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 | Justin
Join Date: Posts: 1 
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2000-12-18 22561
I recently purchased my first diesel tractor.It has glow plugs and a decompression lever for cold weather starting. It starts fine only using the glow plugs. I know the decompression lever relieves the compression so the engine will turn faster. Here's my question: When should I use the decompression device? Also, should I use a diesel fuel additive so the fuel will not gum up in cold weather? Are these additives harmful to the engine? Thanks.
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| Bird Senter
Join Date: Jun 1999 Posts: 962 
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2000-12-18 22564
Justin, my tractor doesn't have a decompression lever, but it was my understanding that if it's equipped with one, it should be used each time you start the tractor to make it easier on the battery and starter. Maybe someone else can tell you more. There are a number of fuel additives on the market that not only prevent gelling, but also may provide better lubrication, moisture dispersal, keep injectors cleaner, etc. I use Power Service all the time, year round, in mine and know some folks use Stanadyne and other brands. Most truck stops would have them. Of course, if you have "fresh" diesel, especially #1 or a winter blend, in cold weather they may not be necessary. Personally, I don't know of any of the diesel additives that could have any harmful effects. ....
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 | Art White
Join Date: Jan 2000 Posts: 6898 Waterville New York 
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2000-12-18 22570
Justin there is no problem not useing the decompression lever but as Bird said it does make starting easier. By useing it you will be able to bring the rpm's up faster so the engine starts easier. On the fuel additives I highly recommend them in Central NY. We use them in our veichles and many of our customers also use them. ....
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| Justin
Join Date: Posts: 1
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2000-12-18 22580
Thanks for the answers. I have one more question: Will the engine actually start while using the decompression lever? If so, how long after it starts should I move the lever back to normal? This is one detail the instruction manual left out. Thanks. ....
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| Bird Senter
Join Date: Jun 1999 Posts: 962
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2000-12-18 22589
Justin, the decompression lever does just that; it reduces the compression in the cylinders to let the battery and starter turn the engine easier and faster to get it started. As soon as it fires, move the lever back for full compression. I don't know for sure how all the different systems might work, but the only ones I've been familiar with relieve the pressure by partially opening the exhaust valve I believe, and if you leave it running with that open, it not only doesn't run very well, and has no power, but you'll burn the exhaust valve. How about it some of you other guys? Is that your understanding of this engine? ....
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| Roger L.
Join Date: Jun 1999 Posts: 0
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2000-12-18 22596
Bird, both of mine have decompression levers. They work as you say, by holding the exhaust valve slightly open. You don't have to worry about the motor starting (or running) when the lever is pulled. They won't ever start when the compression is reduced by that amount. After all, a diesel is a "compression ignition" engine. If it doesn't compress then you have no fire at all.
There is one downside to the decompression levers, and that is that some people get into the habit of shutting the engine off with them instead the common method which is by pulling the throttle all the way to off. This is not a good practice. It doesn't seen to do much damage, but at the least it will end up stretching the decompression wires and levers. The thing that the manuals never seem to mention is that a decompression device is for holding a valve open BEFORE the engine is running. They are not built to open a valve against 500 psi of compression pressure! Rambling on....Turning a motor off is another way where diesels are unlike gasoline engines. On a gasoline engine you usually have the throttle set up so that all the way back on the throttle is the idle position and the engine is turned off by turning the spark off.. On a diesel, pulling the throttle all the way back cuts off the flow of fuel entirely and lack of fuel - not lack of ignition - causes the engine to quit running.
I like to use the decompression for the first start of the day. Here is how to go about it:
With the throttle pulled fully to the OFF positon, pull the decompression lever and hold it decompressed and then crank the engine over with the starter. It will really wind up, but this is fine since there is no unbalanced load on the engine. Just hold it there. I let the starter run until the gauge shows a tiny bit of oil pressure, or the idiot light goes out, or about 5 to 10 seconds pass - whichever is first. But I am hoping to see oil pressure. The drain on the battery is low when I am doing this because it is being held decompressed with the lever. As soon as you see any indication of oil pressure, quickly release the decompression lever and push the throttle to a little over half open - all the while the starter is still running and the motor is still spinning. It is just not firing yet. Now as you let off the decompression lever and advance the throttle, the engine compression will slow the starter motor down at about the same time that the throttle is advanced and she should fires right up. Of course when it begins to run I immediately let off on the starter switch and back the throttle down to a high idle.
The neat thing about doing it this way is that you are prelubing the engine before it ever starts to run. And also if you do it that way you never have to try to crank a cold engine up to starting speed......you already have the engine spun up and all of its spinning energy is available to help the diesel to start.
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| Bird Senter
Join Date: Jun 1999 Posts: 962
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2000-12-19 22604
Roger, of course I know the differences between the spark and compression ignition, but the only engines I've personally used with the decompression feature were gasoline, so they would try to fire and run a little with the decompression lever activated. I kind of wondered about that on the diesel; whether it completely decompressed or left enough that it would try to fire off. ....
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| Roger L.
Join Date: Jun 1999 Posts: 0
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2000-12-19 22609
Well, I kind of wondered where the guesswork was coming from....I know from the many posts that we share that you are quite knowledgeable on wide range of subjects. But on the diesel running with the exhaust valve open....Well, I couldn'resist pulling your chain a bit...for which I apologize.
I wonder why Kubota went away from the decompression lever? Or "compression release" as it is more commonly called...oops, more terminology. Is it still available as an option? They are tricky and take some thought to use, but it is handy to have the compression release for starting. In really cold weather they are a blessing. My own opinion is that having one and using it properly makes an engine last longer, but this is just opinion without a single fact to back it up. The only fact I know is that compact tractor diesel engines seem to last an awful long time. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2000-12-19 22611
Pre-oiling using a decompression lever sounds a little simpler than a fancy gadget that somebody mentioned. My experience with decompression levers is limited to chain saws and BSA 441's, which are gas engines. I guess a major use of the lever on a diesel is to allow easy positioning of the engine when making various adjustments. I don't have a cold weather starting problem, but it does sound like spinning the engine and then releasing the lever might make it easier to start. I believe that fuel cut-off used to kill all diesels, and use of a decompression lever for shut-down isn't good. However, the fuel cut-off mechanism may be different from engine to engine. I didn't find a specific 'kill switch' in a quick scan of my Ford's repair manual. I believe that fuel shut-off is achieved when the pump's control rod is at minimum. The implication for my Ford is that if the throttle linkage becomes out of adjustment, then the engine couldn't be shut-down except by turning off the fuel flow valve. That design idea seems a little shaky, and maybe I’ll find something else in the manual. Other tractors have 'kill switches,' which may take a more active approach to fuel cut-off. ....
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| Bird Senter
Join Date: Jun 1999 Posts: 962
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2000-12-19 22624
Roger, you can jerk my chain anytime; I might learn something (Lord knows, I need to sometimes). And like TomG, my experience with decompression mechanisms was pretty well limited to kick start motorcycles (shows my age, I guess). I don't know, but don't think any of the Kubotas have the compression release anymore. And TomG, I guess your tractor is like my B7100; the fuel shut-off was the throttle lever, while the B2710 now is shut-off with the key (electric solenoid like diesel cars and pickups). ....
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| FarmerWannabe
Join Date: Posts: 1
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2001-02-23 24582
I have an older 7200 diesel.
There is a knob/lever on the left that I use to shut the engine off.
Is this the decompression lever?
When I pull the throttle all the way back my engine IS in idle and continues running.
I don't have any way to shut this thing off other than pulling out this knob.
Do I have an alternative?
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2001-02-24 24643
Kill switches on a diesel usually are fuel cut-offs--the can be solenoids or manual valves. I believe that a decompression valve probably would kill a diesel, but it may not be a good idea if there is a proper kill switch. Using a decompression valve probably puts a few cycles of incompletely combusted fuel into the engine before it stops. You may be able to tell if the knob is a decompression valve by starting the tractor without preheat and at minimum throttle. The engine shouldn't start immediately. If you pull the knob while the starter is turning, the engine should sped up if it's a decompression valve. ....
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| FarmerWannabe
Join Date: Posts: 1
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2001-05-16 28270
Thanks Tom, I was using my decompresssion lever.
Seems that my kill lever just needed some WD40 and now it works great.
Thanks for the help. ....
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| Rick Cosman
Join Date: Posts: 1
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2001-05-17 28315
Well I didn't know I had a problem till I read this thread. I just put a new decomp. cable (factory OEM) on my tractor. I'm in Phoenix so the heat might be a factor but when i pull it out it starts with it out, as it fires I push it in. I'm afraid to adjust it for more decompression as The exhaust valve would be out further and could be struck by the piston (very bad) What do you'all think of that? Rick ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2001-05-18 28321
I think that what ever works is fine. I've never had a diesel with a decompression lever, but words are cheap I guess. The way I've heard of them being used is that the engine won't start with the lever out. The idea is to pull it out, which allows the starter to spin the engine faster, and then, push it in. The combination of high RPM and sudden compression is supposed to start the engine easily. You're probably right that the adjustment isn't releasing much compression. It probably does let the starter spin the some engine faster which helps. I'd be surprised if engineering design would permit an adjustment to bang a valve on a piston, but it's always good to check with a manual or dealer. There are plenty of stories about such things happening to old time 'hot-rodders' who did 'soup-up' jobs on the cheap. They sometimes put high performance cams in engines and also raised compression by milling the heads. In better jobs, proper high-compression pistons that were designed for engines with low head clearance would be bought. As you mentioned, the results were disastrous. ....
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 | Murf
Join Date: Dec 1999 Posts: 7249 Toronto Area, Ontario, Canada
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2001-05-18 28335
I think most 'kill' levers/knobs on diesels built in recent memory use a very simple system, far less complicated even than de-compression. The 'kill' mechanism merely opens a small valve in the injection pump and causes the pump to 'dump' the fuel directly back to the return line, and into the tank. No fuel, no run. The reason this system is preferable to de-compression is it leaves the injectors 'unloaded', that is to say there is no residual fuel pressure bearing on the injectors, lines, etc. Every injector I have ever seen will 'bleed down' over time, especially after they are a few years old, if this happens the fuel drains into the cylinders, and past the rings into the crancase and the oil, NOT a good situation. Best of luck. ....
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