VP44 Troubleshooting

Quick Reference

When doing the following tests, please follow the directions exactly, as they are tried and true and proven accurate from our experience. We want you to get accurate results!

Miss or buck

This usually only happens on really old versions of our FMS boxes and can be diagnosed by shaking the box in your hand and if you hear a rattle, then it PROBABLY has melted the power resistors off of the circuit board. This can be repaired and upgraded to the newer style for about $65.00, plus return UPS shipping. Please call for unique address for where to send it for repair.

Intermittent Miss

This symptom is RARELY caused by an electrical problem with the Injection Pump. It can be a mechanical issue inside the pump caused by WVO or Biodiesel damage or even a blown head gasket. First, be sure that there is no air in the fuel supply and no relevant codes by doing the appropriate tests. If a relevant code exists, like a 336 or 1690, replace the crank sensor first to be sure to eliminate it as the cause of the skip. If you have a 251, 253, 1688 or 1689 code these only pertain to the Injection Pump and are strong indications the electronics in the VP44 are BAD, and that the pump is bad. With one or any combination of those codes, and an intermittent miss, I would replace the pump ONLY AFTER DOING THE TESTS BELOW. A 216, 234 or a 237 code CANNOT cause a skip or miss, so they are not relevant to THIS symptom. If you want to prove the skip is caused by an electrical issue THE MOST ACCURATE WAY, hook up an oscilloscope to the ground side of the fuel solenoid and watch the duty cycle, which is the solenoid energized or closed time. The wire you want to find for the scope probe, is the wire closest to the engine in the top pair of wires coming out of the injection pump top cover plate. This is the same wire used to attach performance devices. You MAY have to increase and hold the RPM at the point to where it is missing, to see if the pulse width changes erratically on the scope. If the pattern on the scope looks different on the oscilloscope when the skip is occurring, then look at the APPS signal wire, which is the blue with a black tracer in the APPS plug, with the oscilloscope to see if that signal is inconsistent, or “dirty” when the skip is evident. If it is, that would prove the VP44 is being told to do the skip because of the APPS signal. If the APPS signal is OK then you have to condemn the VP44 computer and replace the pump.

If you don’t have access to an oscilloscope, and the skip is ALSO evident at idle, and you do NOT have air in the fuel supply, do the Hot Wire test above, and if it idles fine, it means the VP44 is being told to skip. Then you need to diagnose the APPS as best as you can with an analog voltmeter as explained in Dead Pedal, looking for needle quiver at the same frequency as the skip. A scan tool or a digital volt meter DOES NOT WORK for this test, because they have too much averaging or buffering in them to indicate the problem dependably. If the skip only occurs at higher RPMs, then you have to hold the throttle at the point where the skip is evident, and then watch the needle for any quivering. If NO quiver, it most likely is not a bad APPS, and if you have codes that could explain the problem, it most likely is a bad Injection Pump.

Another way to narrow down the possible reasons for the miss or skip WHEN THE ENGINE IS SKIPPING, is to loosen each injector line, one at a time, until you find one that makes the RPM or sound of the engine change LESS than another. The less the change, the less that cylinder is contributing, indicating that that is the problem cylinder. If the skip appears to move from one cylinder to another while doing the crack injector test, that is what I call a rolling skip and if it is NOT an electrical issue, like a bad APPS, it means it is most likely a mechanical issue inside the injection pump, like a sticky piston in the rotor from contaminated fuel, and therefore means a bad Injection Pump. This is why you have to do the APPS test to condemn the injection pump. If it is consistently one cylinder, it might be explained by being a lazy delivery valve on the Injection Pump, but I’ve never heard of it in a VP44, and it may be caused by some mechanical problem in the engine. If you think you have a bad injector, which is virtually unheard of with OEM injectors, swap the indicated cylinder’s injector with the one next to it and redo the test. If the problem moves, it is the injector. If it doesn’t move, it is something else.

Idle Skip

Do the "Hot Wire" test #3 above and if the engine runs smoothly and fine at idle with the big plug disconnected from the injection pump, but runs badly when it IS connected, then you KNOW that the pump is being told to run badly by some other component of the fuel system, UNLESS YOU HAVE AIR IN THE FUEL SUPPLY. Do the "Air in Fuel Test" above for sure if it still runs badly when hot-wired, to be sure it is NOT air. Typically an erratic skip above idle is caused by a "Dirty" or erratic signal from the APPS. Do the "Dead Pedal" test above to see if the skip AND the needle quiver at the same frequency as the skip. If so, then clean the connector to the APPS or replace the APPS


Before doing the voltage tests below, be sure all the silicone boots are securely fastened to the intercooler plumbing, and that you aren’t just experiencing a boost leak somewhere. Don't assume they are fine just by looking at them, as they may be delaminating. Try to twist each one on the pipe to be sure. Look at the intercooler itself, looking for oil stains anywhere in the plumbing indicating a leak. Check the o-ring at the turbo outlet elbow. Check that the sealing plugs under the heating ribbon electrical connections at the intake tube are both there, and check all intercooler plumbing for any signs of a leak. Often, but not always, a leaky intercooler system will make a whistling sound under load, which is a really good indicator of a leak!

RPM Takes Off Without Throttle Application

A few callers have remarked that the RPM goes up on its own, typically above idle, so we have them check that the voltage on the blue with a black tracer signal wire (on Dodges) coming from the APPS, doesn’t go up when the RPM goes up, which confirms it is NOT the APPS. Do the same test if the engine tries to run away as soon as you step on the throttle. No it won't blow up. This test is described under “Dead Pedal” above. If the idle speed goes up on its own, or the RPM runs away when revving up off idle, AND you have a GOOD APPS, then you do have a bad ECM. If it revs up on its own from an idle, you can also do the “No Start” test three, and if it idles smoothly and at the same speed dependably, then it means the ECM is the cause of the problem. If it does it above idle, you'll have to read the APPS signal voltage when driving.

The other strange symptom or indicator of a bad ECM is when the truck won’t start until the wait to start light goes out, or it comes on when driving, or when it shouldn’t.

New for 2012 is a problem where the sensors that get their supply voltage from the ECM are lower or higher than the required 5.0 volts. SOME of the time a bad ECM with any of these symptoms will set a 606 code to make you feel more confident in your diagnosis!

Another new one for the summer of 2012 is delayed start, caused by the ECM delaying the signal voltage to the fuel system relay, which is described in "Hard Start Hot......" above. Do that test and you will know if the ECM is to blame.

These are the only four symptoms that I have seen or heard of, SO FAR, that necessitates replacing or repairing the ECM. The good news is that these unique symptoms have been eliminated every time by repairing or replacing the ECM. You only need one of the symptoms described above to determine that the ECM is bad. The only other way to diagnose an ECM is to try a replacement, if diagnosing a Dodge. It doesn’t seem to matter what transmission, year or engine rating the test ECM comes from, with or without a crank sensor, as long as the above symptoms go away with the borrowed test unit. I have had many callers do it this way, so I feel confident you won’t hurt anything as long as you remember one thing, PLEASE. When installing any ECM, be SURE to ground it to the engine FIRST, before connecting the big plug. This prevents any problem from static electricity or a voltage spike getting into the ECM which can blow away the software and or computer inside. Yes the test ECM may set codes, but if it doesn’t have the above symptoms any more, then you know a replacement or repaired ECM is in your future. I don’t have any experience with Ford or Freightliner ECMs, which may indicate they are better quality, but they are very different and not interchangeable with Dodges as they have twice the number of wires going into them.

Dead Pedal

This is THE MOST COMMON DRIVABILITY COMPLAINT and is an intermittent one that happens most often when the truck is hot or working harder, but can occur when cold too. My experience tells me that 4 times out of 5, Dead Pedal is worse hot, but 1 time in 5 it is worse cold! The symptom of Dead Pedal is rarely caused by the APPS (Accelerator Pedal Position aka Throttle Position Sensor) and 90% of the time it is caused by a faulty computer on the top of VP44 Injection Pump. These numbers are NOT an exaggeration. Computer failures are due to the “Lead Free” solder connections on the circuit board in the computer becoming crystalline over time, which causes an intermittent electrical connection and intermittent Dead Pedal symptoms. Its use is mandated by the Federal Government!

There are no codes that specifically diagnose Dead Pedal or that will condemn the computer and therefore the VP44. This is an instance where a lack of codes is most important, and where you have to prove that the only other component that could cause this symptom, the APPS, IS or IS NOT the cause of Dead Pedal.

The lazy inaccurate way to diagnose the APPS or TPS as the cause of this drivability issue is to scan or read the ECM (not the PCM) to check for any codes pertaining to the APPS, such as a 121 or 122. Codes 121 and 122 only indicate that the voltage going in or coming out of the sensor was outside of the desired parameters, at least once, since the codes were last cleared. Therefore these codes do NOT tell you what happens to the signal when they ARE within appropriate parameters, which is what really matters. If you DO have either or both of these codes you MAY OR MAY NOT need an APPS. To diagnose the APPS accurately you need to use an oscilloscope or an ANALOG voltmeter, one with a needle, to measure and monitor the signal voltage on the blue wire with a black tracer, on a Dodge, in the APPS electrical harness plug. A scan tool or a digital voltmeter has too much averaging or buffering of the signal to be useful for this test. First verify the appropriate voltage range and voltage apply rate with the engine off. Turn the ignition key to the “on” position and slowly press on the throttle and slowly release it. You should see voltages from about .6 volt to 3.5 volts, and not ever see a jump in voltage, or the needle bounce. It should go up and down smoothly, directly related to throttle movement. If it repeatedly or intermittently jumps up or down, then replace the APPS. The adjustment of low voltage at idle, or “resetting” or “recalibrating” the APPS is NOT as important as some people want you to think, and does NOT cause Dead Pedal. The ECM learns the range when you do the install of the APPS correctly and does NOT cause any drivability issue in my experience.

As this sensor can be very intermittent, I strongly suggest you ALSO do the same test when driving the truck to prove the APPS is or isn’t the cause of your Dead Pedal OR DRIVABILITY issue. Extend the signal wire used in the previous test up to the dash of the truck, hooked to the analog voltmeter, and drive it until Dead Pedal or naughty thing happens and look at the voltage on your voltmeter. If you are holding the pedal still and the voltage drops when the engine drops power, or the needle quivers at the same frequency as the stutter, skip, or miss, you need an APPS. If the voltage stays the same and the power drops, you need an Injection Pump!

Truck Dies Going Down The Road

Another frequent VP44 failure is when the truck dies driving down the road for no apparent reason, or when you let off the throttle at high RPM, and the engine won’t restart. This is usually a seized rotor in the Injection Pump and is most common on 1998 and 1999 trucks, or rebuilt pumps that don’t have the upgraded rotor and distributor. The cause of this failure is a poorly "de-burred" rotor according to Bosch. This machining problem has been addressed and apparently solved in later years of production. All of our rebuilt units have these updated upgraded parts. If you run any rotary style pump like a VP44 out of fuel at high RPM you CAN seize the rotor because it runs out of lubrication! This symptom can also be due to contaminated fuel and related corrosion on internal parts of the pump, or an electrical failure of the computer on top of the VP44.

Low Power, Cold

If the truck is gutless when driving for the first little while after start up cold, and then all of a sudden takes off and runs fine, this is almost always a bad computer on top of the Injection Pump. This means replace the Injection Pump. If you have a code 237 with this symptom, address that first by checking MAP sensor signal voltage, as described below. If you can predict when it is going to do this naughty thing, try heating the computer on top of the pump with a hair dryer for a few minutes, and if it runs fine right off, you know I am right. If that doesn't fix it, do the APPS test as described in "Dead Pedal" above to determine if the voltages are doing what they should when cold, to make sure it is not caused by low voltage from the APPS. If the voltage doesn't go up when the pedal is pressed, then the ECM won't tell the injection pump to deliver more fuel, so you can blame the APPS. If the voltage goes up when you press on the pedal, then the APPS is working correctly, and it IS most likely caused by the computer on the top of the injection pump, or some mechanical issue inside it.

Hard Start, Cold

This is typically due to a cracked or broken diaphragm inside the Injection Pump. To test for this try disconnecting the electrical power from the Lift Pump BEFORE turning the key on, and see if it starts better. This happens because the electric lift pump pushes air which is always in the fuel chamber inside the VP44 pump, through the crack or break in the diaphragm, into the mechanical high pressure pump and it becomes air-bound until it rotates enough times to bleed out the air. If it starts better without an electric Lift Pump, it is because the electric Lift Pump hasn’t forced air into the mechanical high pressure pump through the crack or break in the diaphragm, which is what separates the fuel chamber from the high pressure pump. This failure requires a VP44 replacement to fix the problem.

Hard Start, Hot

We have learned that long crank times when warm or hot are not always caused by a worn-out distributor on the injection pump. Before you replace the injection pump for this symptom be sure that the starter cranking speed is adequate. This is important as it is a mechanical pump in the injection pump that makes more pressure and flow the faster it turns, and when the pressure created during cranking is not enough to pop off at least three injectors, the engine will not start. So there may be as many as two causes for this symptom. One, is that it's not cranking fast enough to make enough pressure, the other is the pressure is being lost in a worn-out distributor that is expanded due to heat soak, OR BOTH. Before buying a pump we suggest that you first check electrical parameters for the starting system. A good starter at operating temperature can draw from 450-700 amps, and the battery voltage should never go below 10 volts during cranking. If your starter draws more than these specs, or the battery voltage goes below 10 volts when cranking, you need to fix the starting system, FIRST.

When you have determined that the electrical system is up to snuff, then you can try our clever trick of running cold water from the garden hose over the injection pump when it's hot, for a few minutes, and if that makes it start right away, then you know cranking pressure is the issue, because you shrunk the metal in the distributor and it fits tighter to the rotor and makes more pressure. This indicates a worn out distributor, and a replacement VP44 is needed. If you have just replaced the injection pump and the problem did not go away, then most likely you have a slow crank problem. If you didn't have the problem before the pump swap, then most likely it is a pump issue, BUT,,,, the sad truth here is that a replacement pump may have a worn-out distributor just like the previous pump did. No one wants to put a new distributor on every pump, as it is SO EXPENSIVE. As of this moment in the industry, there is no good test available to the rebuilder to accurately determine if a distributor will perform correctly in hot conditions. The best any rebuilder can do is check pressure and fuel volume from the VP44 at cranking speed on the test stand and if it passes Bosch specs, then it has to be presumed good. This test is performed in a 70° air-conditioned pump room and in the real world pumps are subjected to the latent heat transfer from a 195° engine! The bottom line is you REALLY have to test the starter and batteries first to make an accurate diagnosis for this frustrating symptom.

Another cause of this issue that we have run into recently, is a delay getting 12 volts to the primary side of the fuel system relay. This shows up as a delayed battery voltage on pin seven (red wire with a green tracer on Dodges) in the plug going into the VP44. First test for the delay by removing the big plug on the injection pump and verifying the time it takes to get battery voltage on pin 7 of the plug or on the red wire with a green tracer. To accomplish this, pull the locking slide in the plug toward the fender. There are two half round indents on the slide lock and you will need to pull pretty hard toward the fender while wiggling the main plug with the other hand pulling toward the firewall. When you have the plug in your hand, hold it so it looks like a smiley face, with six pins below the smile and three pins above. Use a test light or voltmeter to verify how soon you see battery voltage on the bottom right pin (Pin 7) after putting the ignition switch in both the “run” and “start” key functions. When you reinstall plug and push in the slide lock, push the plug toward the pump and you will see that when the lock moves in, it pulls the plug in towards the pump. When you think it is on correctly try to remove the plug by pulling on it. If it comes off you didn’t get plug on far enough before you slid the locking slide in. This test tells you when the injection pump is getting electrical power from the fuel system relay in the PDC (Fuse-box under the hood). The ECM not only turns on this relay but this relay also powers another part of the ECM that turns on the lift pump too. The electric lift pump is powered directly from the ECM, not the relay, to control WHEN it comes on. An audible indicator of this delayed voltage or start problem is when you don't hear the electric lift pump come on for 4 seconds when the key is first turned on and for 25 seconds when it is turned to the start function. The relay enable power goes through two connectors, #125 and #130 between the ECM and the relay. The delay problem can be caused by a delayed signal on the brown wire with a white tracer at the ECM connector, indicating a bad ECM, OR a loose connector #125 on the firewall. We have not heard of #130 being a problem yet! Working on these old trucks nowadays is getting pretty interesting, eh?

Long Crank Times

If the engine runs rough for a brief period of time after starting, just a few seconds, or sounds funny when running, this usually indicates air in the fuel supply system, caused by either fuel drain back or air getting into the fuel supply line somewhere. This symptom is NOT caused by the Injection Pump. Please know that good fuel pressure does NOT mean that there is no air in the supply line, as the pressure sensor doesn’t know the difference between fuel and or air pressure! My latest trick to accurately determine if air is a problem or not, is to get a 12 foot section of clear polyethylene or vinyl 3/8” hose from the hardware store and put it in the steel line between the Fuel Filter and the Injection Pump where the rubber section is. For aftermarket plumbing upgrades, figure out how to install it between the Fuel Filter and the Injection Pump. Loop it up under the windshield wiper for easy observation while driving and starting. Bleed the system to get all the air out of the newly installed line, and when you know the engine will be hard to start, monitor the line before, during, and after starting, and even driving, to determine if air ever gets into the pump. Be sure to drive the truck even if you don’t see air under no start or no load conditions, as air can be ingested intermittently from various sources only when the engine is under load. The hose under the wiper allows you to drive and diagnose the air situation under any and all circumstances you like, to see when it does or does not happen. This test positively tells you that you DO or DO NOT have an air issue which can be very important when diagnosing the VP44 fuel system. If you do have air in the clear line, run the engine from a can of diesel in the bed of the truck with a rubber hose stuck in it, connected to the inlet of the Lift Pump, and do the same test under the same conditions that saw air in the fuel, again. If the air goes away, the problem is behind the Lift Pump. If it is still there, the cause is forward of the inlet of the Lift Pump. This can be the Lift Pump itself, if it is a Fass with a leaky o-ring inside, leaking sealing washers, the water drain seal on the filter canister, or it can be leaking o-rings on the “Fuel Tubes” in the cylinder head. This last one is cool, as it shows its symptoms in a very unique way. If you park the truck facing up hill, overnight, on a fairly steep grade, it will start hard the next morning, but if you face it the other way on the same hill overnight, it will start fine! This is because of fuel drain back, and air getting into the system through leaky fuel tube o-rings and going to the highest place in the fuel system. When it faces up hill, the air goes into the fuel filter. When it faces down hill, it goes to the fuel tank! Call me if you need more help determining where the air is coming from.

No Fuel At Injectors


It is very rare, but possible, for a problem with the wiring harness or the CAN Bus wires to prevent the engine from starting, so if you want to be 100% sure it IS the Injection Pump causing the no start, follow the following directions exactly, to be sure of not damaging a possibly good pump. This test POSITIVELY eliminates the possibility of overlooking an electrical problem caused by other components that could affect the start or run function of the VP44, as long as you have verified fuel delivery to the Injection Pump. Remove the electrical plug at the back of the Injection Pump and hot wire the pins on the pump as follows. Get two wires long enough to reach from the battery to the VP44. Install an INSULATED ¼ inch female spade connector onto one end of each wire.Use a set of dykes and cut the flat part and one of the "curls" away from each spade connector to leave one "curl" , which will be about the right size to go over the pin on the pump. Connect one INSULATED connector to pin 7 on the pump, which is the pin on the BOTTOM row of the socket on the Injection Pump, closest to the engine, to preferably fused (10 amp is fine) positive battery power in the PDC (Fuse box under the hood), or directly to the positive battery terminal if you like to take risks!.

Connect the other INSULATED connector to the pin directly above the previous connection, the top row of pins, the one closest to the engine, and attach the other end to battery ground. Now try to start the engine and if it doesn’t start, you absolutely positively 100% need an Injection Pump! If the engine starts this way but NOT with the big plug installed on the pump, you know there is something in the harness or CAN bus wiring to the ECM telling or causing the engine to not start. Call me for help if this is the case.


If you want more proof, or really want to know WHY it won’t start, loosen all of the injector lines at the valve cover. Crank the engine for 30 seconds, and if fuel comes out of only one line, better than the others, this indicates a seized rotor, and the engine will never run again until you change the VP44, because only one cylinder is getting fuel. For the engine to start you need HIGH PRESSURE fuel, AND NOT AIR, to POP OFF at least three of the injectors. If you have only a feeble fuel flow from the open lines, you are looking at only Lift Pump pressure, and the engine will never start. To determine if it is or is not HIGH pressure, look for a puddle on the ground after 60 seconds of cranking. No puddle, no high pressure. If high pressure fuel doesn’t come out of the open lines when cranking, the solenoid pintle valve may be stuck, or the pistons may be stuck compressed in the rotor, due to fuel contamination or corrosion. Low pressure can also be caused by an electrical issue in the computer, where the computer doesn’t energize and close the fuel solenoid to make high pressure, so low fuel pressure going through the injector lines is WHY the engine won’t start. Any of these situations confirms that the engine will not start until you replace the VP44, as long as you have done the other tests above.

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