Q: Fuel rail pressure is all over the place and the truck bucks badley going down the road. Only code that comes up is P215a.

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Fuel rail pressure is all over the place and the truck bucks badley going down the road. Only code that comes up is P215a. The speed sensor on rear differential was just replaced. This all started happening right after I blew the serpentine belt and replaced my powersteering pump

My car has 140000 miles.
My car has an automatic transmission.

The fuel pressure will fluctuate when the motor is bucking. The trick is knowing if the engine is bucking because of fuel pressure, or is the fuel pressure erratic because of the engine bucking. Usually it has nothing to do with fuel pressure. The fuel rail pressure on a diesel is created by the injection pump, which on a diesel engine changes speeds with engine RPM. So I would expect to see fuel rail fluctuation like this.

Your code indicates there is a possible problem with the CAN (computer area network) that monitors and controls the ABS system. The ABS, the TCM (transmission control module) and the PCM (powertrain control module) all talk with each other over this network. To diagnose this system you will need a scanner to monitor the data from these computers. A scanner isn’t the same as a code reader.

It isn’t a common problem for this system to cause drivability issues, but since this code brings into question the CAN system, which can affect every module on the truck, it is something that should be considered. The most common failure for this vehicle is a fuel injector. With the scanner connected, you need to look at the misfire counters for each cylinder. If one is showing misfires but the others are not, you can suspect that injector is faulty.

Injectors are expensive, so I wouldn’t go out an buy one without some additional diagnosis, but this should get you started. If you still need assistance, a certified mobile mechanic from YourMechanic can visit your home or office to inspect your truck’s bucking issues.

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