Does my engine have an IAC Idle Air Control Valve?

Crushin It

Crushin It

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Oct 10, 2006
1,716
Safe Harbor Marina Kent Island, MD
Boat Info
2006 44 DA
QSC 8.3s
Engines
Cummins QSC 8.3s 500HO
Idle Air Control Valve? Engine is 5.7L GM 350 V-8 1988-1995 S/N OF349908. Just started to idle rough. Had new plugs, wires, distributor cap,
rotor installed last year. Trying to figure out the rough idle and stalling below 1000RPM. Fires right up, but won't idle. I'm reading posts
mentioning these symptons could be an IAC valve. I know I have never replaced this. If I have one, can someone tell me where it is located?

Thanks,
 
On the throttle body. Suggest getting a service manual or look at the online part schematics to see where she hides....
 
Stupid question but. you do have fuel injection?

Look at the wiring going to the throttle body. 2 plugs? 1 will be the IAC
 
A bad IAC rarely gives rough idle. It usually just wont hold idle and engine will turn off.

Which leads me to believe you don't have one or is not your problem. You may have accidently swapped your spark plug cables when you servised your engine. If you have verified already check the distributor cap sometime they are finiky.
 
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OK. A little more info for everyone. These engines are carbed not fuel injected. And the engine does try to idle but then dips below 500RPM and shuts down. When assisting with more throttle to keep it running I can see by the fluctuation in the tach and engine sync that their is a miss.....
 
So much for the IAC. Seeing you just tuned it up, the problem is most likely fuel related. Fuel filters done as well, no doubt.

Does it run well at WOT?

Could be crap in the carb

Should have done a compression test when the plugs were out, to eliminate a mechanical problem.

If it runs well at WOT, it's most likely a plugged idle circuit in the carb
 
I'm thinking choke...if it'll fire right up but won't idle on cold start, your choke is probably not opening fast enough. Easy to check, pull the flame arrestor and watch. Move choke with your fingers to see if it helps idle.

Tom
 
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Fuel filters were the first thing I changed. No difference. It does run well at WOT. It does have all new risers and manifolds as of last year also. The carbs have never been cleaned or rebuilt. 600 hours on engines.... Am planning to have a compression test by mechanic. Problem is mechanic can't get there until 2 weeks and it frustrates me that I over maintain and over baby these engines only to have an issue going on.
 
No need for a compression check if it runs well at WOT. I'd still bet on carb issue. Choke issue still a possibility, but now unlikely. Still easy enough to check. Electric chokes fail all the time.
 
Update from Mechanic who arrived - Compression check AOK! Plugs looked good. Suggested Carb to be rebuilt. How much should this cost? Hope to have back in a few days running great. Will keep u posted...
 
If you do it yourself...probably less than $75.00 for a carb rebuild kit. Labor will probably be a couple of hours if you hire someone.
 
You can probably take the carb to a rebuilder, my buddy had his rebuilt for two hundred bucks last year and it was problem solved.
 
Mechanic installed rebuilt carb tonite..Said engine sounds/runs great.. I'll be down this weekend to check out for myself...I can only guess these issues are stemming from E-10.
 
If you are using E10 fuel, stop. Paying more at the pumps will pay for its self in maintenance and repairs.
 
Your carb was 17 years old, can't blame every fuel problem on E10. Crud happens. I dose every fill with stabil, works for me.
 
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