Changed out the thermostats and flushed coolant on UPGRAYYED (our 2002 LB7) the weekend before last. While I was in there, I also swapped out the MPROP (fuel pressure sensor), which I suspected was either leaking or failed and was causing the idle lope with which we’ve been dealing for the last year.

That was the culprit: failed MPROP. With that replaced, her idle both in gear and neutral is now silky smooth. There’s also no smoke on the highway. I’m actually somewhat surprised that it’s possible for a diesel to be so smooth.

Then Monday happened.

Fast forward a week and a day to this morning: Just after I started the truck to head off to work, I lost brakes and steering.

Aside from a brief moment of surprise when the thing lurched into reverse, my thought was something along the lines of, “Odd… I was nowhere near that stuff last weekend.”

Segue:

Why brakes and steering? The Duramax — and several other trucks — use a hydro-boost mechanism rather than a vacuum booster to actuate the brakes. The hydraulic power is supplied to the hydro-boost by the power steering pump.

If the pump fails: no brakes and no steering

Differential:

For neither brakes nor steering:

• insufficient power steering fluid
• loose serpentine or belt failure
• power steering pump failure
• clogged high-pressure from pump
• failed hydro-boost
• engine not running

Okay, I added that last one, but I’m sure it’s in somebody’s troubleshooting manual someplace.

Troubleshooting:

• Shut off the motor and check power steering has fluid. It’s full, although a bit dark: Good.
• Check the serpentine, which, too, looks fine; snug: Good.
• Start it back up and observe that it’s turning the steering pump’s pulley: Good.
• With the engine running, remove the cap from the power steering reservoir to observe fluid movement: No sloshing. No movement. Not good.

Diagnosis:

At a minimum, that’s a failed pump. Could it be something else? Sure, but without any pressure coming out of the pump, there’s no way to know whether the hydro boost is okay or not.

We’re now $170 poorer and at least a few days awaiting delivery of the new unit.

Fortunately, it’s a reasonably straightforward driveway task for a home mechanic.