to keep in mind about current status. 


Thank you, Jan and Alissa. I watched some of the STOL competition with Jan was in that you've recently posted and notice that most are flying with the engine cowls removed. Is that for weight reduction or for better cooling?

I'm getting too hot at idle. and this is with cool ambient temps... so when it does hit triple digits here (which it does in the summer) it will be a problem. I may have to do the radiator upgrade.

- Shawn 

This is just because of weight

I also recommend to join the Viking Engine Owners as prospects page on Facebook. You will have a lot more direct information, customer information and advice and more active answers etc. 

Have you checked temperature by some independent means? I thought my Viking 130 was overheating till I added a separate thermocouple and found a calibration error in my instruments. I now show around 170 deg on the dash during cruise, temperature from can bus data is around 195 cruising. I’ve never had it get over temp on the ground. I put in a Gates 333405 thermostat without any effect. Taking the thermostat out will allow inspection of pump impeller. Another suggestion: I found putting the tail on the ground with the coolant cap off and squeezing hoses will help to burp the system. My coolant level in the bottle will change an inch or so from hot to cold. I also added a small diverter under the radiator to create a low pressure zone from slip stream to enhance air circulation, it also keeps radiator clean as I am on grass.

These are all good considerations. Thank you.

Today I did some extensive testing. First with the cowlings off and the temps stayed reasonable at about 185 max and stable while at idle for 45 minutes. The second run I did with about 20 minutes at a higher rpm of 2600-2700 and the temps went to about 195F and stabilized there. Pulled it back to idle for 10 minutes and the temps went down to 190F. Then I put the canopy back on and the temp went back to 221 or so and seemed to be pretty stable there but I didn't want it to get any higher and shutdown. My conclusion is that in flight it would be fine with good airflow through the radiator. But on the ground in the summer time I'll need to improve the airflow or limit ground time.

So, to improve airflow I am considering a bigger scoop in front to help bring air in, and a "cowl flap" like piece under the radiator to induce a venturi suction and thus aid in pulling air through the radiator. And I think that's exactly what you are describing as a diverter under the radiator, right?

If you are interested in adding a diverter under the radiator I will add a couple photos of how I did mine. I spent hours chasing overheating till I used an independent temp measurement and found I was chasing a non-problem.


Thanks Jim,

I tried a flap under the radiator that would help pull air through the radiator (which is what your pictures show I think) and had no results.

I am concluding that there is not enough coolant flowing to the radiator since the flow into the radiator gets hot, and the flow back to the engine is pretty well cooled (almost to ambient temps). But not enough heat gets transferred away from the engine.

I looked at the impeller when I changed out the thermostat (I tested and both the new one and the old one function when pit into hot water.) The impeller looked fine, and turns with prop rotation.

So now I'm out of guesses that I can test easily.  My only guess is there must be something in the pipes, engine, or radiator that restricts flow.


Hi Jeffrey,

I am taxi testing my Zodiac 650B with a Viking 110 engine. Watch the coolant temp as it warms up. You should be able to see the temp slowly rise and drop back when the thermostat opens. Your checks for heat in the radiator and hoses are good. My Viking View has the coolant temp indicated at 170 F for taxi test runs of 0.5 hrs.

Thanks. Watching mine today it's hard to tell when the thermostat opens except that the temperature increase slows down. It never actually decreases. So far, I have been unable get a handle on the source of the problem.

- Shawn

Can you put an inline flow meter in a split radiator hose and see if it’s flowing coolant? Not sure how many gallons per minute flows through the engine, but this would tell you if it’s meeting that spec.  There’s a ton of cheap flow meters on Amazon that would probably work for testing, then removed once you start flying. Have you tried a different sensor? Maybe as stated before it’s an erroneous reading. I’m just spitballing, and have no expertise here. 

I Thought about a flow meter, but need a cheap one that can handle the temps. I did swap the gearbox and coolant sensors, same temps so the temp readings are correct. My last test was better (upper cowl off so I can see coolant tank and feel the pipes,) with temps raising to 207F at idle but them after running the rpm up to 2400 or so, the temps came down to 200F indicating that a little more air and higher rpm for the water pump may be all that's required. We're going to fly it and see. Then if needed I'll get a flow meter on it. Thanks.



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