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JackC

ONAN twin cylinder head temperatures

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JackC

This probably should be under the engine forum but it may be of a more general interest so I will post it here.

I like the twin cylinder ONANs and want to understand why some fail and in particular why the valve seats pop out or a rod breaks. Some say carbon build up and valves out of adjustment are two of the main causes. I have always suspected that the back cylinder on the ONANs in 400 and 500 series tractors run hotter because the are in a tight spot and failures may occur due to overheating of the back cylinder.

Some have developed vented belt covers believing they will allow more air flow and keep the back cylinder cooler. Also the location of the oil filter blocks some air flow to the head on the back cylinder.

I picked up an inexpensive two channel temperature display unit recommended by SOI and measured some temperatures on the twin ONAN in a 1991 416H with 705 hours. Surprisingly the back cylinder was running about 25 degrees cooler than the front cylinder. The front cylinder was averaging about 270 degrees Fahrenheit and the back cylinder was averaging about 245 degrees. The temperature of the crankcase close to where the engine is mounted to the frame was running about 160 degrees. The air temperature was around 55 degrees.

The back cylinder was running consistently cooler by as much as 30 degrees. i even wondered if the back cylinder was firing and pulled the spark plug wire off the plug to verify that it was firing.

Not sure what it all means but perhaps the difference in head temperatures suggest something is not optimum with one cylinder or the other and the engine needs some service. When the cylinders are not balanced and one has to work harder than the other a broken rod could be the result.

When the compression readings for a two cylinder engine vary by more than 10 or 15 percent that usually means the engine needs service or an overhaul. Maybe when the temperature difference exceeds a certain value that means the same thing. I am going to assume that the temperature difference that i measured today is a warning sign and my engine needs something done before something gives. I just bought it and it probably has never been decarboned or had its valves adjusted.

It is not a particularly easy job to pull those engines, remove the heads and service them but maybe that is what should be done to get the 3000 hours some say they are good for. Maybe measuring head operating temperature differences and cylinder compression test differences are ways to determine when that service is needed.

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dbartlett1958

Jack,

Very interesting results. Not what one would have guessed. Please follow up and let us know more about what you find regarding internal condition of your engine. It would be ironic if after all this time the Onan rear cylinder runs cooler on a healthy engine as well.

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sorekiwi

Interesting Jack.

I think you said you had more than one Onan powered tractor - would it be a PITA to swap the unit onto a different tractor (engine) to see if you get the same results?

Dammit, I think I'm gonna have to get me one of those things. Any chance you could post pics of where you located your sensors so I could mount a pair in the same place pn my Onan? Be interesting to compare notes.

EDIT: Another thought just occurred to me, it would be interesting to get a temp reading off each exhaust pipe - not sure if it would be too hot for the thermocouple though? I do know that the headers on an Indycar are too hot to measure with an Infa-red temp gun!

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JackC

I do have a 20 hp and two 16 hps and I will check them the same way. Where to mount the probes is a good question. I tried to pick up the temperatures off the cooling fins on the sides of the cylinders rather than off the fins on the heads. The temperature readings were higher on the sides compared to the heads. Maybe the exhaust ports are a good spot. I suppose the important thing is to measure both cylinders the same way. I did not take any pictures because my set up was just quick and dirty. The probe wire for the front cylinder was just long enough to rest the display on the motion control cover under the steering wheel.

I would assume for a health engine running on a bench, both cylinders would measure the same temperature.

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JackC

Besides the three ONAN P series engines, I also have two D-160s with the ONAN BF MS engines. Those engines are mounted with the cylinders left and right instead of front and back. I will be collecting data for all of them and reporting back.

I will find out why the 16hp that I measured today is showing the temperature differences that it is and I will report what I find. I will do a compression test soon and then pull the engine and check for carbon and valve adjustment. I will also check the cylinder walls to see if they need to be re-honed for proper lubrication.

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sorekiwi

I would assume for a health engine running on a bench, both cylinders would measure the same temperature.

I would guess so, in an ideal world. But the Onan isnt really that symmetrical the cooling shrouds are very different each end of the motor to accomodate the oil filter on the rear and the starter on the front.

On automotive engines they sometimes measure the temps of each header (EGT's) and use that data to fine tune the air/fuel mixture going to each cylinder. Intake manifold design can make massive differences in the amount of fuel each cylinder requires. But the goal is to get the individual egt's as close as possible to each other.

I guess I am not (yet) convinced that the lower temp you saw on the rear cylinder is necessarily a sign of a sick cylinder. I'd measure the other two tractors you have and see how close those readings are. They cant all be sick can they?? :scratchead:

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MarkPalmer

In my experiences with these engines in many different tractor makes and mounting configurations I don't think cylinder cooling was the main issue behind Onan valve seat failures. That sort of thing would have been well tested during the engineering phases of the design of the tractors the engines were used in. In my time of doing service the most rhyme/reason I found to the problem was the service of the valves and de-carboning of heads and piston crowns not being done at the required intervals. From the "customer" standpoint it seemed expensive to get the work done, and not many kept the tractors long enough for this type of service or if they did they forgot all about it if they even read the manual and understood it.

Early Series 60 Detroit Diesels did the same thing with valve seats- they got beat in to the heads (rather than the blocks of the Onans) if the valve clearances weren't adjusted to specs regularly.

-Mark-

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Callen

I would just about bet you would find a bad valve guide in that cool cylinder. It would be interesting to see the EGT on that engine.

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MarkPalmer

Unfortunately as you figured temperature meaurement alone shows nothing you can base a solid conclusion upon. If you take three different engines with different hours of use and don't know what kinds of loads they were under during thier use and do measurements on days with different outside ambient temperatures and humidity, just taking temp measurements of each cylinder and noticing slight differences isn't going to prove there is a problem and even if it did it will demonstrate a symptom and not the cause.

-Mark-

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Save Old Iron

Jack,

great start to a great thread.

One item to consider is the calibration and response of each of the two thermocouples at the temperature you are measuring. I would encourage you to check the temperature displayed from both the T1 and T2 probes when both are forced to measure the same temperature.This check can be accomplished in several ways. The most convenient may be to heat up cooking oil to the approximate temperature you will be measuring. Immerse both probes into the heated oil, keeping the probes within an inch or so of each in the oil bath. Check the difference in displayed temperature. Any difference will need to be accounted for in your mental math of head temp differences. Calibration factors can also be changed in the meter (there is a calibration sequence listed in the instruction manual of the meter). Allow the probes to recover back to ambient temperature and check the displayed temperature agreement at room temperature.

Remember we are using a measuring device purchased for $23. The K thermocouple probes I'm waiting on cost under $8 each. I expect them to function but I will not blindly trust their performance without some degree of verification.

Another thought. Leave both probes in their current location and swap the probe connections on the meter. Swap T1 and T2 probe connectors on the meter. Any significant change in the 25F delta between heads would then be attributed to the differences in the measuring probe - not actual cylinder head temp difference

I have seen interesting articles on checking proper valve sealing by observing spark line characteristics. If any "turbulence" exists in the combustion chamber from continued movement of charge mixture thru the head, the digital scope will show evidence in the spark line. Erratic mixture flow in the spark plug gap after the initial firing of the plug can indicate improper valve sealing.

http://autolabscopediagnostics.com/spark.htm

.

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JackC

I took some head temperature data on my 1883 C-175 with the Kohler KT17 Series I engine with 285 hours.

The cylinder temperatures were much closer to each other compared to the ONAN P216 with 705 hours.

Both cylinders of the KT17 were running about 345 degrees and the delta was only a few degrees.

Here are some pictures:

1) The Kohler KT17 Series I with 285 hours:

2) The test set up with the $22 two channel temperature measuring unit from eBay. The delta was -1.8 degrees when the shot was taken while the engine was warming up:

3) Probe on front cylinder near the exhaust port:

4) Probe on back cylinder near exhaust port:

The front and back cylinders were both running at 345 degrees. I hit the limit on file sizes so I can't post the pictures.

5) The delta was only a fee degrees and as little as 0.1 degrees.

I am assuming that the 25 to 30 degree difference that I measured on the ONAN P216 with 705 hours is a warning sign of some kind until I can determine otherwise. Better to be safe than sorry.

The measuring unit for only $22 seems like a worthwhile investment to me. I am also waiting for the $15 non contact laser tachometer to make sure my idle RPMs are all up where they should be.

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can whlvr

i would pull the heads and also regap the valves and then see the temps,and let us know,do onans in john deeres and other tractors loose the seat,i know of one that was in a welder that did

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JackC

i would pull the heads and also regap the valves and then see the temps,and let us know,do onans in john deeres and other tractors loose the seat,i know of one that was in a welder that did

I plan to do just that because i don't want to take a chance on having an engine failure. I have a snow blower on the tractor but I am not going to use it until I know the engine is OK. As far as I understand, the ONAN Performer Series engines require period valve adjustment and a decarbon procedure. Some say every 200 hours and some say every 500 hours. If that is not done you are risking a broken rod because the good cylinder is being overloaded with work by the bad cylinder. I thought I read somewhere that the valve seat problem may have been associated with engines produced during a certain time period but I have been able to substantiate that.

I am trying to determine if measuring head temperature is a good diagnostic tool especially for twin cylinder engines.

Was the engine on the welder a Performer Series engine or an older model engine?

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woodchuckfarmer

Dont forget to check the intake manafold , there built in 2 parts. Ive seen them come appart making an air leak. This will make a lean mixtureon one cylindercausing it to run hotter

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leeave96

i would pull the heads and also regap the valves and then see the temps,and let us know,do onans in john deeres and other tractors loose the seat,i know of one that was in a welder that did

So how big a deal is it to pull the heads on these Onans? I recently replaced the head gaskets on a Kohler Command CH18 twin cylinder in another color tractor of mine and had to remove the engine from the tractor to get the tins off of it. In the process, I broke a couple of smaller screws in the aluminum block that held the throttle linkage brackets - MAJOR PAIN IN THE ARSE!!!!!!!! The whole deal made me want to stick with only single cylinder cast iron Kohlers going forward!

So what's involved with pulling the head and re-gaping the valves on these Onans?

Thanks!

Bill

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JackC

Dont forget to check the intake manafold , there built in 2 parts. Ive seen them come appart making an air leak. This will make a lean mixtureon one cylindercausing it to run hotter

That should be an easy check. I will do that first and also do a compression test. I have other P series engines to test to see how they compare.

I wonder why the Kohler (345 degrees) runs about 100 degrees hotter than the ONAN (245 degrees)?

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sorekiwi

Good point about the inlet manifold.

I have done quite a lot of reading about the Onans in the John Deeres, and its an issue that seems to come up quite frequently on their engines. I've had 3 manifolds pass through my place and all 3 seemed fine.

The JD guys dont talk about valve seats coming loose very much...

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sorekiwi

I wonder why the Kohler (345 degrees) runs about 100 degrees hotter than the ONAN (245 degrees)?

Maybe the aluminum block sucks the heat away from the cylinder head quicker? :scratchead:

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JackC

I wonder why the Kohler (345 degrees) runs about 100 degrees hotter than the ONAN (245 degrees)?

Maybe the aluminum block sucks the heat away from the cylinder head quicker? :scratchead:

That is probably it. The Kohler is cast iron and the ONAN is aluminum.

A hotter running engine like the cast iron Kohler KT Series flat heads may not carbon up as quickly as a cooler running engine like the aluminum ONAN P Series flat heads. Maybe that is partly the reason the decarbon service is important for getting long life out of the aluminum ONAN P Series.

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Hydro

Just following this post with a great deal of interest. Wouldn't it be unreasonable to expect a consumer to decarbon an Onan after 200 hours? If as I've heard people say they are a 2000 plus hour engine that would mean it should have been decarboned 10 times. :confusion-shrug:

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Callen

The difference in the temps of the two engines may be the distance your probe is from the spark plug. The spark plug will be the hottest spot in any aircooled engine.

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JackC

Just following this post with a great deal of interest. Wouldn't it be unreasonable to expect a consumer to decarbon an Onan after 200 hours? If as I've heard people say they are a 2000 plus hour engine that would mean it should have been decarboned 10 times. :confusion-shrug:

200 hours would be every 3 to 4 years for me since I have several tractors. I took a quick look at the service manual but did not see any service intervals.

Here is what one vendor on eBay says:

"ONAN VALVE GRIND- DECARBON GASKET SET IF YOU WANT TO EXTEND THE LIFE OF YOUR P-216-218-220 ONAN ENGINE YOU NEED TO REMOVE THE CARBON AND CLEAN THE COOLING FINS AND ADJUST THE VALVES EVERY 200 HOURS OR SO .ALMOST EVERY TIME WE TEAR DOWN AN ONAN ENGINE THAT BLOWS A ROD THEY HAVE BURNT OR OUT OF ADJUSTMENT VALVES ON THE OTHER PISTON THEY LOOSE SOME POWER SO PEOPLE JUST TURN UP THE THROTTLE AND THE OTHER SIDE CARRIES THE LOAD FOR A WHILE THEN THEY TAKE OUT THE ROD,SO THEY REPLACE THE ROD AND DO THE SAME THING AGAIN ,THEN THE ENGINE IS JUST A PIECE OF JUNK IF YOU WANT TO STOP THIS CYCLE A LITTLE PREVENTIVE MAINTINECE GOES A LONG WAY,THIS IS ALL YOU NEED."

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JackC

I bought a 1990 416H with a bad ONAN P216 motor with the intentions of tearing the motor down and repairing it.

The engine had a little over 1,000 hours on it and the rod on the back cylinder broke.

I took the engine out of the tractor (easy job) and took the heads off.

There was a lot of carbon build up that clearly would be a problem with valves seating properly.

The front cylinder was much worse that the back cylinder suggesting the load was on the back cylinder which is probably why the back cylinder rod broke.

The engine did go over 1,000 hours before the failure. I think after 700 hours I would have concern.

At this point, I believe every 500 hours is a good service interval for the decarbon and valve adjustment interval.

I would also try to keep carbon build up down by keeping the idle speed up at 1100 or higher, running the engine at higher RPMs whenever in operation, and using additives like SeaFoam that help reduce carbon build up.

There may be fewer failures in applications where the engine is run harder (generators and commercial mowing applications)?

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Hydro

That would explain or could explain why mine failed at 450hr because it was kept clean especially the air passages, regular oil changes, no vented belt guard mind you but when mowing it would pick up dead grass like crazy and so I made a point of blowing it out but I never did have the valves adjusted, I probably should have. :banghead:

FYI I think this investigation into the cause of Onan failures has been one of the most informative threads I've ever read. The approach to this question is really well thought out. :thumbs: I'm sure many members will benefit from this.

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Save Old Iron

so if lower head temp = higher carbon buildup

and in the Onan, the front builds carbon faster, which stresses the rod in the rear cylinder

is it arguable that the FRONT cylinder should be made to run HOTTER to match the rear head, Yes, belt guard vents MAY cool the rear cylinder, but will additional cooling only serve to accelerate the buildup in the rear head?

What say you Goldilocks?

Not too hot, Not too cold.

Yes, but what is just right?

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