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Frustrating Intermittent Problem with Scag Turf Tiger Koehler Engine

#1

J

JRonascag

Hi! I'm new to this forum, so I apologize in advance if I do something wrong with this post. I have a frustrating problem with my Scag and am sure hoping you can help:
It is an ~8-yr old Scag Turf Tiger with the 27hp Koehler engine, with ~300 hrs. I use it to cut my 5 acre lot.
I've always maintained it carefully and it's been a great mower. Now however, it's developed a very frustrating intermittent problem as follows:
The mower/engine runs perfectly for about the first hour. Then, right out of the blue, it starts sputtering and quickly dies if I don't quickly kill the blades and put the throttle to idle.
If I let it idle for a minute or so, then I can resume cutting for 15 minutes or so with no problems. Then...it starts the bobbling again. If I let it cool completely (i.e. overnight), it will run great for the hour or so and then start the issues again.
I thought I had found the problem: a clogged oil cooler. Embarrassingly, I hadn't noticed that it had developed a small leak at the cooler inlet and oil had dripped onto the fins, allowing it to collect a lot of dust, and the fins were completely clogged. I felt sure this was the problem, causing it to get too hot and shut down (heat or oil pressure sensor I hope?). I removed the cooler, cleaned it and the flywheel housing and fixed the leak, thinking that this would solve the problem. It definitely seems to have helped as it runs considerably longer (about 1.5hrs) before the problems starts again. One other item: I can't tell for sure, but the power also seems to be down a bit overall. It seems that the engine chokes down a little when I do a lot of hard turning and I don't remember that occurring before.
As FYI, I have also replaced the air filter, spark plugs, and fuel filter to no avail. The fuel pump is mechanical, so I wouldn't think it is heat sensitive and this sure seems to be a heat issue of some sort.
So....hopefully you can follow my confusing description and I'd sure appreciate your insight.
Thanks in advance.
James


#2

EngineMan

EngineMan

If you feel that its heat related then I can think of three.

Valve(s)

Ignition Coil (breakdown)

Fuel starvation

let us have the numbers off the engine (model/type)


#3

Fish

Fish

Post your engine's model and spec numbers, it will help isolate possible problem areas,
thanks

Fish


#4

J

JRonascag

I'll get that info posted this evening when I can get it off the mower. Thanks!


#5

J

JRonascag

Good evening! As requested, the model number is STT52A-27CH.
Mower Serial number is 9360106.
It is a Kohler Command engine and.....I don't have it's serial number and mower is at another location. If you need that, let me low and I'll get it ASAP.


#6

EngineMan

EngineMan

Will need the model number of Kohler Command engine.


#7

Fish

Fish

Yeah, we need to get that spec number to see if it has "Smart Spark", I think it does, and that is likely your problem area if it does have it.


Fish


#8

J

JRonascag

Good evening, again. Got the engine info:
Engine number 3324517691
Model CH740S
Spec no ch740-0041
OEM no 482890


#9

J

JRonascag

Btw: I'm certainly anxious to hear if you think it's a smart spark issue or other ideas, but let me add something else that I thought of: when problems occur, I always first try to think "what changed recently that might cause the problem?"
In reading this forum last night I stumbled onto a thread that talked about vapor lock caused by fuel line heating and....it got me remembering...
At the beginning of this season after mower sat up for several months, I had to pull the fuel line and suck some fuel through the fuel pump to get it primed. In the process, the hose end was a mess, so I cut some off its length before putting it back on the fuel pump outlet, making it a much closer fit against the upper part of the motor. AND the heat shield material was badly frayed and I moved what was left of it closer to the carb. I now realize both of these actions left a completely exposed fuel line right where the hot air flow upward after coming through the oil cooler fins and bouncing off the battery box.
So....do you think I could be overheating the fuel line and causing some sort of vapor lock issue?
This scenario really seems to jive with my symptoms, but I may just be grasping. I defer to you experts.


#10

D

dodgefate

I had a motrcycle that would get vapor lock when I was beating the balls off it. It only used to happen at about 120 mph. While riding I used to take the key out, open the gas tank and restart the bike. So if it is vapor lock, just unscrewing the fuel cap should be an instant fix/diagnosis. If that dosent work I would recommend caring around a can of carburetor cleaner. If it starts to bog down, spray some in the intake and if it steady out you know its a fuel starvation problem. or you could just choke the motor to see if it stays running.
Only 3 systems you have are fuel spark and compression so if you narrow out the one then that leaves two left.


#11

EngineMan

EngineMan

Start with the bad fuel and heat shield, renew the fuel pipe and fit some heat shield material between the pipe and engine and then try it, have you got a manual for that engine..?


#12

J

JRonascag

I have the basic manual that came with the mower. I replaced the fuel line and applied heat shield tape over whole length. Now I just have to wait for the grass to get high enough to challenge the mower. Will be a couple weeks dang it...
Thanks to all for the great help and I'll keep you posted.


#13

EngineMan

EngineMan

This may help you.

https://www.dropbox.com/s/7nsje7myszbbovb/SM_2469006_CH18_thru_CH750_REV1_12.pdf


#14

Fish

Fish

Well, your engine has "Smart Spark" or the S.A.M. module, so hopefully you found your engine's problem. If not, you might just take a quick look at the backside of the keyswitch, many mowers that have had on-off SmartSpark troubles had badly corroded keyswitches.


#15

J

JRonascag

Wow! That manual is MUCH better than what I had. Thanks EngineMan!

And Fish, I'll give the ignition switch a look next time I'm at the mower. Sure hoping the fuel line was my problem, but will let you know how it comes out.

I'm curious though: Apparently the Smart Spark system has significant issues?

Thanks again. This is a great Forum!


#16

Fish

Fish

JRonascag said:
Wow! That manual is MUCH better than what I had. Thanks EngineMan!

And Fish, I'll give the ignition switch a look next time I'm at the mower. Sure hoping the fuel line was my problem, but will let you know how it comes out.

I'm curious though: Apparently the Smart Spark system has significant issues?

Thanks again. This is a great Forum!
Click to expand...

Yeah, it is a different critter. Unlike all of the other ignition systems, this one requires voltage to be supplied to the S.A.M. module for it
to function correctly, and old corroded connections can wreak all kinds of havoc.

They just came up with a system where it has an extra hundred things that could go wrong......


#17

J

JRonascag

Interesting (and somewhat discouraging). So here's a possibly dumb question: If proper voltage is critical for the Smart Spark function, does that mean that a fully-charged battery is a key to this engine running properly?


#18

Fish

Fish

JRonascag said:
Interesting (and somewhat discouraging). So here's a possibly dumb question: If proper voltage is critical for the Smart Spark function, does that mean that a fully-charged battery is a key to this engine running properly?
Click to expand...

It can be a problem, which is why I ask questions about the mower, like if it has an electric pto, as that can drain a lot of juice.

The Smartspark requires a lower voltage minimum to operate correctly, like 10 volts or so.

But corroded connections or damaged wires are usually the problem, as they can drop voltages as well.

If the overall voltage drops too low, the pto usually starts cutting out first, but not always....


#19

K

Keith O

JRonascag said:
Hi! I'm new to this forum, so I apologize in advance if I do something wrong with this post. I have a frustrating problem with my Scag and am sure hoping you can help:
It is an ~8-yr old Scag Turf Tiger with the 27hp Koehler engine, with ~300 hrs. I use it to cut my 5 acre lot.
I've always maintained it carefully and it's been a great mower. Now however, it's developed a very frustrating intermittent problem as follows:
The mower/engine runs perfectly for about the first hour. Then, right out of the blue, it starts sputtering and quickly dies if I don't quickly kill the blades and put the throttle to idle.
If I let it idle for a minute or so, then I can resume cutting for 15 minutes or so with no problems. Then...it starts the bobbling again. If I let it cool completely (i.e. overnight), it will run great for the hour or so and then start the issues again.
I thought I had found the problem: a clogged oil cooler. Embarrassingly, I hadn't noticed that it had developed a small leak at the cooler inlet and oil had dripped onto the fins, allowing it to collect a lot of dust, and the fins were completely clogged. I felt sure this was the problem, causing it to get too hot and shut down (heat or oil pressure sensor I hope?). I removed the cooler, cleaned it and the flywheel housing and fixed the leak, thinking that this would solve the problem. It definitely seems to have helped as it runs considerably longer (about 1.5hrs) before the problems starts again. One other item: I can't tell for sure, but the power also seems to be down a bit overall. It seems that the engine chokes down a little when I do a lot of hard turning and I don't remember that occurring before.
As FYI, I have also replaced the air filter, spark plugs, and fuel filter to no avail. The fuel pump is mechanical, so I wouldn't think it is heat sensitive and this sure seems to be a heat issue of some sort.
So....hopefully you can follow my confusing description and I'd sure appreciate your insight.
Thanks in advance.
James
Click to expand...

I have the same problem and it has gotten worse. It is a fuel delivery issue for sure. I have a clear fuel filter and when the motor starts to shut down the filter is empty and bubbling. The mower is running out of fuel! When it shuts down I can see the filter fill with fuel again. When I crank it again it takes a bit for the fuel to pull into the motor and runs till the filter is empty again and starts shutting down. I suspect there is trash in the tank, fuel line and or both restricting fuel from being pulled up to the motor. What are your thoughts?


#20

L

Luffydog

Drain the fuel out of the tank and take line apart and blow air back into the tank it sounds like it is clogged up in the tank where the fuels comes out. Take a bright light and shine inside the tank and look for debree. Give that a go and let us know how it runs afterwards. The fuel should come out of hose the same size as of the fuel line.


#21

J

JRonascag

Keith O said:
I have the same problem and it has gotten worse. It is a fuel delivery issue for sure. I have a clear fuel filter and when the motor starts to shut down the filter is empty and bubbling.....What are your thoughts?

I suggest you look back at my post in this thread on 8-6-2013 (9:45pm). My issue turned out to be vapor lock in the fuel line and your description of “bubbling gas” in the filter makes me think yours could be same. Mine began occurring because I’d shortened the fuel line, getting it closer to the hot engine, and moved the remaining fuel line heat shield to the wrong location. A longer fuel line and renewed heat shield completely solved it. Been great ever since. Hopefully your fix will be as simple.
Click to expand...


#22

Fish

Fish

If I remember correctly, the S.A.M. module requires at least 11 volts to function properly.


#23

J

JRonascag

Thought I’d show a picture of the fuel line fix. Note that it’s the fuel pump outlet side that goes up over the HOT engine.

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