A Rookie Mistake

Ever change the wrong part? I did recently and while embarrassing, it pointed out a pitfall of experience … mainly that you can’t always trust it. The under hood noise from my long-suffering F-150 had increased in volume and it was clear that something had to be done if I wanted to avoid issues mid-winter. It was clearly coming from the accessory drive and with the recent change of alternator, idler, tensioner and belt, the hunt logically moved to the water pump. On this vehicle, that’s not a big issue, and besides, it was the oldest engine accessory, with some 200,000 kilometers on it. Of course it’s the water pump! Good thing it’s a fast and easy change, because the noise was still there after the “re-and-re.” A little belt dressing cured the noise for about a minute, but it came back, louder than ever. Louder? It actually makes sense. The noise is caused by belt slippage, so the dressing, which increases friction between belt and pulley, eliminating the noise. It’s a Band-Aid temporary fix if there’s a weak tensioner, but with modern serpentine systems you’re usually going to just change belt and tensioner. So why would the noise get louder?  Because the accessory that’s causing the slippage in the first place is dragging the belt, and if the resistance of its pulley to the motion is strong enough, the dressed belt will just squeal more as its added friction can’t overcome the pulley’s resistance to rotation.

What happened? Basically, I relied on my knowledge of the vehicle to deduce that the oldest part on the engine dress just had to be the cause … except it wasn’t. The culprit was the power steering pump, which showed none of the usual symptoms of going bad. Power assist was still smooth and progressive with no hesitation or tight spots and full function at all engine speeds, including idle. As is often the case, however, I hadn’t looked at it in years, mainly because the system didn’t leak and the fluid looked a healthy red colour. A quick poke with the stethoscope confirmed the trouble, but the clincher was the fluid itself. A small amount drawn off the reservoir was red all right, but smelled burnt, like bad transmission fluid. If this was ATF I’d expect a transmission rebuild on the acrid stench alone. The power steering system is, of course, a hydraulic system, like brakes and automatic transmission; but like so many consumers, I had never flushed it or changed the fluid. At closer examination, the reason is that previous high-mileage cars and light trucks I’ve owned always leaked a little fluid, which meant occasional topping up, and effective fluid change every three or four years. My modern F-150 was tight as a drum, so the old, bad fluid was trapped in there forever. On closer examination, sludge was restricting fluid flow from remote reservoir to the pump, causing oil starvation and probably cavitation, foaming the oil that does reach the vanes. That might be noticeable in an integral tank design like the venerable Saginaw unit, but the remote tank hides the effect. The moral of this story? Flush the power steering system regularly, and if installing a reman unit, clean remote reservoirs and check for plugged screens. Even better, install a remote filter in the low-pressure return line. Power steering has become so good, it’s no longer front of mind for routine maintenance … but it should be.