Evo versus standard power steering

[east302]

I’ve been having an issue with the power steering pump on my 98 Tahoe. I swapped out the leaking Evo power steering pump with a Delco reman and bought the GM Evo bypass for it.

Here’s the GM bypass kit...

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Something was wrong with the pump, it kept bypassing and I had little pressure to the gear box. So I bought an Oreilly Auto power steering pump that had the bypass already installed.

It leaked at the pressure fitting on the back and the steering was pretty heavy.

So I grabbed a junkyard pump from a 99 Yukon and put the Evo fitting onto the Oreilly pump. It’s better now.

But, the pump with the bypass already installed had a different valve than the Evo pump.

Here’s the valve from the Oreilly pump...

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...and the one on the right is what came out of the 99 Yukon. I borrowed the photo from the internet, but the valve from the Evo pump has a shim on the end under the nut. The one on the left doesn’t have the shim which is what the Oreilly bypass-equipped pump had.

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So, which one is correct? The GM kit says nothing about swapping the valve, so why would a “bypass-equipped” pump have a different valve unless they screwed up doing the rebuild.

Would a pre-97 have the same valve as a 97+ with Evo?

Combined with evo, it apparently makes a big difference in steering feel.


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[sewlow]

Those valves have nothing to do with the EVO, but everything to do with the feel/weight of the steering.
Personal opinion, I'd delete that POS EVO system. GM makes the delete kit for a reason. They've figured out that the EVO is problematic. Both of my trucks have had the EVO fail at the most inopportune times.
Now...what those valves that you have do is to mess with that spring & puck in front of them inside the S/box.
It's that spring that decides how the steering feels.
On the F-body & G-body forums, guys have been modding that spring in order to adjust the boost point which affects the weight/feel of the steering.
The reason being is that since about the mid-'70's, the Saginaw S/box is over-boosted, giving a lighter feel at slower speeds. (For the Soccer Moms.) But then, at speed, the steering is too light giving a numb disconnected feel from the road.
The spring mod that the F & G-body guys are doing is to clip off some of that spring. This changes the pressure point of where the fluid is routed though that valve. Yes, the steering will feel heavier at slow speeds. Alternatively, at speed, the numb disconnected feel is lessened, resulting in a more positive weight/feel.
The difference in the valves that you have is that they are doing pretty well the same thing as clipping the spring length down.
That adjustable one you have is kinda trick. I've never come across one in my steering box experiences. A lot more forgiving of a mod than clipping that spring. Cut down too much of the spring & now you're hunting down a new one to start the process all over again, whereas if you go too far with that adjustable one & it's a simple matter of removing & resetting it more or less to your liking.
It can be a time consuming process to get the weight/feel of the steering set to your preference, but worth it in the long run.

 

[east302]

Thanks, that helps explain things. I’m not sure why a reman that was supposedly for my truck would not have the shim that the original pump had.

Is the shim used to keep the spring further compressed, making the relief open at higher pressures and allowing for lighter steering feel?

I’ve never driven a stock non-EVO and don’t have a point for comparison, but have heard they had a heavier feel to them.


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[sewlow]

"Is the shim used to keep the spring further compressed, making the relief open at higher pressures and allowing for lighter steering feel?"

Yup! That's it exactly.

 

[mr98z71]

Anyone reading this now, the relief valve shown in the pictures does not dictate how the steering will feel, it is a relief valve. If you hold your steering wheel at full lock, this is the valve that prevents you from going beyond that point because it will relieve pressure. A lot of people end up modifying this valve to give them more road feel (less assist) but that is wrong. This valve only comes in to effect when the pump is making around 1100 psi, the pump is not making near this kind of pressure cruising down the highway. Modifying this valve to give you good road feel on the highway will result in very little assist when parking.

What should be modified is the Flow Control Valve, this valve controls the amount of flow going to the steering gear and will decide how twitchy or heavy the steering feels. On 3500 trucks with Hydroboost, this orifice would have been 5/32nds, on 1500 truck, about 1/8". New flow control valves can be bought online or you can modify yours by welding and redrilling it.

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[mr98z71]

Thanks, that helps explain things. I’m not sure why a reman that was supposedly for my truck would not have the shim that the original pump had.

Is the shim used to keep the spring further compressed, making the relief open at higher pressures and allowing for lighter steering feel?

I’ve never driven a stock non-EVO and don’t have a point for comparison, but have heard they had a heavier feel to them.


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The shim would make the relief valve open sooner and give you less pressure. Looking at the piston on the left with the blue spring, that's not a gm spring and is why the nut is bottomed out. Gm used shims or no shims depending on if you ordered a Yukon vs a 1 ton truck with a plow package, the 1 ton truck would have needed more pressure to turn the same tires with the added weight of the plow.

What did the Evo fitting on the back of you pump do? It changed flow based on whether you were on the highway or city, city driving, you had and overassisted feel, get on the highway, steering got heavy, nothing changed inside that relief valve, however the amount of flow was reduced.

 

[98 Nitro]

Do the 95/96 trucks have the same steering feel as a 88- 94 truck or do they have a little more assist? I have a 94 and a 98 and was considering the 96 steering box for the 98.

 

[smdk2500]

Do the 95/96 trucks have the same steering feel as a 88- 94 truck or do they have a little more assist? I have a 94 and a 98 and was considering the 96 steering box for the 98.

From what I can remember when i had my 94 the steering felt about the same as my 95 stiffness wise. The 94 was a 1500 and my 95 is a 2500 and has about 130k more miles on it. But both where 4x4.

 

[5speed Hoopty]

I could use a little help. I have 1990 c1500. And just recently, while looking for an electrical short I noticed my evo was unplugged. However, looking at the delete that several members have posted (amazon) I could not wrap my mind around how the smaller delete line would fit between the two fittings.

Here is the picture of my power steering evo line.

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Basically, the replacement line look sig smaller than the evo line its replacing (and I mean specifically between the two connection points not over all length...am I missing something?

thx

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[Dropped88]

gm didnt have EVO in 1990 and it was on back of pump. that looks more like a pressure switch