Loss of steering assist near idle

[spark3542]

New to the forum, fairly new to diesels, new to turbos, but fairly knowledgable about cars in general.

I just purchased a '97 burb K2500 diesel.

I noticed my steering assist drops away at slow speeds, like turning into a driveway. Looking under the hood, I noticed that the normal GM power brake assist is missing, and in it's place some type of assist unit that is driven by the power steering pump. Level of fluid in PS pump is normal and is a nice red color.

I'm wondering why there is not a normal power brake assist unit. I'm guessing because the turbo would have ill effects on the amount of manifold vacuum available, or is it somehow related to it being a diesel?

It doesn't seem normal to have to sacrifice steering assist when brakes are asking for assist at the same time. Is this a common problem and what is the cure?

Thanks

Mark in MA

[DmaxMaverick]

Welcome to the Forums!

A Diesel engine has no throttle (intake is unrestricted from the filter to the cylinders), so too little vacuum. The hydroboost (that's the thing in place of a vacuum booster on a gasser) system has been used on GM Diesels forever. They are generally reliable, but some do have problems, just like any part.

The steering assist will normally decrease slightly with moderate to heavy braking. If you are losing all assist, something is wrong. It could be a tired pump or hydroboost unit. Modulation of the pressure/flow of fluid for the booster and steering box is controlled by the booster. The booster gets priority, but it shouldn't remove all assist from the steering. If you have no reserve braking (after shutdown, you should have 3 or 4 full braking applications from an accumulator....that little canister looking thing on the inboard side), the hydroboost unit needs repair/replacement. Unless you have extensive hydraulic repair experience, I suggest you do not try to repair it. They're not cheap, but should last a long time.

[Hubert]

My truck and have read of other complaints (I would say its a common phenomenon). The anti lock brake cause a hydro assist dump too. Say braking at a stop sign with a rough patch of road. The rear wants to skid but the antilock brake dumps pressure. If you hit a bump (curb) while turning in your driveway while braking thats probably what triggered it. I get the dump of assist usually going slow last 10-20 ft before a stop sign where pavement is wrinkled and usually has a bit of gravel that makes the rearend loose traction. I loose firmness in brake pedal and have to stand on brakes to keep from over shooting the stop sign - its not a good feeling. It only happens when I am braking kind of hard. The steering wheel also feels different but I have never been turning real hard at the time.

Maybe new shocks would help but I don't know. Test it a little and see if you can make it come and go. Brake a little hard and turn while loose traction and some bumps and see if it repeats the feeling you experienced before trying to throw money at it. Unfortunately it may be normal.

[CareyWeber]

New to the forum, fairly new to diesels, new to turbos, but fairly knowledgable about cars in general.

I just purchased a '97 burb K2500 diesel.

I noticed my steering assist drops away at slow speeds, like turning into a driveway. Looking under the hood, I noticed that the normal GM power brake assist is missing, and in it's place some type of assist unit that is driven by the power steering pump. Level of fluid in PS pump is normal and is a nice red color.

I'm wondering why there is not a normal power brake assist unit. I'm guessing because the turbo would have ill effects on the amount of manifold vacuum available, or is it somehow related to it being a diesel?

It doesn't seem normal to have to sacrifice steering assist when brakes are asking for assist at the same time. Is this a common problem and what is the cure?

Thanks

Mark in MA

Mark,

My 97 will do that one in a while like once every 5 to 6 months this started when I had about 100K miles on the truck now I have +208K miles. I think it's related to the speed contolled steering AKA EVO that was used in 97 - 98 (don't know about the 99's or 00's).

From what I gather I think when mine goes I'll get a 96 power steering pump to avoid the EVO $$$$. IIRC the hoses are different too, but in my case if I swap I'll replace the factory hoses their due with +208K miles on them.

Carey

[spark3542]

Thanks for the replies. I'm very confident there is no ABS engaging when I lose my steering assist, the speeds I see it at are very slow, road surface is very smooth, and also I don't hear the low groan of the ABS unit.

I'm not pumping the brakes or braking hard when it happens.

If this vehicle has speed-related variable assist, it almost feels like it is working backwards...meaning more assist at high speeds, less at low.

I'll continue to troubleshoot to determine exactly what scenario is present when I get no assist.

Thanks

Mark in MA

[CareyWeber]

Thanks for the replies. I'm very confident there is no ABS engaging when I lose my steering assist, the speeds I see it at are very slow, road surface is very smooth, and also I don't hear the low groan of the ABS unit.

I'm not pumping the brakes or braking hard when it happens.

If this vehicle has speed-related variable assist, it almost feels like it is working backwards...meaning more assist at high speeds, less at low.

I'll continue to troubleshoot to determine exactly what scenario is present when I get no assist.

Thanks

Mark in MA

Mark your speed matches mine too when it happens.

Carey

[spark3542]

I found that it loses assist at engine idle with zero throttle applied. I had zero brake applied as well. No squeaking like a slipping belt. No groan like low fluid

It turns into "armstrong" steering at idle while rolling, then assist comes back as soon as any throttle is applied.

Does this sound like a bad pump?

Mark in MA

[CareyWeber]

If this vehicle has speed-related variable assist, it almost feels like it is working backwards...meaning more assist at high speeds, less at low.
Mark in MA

Mark,

I think the EVO valve gets stuck in the high speed mode ie less assitance for more control at higher speeds.

It could be the pump, but mine has been doing it off and on for a long time. If it were the pump I think it would have died long ago.

Carey

[spark3542]

Where exactly is this EVO valve? My Chiltons is woefully inadequate in the steering section..."loosen bolts, remove pump" is about as deep as it gets. There doesn't seem to be a troubleshooting section at all for steering.

Since the loss-of-assist seems to be unrelated to the brake, I'm assuming the hydroboost is ok, and the problem is either: PS pump is weak, especially at low rpm, or variable assist is failing. Is the variable assist valving inside the hydroboost for the brakes?

Thanks

Mark in MA

[CareyWeber]

Where exactly is this EVO valve? My Chiltons is woefully inadequate in the steering section..."loosen bolts, remove pump" is about as deep as it gets. There doesn't seem to be a troubleshooting section at all for steering.

Since the loss-of-assist seems to be unrelated to the brake, I'm assuming the hydroboost is ok, and the problem is either: PS pump is weak, especially at low rpm, or variable assist is failing. Is the variable assist valving inside the hydroboost for the brakes?

Thanks

Mark in MA

Mark,

If you look at the back of the power steering pump you can see where the wire connect to it.

http://www.dieselplace.com/forum/showthread.php?t=83874

http://www.asashop.org/autoinc/april2005/techtips.cfm

Search on posts started by asnowsquall (http://www.dieselplace.com/forum/member.php?u=15498) on deiselplace.com.

Carey

[spark3542]

I did find the connector on the back of the pump before I read your post. I disconnected it an did a test drive...sure enough max assist at all times. As far as I'm concerned, I don't need the evo function, I'm happy to have max assist at all times.

Thanks for your help.

Mark in MA