Learning 101: What is an IAC valve and what does it do when it fails?

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From the Sea Ray Boater's forum: The IAC valve regulates the air flow at low RPM's or idle by bypassing the throttle plate thus allowing the engine to operate at the low rpms which you need to do on fuel injector engines. "So and So" carries a spare IAC VALVE with him at all times....

I thought I would buy a newer boat thinking the technology is better:


My 2013 MerCruiser 377 Mag EFI engine encountered problems upon startup half way through the season. I couldn't get it to warm up without physically hand holding the throttle to keep it running. Once it warmed up I could drive the boat fine but the idle was very low and seemed like it would stall out, but it didn't . It got right up on plane and ran great but when I put it in neutral it ran around 350 rpms. Earlier in the year, I got an Engine Misfire light that never went away. I though I found a loose spark plug wire and pushed it back on but the warning stayed on. Apparently a dealer with the Mercury $6,000 diagnostic software package can diagnose the cause, repair it and re-set the computer.

I called Mercury, bought a service manual, watched U-Tube Videos, bought a new distributor cap and rotor, bought and replaced the "Cool Fuel" paper filter inside the canister, found out a pancake filter underneath the canister was never replaced and not listed in the service manual, so I replaced that and replaced the IAC filter, some call it a slinger and others call it a muffler. The IAC filter is a 1" x 1" x 1/2" piece of foam that inserts into the throttle body and costs $10. I also toted 14 gallons of rec fuel and added Stabil to it thinking maybe I had bad gas. Since it jumped right up on plane, It couldn't be bad gas, could it? All in all, it cost me $500 and the problem was still there.

Since it was the end of the season and since my dealer was going to pull the drive off and service the drive ($400) and winterize and shrink wrap the boat ($473), I reluctantly asked them to check out the stalling problem. They diagnosed and replaced the faulty IAC valve ($187) , added spark plug wires ($145) , replaced spark plugs ($156) , diagnosed it on the computer and determined it was fixed ($625 labor). All of this work came at a hefty price on top of the $500 I spent getting no where. I could have done the spark plugs and wires, but they had me hooked... I had a weird noise, like a howl, that the mechanic said was the fuel pump and it's normal. Maybe the low idle played a part in the FP howl...Thank goodness the fuel pump wasn't bad.

I had older boats with carburetors and all of this PCM (power control module) and fuel injected technology is new to me. Is it possible to work on your own boat if its a computer guided engine? Everything is sensored, labeled and coded in the Power Control Module to shut the engine down if something goes wrong or give you a warning. All I could do is throw a bunch of parts at it, but now i realize i went down the wrong path...

I am hoping this helps someone who may have the same problem and they don't waste a bunch of dough like i did..
 

Janky

New member
Decided to do a little research... I'm fairly confident I found the Volvo Penta part for my 5.0 GXi and it's about $280. I also found a Sierra equivalent for $70. Then I found a no-name brand on Amazon for $35! Crazy. I'll gamble for $35
 

Doc

Liquid Addiction
Staff member
A friend bought the Amazon one and fixed his boat two years ago. I had a different issue but we thought it might fix mine also. Seemed worth the gamble of a few bucks. Bought a two pack on Amazon for 47 bucks. did not fix my issue but I have a spare now if / when I need one. :D
 

Iggy

Active member
A friend bought the Amazon one and fixed his boat two years ago. I had a different issue but we thought it might fix mine also. Seemed worth the gamble of a few bucks. Bought a two pack on Amazon for 47 bucks. did not fix my issue but I have a spare now if / when I need one. :D
I would put the original one back since there was nothing wrong with it. Rather than depending on an off brand.

On my old boat, my fuel pump was going. A new Volvo one was close to 800. On Amazon it was half the price so I bought it knowing that I could easily return it!. When I got it, it came with a fuel filter. BUT the fuel filter looked the same but used a different thread type. Then I noticed there was paint inside the fuel ports. It went right back!!
 
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Doc

Liquid Addiction
Staff member
I would put the original one back since there was nothing wrong with it. Rather than depending on an off brand.

On my old boat, my fuel pump was going. A new Volvo one was close to 800. On Amazon it was half the price so I bought it knowing that I could easily return it!. When I got it, it came with a fuel filter. BUT the fuel filter looked the same but used a different thread type. Then I noticed there was paint inside the fuel ports. It went right back!!
My fiend who replaced his bad one, and mine both ran fine for two summers without issues. His is still going and as far as I know the one in my boat is still working. I sold the boat in July of 2020 and talk to the new owner once in awhile. No mention of that going bad, but he has told me of other issues.

I agree not all the cut rate replacement parts should be relied on but in this case it seems to have been okay for what it did.
 

Janky

New member
My fiend who replaced his bad one, and mine both ran fine for two summers without issues. His is still going and as far as I know the one in my boat is still working. I sold the boat in July of 2020 and talk to the new owner once in awhile. No mention of that going bad, but he has told me of other issues.

I agree not all the cut rate replacement parts should be relied on but in this case it seems to have been okay for what it did.
I would say it also depends on the ease of replacement at sea. I got the $35 IAC valve and will try it, but I will most certainly keep the old one in the toolbox. I would not do the same 'gamble' with something like a raw water impeller. Though I would trust a known aftermarket brand like Sierra.
 
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Denny

Well-known member
Gold Site Supporter
No short cuts with my girl. Tows and a break down in bad weather could cost you more then you really want to pay.
 

Iggy

Active member
What happened to me was that the idle went very high. I was 50 miles from home when it happened. So just putting in gear, there was a big bang. So an after market part like that. As Denny said, it could cost you more. Oh well.............
 
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Janky

New member
What happened to me was that the idle went very high. I was 50 miles from home when it happened. So just putting in gear, there was a big bang. So an after market part like that. As Denny said, it could cost you more. Oh well.............
Thanks for the warning. Well, at a minimum, the $35 is probably the cheapest one can ever pay for a quick diagnosis. If it resolves my issue of stall coming off throttle, then I know that part was root cause. If not, look elsewhere. But I will think twice about leaving it in if it does solve the problem.
 

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Janky

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Finally a follow up. Works like a charm. Had it out about 15 times so far this season. I will say, the motor still dips down in idle as soon as I get in neutral after slowing down, but it doesn't stall like it used to.
 
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