Mack DM stalling (fuel problem?)

[Lucky dogg]

I have a 1995 mack DM.  It is an oil and gas service rig.  The mack motor runs the service unit body also.  Recently, I sent the mack to a local mack garage bc it had a loss of power and was stalling.  They told me it was the charge air cooler and fixed the air compressor unload valves.  This did not solve the issue and they had it over a week.  The truck starts and runs.  It idles fine, does not have a miss, and the service unit seemed fine also.  We drive it down the road and it acts like its out of fuel and stalls.  It takes several cranks to get it back started also.  As time progresses it is getting worse and worse.  I had the crew do the normal (changed fuel filters, air filter, check battery cables, checked fuel tank cap, pick up tube in tank, ect).  We need this rig to run bc unlike a normal semi, my 3 guys who work on the rig daily go with out a check when its broke.  It is an econodyn.  Fuel linkages are manual and does not appear to have a computer on the frame.  Please help as we are not sending it back to mack unless it is absolutely necessary.  Thanks all! 

[j hancock]

Install a pressure gauge into the fuel line and check that the fuel transfer pump is getting at least 20 psi output to the injection pump.  The transfer pump may be getting tired or the fuel hose may have deteriorated internally and causing a flow restriction.

[Lucky dogg]

Thats the first thing we will try Monday.  Thanks.

[Rob]

While this is a good suggestion you need to go further. Plumb your gauge into the system and measure the pressure at low idle; then measure the pressure at about 1800rpm. Then with a helper monitoring the pressure gauge and the brakes set on the truck, slightly accelerate the engine while slipping the clutch to load the driveline. Not hard enough to smell the linings get hot but enough of a draw on the engine to load it. Ensure your monitored fuel pressure remains above 18psi at all times. Continue to monitor this for about a minute then let off, shut down, and allow things to cool about 15 minutes. At the end of 15 minutes, look at the pressure gauge without starting the engine. If it is a zero reading, consider rebuilding or replacing the transfer pump mounted to the side of the injection pump. Chances are the valves internally are compromised, or the piston is worn in it's bore which there is no field repair available; only replacement.

Use a liquid filled pressure gauge or it will not last due to engine vibration.

 

[fjh]

I have a gauge built with a long whip on it 6 ft or so put it thru the vent window and drive it to get (REAL TIME) DATA in todays lingo ! Poor mans dyno!

Drive around the block in a higher gear foot on the brake and throttle to the floor the gauge as stated shouldn't drop much below 20 under this condition

if so you have either a restriction bad hose ,Bad pump    OR (A TANK VENT PLUGGED)     the last thing there is commonly overlooked ! Got burned on that the odd time! that also will show up on the gauge it  will fluctuate! 

symptom of that is similar to yours runs good for fifteen minutes or so then dies out!

just something  else to look for.

[Rob]

That is the way I t/s also in real time. I do prefer the remote gauge via hose myself but I don't want to get the gent stuck alongside the road someplace while trying to isolate. Plugged tank vents, swollen, or compromised hoses on older equipment, and worn parts are all possible contributors. Basically if the transfer or lift pump pressure cannot be maintained under loading and falls off, the fuel volume delivered to the injection pump gallery is also falling off and the engine will starve which is seemingly the scenario experienced.

Easy enough to remove the suction hose on the inlet side of the primary fuel filter and blow compressed air through this line listening for gurgling in the fuel tank. This ensures the fuel line is not plugged or restricted. Most R models had lower sumps in the bottoms of their fuel tanks and would be a good idea to drain this low point to remove any contaminants too. Physically look at the lines to ensure they are dry. If they are leaking any fuel, they are also allowing air entrainment of the fuel supply which is not good.

I'd wager he'll be replacing the transfer pump from my past observations. PAI still markets them so not so bad in cost and easy to change in the driveway if need be.....

[theakerstwo]

Any diesel needs the fuel pressure tested under load.I have a tee i made for putting inline before the injection pump that i made in the mid sixtes and still have it.I miss placed it a few times but always find it and yes a #4 hose will connect to it and you connect it to a wiper arm.I have tee i made also that will go in some of the suppy lines between the filters and supply line with a vacumin gauge connected to it.If you have a power problem that fiest thing i use and some times it will take a few miles under load to slow you something that cant be seen by just running the engine up.

[Rob]

Exactly. Seat of the pants feel usually means nothing if not backed up with analytical data resolve. Can't do that without test equipment properly calibrated and the output information interpreted correctly. Testing diesel fuel pressure at low idle or no load at high idle will prove nothing as the fuel loading/draw in minimal. Best to throw 20K load onto a truck and drive it hard while watching test equipment for interpretation. Lots can be revealed this way.