ONAN twin cylinder head temperatures

[JackC 616]

A simple test would be to measure both cylinder temperatures before and after obstructing the flywheel screen. I will try that soon.

An obnoxious audible alarm would be good if a set temperature was exceeded.

I am not sure how helpful temperature delta is but it may indicate something is out of order causing an imbalance which could lead to a failure of some kind.

The ONAN engine will even start and run with a dead cylinder.

[sorekiwi 761]

An interesting thread.

The ONAN engine will even start and run with a dead cylinder.

I bought my 520H with a broken rod (rear cylinder) and it ran fine, idled great, but was gutless as hell. I'd actually assumed it had a valve seat problem and was actually pleased to find it had a broken rod- a broken rod is a relatively easy fix, a valve seat is always a little bit of a lottery as to whether the repair will stay together.

My conclusion on the broken rod was that the motor had been run low on oil - scored main bearings.

I have always said that I thought the grass or leaf buildup on the intake screen was more of a problem than the non-vented belt gaurd. I have actually disabled my seat switch so I can lean forward and sweep the debri off the screen when I need to (without having to stop). The tractors that have the Onans mounted across the chassis probably dont have this same problem, and also seem to have less valve seat issues.

I am not sure that you will hear a beeper when mowing at 3600 rpm. I would be tempted to put a BIG light on the dash somewhere - maybe something like the rectangular side lights off a trailer up on top of the dash. Many years ago it was common for race and rally cars to use a tailight with a 35 watt bulb as an oil pressure light and it seened to work well.

[Hydro 130]

The one other observation I make is how many manufacturers do we know that drove a 60" deck with 20 HP? Is it possible you could run a 48" deck and never encounter the problem with a 520? I have to wonder if there is a narrow margine between high temp and a failure temp given the two dissimilar metals.

[leeave96 487]

Not being an engine guy, and not having an Onan I do not know if this makes sense, but being an analyst I do have a couple of observations here.

The temperature has always been stated as the culprit.. do we have any idea where this notion came from? Did someone do some research about it in the past that has been forgotten?

The observed differential does not seem to account for much at all. Even if we discount measurement errors, a 25 degree difference does not sound like much. I do not have an Onan manual, but other engine's spec sheets often give much larger ranges for normal operating temperature.

Are the front and rear rods and valve seats identical? (or other parts of the mechanism? ) Maybe the issue is a design or manufacturing one.

We also have two different things that happen, the valve seat coming loose and rod breaking. While the temperature/carbon theory would explain both effects, there is nothing that says that these two things are related. Maybe there are two issues that result in different failures?

This list is not based on any knowledge of anything, just in listening to the discussion here. it might be worth some interested party's time to make a list of the questions, and the group can collectively figure out how to answer them. Afraid I am not going to volunteer, because as I said, I do not have one of these.

FWIW, I think the issue with the valve seats coming loose is dissimilar metals. Steel seats and aluminum block - different rates of expansion with temperature. The hotter the engine, the more potential movement. I think as you heat aluminum, the valve seat hole would expand at a higher rate than the steel seat and the seat comes loose.

OTOH - I could be completely wrong - IT HAPPENS ALL THE TIME!!!!!!!!

Thanks!

Bill

[sorekiwi 761]

I dont think you are wrong, it is the heart of the problem.

BUT it can be done right - every automotive aluminum cylinder head has some sort of ferrous valve seat installed in it, and its not very common to have one come loose. Watercooled engines dont see the temperature extremes that air colled engines see, but then look at how many motorcycles have aluminum heads / steel valve seats.

[Save Old Iron 1,563]

I am not sure how helpful temperature delta is but it may indicate something is out of order causing an imbalance which could lead to a failure of some kind.

At what delta would you have the operator alerted? 50F? less?

[Save Old Iron 1,563]

These will work as sensors. They are not thermocouples and do not require $$$ of amp circuits to trigger a warning at 300F+.

I'll grab a couple and see how they perform.

http://www.ebay.com/itm/2x-100K-High-Stability-NTC-Thermistor-3-6mm-RepRap-Prusa-Mendel-Bed-and-Hot-End-/221141783539?pt=LH_DefaultDomain_0&hash=item337d13d3f3

[Save Old Iron 1,563]

I am not sure that you will hear a beeper when mowing at 3600 rpm. I would be tempted to put a BIG light on the dash somewhere - maybe something like the rectangular side lights off a trailer up on top of the dash.

It is possible the current LED indicators on a 520 could be used to indicate a temp failure. Wiring the alarm circuit to the TEST LIGHTS switch of the current panel could be setup to cause the whole panel to light up and flash if the head temps went too high.

or

we could kill the ignition just as easily and bring the whole issue to a halt

or

a 2 stage plan. Warn the operator at 320F and shut off the engine at 350F ??

Gotta love electronics. You can do whatever the heck you want !

[JackC 616]

"My conclusion on the broken rod was that the motor had been run low on oil - scored main bearings".

If the engine is run low on oil, I would assume the head temperature would increase due to friction and an over temperature alarm could have saved the engine.

"These will work as sensors. They are not thermocouples and do not require $$$ of amp circuits to trigger a warning at 300F+."

Sensors with ring terminals should be easy to attach somewhere such as where the engine tins attache to the heads or under head bolts.

"It is possible the current LED indicators on a 520 could be used"

I would try to come up with a device that will work for any single or twin cylinder application not just the 520, or make different versions adaptable to specific applications like the 520.

"we could kill the ignition just as easily and bring the whole issue to a halt"

I think I would leave the kill decision to the operator at least initially. If the tractor is in the middle of the road when I am blowing snow I would not want it to stop there. There are alarm horns that should be louder than the equipment that is running. That way the device could be mounted out of sight under the hood and not compromise the appearance of some of our "beauties".

"At what delta would you have the operator alerted? 50F? less?"

Don't know yet. 50F difference sounds like there could be a lurking problem to me. We need more data to try to see what is "normal". Anyone out there with temperature measuring devices like the infrared units should be able to measure head temperatures and report in to help us develop a data base. I have more engines to measure but have not had the time to do it yet.

It seems to me that a temperature monitoring device that is easy to install and that allows the user to set alarm points for over temperature, and delta on twins, should be valuable to have and applicable to many applications including tractors and generators. There is nothing currently out there that I am aware of that fits that description.

We can't afford to lose any more of those sweet running ONANs, especially the ones on our Wheel Horse tractors.

[roscoemi 245]

Race cars use shift lights that come on before the rev limiter hits. A bright red led that is easy to hide would do the trick and should be cheap.

[Brian K WheelHorse 28]

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

After getting my 1997 20HP 520H this year I decided to go ahead and check the heads even though it only had 366 hours on it. There was carbon build up but I've seen worse. I attribute some of that to the crappy ethanol gasoline being sold now and I ran 3 back-to-back sea foam treatments in it. Just never sure exactly how the previous owner treats an engine. The heads aren't too bad to get too - just a pain to torque the head that faces inward. I ended up removing the cross bar that is situated near the battery (in addition to all the engine covers, manifolds, etc.). I got the gasket kits from Parts Tree I think. I was fairly deliberate on reinstalling and re-torquing everything. I was surprised that the lubricant for the bolt threads was regular motor oil (called out by the manual). I think anyone with average mechanical skill can tackle the job. Adjusting the valve gap is pretty easy, although the access for these is under the manifolds. The specs are in the motor manual - set the piston to the top and set the gap. There's a couple of 7/16th nuts on those I think. Not bad. I always thought the 520H was the "Cadillac" of Wheel Horses so when I got mine - I wanted to take some time and a few bucks in gaskets to make sure it was right. I will be using additive in the gas very frequently and I won't be leaving any of the crappy ethanol in it over the winter - I will run it all out. Especially since I put a brand new carb on it.

[Hydro 130]

Brain K, just go to Hi Test, no ethanol in it, solves a lot of problems.

[tiggercl 28]

Hi Brian,

Did the Onan valves need much adjustment for 366 hours?

Paul

[Callen 64]

We run our Blue printed 5hp Briggs race motors at 7,000 RPM and 430 degrees every time we go on the track and never have a valve seat come out. On the other hand, we used to run a lot of moded engines with over sized seats and had 2 come out. Also heard from engine builders that they have found some that fell out of brand new never started engines.

[sorekiwi 761]

Brain K, just go to Hi Test, no ethanol in it, solves a lot of problems.

+1 on this, I've been running mine on 100% gas for the last few months and the butt-dyno says it runs much better. Havent had the heds off so I cant comment on carbon build up.

Yes it costs about $4.20 a gallon, but in reality thats only about $2.50 extra a weekend of mowing for me.

[Hydro 130]

"Yes it costs about $4.20 a gallon, but in reality thats only about $2.50 extra a weekend of mowing for me"

Pay now with Hi Test or pay later with regular.

[mrc 727]

hello all, i currently do not own an onan powered wh. super interesting thread! i think the way you are trying to get to the root cause of the problem is fantastic! sorekiwi brings up a good point, and that is the john deere guys don't seem to have this problem. therefore i might suggest locating someone with an onan powered jd and get readings from that machine or several jd's would be even better. i am staying tuned to this. good luck. mike in mass.

[Brian K WheelHorse 28]

Brain K, just go to Hi Test, no ethanol in it, solves a lot of problems.

Yeah - it would be worth it to avoid any sludge problems.

[Brian K WheelHorse 28]

Hi Brian,

Did the Onan valves need much adjustment for 366 hours?

Paul

Strangely enough - 2 of the valves were adjusted slightly in too far (that made me a bit suspicious about the previous owners care for it. Nothing to get worked up about and 2 of them needed to go in a bit. I was VERY deliberate on the valve adjustment. Only a few thousandths though. So It would have likely been ok without the valve adjustment but it's really an easy check once you have the

manifolds off so ya might as well. And Someone said it before...when these things run good....WOW...it's worth it. If I had to tear the whole thing down to the crank and rebuild it I would. Great runner. Most aluminum blocks/heads are a bit fussy. My parents old 84 Cadillac gave them a ton of grief...

[Hydro 130]

hello all, i currently do not own an onan powered wh. super interesting thread! i think the way you are trying to get to the root cause of the problem is fantastic! sorekiwi brings up a good point, and that is the john deere guys don't seem to have this problem. therefore i might suggest locating someone with an onan powered jd and get readings from that machine or several jd's would be even better. i am staying tuned to this. good luck. mike in mass.

The small engine mechanic I go to worked for a John Deere dealership here for over 20 years. He tells me the Onans were just as bad for John Deere and suggests that was one of the reasons for the switch to Kawasaki. He says Deere replaced or repaired a lot of Onans under warranty during that time. He believed they overheated due in part to the fact they were shrowded in so much and quoted the 300 series as an example with valve seats being the cause of the failure.

It is interesting when you look at the Small Engine Warehouse website and the replacement kits they have available. They sure have a lot of replacement kits for the Onan application and I am not just talking Wheel Horse only tractors. They don't seem to have any kits for Kohlers in Wheel Horse.

[tiggercl 28]

Just a thought, WH operater manuals etc. make no mention of any prescribed 'cool down' periods for the Onan engines.

It would be interesting to note the change in temps from 3600 rpm hard running then unloaded at say 1700 for a 3 minute cool down period...

Lycoming engines in the smaller helicopters have mandatory shut down procedures... (different metallurgy though..)

Paul

[JackC 616]

Perhaps overheating is one of the primary causes of the ONAN engine failures which justifies developing temperature monitors and alarms.

I just picked up an overheat alarm that was used on the Wheel Horse 724-Z with the 24 hp ONAN.

Apparently Wheel Horse, or Dixie Chopper who designed them for Wheel Horse, decided it was a good idea for this zero turn mower.

Maybe the alarms should have been put on a lot more ONAN powered machines or at least offered as an option.

Not sure what I will do with it yet. Maybe install it on one my ONAN powered tractors.

Here is the alarm. Looks like it would be pretty loud. Not sure what the trigger temperature is:

Here is an example of a 724-Z for sale that they were used on.

http://fultontractor...rse-724-z-turn/

[Brian K WheelHorse 28]

Wow - looks like it just "makes" that temp switch and it starts howling. That's cool - never seen one before.

[JackC 616]

The temp switch may be the same one that is on the bottom of the rear head on later 520H ONANS so I may be able to hook the horn up to the head on my 520HC or on one of my 416Hs.

We all know how quickly the flywheel screen gets plugged with grass when mowing with the twin ONANS on a wheel Horse 400 or 500 series tractor. Has anyone ever noticed an overtemp light coming on? Here are some comments from a former 724-Z owner. Note the comments on cooling and the horn.

"I had a 724-Z for a while and it is a great mowing machine. Lots of power, very fast, and amazingly nimble. They were offered first around 1988 and thru at least 1995 in Toro guise. Perhaps longer. They were replaced with the Toro 222 (much improved, but drastically different) for one year before Toro took on the Exmark line of zero turns as their own.

The 24 horse Onan P224 has more than enough grunt to drive that 60" deck thru almost any grass at nearly the tractor's top speed. The fact that it sits out on the back of the tractor and is largely unobstructed means that it cools well and is less apt to shed a valve seat like some designs that enclose the engine and don't let it breathe. These tractors had a thermostatically driven horn on them to warn of overheat. Very smooth, very powerful, and very thirsty."

[Brian K WheelHorse 28]

LOL - Jack - mine turned on today! I took the day off and was mowing the back (about 2 acres) and I cleaned the screen off once. I had a few more passes to make and I looked down and sure enough - the high temp light was on. Of course the screen was packed full again. I'm going to make a "screen sweeper" for mine and possibly put in an alarm as well. I'll just find a relay that has a 12v coil and put on a car horn or something. Maybe there is a already a spare set of dry contacts in the electrical system I can use some where. What a ka-winky-dink. First time it happened to me - it's those darn leaves right now.