[FIXED] '94 Chevy Lumina 3.1L MPFI Random, intermittent, cranking no start.

I’m pretty much stumped on what direction to go with this thing, thanks in advance for any input.

1994 Chevy Lumina, 3.1L MPFI V6
I have an intermittent, cranking no start.

When it does this, it will start and run if I give it some throttle, but will immediately die if I release the throttle. There is a fuel smell from the exhaust when I do this, so it appears it’s getting too much fuel and flooding. It happens randomly; any engine temperature, and any amount of time after driving.
When it starts, it starts and runs great, nothing out of the ordinary.
The longer it sits after driving, the more likely it is to start, but it will sometimes still do this even after sitting for days.

Things I’ve done / Other information that might be useful:
-No trouble codes.
-I have spark. When it is not starting, I still get spark between the coil towers when I pull two wires off a coil. (This is a waste spark ignition system)
-Tested the impedance of the injectors, and they seem to be within spec. The injectors are under the intake plenum in this engine, so I had to measure their equivalent impedance in parallel and get an average, which was within spec. (This is a group fired system, so all the injectors are wired in parallel, and they all spray at once)
-Fuel Pressure is within spec with key on, and holds.
-The fuel pressure drops when I apply a vacuum to the fuel pressure regulator, (as it’s supposed to), and no fuel leaks from the regulator.
-Throttle position sensor is good. I back probed the connector and the voltages for closed and open throttle were what the service manual said they should be.
-Coolant temperature sensor appears to be good. I pulled the harness and measured its resistance as the engine warmed from ambient temperature to operating temperature. The service manual has a table of resistances and temperatures, and it was consistent with those.
-MAP sensor is only about 5 months old. I had the same problem in February, and the sensor signal voltage was about .5V off of what it was supposed to be for no vacuum, so I replaced it. I thought it fixed the problem because it didn’t happen again until a couple weeks ago, but I guess not… I will probably check it just to be sure.
-I can’t find any vacuum leaks. This engine is known for bad lower intake manifold gaskets, so I’m wondering if this could cause a sneaky intermittent vacuum leak bad enough to cause this problem?
-I put iridium spark plugs in a few months ago, (Didn’t have this problem until about 2 weeks ago), and I was wondering if the smaller electrodes were more prone to fouling, so I replaced them with the factory recommended AC Delco copper plugs in hopes that that was the problem. There was no change.

Lumina_of_doomina wrote: it will start and run if I give it some throttle, but will immediately die if I release the throttle.

Have you cleaned the throttle-body/IAC?

pole71 wrote: Have you cleaned the throttle-body/IAC?

I had cleaned the throttle body, but I overlooked the IAC. I just went out and pulled the IAC to clean it. The plunger was fully extended when I removed it, (Which I don't think is where it is supposed to be), and didn't go back in all the way when I tried to re install it. (I was careful to not move the plunger when cleaning). Thinking this was odd, I turned the key on with the IAC out to see if it moved. It moved, and then the plunger launched out of it. (I did this before to test the IAC ~5 years ago for a different problem, and it did not do this.) I replaced the IAC with a good one I had lying around from when I was a 16 year old parts changer. The car has since started twice in a row. I'm not going to call this fixed yet in case it was a coincidence, but I'll try to take it to work tomorrow and see how it goes.

If that was the problem, I'm going to feel dumb. Thanks for the suggestion. I can't believe I overlooked that.

There is no dumb! Knowledge!!! That is why we are here!!!!

Kirby wrote: There is no dumb! Knowledge!!! That is why we are here!!!!

Good point. Lesson learned: Never overlook the obvious or assume previously changed parts are good. Guess they don't make stuff to last anymore...
It has started every time since Wednesday night when I did that, so I'm gonna call this fixed. Thanks again!