I have a late 90's 935 1800 hours and have been experiencing EASY start but will run rougher than crap for the first 3-5 minutes then is smooths out. (similar to low compression problem) BUT if you shut it off and restart it right after it smooths out it goes back to rough running for another 3-5 minutes. <--------a low compression cylinder after warming up wouldnt do that. Go to engage mower and it will nearly kill it. It has LOW power. Oh I also pulled the oil cap while running and it has little to none blow by...So the obvious was to run compression and I was expecting a dead hole with one marginal but what I found was even # across the motor 155, 157, 159. Is it possible that it jumped time or has a valve problem? Anybody have any experience here that could lend a hand?
Yanmar in a JD F935 mower problems compression 155#
- Thread starter Himark
- Start date
bruce1966us
Platinum Member
those are very low compression numbers. I think that mower has a 3TNA72 correct??
yes they are. I was thinking a good motor should be around 380? Anyway a typical diesel with that low shouldnt even start and this one pops right off. A bit puzzling.
normde2001
Elite Member
- Joined
- Mar 10, 2002
- Messages
- 4,143
I would suspect a faulty compression gauge and look elsewhere. It shouldn't even start with that kind of compression. Sounds like possibly plugged exhaust.
My F935 had similar problems on starting until I replaced all the glow plugs. Also had a collapsed air inlet hose and the valve clearences were way off. Easy to adjust valves, new air hose and new glow plugs. I bought a digital tachometer from Harbour Freight and found the the low idle rpm was too low and the high rpm limit was too low by the same amount. The air main filter in this machine seems to clog up very fast, too. Mine has sort of a vacuum gauge checker deal for this issue.
air filter was not an issue, all glow plugs are perfect and it fires on first hit. It just dies right away and you can restart it but its running very rough. I mowed with it again Saturday and it is now having enough power troubles to engage the mower deck. I can only think it must be right on the compression but it still puzzles me why it starts so easily.
JL Sargent
Silver Member
- Joined
- Jun 22, 2008
- Messages
- 114
Maybe the glow plugs are playing a big part in the easy starting. I would suggest a second compression test with a different gauge. If compression is low there is just no getting around it. 
**update**
I went over to a friends house and he has same mower, hooked up my compresison gauge to his and his readings were 170#. So my gauge is screwed...<---good thing. So since his mower runs fine I can assume mine is in OK compression range. This must be a fuel issue? another symptom I forgot to mention is when it starts and is running rough you can full throttle it and make no difference. So what are the symptoms of a bad pump? or should I just pull it off and the injectors and send them all in for testing?
I went over to a friends house and he has same mower, hooked up my compresison gauge to his and his readings were 170#. So my gauge is screwed...<---good thing. So since his mower runs fine I can assume mine is in OK compression range. This must be a fuel issue? another symptom I forgot to mention is when it starts and is running rough you can full throttle it and make no difference. So what are the symptoms of a bad pump? or should I just pull it off and the injectors and send them all in for testing?
clemsonfor
Super Member
Another thing not sure of this mower, but i guess it has a governer like our tractors, maybe the governer is screwed up? But isnt that part of the pump?
Help...engine starts and runs rough for 2-3 minutes without any response to throttle input. After it runs this way for that time it smooths up and runs fine. Shut it off and it restarts fine. Let it sit for a couple hours it still starts fine. Let it sit overnight and same rough start. It is like you have it at a low idle point where it cant smooth up. Governor??? I was chasing a fuel issue but am currently running a clear fuel line to visually see fuel is being delivered bubble free.
Still fighting this mower. Update,
New injection pump
new air filter
tested all 3 glow plugs OK
bench tested injectors OK right on pop off specs
fuel filter replaced
fuel pump runs to specs, runs over 7 oz in 30 seconds and above the 3 PSI
compression is good and even
fuel line to inj pump is clear and bubble free
Still fighting this mower. Update,
New injection pump
new air filter
tested all 3 glow plugs OK
bench tested injectors OK right on pop off specs
fuel filter replaced
fuel pump runs to specs, runs over 7 oz in 30 seconds and above the 3 PSI
compression is good and even
fuel line to inj pump is clear and bubble free