1999 GMC Suburban • V8 4WD Automatic • 160,000 miles

I have a 1999 K1500 GMC Suburban with approx. 190K miles. I started intermittently losing power assist on my steering at low rpms a few days ago. It would kick in and out, no it is staying mostly out.

Its not a complete failure, but rather one related to rpms. For example, in typical 2 wheel drive, anytime the engine drops below about 1000 rpm, the power assist becomes very weak or completely gone. If you rev the motor up between 1200 and 2000 rpm, it kicks back in. To confirm those symptoms, I put it in "4-Lo" and drove it in first gear. Since this kept the rpms up even at very low speeds, the power assist continued to work. (However, when sitting still and idling below 1000 rpms, it obviously didn't work in either scenario.)

Therefore, it is not speed related, but rpm related. I have checked the belt and made sure it is tight, and as far as I can tell, the steering pump pully is not slipping when the steering wheel is turned.

Do you think this is most likely the steering pump. The speed sensor, or some other component in the power steering system?

Thanks in advance for your help.
March 12, 2013.

How many miles since the belt and tensioner was replaced? You cant test the belt with it moving and thetensioner may not be keeping it tight as it shold at lower rpm. Then have your power steering pump tested, it may be weak and need replacing. Finally, if you replace the belt and tensioner (reccomended at 60K) then try taking off line from cooler or return line and put new fluid in it. Make sure itis is power steering fluid and not transmission fluid.

Mar 12, 2013.
Belt has less than 10k on it, but I don't think tensioner has ever been replaced. Could a tensioner go all at once like this, or does it usually go bad slowly over time?

Mar 12, 2013.
It goes slowly over time and loses tension.

Mar 20, 2013.