8.1 Engine- Starts but low rough idle then quits

Erik R

Member
Mar 15, 2015
120
Lake Lanier Buford Georgia
Boat Info
380 Sundancer 2003
Engines
8.1 Mercruiser Horizons
This has happened 2x now. Go to start the engines in my 2003 380 Sundancer and the port fires and idles, then I crank the starboard and it fires up but it's running at a low idle, has a uneven rumble to it, then it dies. If I crank it again and give it some gas and run the RPMs up a bit, that seems to help get it going. Once it's up and running and I let it warm up, she's good. Any ideas very much appreciated? Spark plugs? Spark plug wires? Fuel filter?
 
Plugs usually cause the transmissions to rattle in gear at idle speeds, but even with a bad plug the 8.1 will accommodate other cylinders to even it out. Plug wires may be the issue, but it's low on the probability list. Fuel filters will usually have more issues with the engine under demand - not so much at idle.

My guess would be the idle air control (IAC). Its a GM part that you can get from any supplier and swap out yourself. This little control determines the correct amount of air allowed into the intake at idle to just above idle (while the throttle plate is closed). There is a tube attached to the throttle body that extends over the top of the engine to the IAC on the opposite end of the engine. It's attached with two bolts and needs a new gasket when replaced to prevent air leaking into the intake. Once this control fails to operate properly its real difficult for the engine to idle, but once past 1000 rpm or so it doesn't come into play at all.
 
I second IAC. Easy to replace.

If that doesn’t fix it, I’ve seen multiple ECM failures that stop operating IAC correctly. Just had it happen to mine last year, sent to get rebuilt and they were unable to, but they gave me an excellent deal on a new one. The company was OBD repairs in Redondo Beach CA.
 
Thanks. Is there any way to test the IAC? Or is it cheap and easy enough to just replace it?

I guess you could swap the two IAC's between engines and see if the problem follows the IAC location, but you'd have to replace the gasket on each one. Probably better to just get a new IAC and give that a try. Just do a quick google search for Idle Air Control Valve. I found some on E-Bay in 2 seconds as low as $46 with free shipping (including IAC and gasket).

https://www.ebay.com/itm/OEM-Idle-A...m=183098336308&_trksid=p2047675.c100005.m1851

There are also videos on youtube showing how to replace.
 
Most likely the iac. Had same issue. I used a small amount of wd40 to clean the actuator pin inside. Moved it back and forth until it was smooth as butter. Put it back on and back to a smooth 650 rpm. I bought extra gaskets, cheap. However I got lucky and the original gasket stayed intact.
 

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