Having an issue that is most present in colder temperatures (under 50 degrees F). 1994 B250 5.2l conversion van.
Engine will idle ok for a few seconds, and then you need to modulate the throttle every second or two to prevent it from stalling (engine rpm drops really low and it dies). Starts right back up.
On warmer days (50-65 deg F), it will usually idle by itself, but once it gets into closed loop or so, it will stall at idle.
In the summer, engine runs great without stalling. Occasionally, you can get it to stall if you shift it into reverse when the engine it still warming up.

-I got the check engine light to come on once after a stall. It was code 23, intake air temp sensor error (aka charge temp). I replaced this but stalling continues. Resistance of the old and new sensor is within spec, but old sensor was really gunked up. Code never reappeared.
-I cleaned the throttle body and the idle air control valve, but it didn't make a difference. I did unplug the battery to reset the the pcm.
-It does seem the IAC valve moves a little erratically and opposite of what it should be doing. For example, at (cold) idle it seems to be almost completely shut and when I press the throttle it opens up. You can hear a very loud hissing/sucking sound coming from the IAC, when pressing the throttle and also randomly at idle. Since it does move, I think the IAC is ok but the info it is getting from the pcm is incorrect. Or does this sound like a failing IAC to you guys?
-I also removed the IAC and moved the pintle in to allow more air to bypass the throttle body at idle. I reinstalled without the electrical connector. Engine idled perfectly without stalling.
-I can't get to the coolant temperature sensor although I would love to check it. There's a sensor just for the temp gauge on the dash (one wire) next to the air charge sensor, and there should be another two-wire sensor for the pcm next to the thermostat/coolant hose if my searching is correct. The service manual makes no mention of 2 sensors and does not give the exact location. I can't even find the two-wire sensor, am I looking in the right spot for a 1994 5.2? I think OBD2 1996+ has only 1 sensor.

Other quick diagnostics I did:
-Unplugged the O2 sensor, no change. PCM only uses O2 sensor input in closed loop mode and I wanted to verify we are staying in open loop.
-Unplugged EGR electrical connector, idle got worse. With EGR unplugged, valve is open at all times, verifying EGR is operating and staying closed when at idle.

So, I'm thinking it's either the IAC, bad signal to the IAC, or coolant temperature sensor. Or maybe MAP or TPS sensor?
Any tips? Also, any suggestions for OBD1 scanners that could show me the coolant temperature the computer is getting or possibly adjust the iac incrementally?
Thank you!

 

An IAC valve doesn't last forever. Even if it's clean, that doesn't mean to plunger is moving like it should. I would start with a new IAC.

 

Thanks for the response Alloro. IAC is moving and it seems like it can travel it's full track. It just moves a little erratically and opposite of how it should. Does this sound like a failing IAC or is it getting wrong output messages from the pcm?

I would love throw parts at it but it can quickly become more expensive than just having a repair shop do it. Had a bad experience on this forum where an admin told me my new aftermarket part doesn't work with Dodges. Replaced with a genuine part, didn't fix the problem and I was out $90.

 

The movement should be smooth, since it moving erratically that could mean the plunger is getting hung up due to dirt or debris having gotten inside of it. Have you tried the finger trick yet? With the engine running and the air cleaner cover off, using your finger, repeatedly cover and uncover the hole in the air horn/throttle body that allows air past the IAC valve. Doing this forces the valve to open more and then close when you remove your finger. Doing this can sometimes force a reset of how the ECM controls the IAC.

The only OEM parts I would suggest as being mandatory are the crank sensor and the fuel pump. History has shown many failures with those two when using aftermarket products.

 

Welp, it wasn't the IAC. Any ideas?

 

Have you done a fuel pressure test?

 

Fuel pressure is at 39 psi with key on and at idle.

According to the factory service manual, pressure should be at 31 psi. It goes on to say that if you disconnect the vacuum line to the fuel pressure regulator, pressure should increase to 39 psi. Does this mean that my fuel pressure regulator is bad? Anyone have experience with fuel pressure readings on the 5.2l engine?

 

94 should be port fuel injection, and the fuel pressure regulator should be in the tank. So, no vacuum line to it. Is that not what you actually have??

 

I'm wondering if the heat riser tube is still on the engine? Yes, you do need it in some cases to avoid ice choking the throttle body. I've seen ice form in 50 degree weather.

 

You're absolutely correct, and the factory service manual is wrong! Looks like the fuel line comes directly from the fuel tank into the fuel rail. I couldn't find the pressure regulator in the engine bay area, even after looking at the diagram they have in the book. This explains it!

So, looking at a 1995 (year later) manual that shows the in-tank fuel pressure regulator, spec should be 35-45 psi. Mine is in spec at 39 psi.