OT power steering problem

[Old Time]

I just got a nice clean 2001 GMC Sonoma pickup. It has the 2.2 liter engine and runs and drives great. My problem is occasionally when parking the power steering doesn't work, after you have a death match with the steering wheel to get out of the parking space, it works. The local shop swore it was the fan belt, they replaced and no change. When I took it back they shuffled their feet and admitted they didn't have a clue why it does this. The strange thing is it makes no unusual noises when it quits, (belt squealing from slipping). Does anyone here have any ideas or had this happen to them? Any suggestions would be appreciated.
Old Time

[Joe Rogers]

If it always fails in one turning direction it is the steering gear box. If it fails either direction it is most likely the bypass valve in the pump is sticking in the bypass direction. I have to say that loss of assist due to the gear is extremely rare. Any other restriction in the pressure side will cause a noise change as the pump cycles to max output and then bypasses.
Joe

[Mike Nash]

This may not be the same thing at all, but we have a 1999 Chevy Astro with 4.3L. I changed the power steering pump when it started acting weird (had to do it again later because the first replacement was always groaning and we were tired of it.) On ours, it WAS the serpentine belt. For that model the giveaway is the brakes will also start pulsating (even when not moving) when applied. I did not realize at the time that both are run from the power steering pump (Hydroboost).

Anyway, the real problem was the belt length for that model had been discontinued so all you can find are too short to go on, or too long to work for very long at all before the tensioner is on its stop. None of the parts suppliers or catalogs mention this. It took me a very long time to discover the discontinued length issue. I found a NOS off ebay correct length belt finally.

Totally unrelated but I finally got rid of the persistent belt chirp (from the time we bought it used through umpty eleven belt changes) by removing the crank pulley, cleaning the slight rust off the back and putting it back on straight. It wasn't so much anything you could see, just another internet discovery.

[Forestgnome]

Is it a serpentine belt? May the tensioner is weak?

[Willy]

Does the power assist fail in both directions?
Fluid is up, hoses are tight from reservoir to preclude the ingress of air into the system?
Belt is as mentioned, the correct length? Does this happen hot, cold, damp, all the time?
Can you detect if the engine is being placed under load or not from the pump when assist is lost?

If it were me I would tee in a high pressure hydraulic gauge into the system that I could observe while driving in order to diagnose the loss of assist as it happens. Or better yet find a shop that has a power steering analyzer gauge and knows how to diagnose the system in order to isolate the problem to either the pump or steering box/rack.

Fixing intermittent problems can be a challenge due to the fact that it's hard to fix what isn't broke. Chances are you can leave it at the shop for three days and the system functions perfectly until you drive it home.

[ahidley]

Bad rack

[daGrouch]

Since it only seems to happen while parking I am going to assume it's not enough pressure at low RPM. I would suspect a weak pump.

But rather than throw parts at it or buy a gauge and fittings, I would grab an accomplice and duplicate the problem with the hood up. Have said accomplice watch the PS pulley to see if it stalls. Stall = belt/tension issue.

If no stall or slipping is found then check the return hose for stiffness (not running). If it's squishy and floppy it could be collapsing.
When the PS dies throw it into neutral and rev the motor. If the PS works at higher RPM then:
Pump is weak
Pulley is too small

Since it's a used truck there's no telling if the pump was replaced with an almost-but-not-quite-correct pump or some dealer/owner overdid the old brake fluid stop leak trick and swelled the regulator seals so much they are binding. It's possible the piston seal on the gearbox is worn but I can't recall that ever happening on the dozen or so GM trucks I've had.

Maybe replace the fluid mainly to see if any crunchies are in the old fluid. A filter screen could be clogged to missing. I think there is a screen in the return port on the reservoir.

[J Tiers]

Clogged could be it... parking is a LOT more volume flow than general driving, and when you are driving, you almost do not NEED the assist, so even if it is a bit weak, it might not be noticed

[Willy]

Since the loss of assist is of an occasional nature it could be a disruption of flow and pressure. This could emanate from the pump either not from it not being properly driven, or an internal pump issue. The temporary loss of assist could also originate from within the steering box itself due to a malfunctioning valve body.

It may also be something as simple as a hose that has separated a layer internally and is thus restricting flow.
This is why I would choose to use a pressure gauge and preferably a proper power steering analyzer gauge setup to pinpoint the issue. Doing otherwise is hit and miss and could get expensive if you needlessly replace components. Or heaven forbid you expectantly loose assist while negotiating heavy traffic.

[dave_r]

The full size trucks have a check-ball/valve on the output port that sometimes get stuck/jammed and can be replaced separately from the whole pump and you don't have even remove the pump from the truck to do it, and it can cause a problem like the OP has. His PS pump may have the same kind of valve that can be replaced.

[Old Time]

Thanks for the suggestions, it seems to do it after its driven about 15 to 20 miles. I tried putting it in neutral and speeding the engine up, didn't make any difference. I'll check the tensioner and the hoses tomorrow. Thanks again.
Old Time

[Joe Rogers]

The full size trucks have a check-ball/valve on the output port that sometimes get stuck/jammed and can be replaced separately from the whole pump and you don't have even remove the pump from the truck to do it, and it can cause a problem like the OP has. His PS pump may have the same kind of valve that can be replaced.

The vehicle does not have a rack and pinion. It has a 600 series Saginaw gear. Nearly bulletproof if not post accident. The pump does have the bypass valve but replacing it may not fix the probllem if the bore in the pump housing is pooched in some way. Better to hang a new/reman pump and replace the hoses. The flow test on this simple system is a waste of time. I have only done it to appease the warranty gods.
Joe

[Arcane]

I have no idea if this is the problem or not but it seems to have been for a lot of people so I'm posting this as just another possibility to consider.

http://s7d9.scene7.com/is/content/Ge...58222pdf?$PDF$

Also, there's a PDF @ http://tinyurl.com/evoreplacement

[Joe Rogers]

EVO wasn't an option on the S/T trucks in 2002. Doesn't apply.
Joe

[Forestgnome]

Thanks for the suggestions, it seems to do it after its driven about 15 to 20 miles. I tried putting it in neutral and speeding the engine up, didn't make any difference. I'll check the tensioner and the hoses tomorrow. Thanks again.
Old Time

If you get air in the lines it wil do the same thing, will clear up after driving a bit. A small pin-hole leak will allow air to draw in when the system is off.