Diagnosing an issue - Question about IAC

[yoster]

Hello everyone,

I have a question about the IAC. Is it a device that can be "going out" or is it a binary "working" or "not working" type of thing?

I've been dealing with a set of symptoms for about 2 years now that I can't quite crack.

Specs:
2004 Larson 212LSi
2004 Volvo Penta 5.0 GXi, SX-M 1.60

Symptoms are as follows:

• When idling, if I rev it up in neutral and then QUICKLY pull back to idle, the RPM will drop low, almost to stall, then it will 'catch' itself and idle smoothly. Never causes an issue on the lake as I don't come 'instant' off plane anyway, but I can definitely replicate the issue on the trailer over and over again.
• This will also happen when starting cold. It will rev high, drop down and almost die, then catch itself and idle 100% fine from then on.
• On very random occasions, when warm out on the lake, it has a hard time starting. It tries, coughs, but won't catch. Almost like vapor lock but it isn't. Once it starts, it idles fine. And it always DOES start just by cranking more and more until it finally decides to catch and go. But it always starts. And once it does, 100% normal idle.
• Last year, completely out of the blue, while idling on the trailer, the engine just died out of nowhere. I mean going from a strong idle, to off. As if you turned off the ignition. Happened 3 times in a row, then ran 100% fine the rest of the day/season. It was the strangest thing. Has never happened since.

Here is what I have changed during this time as general maintenance, none of which have affected the issue in any way whatsoever:

• Fuel pump assembly. Also pressure-tested.
• Pressure regulator
• Cap & rotor
• Spark plugs & wires
• Pressure-tested the injectors for leak-down (they held fine)

My hypothesis is the following, which I'd love someone here to approve or deny. I'm wondering if the IAC is 'going out' or 'sluggish' - and not retracting FAST like it should. Which would explain when I come QUICK off a fast idle, it almost dies then recovers. It also might explain why on a cold start, it starts and revs REAL high, then almost dies, and recovers. It (could) also explain my very random engine stalls last year on the trailer.

Is it possible for an IAC to be 'going out' or intermittent to some degree?

Otherwise this engine is smooth as silk, accelerates like brand new, etc.

 

[ESGWheel]

I had similar issue with my 2005 5.0 GXi-E. Was kicking my butt including my marina’s mechanic who took it for a number of test runs all to no avail. My symptoms were more severe then you describe > it would mostly stall post throttling down quickly but otherwise the same.

A separate - and at the time I thought unrelated - issue I was having was dreaded belt dust. Previous owner had this dust issue as well based on the mess in the engine compartment. I was chewing up a belt a season until I finally got it resolved.

Back to the stalling issue. I was done with it so decided to shotgun it > started replacing one by one all the various sensors staring with Throttle Position (no change) IAC (no change) and then went after the Manifold Absolute Pressure (MAP) sensor. Pulled it out and it hit me like a ton of bricks: it was all crapped up with belt dust! I just cleaned it and left the new one in the box to see. I have never had an issue sense, purrs like a kitten.

If not familiar with these, MAPs are sensitive devices and should only be cleaned with the correct spray cleaner.

If you have belt dust, check your MAP senor. Heck even if you do not have dust check it anyway

 

[yoster]

Hmm well, the engine compartment is pretty darn clean, and it's very low hours (about 70) - BUT - I'll still check it! The engine sat for YEARS before I got it and got a bunch of mold in the engine bay. She's spotless now but, I'm wondering, if that device got some crud on it in all the years of sitting. Worth checking out - thanks for the idea! I just so happen to have MAP sensor cleaner spray on hand too