Power steering problems

[another Todd]

Symptoms:
At idle, there is no detectable power assist.

Parked at 1000 RPM steering works fine, except that if I hit the brakes while turning the wheel, the wheel stops turning for a second then resumes.

On the highway, there is too much power assist and it wants to steer itself, it seems like you turn it just enough to change lanes and the wheel wants to keep turning that direction, you have to hold the wheel tight or it will keep turning. Doesn't seem to center itself.

Vehicle: 1997 Suburban 4WD, 454, hydroboost, variable assist power steering. I have flushed the system, replacing most of the fluid with zero change.

 

[Ike_Clanton]

Is this a case of a bad EVO sensor, or a bad pressure releif valve in the pump?

My GM tech buddies said a thing called the EVO sensor on those trucks go bad and cause all sorts of scary steering antics.

 

[another Todd]

Not sure what or where an EVO sensor is. Without specialized tools, how do I test the pressure relief valve? How do I test the EVO sensor, and where is it?

 

[another Todd]

OK---electronic variable orifice. On the back of the PS pump. How is it tested?

 

[mechtech2]

I'm thinking that there should be a hidden code in the computer for this.
One that does not trigger the dash light.
I'd have it checked - some places do this for free.

A shop or dealership will probably place the whole unit, no matter what is wrong.

 

[another Todd]

I hooked up my scan tool already, there are no codes. When I first started looking into this, the EVO plug on the back of the PS pump was disconnected, which according to the GM manual causes full pressure to be applied all the time, the steering was the same when I plugged it back in and there were no codes before or after.

 

[brianl703]

The EVO isn't a sensor, it's a valve. It's controlled by a variable duty cycle signal. The purpose of this is to reduce power steering assist at higher speeds, which is accomplished by increasing the duty cycle of the signal applied to it as vehicle speed increases.

You shouldn't notice any difference by unplugging it at idle because full pressure is called for at idle. At highway speeds you should notice a difference however.

 

[another Todd]

That's the problem, I get no assist at idle and too much at highway speeds. Is there a typical problem that causes this? I'm not completely familiar with hydroboost, so I am no sure where to even start looking. I hate throwing money at it by guessing.

What kind of signal should I get from the EVO plug? Can the EVO valve be removed and cleaned/tested? Does it just unscrew from the back of the pump or is it pushed in with o-rings and some kind of retainer?

 

[another Todd]

Bump. I am considering replacing the power steering pump with a remanufactured unit with the EVO installed. The cost is about $150, which I imagine is what a dealer woulod charge me just to diagnose it. And of course, dealers are not always right in their diagnosis. Would this be a reasonable first step. My theory is the pump may not be putting out enough pressure, I have no way of testing it, but it does put out a lot of volume.

 

[Grebbler]

Inside the bottom of the steering column is a ring/donut shaped sender for the EVO. Unplug the harness to it and see if the freeway issue goes away.

 

[another Todd]

I pulled the EVO/Passlock fuse instead. Would this have the same effect? The issue did not change, no power steering at idle, too much at speed.

 

[another Todd]

Grebbler, I found the EVO sender, the wiring was hidden above the coluum, so I didn't see it when I posted above. DC'ed the sender and test drove, seemed a whole lot better at speed, did not seem to "over"steer. Of course, no change at idle though.

Should I replace the sender or just leave it disconnected?

Now then, the no PS at idle issue. Pump?

 

[Grebbler]

It's not an expensive part if you look around the net. You can leave it DC'ed if you want.

Since it's fine at speed I would have to agree that the pump may be weak or you have a partial line clog.

I have an '02 Tahoe and it's kinda stiff when not rolling and I think there is a GM caution not to turn the wheel unless in motion. Some tires and/or low pressure will effect it also. Do you mean idle and stopped? How about idle and coasting?

 

[another Todd]

If you are rolling a little it is better I think, but still hard. If you roll up to a turn say at 10 mph and try to turn fast, it's like there is no PS, you have to really put some muscle into it. I will order the part (Dorman 44276 from car parts discount is $26.95) and put a new pump in it and see what happens. Thanks for you help.

 

[another Todd]

Finally I got around to replacing the pump. Solved all steering problems but created a new one. The new problem is now my wife loves driving it, and wants to sell her trailblazer and drive my suburban full time.