Scag / Kohler engine has me stumped

[aloha]

I have a Scag Turf Tiger ZTR with the 27 h.p. Kohler CH740 engine that has died and defies troubleshooting. The grass is growing and I am desperate for suggestions.

The Scag is 6 years old, 560 hours, and has been completely reliable up until now. 3 days ago I had just started mowing, and about 3-5 minutes after having started the engine the engine backfired very loudly and immediately died. (I had just started a right turn after having mowed a straight line for about 2 minutes when it happened, probably just a coincidence, but it did happen at the instant I started the turn.) When I looked at the engine the air hose from the air cleaner was off the carburetor, presumably blown off by the force of the backfire. When I crank the engine it fires and sounds as if it is going to run, but when I release the key from the start position the engine it dies.

I have confirmed fuel is being pumped and the carburetor fuel shutoff solenoid is operating correctly. I pulled sparkplugs and one was damp with a gas odor. Visual inspection of the carburetor shows nothing amiss, linkages are working correctly, although the throttle valve moves little from the full open position, which is where I normally run it. It appears that the fuel supply is not the cause.

I have checked that there are no obstructions to the air flow and attempted starting with the air cleaner removed. I find no problem with the air flow.

I have replaced both spark plugs. I have used a spark plug tester that shows both cylinders are sparking.

The local Scag guy said he has seen similar symptoms with a broken valve push rod. I pulled the valve covers and all push rods appear sound and straight. Pushing on the rocker arms showed all valves moving freely.

I have gone through the troubleshooting steps in the engine service manual, http://www.kohler-engine-parts.opeengines.com/pub/SM_2469006_CH18_thru_CH750_REV1010EN.pdf, and find no clues in those things I can do without special tools and which seem even remotely possible. I'd like to get a timing light on it, but it appears that would require pulling the engine from the Scag to get access to the timing marks, and with the engine not running except by the starter turning it there might be little to be learned.

The local Scag guy has 2 to 3 weeks of repairs backed up, so I am pretty much on my own. And having determined gas apparently ok, air apparently ok, spark apparently ok, and the main mechanical components apparently ok (at least those that can be seen without seriously tearing into the engine) I have run out of ideas.

So, someone please show me that I am an idiot and have overlooked something easy. Or even something not so easy. All ideas are welcomed. Thanks.

 

[Fixemall]

Unhook the wires at the keystart and jumper the solenoid, sounds like a hot wire is grounding...if no luck try disconnecting everything but the battery main and jumping it again.

 

[Gator6x4]

A backfire that blows the air hose off the air cleaner was a backfire. Have you checked the compression on both cylinders? The backfire could have dislodged a valve seat. The backfire could also have sheared the flywheel keyway or caused it to slip. Have you checked the seat safety interlock switch? Are the other safety switches, deck and gearshift etc., still all plugged up?

Most Koehler’s use a white wire to ground the ignition coils, can you temporarily disconnect the ignition coils ground wire to see if it will start?

Will your Dealer provide a copy of the electrical schematics for the machine?

 

[Gary Fowler]

My Craftsman ZTR with the 26 HP BS engine died recently while mowing (still under warranty) and it was the parking brake safety switch. It was running well and just quit but mine would start up then die when I tried to move it. Yours could also be a safety switch problem although when mine are acting up, the engine wont turn over unless all controls are set properly.

 

[dodge man]

Are you sure your getting spark? I'd hold the spark plug near the head or something and make sure you see spark. If you are sure your getting spark, try spraying a little starting fluid in the carb and see if it wants to run better. If it starts and runs a little bit with starting fluid, you might be getting gas, but not enough.

If its not any better with the starting fluid, and you are getting gas and spark, that means you are getting spark at the wrong time, and this is my best guess what is wrong with your engine. You said something about timing. How are these things timed? Is there a way to adjust it? New ingnition module? Of course it could also be mechanical, broken piston, bad valve etc, but this would require a tear down to find.

I'm just throwing out ideas and guessing a little, good luck.

 

[aloha]

Thanks to all for the thoughts and suggestions. I may be on track to sorting this out.

Compression looks good -- about 143 psi in both cylinders. The Scag service manual does not say what it should be, but no obvious problem here.

I have tried starter fluid. If anything, the engine fires less when I spray a good shot of ether in the carb, which I would say is impossible if someone else said this was happening and I had not seen it myself.

When I look down the carburetor throat while cranking the engine and the choke open I can see fuel pulsing in.

It does not appear to be a problem with any of the safety switches since the engine cranks and spark plugs fire, the engine just does not maintain speed when the key returns to the on position. I have confirmed the spark both using an inline spark plug tester that lights up when there is a spark, and pulling the plugs, grounding them, and seeing what appear to be good sparks across the gap.

I am unable to check spark timing since the engine won't run.

What I have come to suspect is that it is electrical, as suggested by Fixemall and Gator6x4. The clue being that when I crank the engine it fires and sounds as if it is going to run, but when I release the key from the start position and it returns to the one position the engine it dies. This seems to suggest that there is a problem in the switch or the kill circuit, and the engine is being killed with the switch in the on position. Sorting through the wiring, and trying to make sense out of what the service manual says, is proving very time-consuming. I will have to get back to it tomorrow.

In the meantime, any further ideas and suggestions would be most welcome. Again, thanks to those who have responded.

 

[gardencarer]

i think it was just due to a little mistake

 

[dodge man]

From what you describe, it almost sounds like you are not getting spark when you let go of the key. In other words, the run position is not run but off. Is there a way to check to see if you get spark on the run position? Can you bypass the keyed switch in some manner and get the thing to run? If it were me, I'd try to bypass the key switch in some manner and see what you get.

Did I say I hate electrical problems? Good luck!

 

[aloha]

I, too, hate electrical problems.

One local mechanic opined that it sounds like a problem with a safety switch. I must disagree since the engine cranks and spark plugs fire, the engine just does not maintain speed when the key returns to the on position, and I have checked all safety switches and all appear to be operating correctly. Fuses are good. All wiring connections seem to be attached.

I have confirmed the spark both using an inline spark plug tester that lights up when there is a spark, and by pulling the plugs, grounding them, and seeing what appear to be good sparks across the gap. However, the spark stops immediately when the ignition switch goes from the start position to the run position.

When I disconnect the kill leads from the ignition modules I get no spark at all.

I need to think it is an electrical problem since the difference between the spark plugs sparking and not sparking is whether the starter switch is in the start or run position. I am struggling to make sense of the ignition switch so I might try using jumpers between wires to try isolating the problem. The Kohler and Scag wiring diagrams are not as helpful as they could be, in part because the Scag diagram shows wires going to "engine" without indicating what engine components the various wires connect to. There is a part called "electronic module" which appears to do magic with various circuits including the safety switches, but appears to be untestable.

I am getting a bad feeling that unless I get this figured out myself, what passes for an expert mechanic around here may spend many, many hours on the troubleshooting as I have and may hand me a bill calculated to fund his retirement.

 

[Fixemall]

Generic keyswitches are like $10, I keep several on hand...might start by replacing it to rule that out before diving into the wires. autozone carries them.

 

[Boit]

After reading the initial problem description( which was very good), I'd like to throw in my opinion. It appears that the backfire was from unburnt fuel in the exhaust that just so happened to ignit when the valves were open and sent excessive pressure back thru the intake hence the tube being blown off the carb. I would guess that something caused the ignition to be cut off(interlock switch maybe) and the raw fuel was ignited in the exhaust from normal heat accumulation. Now, what caused the ignition to be killed is up for discussion. There are a few safety switches in the system, such as the obvious; seat, brake release.etc. Since the number of switches are limited, I'd look at those first. Good luck and please report back when you have solved this problem.

 

[MiserableOldFart]

I'd still give a crack at that key start. If it's the old standby "Indak" type switch, they can corrode and stuff gets worn and they can cause problems. They are, as mentioned above, inexpensive and easy to replace, and if it's not the problem, you will have a spare..

From there, I'd check the safety switches, especially the seat one and if there's one on the blade clutch. They are generally very easy to get at. Then wiring, then ignition and timing.

Sometimes the contacts in the back of the key switch corrode and can be cleaned with a sharp knife or file. Had that one happen this year on a much older switch.

I've even had one or two occasions when a safety switch was got to work again with a shot of WD40.

 

[Kernopelli]

Not saying there aren't some great guys here on TBN but Lawnsite.com repair forum has some Kohler gurus that are real gems and Kohler certified mechanics that have helped me immensely in the past. Might join the site and see what they have to say.

 

[aczlan]

Page 25 of the Kohler PDF :

Engine Starts But Does Not Keep Running
1. Restricted fuel tank cap vent.
2. Poor fuel, dirt or water in the fuel system.
3. Faulty or misadjusted choke or throttle controls.
4. Loose wires or connections that short the kill
terminal of ignition module to ground.
5. Faulty cylinder head gasket.
6. Faulty carburetor.
7. Vacuum fuel pump malfunction, or oil in vacuum
hose.
8. Leaking/cracked vacuum hose to fuel pump.
9. Intake system leak.
10. Diode in wiring harness failed in open circuit
mode.

I would guess that 4 or 10 is the issue here.

Aaron Z

 

[aloha]

Thanks for the additional suggestions. I have posted on lawnmowerforum.com and will try lawnsite.com if I don't get it fixed.

aczlan's guess may be on target. I had concluded that this must be an electrical problem because the only difference between the spark plugs firing and not firing is the position of the ignition switch -- they fire when the switch is in the start position, stop firing immediately when the switch goes to the run position.

Today or tomorrow I should receive a new ignition switch and a relay/diode. Logically, the problem seems like it has to be in the ignition switch (something that could be likened to a connection shorting the kill circuit), but testing the diode indicated it had a short. Because all parts need to be shipped in I figured that I needed to order both rather than risk letting the grass continue to grow if the first part did not fix it. I will report on the results.

 

[retiredmgn]

I had a kohler 6hp on a machine. Went from running fine to dead. Finally found the coil had died. Never seen such a thing before on any small engine. Put in a new coil, no problem since.

 

[aloha]

As previously posted, I thought I had the problem narrowed down to either the ignition switch, since the spark plug stopped firing when the switch went from start to run position, or a diode, since my multimeter reported the diode was bad. Replacements for both parts arrived today. There is no change in the symptoms.

A quick recap:
The engine fires and runs with the ignition in the start position, but the spark stops instantly on changing the ignition from start to run.
Disconnecting the wire for the kill circuit makes no difference.
All safety switches check out ok.
All wiring connections appear good.

I am well and truly stumped by this. There is an electronic module that apparently does some magic involving the ignition switch and safety switches, but the manuals give no clue on how to test it, and it is too expensive to get a new one on the off chance it is bad. Troubleshooting is complicated by the Scag and Kohler wiring diagrams not exactly matching what is on the machine (even though wiring is all original).

All suggestions will be most welcome. Thanks.

 

[sld]

Could it have jumped time from the backfire? Does it use a metal or plastic timing gear?

 

[aloha]

sld - thanks for your thoughts. If I understand your question, the answer would be that it is unlikely the timing changed. Timing is controlled by magnets mounted on the flywheel which pass near ignition modules to trigger the spark.

The sparks seem good and strong and correctly timed while the ignition switch is in the start position, but no spark as soon as it goes to the run position. I will not be able to get a timing light on it to confirm timing until I can get it to run without the starter running at the same time.

 

[aczlan]

The sparks seem good and strong and correctly timed while the ignition switch is in the start position, but no spark as soon as it goes to the run position. I will not be able to get a timing light on it to confirm timing until I can get it to run without the starter running at the same time.

What happens if you pull the trigger wire going to the starter solenoid while leaving the key in start?

Aaron Z