1993 6.5 DB2 mechanical pump. Noticed an issue on a highway drive yesterday. Searched on the 6.5 forum and found quite a few threads with the same symptoms but no answer as to the cause. If I crested a rise on the interstate at 65mph and backed off the pedal so as not to increase speed down the other side, when the rpm's got to about 2000, the fuel would surge/hesitate/surge/hesitate with no change in the pedal position. Symptom would disappear if I let off the pedal completely and it never happened when accelerating or holding a steady speed on the flat or uphill. It only happens when unloaded on a slight downhill. It's not a big problem but it's irritating. Anyone here have any idea about what might be happening? It's almost as if the connection between the pedal and the IP has slop in it and it's causing the IP to oscillate between adding fuel and backing off.
There are threads on the 6.5 forum with the same symptoms but with the electronic pump. Everyone blames the PCM when it happens then. But with the exact same symptoms on a mechanical pump, I'm thinking something else has to be going on.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Is there something on the Linkages of these Pumps that slows down the travel of the Linkages so it does no cut the Fuel Back fast?
Old Gasser Cars used to have a Dashpot on the Carburetors of Cars with Automatic Transmissions for that purpose.
Do you have an Automatic Transmission.

What operates the EGR on Your Vehicle?

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84 300D, 82 Volvo 244Gl Diesel

I'll have to look and see. I haven't heard of anything like that. The electronic pumps are drive by wire but the DB2 is a mechanical connection.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I think 6.5's have basiclly the same stanadyne pump as 6.2's. (what I have). It is refered to as a 'min - max' fuel governer (inj pump). When you step on the 'gas' it accelerates at maximum throttle till reaching your set (where your foot is) throttle position. when you back off the pedal, throttle drops to idle till reaching that 'foot position' again. Maybe its possible that its surging at a certain speed, trying to achive some sort of speed/throttle balance.

I took a class in diesel maintanence / tuneup etc. from the local jr college back in the 80's after getting screwed by the local chevy dealers, about 80 bucks in parts (1988+-) 3-1/2 hrs actual labor time (in in the morn done before noon) 850 bucks. Screw them 'forever'.

The 6.2 pumps are EASY to rebuild. Gettin' 'em off & back on is the big trick.

edit

Incidently, with all that linkage between your foot and the pump, ALL you are doing is compressing a relatively small spring in the top of the pump

Yes, it is roughly the same pump as on the 6.2. It does seems as if the surging is an attempt to reach some kind of equilibrium in speed/throttle.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Surging at a neutral throttle on that pump usually suggests weak governor springs.

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1984 300SD turbo 126
"My true love"

God made me an atheist and who am I to question His wisdom

I guess that suggests a rebuilt pump unless there's some exercise program for governor springs.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I'm not an expert but I also have a '93 truck with a 6.5 and I've been reading up on it. The truck will run without the lift pump - the IP alone can draw enough fuel - but that that puts a strain on the ip. It is recommended to check for good lift pump ressure as a means of protecting the operation a nd the life of the IP. The lift pump gets its current through contacts in the oil pressure sensor and that is a weak link for current flow. I plan to put in a new oil pressure sensor and also install a relay for better current to the lift pump (while still maintaining the safety feature of the oil pressure sensor shutting off the lift pump).

"EASY to rebuild"; people keep throwing that term Rebuild or Rebuilt around to the extent that it is extremely misleading.
When you are done rebuilding something everything is supposed to be setup just like the Fuel Injection Pump was new.

Just like with the Turbochargers People say they rebuilt. When someone goes in and changes the Bearings and the Seals on a Turbocharger that is not a rebuild. Nor is taking the Engine apart and putting New Rings on the Old Pistons and installing new Bearings and Seals rebuilding the Engine Block.


During a Rebuild of a DB2 besides replacing any worn parts they would check to see if the Bore or the Advance Piston was worn (and reamed or honed to the next oversize advance Piston) and for wear on the Housing where the Cam Ring Makes contact (if deeply grooved the Pump Housing needs replacing).

The sort of Heel area on the Governor Weights would have been checked for wear and if needed replaced; the same with the Rollers and Shoes.
After the worn parts have been replaced it is on to the calibration of the Pump on the Test Stand where the Lift Pump Pressure, advance, and Fuel quantity would be checked. Not to mention the Governor adjustment. And, so on.

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84 300D, 82 Volvo 244Gl Diesel

I rememberd this while I was writing all that stuff below. Just like on Mercedes the Tank needs to be vented or it caues problems with Fuel Delivery.
On the Chevy Diesel cars in the late 1970s and one Truck the Tank Vent at that time was in the Fuel Fill Cap and something got inside of them and plugged them up.
On the fist one I discovered that by accident. I was trying to figuer out why the Car was running poorly but the Fuel was low and put Fuel in from a 5 Gallon Can; forgetting to put the Tank Cap on.
I started the Vehicle and it ran fine.
My Boss started questnioning Me as to what I did and He guessed it was the Fuel Cap (which had the tank vent inside). Sure enough when the Cap was put back on after it ran a while it started having issues again.


The OP should try driving with the Fuel Fill Cap loose or removed and see if it solves the problem.


The reason for the 2 pieces is there is an adjustment with a small Special Tool to set the "Governor Gap".

It is not unknow for that screw to become loose enough it allows the forward (on the Vehicle it would be towards the rear of the Vehicle) end of the Linkage to move around and that is connected to the Metering Valve so that would mess with the Fuel output of the IP.

The last time I worked on a DB2 was sometime in the late 1980s.
The Pump is designed to have a certain range of Fuel leak up into the top area and there is a Valve to keep the Pump Housing at a specific pressure.

However, as the Metering Valve wears and the Head in the Pump wears, the Advance Piston Bore wears and there is sort of an oriface with a Wire in it so the Pump is self air bleeding and over timey you get more leak back the you are suppsed to.

Nearly all of the IPs I rebuilt had more than the specified leak back and I would have to change the Oriface I spoke of to reduce the volume of leak Back.

The importance of the leak back is that if the Volume of leak back is too much to pass out of the Housing Pressure Valve/Fuel Return Valve at the top of the cover the Housing pressure can get too high and it effects the IP advance and the amount of Fuel.
You can shut the pump down Fuel on those IPs by putting your thumb over the Housing Pressure Valve/Fuel Retun Valve.

Also any restriction in the return Fuel Hoses can effect cut the Fuel off.
Also if there is a problem inside of that Valve it effects the Fuel amount going out of the Pump.

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84 300D, 82 Volvo 244Gl Diesel

I have a fuel pressure gauge and it shows the lift pump is producing a consistent 7-8psi all the time, including when symptoms occur. The tank cap vent is an interesting hypothesis. It does have the diesel labeled cap which is supposed to vent but I do hear a hiss when I remove it. The fact that I noticed the symptoms on long drive gives credence to the idea that it could be caused by vent problems. I'm out of town for a couple of weeks so I won't be able to experiment to see what happens with a loosened cap.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

when you get a chance, remove the o-ring on the cap, and see if it still does it.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
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