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View Full Version : What are the symptoms of a bad IAC?



Nehalennia
11-13-2008, 05:29 PM
I believe Dave S carries a spare IAC on board, so I was wondering what the symptoms of a bad IAC are and what does it do exactly?

I had mine replaced when the idle didn't come back down below 1000RPM after being on plane or above 2000RPM.
I know it stands fro Idle Air Controller but are there other symptoms that anyone else has had?

mvoellinger
11-13-2008, 05:40 PM
I know on cars it can be any number of symptoms involving idle. A hunting idle where it bounces around, inability to idle cold without giving it some gas now and then, etc.

Dave S
11-13-2008, 06:00 PM
Todd

I had two IAC failures and in both cases there were no advance warnings that I remember. One minute the boat would idle fine and the next minute it stalled and couldn't be re-started without advancing the throttle. As soon as the throttle was returned to idle, the motor would stall. However, I do recall a number of other threads on CSR where the symptoms were a bit different and a new IAC fixed it.

Dave

wish2fish
11-13-2008, 07:25 PM
my situation was like dave's above. Worked fine all weekend and when I came off plane to pull the boat on Sunday the boat stalled.

Fishnut
11-13-2008, 07:52 PM
I had one fail. The engine will stall in neutral or without the throttle pushed forward. You may also get an alarm of the every 2 minute type.

As I understand it, the IAC 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.

eigcpa
11-13-2008, 08:24 PM
I had to have mine replaced this past summer. The engine alarm beeps, then engine would not idle. you could start and run it if you push the throttle forward without engaging the gears. As soon as I brought back to neutral it would stall. My marina confirmed its failure using the computerized diagnostic tool.

Nehalennia
11-13-2008, 10:12 PM
Thanks guys. Just curious what to look for since it seems to be a failure with somewhat common symptoms

mwph
11-13-2008, 11:33 PM
And the cost of said controler is..................?
And the difficulty of said install is................?

Still LauCo
11-14-2008, 06:57 AM
I had to replace mine on the starboard side when the engine wouldn't re-start and when it finally did there was backfiring when I would try to advance the throttle.

MWPH, I think it was about a $50 part, and install is very easy.

Presentation
11-14-2008, 07:03 AM
And the cost of said controler is..................?
About $100 (I think I paid $89 at a dealer last year) or $200 if you are smart and buy a spare. (buy a new gasket also, couple of bucks)




And the difficulty of said install is................?
It’s a DIY (do it yourself job) and takes about 4 hours.

Time breakdown for the novice:

1 minutes to grab the newly purchased IAC and Allen wrench kit and walk from your car down the dock to your boat.

3 hours and 53 minutes to stand on the dock with your new IAC in your hand as you talk to fellow dock mates explaining what you are going to do, your symptoms, how you read about doing this here at CSR.

1 minute to open the hatch, remove the flame arrester, that thing that looks like a air filter on the top of that big black thing called a engine, 2 minutes for frustration as you cant find a silver colored object like the new IAC you are holding in your hand, then you get a few seconds of “Oh” as you realize its right there in front of you, its just been painted black like everything else, a minute to find the correct Allen wrench size from your kit, a minute to unscrew the screws, unplug the plug, remove the old IAC, replace with new IAC, re-screw, re-plug, and replace the flame arrester. 1 minutes to sit there and enjoy your job well done.

Now, at the end I recommend you leave a few hours to stand by your boat with the old IAC in your hand to tell the new people coming down the dock what you did and how much money you saved by doing it yourself but since the job is done you can not count this time.

carterchapman
11-14-2008, 07:39 AM
About $100 (I think I paid $89 at a dealer last year) or $200 if you are smart and buy a spare. (buy a new gasket also, couple of bucks)




Its a DIY (do it yourself job) and takes about 4 hours.

Time breakdown for the novice:

1 minutes to grab the newly purchased IAC and Allen wrench kit and walk from your car down the dock to your boat.

3 hours and 53 minutes to stand on the dock with your new IAC in your hand as you talk to fellow dock mates explaining what you are going to do, your symptoms, how you read about doing this here at CSR.

1 minute to open the hatch, remove the flame arrester, that thing that looks like a air filter on the top of that big black thing called a engine, 2 minutes for frustration as you cant find a silver colored object like the new IAC you are holding in your hand, then you get a few seconds of Oh as you realize its right there in front of you, its just been painted black like everything else, a minute to find the correct Allen wrench size from your kit, a minute to unscrew the screws, unplug the plug, remove the old IAC, replace with new IAC, re-screw, re-plug, and replace the flame arrester. 1 minutes to sit there and enjoy your job well done.

Now, at the end I recommend you leave a few hours to stand by your boat with the old IAC in your hand to tell the new people coming down the dock what you did and how much money you saved by doing it yourself but since the job is done you can not count this time.

Doug - that is great - ROFLMAO - 3+53 of the 4+00 standing on the dock talking!

Great post!

Carter

douglee25
11-14-2008, 07:53 AM
The IAC valve is basically a controlled vacuum leak. It regulates the idle by allowing the intake plenum to vent to the atmosphere. Have you ever taken a vacuum hose off an engine while the engine is idling? What happens? The idle increases. The same principal is applied here.

The IAC can fail in a couple different ways...

1. The IAC can fail in the closed position. The engine will not idle (may just die when coming off plane) and more than likely, will not start without the throttle being cracked open.

2. The IAC can fail in the open position. Mind you the IAC is not a discrete valve (on/off) but rather an analog device that varies with voltage. In this case if it fails, you will get an engine that has a high idle which will never return to its normal state.

Hope this helps.

Doug

Larry&Leisa
11-14-2008, 09:40 AM
Is there any point in changing one out after so many hours or is it just something that either it works or it doesn't...?

Nehalennia
11-14-2008, 09:44 AM
This is great info guys, I'm glad I asked. I may go pick up a spare.

Turtlesboat
11-14-2008, 09:51 AM
yeah, it's one of those... spend a little now or a lot later, things. My first one went with a warning, second one went with no warning.

Vince_nj1
11-14-2008, 10:29 AM
My boat is a 2006 with 6.2 Horizons which I believe are fuel injected. The last time I was on my boat I tried to start the engines without touching the throttles and they struggled to get get started. I featherd the throttles, got them started and then went back to idle and they stalled. I restarted them, got the RPMs up to around 1200 for about 30 seconds and then went back to idle and they stayed running.

Is this normal for a fuel injected engine? Could I be having AIC issues? They run fine when warm, but I find it was odd that they needed a little help to start and stay running when cold.

douglee25
11-14-2008, 10:29 AM
Is there any point in changing one out after so many hours or is it just something that either it works or it doesn't...?

I believe your 7.4's are carbed, correct? IAC valves only apply to fuel injected motors.

Doug

douglee25
11-14-2008, 10:42 AM
My boat is a 2006 with 6.2 Horizons which I believe are fuel injected. The last time I was on my boat I tried to start the engines without touching the throttles and they struggled to get get started. I featherd the throttles, got them started and then went back to idle and they stalled. I restarted them, got the RPMs up to around 1200 for about 30 seconds and then went back to idle and they stayed running.

Is this normal for a fuel injected engine? Could I be having AIC issues? They run fine when warm, but I find it was odd that they needed a little help to start and stay running when cold.

They could be on their last leg. There are several different styles of IAC valves, but typically there is a pintle that moves in and out via a stepper motor. The pintle binds due to carbon buildup. That could be what's taking place on your setup. Try removing them and bench cleaning with some carb spray.

Doug

Larry&Leisa
11-14-2008, 10:54 AM
I believe your 7.4's are carbed, correct? IAC valves only apply to fuel injected motors.

Doug

on the 370EC correct.... but the 240SD :smt001




(but thanks.. I would have been looking all over... :smt100)

Turtlesboat
11-14-2008, 11:03 AM
I was led to believe that they generally last 2-4 years. one that went last year was original which meant it was 5 years old. the other one that went had already been replaced 3 years ago.

douglee25
11-14-2008, 11:07 AM
I was led to believe that they generally last 2-4 years. one that went last year was original which meant it was 5 years old. the other one that went had already been replaced 3 years ago.

I wonder if it's the salt atmosphere on boats that make them wear out fast? I've had fuel injected vehicles from the late 80's and they were on their original IAC valve. That's almost 20 years! I don't know if mercruiser recommends cleaning on a scheduled plan, but I think that might be something to add for your FI guys.

Doug

wish2fish
11-14-2008, 12:23 PM
My boat is a 2006 with 6.2 Horizons which I believe are fuel injected. The last time I was on my boat I tried to start the engines without touching the throttles and they struggled to get get started. I featherd the throttles, got them started and then went back to idle and they stalled. I restarted them, got the RPMs up to around 1200 for about 30 seconds and then went back to idle and they stayed running.

Is this normal for a fuel injected engine? Could I be having AIC issues? They run fine when warm, but I find it was odd that they needed a little help to start and stay running when cold.

what are the chances both would be going bad at the same time? Plus whether warm or cold they would normally still stall. I'd be thinking something else.

carterchapman
11-14-2008, 01:44 PM
I wonder if it's the salt atmosphere on boats that make them wear out fast? I've had fuel injected vehicles from the late 80's and they were on their original IAC valve. That's almost 20 years! I don't know if mercruiser recommends cleaning on a scheduled plan, but I think that might be something to add for your FI guys.

Doug

Doug,

You are absolutely right about the GM auto IACs. I've had the same experience. I don't know if the Mercruiser part is the same as the GM part, but it brings up a question - I've used a can of Chemtool B-12 added to my cars' fuel tanks each oil change. Is anyone adding injector cleaner to their Mercruiser MPI's?

Presentation
11-14-2008, 02:55 PM
Doug - that is great - ROFLMAO - 3+53 of the 4+00 standing on the dock talking!

Great post!

Carter

I agree it was funny; however its a pretty accurate breakdown.

Glad I made you laugh.

mwph
11-14-2008, 09:10 PM
Thanks for the great info guys..........and the breakdown of the install........priceless!:smt038

dsteele1
11-15-2008, 12:45 AM
Here's a previous post with a picture of an IAC...

http://www.clubsearay.com/forum/showthread.php?t=11308

BON DIA
11-15-2008, 09:27 AM
vince nj1 wrote ... My boat is a 2006 with 6.2 Horizons which I believe are fuel injected. The last time I was on my boat I tried to start the engines without touching the throttles and they struggled to get get started. I featherd the throttles, got them started and then went back to idle and they stalled. I restarted them, got the RPMs up to around 1200 for about 30 seconds and then went back to idle and they stayed running.

Is this normal for a fuel injected engine? Could I be having AIC issues? They run fine when warm, but I find it was odd that they needed a little help to start and stay running when cold.

We have 6.2s also & your description is typical of our winter initial start procedure, air temp < 45, both engines. No problem in the summer. We did have an IAC failure last summer, engine died when throddle closed.

Chris R
11-15-2008, 10:54 AM
I'm assuming DTS has no effect, one way or another, on this?

This is another of those things I simply scratch my head and say "why" on. For this to be so common people carry spares I mean. I cannot imagine carrying spare parts around in any of my cars. Harsh environment or not, I think of what my boat cost versus my most expensive car (Jaguar) that has never been near a repair shop.

Thanks for listening!