Recently out on the twisties and unexpectedly went wide on a turn.
Steering felt like it hit the lock. Later while testing I encountered the same event sitting in the driveway.
If I held the wheel on the stopping point and used a bit of force the steering continued to turn to the physical stop.
Fluid is one year old, level is good.
Both rack and pump have 150K on them.

I'm guessing the rack, replacing is not something I'm looking forward to...
Maybe if I replace the heater core first this will make the rack look easy.

 

Sounds like rack. Pump doesn't know where the stops are.

 

Check the tie-rods and the mounts

 

I have done both the rack and the heater core on my LT4 and they both suck to do but I would say the rack is much easier. The biggest PITA is sliding it out. you have to twist, turn, pull, yank and curse a lot. There is a company called turn one that rebuilds them and they have a solid rep but it is a bit costly. I'll be sending mine in soon. I'm not much of a mechanic but at 150 k maybe inner tie rods on the rack. I changed out everything on the front, shocks, bearings, tie rods, ball joints and it made a big difference but I still get some play in the wheel so I am going to get the rack rebuilt and also I think the intermediate shaft can loosen up and give you some play too. Hope it all works out. I used moog and timken and went with KYB shocks. Rock auto had the best prices. good luck

 

Anyone have a Z51 steering rack for sale or trade?

 

Try putting in Lucas power steering treatment. It works for a lot of things

 

Its a simple case of "morning sickness" that's been common to the C4 since its inception. Probably the most common complaint to appear about the C4 back in the 80s and 90s.

Lucas will fix it.

This is what IS happenin..

As the rack wears there is no place for debris to go, so it ends up getting into the tiny orifices of the hydraulic control block/assy and causes temporary sticking or momentary absence of power assist.

Its worst when stone cold. IT CAN happen at anytime and CAN be quite dangerous if you are unable to take hold and compensate for the sudden wheel jerk in a turn.
I once bought a C4 from a car lot by beating up on the sales guy after this problem showed up during the "test drive". Got a fair discount because the car "needed so much work" to make it safe. Drove it directly to the parts store, poured in 1 pt of Lucas R&P conditioner and stop leak and the problem went away within seconds...as soon as the goo can circulate in the system, its fixed.

My PS reservoir is now mostly Lucas since my rack is getting old. The stuff is very thick so it takes a few minutes warm-up in 30* temps but its fine after 1 or 2 min idle, which should be done anyway on a cold start.

If the FSM directions are followed, racks are not that bad. People TRY to swap the rack without removing other things and THAT is what makes it tough. Just drop the sway bar and you have lots more working room. The worst part if the secret PS hose fittings that GM tried to hide from your view as best they could...
the kind that are 1/4 turn of a flare nut wrench for about 8 turns....genuine GM PITA kinda fittings.

Its never a bad idea to replace the PS hoses if they are old and IF the pump is as old as the rack, I'd change it as well. Pumps are NOT expensive but they ARE a 8itch to change when your car is dead in the water. May as well do it as preventive maint, 'cause if the rack is acting up the same trash has been sent thru the vane pump as well.

 

OK I'll try it.

Switching to synthetic PS fluid cured the morning sickness years ago.
This event is more of a hydraulic malfunction.

 

well, if the power is still there, but in/out, its basically the same thing...sticky valves in the PS control block. It MIGHT have a damaged pressure spring...The FSM has a good detail of this and how to remove it. Try the Lucas..thats a $10 attempt that MIGHT help. The $hit works miracles.