s750 losing power

yellowdogsvc · Oct 16, 2012

I have an s750 M series loader. It's been running fine until today when I noticed that it was losing power when digging and driving and/or operating auxiliary hydraulics at the same time. I wasn't able to diagnose it and just replaced the fuel filter about 15 hours ago. I had just topped it off at the gas station with about 17 gallons of diesel. Machine only has about 500 hours and had a new fuel tank put in last year so I don't think I have debris problems.

I decided to have Bobcat come out. They checked my fuel lift pump and it was showing 4 to 4.5 psi which he said was adequate. We changed the fuel filter again, looked for water, etc., and found nothing. The only thing he could come up with was "bad fuel." This tech is especially sharp so I want to trust him but I have my machine sitting on my trailer running at 2k rpm trying to burn off fuel so I can try and fill it up in the morning. The fuel doesn't smell bad and I know it's relatively fresh (at least brought to the station recently). He told me to run the machine and burn off as much as I can in the tank and then go to a different station and get new fuel and see if it makes a difference. To me, the motor just doesn't sound right.. at 2000 rpm, the exhaust doesn't sound normal. It is faint but I can hear it running rough though the RPM's are steady.

On the off chance that someone sees this post and has had something similar happen with the bigger Kubota motors, could you share your thoughts? Does this sound like "bad fuel"? Thanks in advance.

SSdoxie · Oct 19, 2012

Any results on change of fuel?? Did it help?
I just had a thought that excaped me when I first read this, check the drive belt tension. Although rare, I have had couple similar problems and found the belt was slipping slightly, not enough to squeal but enough to reduce hydraulic performance and not pull the engine down far enough to force the governor to kick in to extra fuel. The adjustable stop on the tension idler should be about 1/4" clearance from contact to the idler when engine is off.

yellowdogsvc · Oct 20, 2012

SSDoxie.. didn't check the belt tension. Fuel didn't make a difference. The tech came back out and thinks he solved it by putting a new fuel lift pump on. He said it wasn't pulling enough vacuum (he had only checked the other side) on the old one. I still have to try the machine out but it sounded better and seemed to be more responsive at the shop.

This machine can usually climb a wall.. Having it bog down and just not "hum" right told me there was something wrong. Since I use this machine for mulching and lifting really heavy rocks, I can definitely tell when it's a power loss.
I will keep the thread posted. I appreciate the feedback!

SSdoxie · Oct 21, 2012

Did the tech check the fuel line back to tank and pickup tube to make sure there was no debris stuck in it, especially at the 90 degree fitting going into the tank? That type of failure in the lift pump makes me think there was something stuck in it holding the check valve partially open.

yellowdogsvc · Oct 21, 2012

About a year ago, I had a fire. First one in 16 years and I always keep my machines clean. Freak thing. The fuel tank was replaced but when I got the machine back I went to drain water out of the fuel filter as part of my morning inspection and debris came out. They hadn't changed the fuel filter though my old tank had been contaminated. Your theory makes sense and if it acts up, I will bring that back up however it has run over 300 hours with no problems until last Monday. It's possible it was slowing getting worse and until it failed, I didn't think much. I do not dig very often so I don't get to push the machine. I generally run the mulcher at wide open throttle or I'm grappling brush which is variable rpms and not that hard on the machine.
SSDoxie, you really know your machines and I'm sure a lot of us appreciate you being on the forums.

yellowdogsvc · Oct 22, 2012

Back to the drawing board. Started the day with machine running fine. had 3/4 tank of fuel. Right about the time I hit just under half a tank, it started acting up again. Three guys on jobsite stopped me to tell me my exhaust sounded bad. Was making that high rpm sputter sound but RPMs were steady except when I open or closed my grapple or tried to turn the machine. Then the rpms would drop by up to 1000 RPM and the machine would bog down. I've run the fuel down to where the fuel warning stayed on for an hour. I brought the machine back home, opened the drain on the fuel filter and checked for water or debris. Not much out of the ordinary.
After opening the drain on the fuel filter, however, the machine wouldn't start. Had to prime it. It did this the other day but has never done that in the past simply from opening the drain.

I guess my next step is to drain what's left of the fuel in the tank and have them check the lines. If it was an injector, it would run rough or have a power loss all the time, right?

I checked the belt as best I could and it seemed tight. Service manager seems to think it is still something with the fuel. Nothing smells out the ordinary but the exhaust doesn't sound right. The only coincidences are that the muffler was pulled last week to install a guard that had broken free. I already asked the tech about the muffler causing problems and he said there isn't anything that could be restricting it and it just bolts back on.

I have run an entire tank of fuel through this machine since I supposedly got bad fuel. I used a glass jar to capture fuel this afternoon and so no evidence of water or other contaminants. Primer bulb doesn't collapse, either. Could this be an air in the system issue?

Thanks for any help.

SSdoxie · Oct 22, 2012

Check your fuel cap, make sure the vent is open. It should hold slight pressure when blowing through from tank side but draw air easily when sucking on the threaded end and you should hear the vent open and close when sucking or blowing. Not the nicest way to test it (tastes like crap, but is quick and simple). If it forms a vacuum in the tank, can't draw adequate fuel and when you open filter drain it gives air chance to draw back inot tank forcing need to prime the system.
Sounds like something a new fuel cap will resolve.

yellowdogsvc · Oct 22, 2012

Just tried it. It doesn't pull air easily compared to the fuel cap on my toolcat which is parked next to it but I can hear the vent closing and some air moves.

What I find strange is it's run fine for 300 hours since my fuel tank was replaced. If debris was in the lines, it surely would have moved through or gotten stuck by now. It's possible that it was bad fuel and something heavier than diesel is at the bottom of the tank whether it's debris or water but I have found no evidence of water. I have refilled twice so far.

I can try switching fuel caps with my excavator or toolcat and see if that makes a difference. I can still drain tank just in case and check all lines.

I guess I can have them look for compressed fuel line too that might not be letting enough fuel flow to the machine. But again, it only does this after I ran it half a day or so it seems. And that brings me back to the fuel cap.. running it half a day is enough time for it build up a vacuum.?. later, the vacuum escapes and it runs fine for awhile until it happens again. Does that sound plausible?

SSdoxie · Oct 22, 2012

yellowdogsvc said:
Just tried it. It doesn't pull air easily compared to the fuel cap on my toolcat which is parked next to it but I can hear the vent closing and some air moves.

What I find strange is it's run fine for 300 hours since my fuel tank was replaced. If debris was in the lines, it surely would have moved through or gotten stuck by now. It's possible that it was bad fuel and something heavier than diesel is at the bottom of the tank whether it's debris or water but I have found no evidence of water. I have refilled twice so far.

I can try switching fuel caps with my excavator or toolcat and see if that makes a difference. I can still drain tank just in case and check all lines.

I guess I can have them look for compressed fuel line too that might not be letting enough fuel flow to the machine. But again, it only does this after I ran it half a day or so it seems. And that brings me back to the fuel cap.. running it half a day is enough time for it build up a vacuum.?. later, the vacuum escapes and it runs fine for awhile until it happens again. Does that sound plausible?

Sounds very plausible. Fuel cap is same on all 3 machines, try 1 of the others then get a new one if needed.
PS: ran into almost same scenario last week and cut the fuel cap in half, was suprised at amount of dirt/dust collected around the vent check.

yellowdogsvc · Oct 22, 2012

given that I mulch and we've been in a drought until recently, I can picture the dust. Like I said, the toolcat's cap was easy to pull air through. Not so much with the S750's.. Thanks again. I will try it out again tomorrow.