Our van has had an irregular idle for a while now, sometimes low, usually high. I’ve tried several things I found online after running the check engine codes. Cleaned the mass airflow sensor, cleaned the throttle body, ran catalytic converter cleaner through it.
It went away for about two weeks and is back now. One thing I read and haven’t tried yet is the power steering pressure switch. I bought a new one, but can’t seem to find it under the hood to change it! 
Any other ideas??
Also where in the world is this pressure switch???
Who makes your Express 3500?
My Corolla had an idle rpm problem a few years back. That turned out to the be idle air control device was defective. If your engine uses that part (IAC), take a good look at it, making sure it isn’t covered w/gunk and binding. Tapping on it with the plastic handle of a screwdriver can sometimes provide a clue. Other common reasons for idle rpm problems are
- dirty throttle body causing throttle valve to stick
- engine computer thinks coolant is cold when it isn’t, so it increases idle rpm when it shouldn’t
- vacuum leaks of all sorts. ask your shop to do a thorough vacuum system check. sometimes spraying starter spray in one location will cause the engine rpm to increase, indicating the vacuum leak is in that area. the power brake booster, when it fails, can cause big vacuum leaks.
- faulty idle up-devices. the power steering vacuum switch is one such example. on my Corolla that is part of the PS pump ass’y. there’s two vacuum-sized tubes attached to the ass’y. I’d tend to discount that as the cause, those are usually pretty reliable.
My guess, faulty or gummed up IAC.
Chevy and it doesn’t have an IAC that year.
What he wants to know is where the power steering pressure switch is.
I think replacing the pressure is a stab in the dark and won’t fix your problem. Cleaning the MAF and the throttle body made the problem go away for a while. Why would that have any effect on the power steering pressure switch? It doesn’t.
If you post the check engine codes here, that would be a great help as would the engine size, miles on it and how long this has been going on, did it steadily get worse from nothing or did it magically appear one day.
Lot of miles 270000
Other than this it runs good
6L V8
It started this early spring doing it every once in a while and slowly got worse to the point where it’s pretty much all the time now. I’ll get the codes and post them later today
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If you have an obd reader that can scan and graph both the commanded and actual throttle opening, you can see if there is any “noise” in either signal. If the throttle body is worn out, it may “hunt” trying to make the engine idle. If the throttle pedal sensor is worn out it may be comanding a jumpy idle.
You have enough miles on the engine that the throttle body may actually be worn out. Same could be said for several cylinders. A compression test would tell you a lot.
P0507 is the only code it’s giving me now
The P0507 is pretty much telling you what you already know! A P0550 would be a failed steering pressure sensor… so the truck thinks that is still OK.
From the interwebs…
A P0507 DTC trouble code may be caused by one or more of the following:
(my comments in bold brackets)
A vacuum leak - could be a cracked intake manifold
Leaking air intake after the throttle body (could be a leaking gasket, easy to inspect, cheap to change)
EGR valve leaking vacuum (EGR gasket, maybe?)
A faulty positive crankcase ventilation (PCV) valve (easy and cheap to replace, just do it and check the hoses, too)
Damaged/failed/dirty throttle body (OK, could be a bad throttle body, save that cost for last)
Failed EVAP system (no evap codes, I doubt that is the issue)
Failed IAC (idle air controller) or faulty IAC circuit (you don’t have an IAC so ignore this!)
Read more at: https://www.obd-codes.com/p0507
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