Need some help/tips on getting air out of a power steering system. Truck's a mutt, '97 7.3, with a 6.0 power steering pump, and 2007 gearbox, stock 245/75r17 tires. Still has vacuum brakes, no hydroboost. All new lines/cooler/reservoir in last year. Everything functioned well, had no issue bleeding when I originally did the gearbox/pump swap last year.

This week I decided to replace the previously installed '07 steering box with another new one, as the used one had a lot of slop.

Replaced gearbox. Bled air as usual (on jackstands), felt normal. Went for a test drive and lost power steering before the first block, and had to rev engine up to 3k to turn the wheels. Back home, tried to bleed some more, would be fine, go for a drive same thing no steering. Disconnected drag link to get some more throw/turning radius, and tried to bleed air again. Turning the wheel dozens of times with engine off, and running. Go for test drive, same thing, gave up.

Returned/exchanged gearbox, as I've never had this issue before, and also bought a pump thinking maybe I ruined it even though it always had plenty of fluid in the reservoir.

Replaced box again along with a new pump, and I'm having the same exact issue. I've turned the wheel over 50 times (counted to 50 left stops, and 50 right stops) with the engine off, along with dozens of turns with the engine running (on jackstands). Was fine stopped, but after a test drive, no steering. Bleed some more, but then lose steering again. There has to be an air bubble in the box or something, but I cannot for the life of me get it to bleed out. Again, the pump was never ran dry, fluid isn't milky/aerated, I don't see any air bubbles when turning the wheel (although it's just me and the reservoir isn't the easiest to see into, F7UZ-3A697-AB), nothing's leaking and all fittings are tight.

Anyone have any tips?

Box is a Motorcraft stg-263rm
Pump is a Motorcraft stp-168rm ("old" and replacement)

Thanks in advance!

 

Yes, you can get Mityvac attachments and vacuum bleed the system. Basically you just apply vac to the reservoir and bleed as usual (I personally only ever do running, wheels unweighted).

The pump will be noisy when under vacuum, and I always felt like it's starving the pump of fluid a bit, so I don't do it too long. I'm not sure my theory is correct -- it's just unnerving to hear it complain so.

I've had a couple pumps that still whined a little after conventional bleeding and were silent after vacuum bleeding.

There are several videos on YT -- I had to educate myself before trying. It's not like traditional brake vacuum bleeding where you expect to extract fluid. You're just trying to "suck" the air to the reservoir-- and I can say it does (or at least, can) work!

The Mityvac attachments are yellow plastic.

LET US KNOW WHAT YOU DO!

 

I've never had a vehicle not bleed simply by turning the wheel. But I only do 10 turns unloaded and then a few turns loaded.

But in all cases after I've opened it up, made a repair, and re-filled, I would let it sit for a while. Refill to the line and then go mow the lawn or something. Let it sit for a good while and re-check the fluid level. Most times it self bleeds. Your reservoir is the highest point of the system.

 

Well, the truck's sat since I made the post. Purchased a vacuum cap adapter, pulled 20in (most i can do until pulling fluid), turned wheel 20 or so times. Vacuum didn't go down much, maybe 5in, pumped it back up and let sit for a little while, and it stayed at 20. Removed, started truck, turned wheel 10 times. Set the truck down, and steering felt great/effortless. Revved it up to 1500 rpms, and lost power steering, unless revved up to 1500-ish. So confusing, essentially the same issue as prior.

 

Wow I dunno! Normally I might wonder about the shuttle valve in the box but you're on your second box this go-round.

You probably had to turn in the original box as a core? If not I'd be curious if the problem remains with the first, sloppy box installed.

Let us know what you do.

You might try vacuum bleeding again because it's free. I once had this happen on an '03 Caravan after rack and pump replacement. Back then I finally took it to a shop to diag. They declared the pump faulty but also said they "pressure bled" the system, which they said didn't fix it. I don't know what kind of bleed they actually did. I paid for the diag and said I'd consider my options, but after that it was completely fine. All I can figure was a VERY stubborn air pocket somewhere in the system.

After that I actually bought a used Kent Moore p/s pressure gauge because I wanted the ability to SEE what a pump was doing, but I've never had reason to use it since. Helpful, eh??

 

Actually sorry just saw the 1500 rpm thing. To me that's a pump issue.

Doesn't Lucas make something for p/s systems? I'd be curious if there's a hack to increase viscosity a bit -- not as a long term fix but in the name of science to see if a thicker fluid increases pressure and your problems go away or improve.

You have a mismatched box and pump -- I wonder if the box's shuttle valve has a different threshold? I KNOW you said it all worked originally but maybe that was just used or weak valves/springs/parts.

Can you fit a Sag to this application? Is it already a Sag pump?

 

I had a similar issue when I converted my '79 to hydroboost using parts from an '01 superduty.

After installing new steering gear, pump and HB unit I went through the bleed procedure as recommended by redhead which amounted to about 30 cycles back and forth with engine off, tires off ground. After doing that, fired truck up and had p/s for about a second, then I could only get assist by revving the engine.

I too purchased at vacuum cap bleeder and played with it for days pulling about 20 inches of vacuum. Still didn't fix the issue.

As a last resort I pulled my brand new p/s pump apart to discover the bypass valve was stuck in the open position, that's why I had a little bit of p/s at higher RPM but not at idle. The valve I am speaking of is where the pressure hose threads into the pump, you may notice you can fit (if I recall correctly) a 7/8" wrench. I left the p/s pump bolted to the bracket so it was held secure, removed pressure hose, loosened the fitting with wrench then slowly removed by hand (there is a spring behind the piston so use caution). When I pulled the fitting out, the valve/piston was jammed. Using my finger I was able to get the piston freed up and the spring pushed it out of the housing. Once I got the piston/valve moving freely I reassembled the pump, filled with fluid, put vacuum cap back on and bled the system pulling 20 inches and letting it sit for 10 minutes or so.

Fired the truck up and VOILA! I had full power assist at idle.

I did some additional reading after I got it working and apparently that valve can get wedged open if you fire the engine up and there is still a lot of air that needs to be bled from the system.

I had swapped many p/s pumps in the past and never had these kind of problems. My guess is that the combination of new pump, steering gear, HB unit and hoses there was a lot more air in the system than I thought when I initially fired it up. By the time I attempted the vacuum cap to bleed I will guess most if not all the air was already out of the system, but since the valve was stuck open it didn't matter.

Hope that helps. Good Luck!

 

Well, same issue with original gearbox. Works fine on initial startup, rev and lose power steering. So gotta be an air bubble somewhere, and just airlocks the system when revving.

Put the reman back in. Might be hearing some air bubbles this time about 1 turn from the right stop, but hard to tell with my hearing. Spent a while turning the wheel to no avail, so just left the mityvac on it for the night (false hope).

The gearbox and pump shouldn't be mismatched, both fit the same '07 Vin I've been using. Also wouldn't make much sense to now have the same issue with old box.

I had actually pulled the relief valve on the original pump/the first gearbox, thinking maybe it was stuck, but it was free. Though potentially it unstuck itself during removal. Wouldn't hurt to check this pump too I suppose.

Was thinking of pulling the p/s cooler down and giving it a spin (still stock tube "cooler" on the engine cross member), but I don't see the return side causing an airlock. I have removed and lifted the reservoir up above the fender (though it's already mounted about level with the fender up by the hood hinge), but that didn't help any.

 

Wow! In an act of desperation, I've never heard of reverse bleeding a p/s system but I wonder if it's possible? Force fluid from the return line to the reservoir via the pressure line?

Let us know what happens, it's gotta be possible to resolve this.

 

I'd thought about that, but reverse bleeding shouldn't be possible. The gearbox has check valves (or just the function of the spool valve, not entirely sure) to prevent the fluid from going in the opposite direction. Which is also why rotating the steering back & forth w/ the engine off bleeds the system, rather than just pushing the air bubble left/right, as fluid will only flow in one direction.

But I believe the situation is resolved.

The pressure valve in the reman pump was partially stuck when I checked it today. Pushed the valve down, and was fine again until revving the engine. Smacked the pump with the engine running a few times, and I'd get power steering back, but would lose again once revving up. Went back and forth a few times with that, to no avail.

Reinstalled the original pump, and I have power steering the whole time, no issue revving the engine, took a drive to town, all good!

So my assumption is that the gearboxes were all good. The relief valve on the original pump, after first installing the gearbox must have become stuck on first startup (likely an air bubble, as mterickson had described above). And then I swapped the pump with one that would continually stuck the relief valve, showing the same symptoms.

Now I just need to resolve the steering being a bit "twitchy" when driving straight and have to turn the wheel just a touch on a minor curve. Presume bad ball joints/tie rod ends (know the upper bj's are bad as I ignored them when installing caster shims, haha), as it was kind of like this with the old gearbox, just much more noticeable now. Hopefully that's just due to the gearbox not having a 1/4 turn of play.

Appreciate all the help and ideas from each of ya!

TL;DR: pressure relief valve on the pump was sticking open (bad pump)