IAC questions

I have a 94 GMC Suburban 2500 7.4l....I think my IAC is failing. I have cleaned it and the bore, set idle by disabling the IAC. But when I start the truck, it will stumble then recover and stumble again, then recover. does this a couple of times. Once its warm, and I shift into gear, it will stumble then idle will go up and down between 200-1000rpm until I give it gas and go. Coming to a stop it will do the same thing. I looked up a new IAC motor and 2 are listed...a 10mm pintle and a 12mm pintle. The one that's in the TBI now has been replaced before...my question is.... HOW DO I KNOW I HAVE THE CORRECT ONE?..
sorry, wasn't yelling, just wanted my question to stand out.
Thank you for any advice.

I don't know exactly where gm measures them so I only go by bigger and smaller. My 94 7.4 has bigger, my 93 7.4 has bigger and a tbi off a 92 I have is bigger. The outer edge of bigger is over 1/2 and smaller is under 1/2. Also info would be did it just start doing this with no changes or did you make some bin changes. Do you have data logging setup?

Thank you. I just bought this thing and am trying to get it ironed out.

Have you observed IAC steps when this happens?

I doubt that the IAC is your issue.

When's the last time a full tune up was done? What is the timing at? What is the fuel pressure? etc...

I just did a full tune up. cap, rotor, wires, plugs, timing set at 4*. Just got a new fuel pump in May. I need to get the adapter to check fuel pressure. When I took the IAC out, the pintle moved in and out like a PCV valve. Ive been told its bad.

You still didn't mention if this problem has always been there or if say it started after you did the tune up. When you go to check fuel pressure the 94 should be 28-30. The 7.4 has a different fuel pump than a 5.7.

The problem existed when I bought the truck last week. I did the tune up and it still existed. Removed the IAC cleaned the bore and valve, problem still there. But a friend of mine told me the pintle should not move in/out freely.

Last night I drove it and at each stop sign, it stumbled, it did die once(5 stop signs)...but did stumble at every stop.

You can push and pull the pintle but it takes a little pressure. It won't just flop in and out with no pressure. Do you know when this problem started with the previous owner. Is it possible it started after the fuel pump change.

I do not know when it started. They didnt give me much info. They were asking 1800 for the truck and I bought it the next day for 500 because they didnt want to pay for registration again in a few days. Yes the pintle just kinda flops around.

Just as a check I shook all the spares in the shop and non of the pintles move while shaking. The iac is 30-40 $ so with your good price it might be a cheap fix. My 2 bits would be for the 12mm. Also might still be a good idea to test fuel pressure.

So, I pulled the TBI off the truck and found a few things. It seems to be a rebuilt TBI. An injector spacer was added to it, the injectors were sealed into the pod with silicone, the TPS 'gray' wire was corroded, and the return line nut was stripped. So I cleaned the gray wire and filled the connector with dielectric grease, replaced the IAC motor with Standard AC27, went to the wrecking yard and pulled a return line from a 5.7 truck. NOT THE SAME. I just cut off the nut, cut MY return line installed the new nut and flared both ends of the cuts and used a piece of 30r14 fuel hose to splice it back. After I replaced,cleaned and fixed everything, I took it for a drive. It stumbled once, but other than that, it ran great. I still want to check fuel pressure

Does your cel work and if so what is it doing?

Right now I have a code 36. Which ut has always had.

36 is the pcm controlled vacuum kicker right at the throttle arm. Does your truck have that? I would disconnect the battery to cancel the codes and start fresh. Years ago I disabled the kicker and found with the bins these trucks run better with the kicker enabled and working. It is used for cold start but some also say it is used for when you shut off hot it prevents throttle blades sticking after cool down. I am pretty sure it is also used as part of throttle following but info on that is hard to come by. It has the throttle kicked slightly on all starts hot or cold so check it out. With the engine off you should be able to see it holding the throttle arms open slightly. You can adjust the amount by turning the screw on the can.