Inside The Bosch P-Pump - Diesel Tech

P7100 Pump Timing
The port-helix P7100 injector pump used on '94-'98 Dodge Cummins engines is timed by the phasing port closure of the #1 cylinder. The injection pump must be accurately timed to the engine. This means phasing pump port closure to a specific number of degrees BTDC on the cylinder to be timed to a specification that can be no more than one degree of crankshaft angle off. The injection pump timing needs to be very accurate and precise for the engine to run its best.

To review the timing procedure, Diesel Power traveled to the University of northwestern Ohio (UnOH) and worked with diesel instructor Bill Sergent and his class. In addition, we visited with Brad Anderson of north West Fuel Injection Service, a premiere injection pump rebuild shop in Columbus Grove, Ohio. We were exposed to not only stock rebuilds of injection pumps, but also the theory behind modifying the fuel delivery for a hopped-up diesel. Anderson told Diesel Power that injection pump performance is something that many enthusiasts overlook when modifying an engine. They assume the timing and operation are correct. If the injection pump is not functioning properly, then all the speed parts in the world will only have a minimal impact on the engine.

There are three methods to time a Bosch P 7100 pump-spill, pin, and dial indicator. The most accurate and effective method of timing is the spill theory. It identifies in crankshaft degrees when helix-port closure occurs. It is also the easiest method for the enthusiast to use since it requires little in the way of special tools, but you may need to invite a few of your buddies over to help.

As with any procedure on a diesel engine, the factory shop manual should be referenced. The following is an overview of the steps to time a mechanical injection pump on a P7100-equipped Cummins diesel.

Do-It-Yourself P7100 Timing
First, check the injection pump specification plate on the engine for the port closure value. Then, with a breaker bar on the crankshaft bolt, manually turn the crankshaft in the engine's direction of rotation to position the #1 piston on the compression stroke. Locate the engine calibration scale that is usually found on the front pulley, harmonic damper, or the flywheel. Position the crankshaft approximately 20 degrees before the port closure specification. For example, if the specification is 10 degrees, you would set the crankshaft at 30 degrees.

Remove the high-pressure pipe from the delivery valve on the injection pump #1 cylinder. Unscrew the delivery valvebody and remove the delivery valve core and spring. With a scrap piece of injection line from a salvage yard or engine shop, install it as a test spill tube on the injection pump. A discarded high-pressure pipe cut neatly and shaped into a gooseneck is all that is required.

Use the hand primer pump to charge the gallery. The pressure created by the pump will be insufficient to open the delivery valves, which means the fuel will exit through the spill tube you have attached to the #1 port. The fuel should exit in a steady stream, so have a friend hold a coffee can or other container under the tube.

Then, slowly and smoothly turn the engine over with the breaker bar in the direction of rotation while observing the stream of fuel from the test tube. When the injection pump plunger leading edge rises to trap off the spill port, the steady stream of fuel exiting the test tube will first break up into droplets and then cease as the plunger passes the spill port.

The critical thing here is to locate the pump precisely (through the movement of the crankshaft) at port closure. At that time, the flow at the test tube should exist but be minimal; 2 to 6 drops per 10 seconds.

Now check the engine timing marks. Where the crankshaft position produced the drip from the test tube is the port closure. The crankshaft should be within one degree of the listed specification if the injection pump is timed properly.