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RPM drops after throttle opened

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Paul_in_NS
Join Date: Jul 2003
Posts: 7 Halifax, Nova Scotia
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2003-01-26          48195

I have a 1988 JD855 with 1700+ hours on it. Since I got it 2 years ago it has had an odd hesitation when opening up the throttle. The revs increase as you would expect and then sometime after a few seconds to about a minute they drop back again. Advancing the throttle further accelerates the engine without any further hestitation or drop in revs.
Back in the summer I tried a different board (hadn't found this one yet) and got afew suggestions which I followed. I have changed the air filter and fuel filter. I also checked the throttle linkage which appears to be working smoothly and the handle does not back off as the revs drop. After all that the engine behaves exactly the same way.
Anybody have any insight or suggestions for this?


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RPM drops after throttle opened

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Jeff Earthwerks Unli
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2003-01-26          48203

Does it have lift or transfer pump which could be electric or mechanical?

What color and density is the exhaust during the event?

At "first blush" sounds like it could be a faulty injector pump or worn out or leaking injector(s). ....

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RPM drops after throttle opened

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TomG
Join Date: Feb 2002
Posts: 5406 Upper Ottawa Valley
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2003-01-27          48236

If I understand the problem, there is a hesitation and then some drift near a particular rmp and the problem disappears at higher rpm's.

Jeff's comments seem a pretty good bet especially if there's abnormal exhaust. I'm off on a slightly tack for the moment. I wonder if the tractor holds a steady slow idle (800 rpm or less) and if the max no-load rpm is in adjustment.

I think I'd check if there is service routine for the governor that is appropriate for owners. Many repair manuals say that governors aren't serviceable except by specialists. Some manuals indicate that periodic lubrication of the governor weight pivot points should be done.

The difficulty here is that the problem may be what my JD FOS Engines manual calls 'speed drift,' which is a governor problem. It would be good to eliminate that problem from consideration before chasing exotic possibilities, but it might take a diesel shop mechanic to say the governor is OK for certain.
....

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RPM drops after throttle opened

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Billy
Join Date: Oct 1999
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2003-01-27          48241

"The revs increase as you would expect and then sometime after a few seconds to about a minute they drop back again. Advancing the throttle further accelerates the engine without any further hestitation or drop in revs"

This part sounds like the friction disks need to be tighten on the throttle. Even though you say "the handle does not back off as the revs drop", it would be so slight I doubt you could notice.

Billy ....

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RPM drops after throttle opened

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Art White
Join Date: Jan 2000
Posts: 6898 Waterville New York
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2003-01-27          48244

Yours is not a major problem yet unless it is a 500 rpm drop. I do beieve it is just a lazy govenor. Many engines will run off a couple of hundred rpm when you do what you said. If you were to gradually bring it to full throttle slowly increasing it will it still do it? ....

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RPM drops after throttle opened

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Peters
Join Date: Feb 2002
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2003-01-27          48264

My 955 would do the same If I was at PTO throttle rpm. I was convinced that it was the friction disks as at max throttle it would back off slightly and I would have to move it back up. (955 is the same tractor with more HP).
Peters ....

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RPM drops after throttle opened

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Paul_in_NS
Join Date: Jul 2003
Posts: 7 Halifax, Nova Scotia
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2003-01-27          48268

Thanks for all the the responses.

Jeff - There is no lift or transfer pump as far as I know. The exhaust just makes the usual little puff when accelerated but nothing at all when it drops. Most of the time there is no noticable exhaust except when it is really lugging (not often).
TomG - The manual calls for an idle speed of 1450 which is what it is. It holds that jsut fine.
Based on the other comments, I think I am going to start with the throttle friction plates before getting involved with the governor options. I've lived with it this long I think I will finsih the winter as is and get to it in the warm weather. I just spent 5 hours clearing 4"-6" of slushy snow off the driveway (1 kilometer long) with the loader, the snowblower wouldn't touch it.

Paul ....

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RPM drops after throttle opened

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TomG
Join Date: Feb 2002
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2003-01-28          48306

Paul: I know the old slush out the chute problem. Fortunately it hasn't happened here this winter, but there is freezing rain forecast for Saturday. Generally my blower will throw slush 10' or so and doesn't clog too often unless I clutch the pto before the auger is cleared. I've never tried various 'non-stick' sprays you sometimes hear about. Usually my loader does the job OK when the blower doesn't.

I asked about the slow idle because a gimpy governor sometimes will show up at slow rpm's. Many manuals spec for both max and min no-load rpm's, and each can be adjusted. 1450 rpm sounds like what is called a fast idle for many engines and a min no-load would be less. Many diesels won't warm up at much below 1500 rpm or so.

I think most engines can be expected to hold steady speed at lower rpm's and there may be specs in repair manuals rather than owners' manuals. It's often good to give an engine a few minutes cool down at low rpm's after hard runs.
....

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RPM drops after throttle opened

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Jeff Earthwerks Unli
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2003-01-28          48317

Lift or transfer pump: From my experience there are two fuel delivery system types:

a. Lift or transfer pump which ususally indicates that the fuel tank is lower than the fuel system and thus needs to "pushed" or "pumped" or "transfered" to the injector pump. The pump may take the form of the old-fashioned lever-and-diaphram type fuel pump driven off the camshaft.... or....an electric motor/pump assembly or an in-line solenoid push/pull type

or

b. Gravity feed whereby the fuel tank is higher than the injector pump therefore not requiring pumping per se.
The fuel line is plumbed to the primary filter.


....

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RPM drops after throttle opened

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Art White
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2003-01-28          48327

I would be curious as to if the lever is actually coming back. Many of the diesel engines will surge out beyond on the govenor and then settle back. The newer stlye pumps normally do not do this. Many will do this and I wonder if any action is to much unless the lever is actually moving back. ....

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TomG
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2003-01-29          48354

I'm curious about it too. I imagine the lever Art referred to is the lever on the injector pump control shaft rather than the hand throttle arm. Governor linkages also have a few levers.

The pump throttle control arm is moved by the governor linkage in response to changing loads or hand throttle positions to change the throttle position and keep constant rpm. If the governor is pulling back the engine after throttle increases, then you should be able to see the control arm on the pump move. If the arm doesn't move then the engine has surged above the governor and is just settling back. Of course, it might be kind of tricky to see the lever if the problem only happens when driving.

It's something that probably can wait till spring. The problem may prove to be the hand throttle friction plates or other problem, but most governors have several adjustments, and there are several springs that can become worn. I think I'd lubricate all moving points on the external linkage (but not the hand throttle friction plates as mentioned) and inquire from a dealer if replacing the springs or other governor maintenance are appropriate owner maintenance procedures. The adjustments aren't ordinarily owner procedures.
....

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RPM drops after throttle opened

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Art White
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2003-01-31          48461

Tom many govenors will surge some. Some injector pumps are more noted for it. It is internal not external and cannot be adjusted out. Quite often to open a tractor to full throttle they will exceed the recommended rpm and than fall back to where they are rated. It could be the friction discs in the control lever that holds it in place,but you should see the lever move back and chances are it will do it at any position. Some dics are adjustable often by tightening a bevel nut on the bottom of the linkage. ....

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RPM drops after throttle opened

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TomG
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2003-01-31          48475


Art: I believe that as you say some governor pump combinations do surge and not much can be done. However, governors are matched to pumps. For mechanical types, spring tensions and governor weights are adjustable in the sense that pumps are calibrated.

According to my JD FOS manual, some governors have adjustable speed droops. Speed droop reduces overshooting and may improve general stability of the engine. Adjustment may reduce the problem if the governor is overshooting. The speed droop adjustment changes the length of governor and fuel control rods so more or less of the full governor range of travel will change the throttle from min to max.

Of course, none of this means that us owners or even basic dealer techs can or should go out and adjust their governors. Mechanical governors also are far from perfect in their governance and as mentioned there's likely going to be some surging and hunting going on no matter what is done.

....

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RPM drops after throttle opened

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bullworker
Join Date: Feb 2002
Posts: 22 western mass
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2003-01-31          48478

did you happen to run this tractor out of fuel at any time?air trapped in the fuel system can cause many problems including power loss, fluctuating rpms, bleeding the system allthe way to the injectors may help. ....

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RPM drops after throttle opened

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bullworker
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2003-01-31          48479

just another thought, air can also enter the system through a worn out fuel line, almost imposible to find. changing any suspicious fuel lines may help. ....

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RPM drops after throttle opened

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Paul_in_NS
Join Date: Jul 2003
Posts: 7 Halifax, Nova Scotia
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2003-04-09          52873

Well, I actually got enough warm weather to tackle the ideas you guys offered here. It turned out to be on the simple end of the problem. It was simply that the machine screw that clamps the end of the throttle cable to the actuator arm of the fuel pump was loose. Of course, I managed to shear the head off tightening it and, since it is metric, had to go to the hardware to get a new one. There was enough of a kink in the throttle cable that it operated the lever through most of its range of motion but the cable was able to slip back and forth through the clamp about 3/8".

At least it was cheap, 45 cents and I now have 9 spares!

Paul ....

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