Faulty IAC or something else

[badrano]

I had some intermittent stalling issues at idle in gear and some rich smelling exhaust. Running on plane, boat behaved great.
I got me the Diacom software and plugged in to see what was going on.
Everything but the IAC duty cycle seemed fine. BTW, all testing done running on muffs.
With the software, I selected "System Information" and saw what looks like idle RPM set points/limits. 550 RPM in neutral, 600 RPM in gear.

Here's what I tried working with another boater who has a little more expertise -
-Engine at idle, IAC duty cycle at 99%. RPMs ~550
-Added throttle to about 900 RPM, duty cycle drops to around 40-50%.
-Go back to idle, duty cycle drops in to the 30's and then over many minutes, it creeps back up to 99% all the while RPMs staying around 550.
-The guy pulled the air line off the IAC (IAC to manifold, not throttle body to IAC) and RPMs increased....seems like a reasonable reaction.
-He then went to touch the IAC motor housing and it was really warm, almost hot to the touch. He broke out a laser temp gun and the temp was around 95 F.
Consensus was I had a bad IAC.
-Bought a new IAC from Advanced Auto (Merc # crossed over to an IAC for a Ford Winstar). Compared the old and new IAC and the new IAC has a spring in it (like the attached pic) and the old one doesn't. Not sure if it's a new design or the old IAC lost some parts that I can't seem to find.
-The behavior with the new IAC seems to be the same.
Also did the IAC test and did not see any noticeable change in RPM going from -100 to +100. This test was done against both IACs.

What's the normal IAC duty cycle at idle and does the above behavior make sense?
Do I have some missing parts somewhere from the old IAC? Don't know if the parts can get sucked through the air tubing between the IAC and manifold.
Do I have a wiring issue between the ECM and IAC?
All in all, it makes sense to me that if the IAC can't open up to let more air in, the computer is going to dump more fuel, hence the stinky exhaust.

 

[alldodge]

Your testing is correct and the IAC has changed. New style is good

Normal IAC should be around 40

 

[tpenfield]

It would be good to run the tests under load rather than running on the muffs. You can do a recording of the data with the Diacom software.

 

[badrano]

Your testing is correct and the IAC has changed. New style is good

Normal IAC should be around 40

Not sure what you mean by all my testing is correct. Are you saying that the assumption of a bad IAC is correct?

 

[alldodge]

Your post list where you take readings and how the motor reacts to each change, to me that's part of testing.

The old and new IAC are doing the same so while the new style is better then the old, the old was not the issue from what I read.

What does Diacom show for injector pulse time, does it increase/decrease as the IAC changes?
Don't know what motor, but does the fuel pressure change any and what is it?

 

[badrano]

Your post list where you take readings and how the motor reacts to each change, to me that's part of testing.

The old and new IAC are doing the same so while the new style is better then the old, the old was not the issue from what I read.

What does Diacom show for injector pulse time, does it increase/decrease as the IAC changes?
Don't know what motor, but does the fuel pressure change any and what is it?

I guess my signature is not showing....I have 2005 Merc MX 6.2L MPI with B3 with the ECM-555. Serial number starts with 0W061xxx.
I don't see Injector pulse listed as a parameter. I need to pick a fuel press gauge to see what's going on there so that will be next.

 

[alldodge]

My bad, your motor is in your sig, FP should be 43 psi at 1800 rpm

 

[badrano]

Looking at the system info the engine may be a 2004.
"04MY 6.2L BRAVO"

 

[badrano]

What does Diacom show for injector pulse time, does it increase/decrease as the IAC changes?
Don't know what motor, but does the fuel pressure change any and what is it?

I don't see injector pulse time on the parameter list.
Fuel pressure settles to around 40 psi once the pump finishes priming. Sits around 48 psi at idle and around 45 psi @ 1000 rpms. I'm running on muffs so I'm not sure if I can go as high as 1800 rpm.

 

[badrano]

To continue my investigations, the duty cycle started to act differently this time around and there was no real smell of rich exhaust this time.

Run #1
-Engine at temperature
-RPM 550-580ish
-Removed the air hose between the throttle body and IAC and RPMs jumped closer to 600. I guess this makes sense because taking out that air hose removes some level of flow restriction.
-Duty Cycle still at 99%
-Reinstalled the air hose, rpm dropped back down to 550-580
-Duty Cycle still at 99%
-Added throttle to about 900 rpm and duty cycle dropped to 40-50% range
-Back to idle, duty cycle dropped into the 30 range and started to creep up
-Removed the air hose, rpm went back up and duty cycle started back down until it bottomed out at 0%. No change in rpm noted.
-Reinstalled the air hose, rpm dropped back down as expected, duty cycle still at 0%

Run #2
Same sequence of events, but after reinstalling the air hose, the duty cycle started to drop. This behavior is different than run #1. See the attached pic of the graph.

This behavior doesn't make any sense. How can duty cycle be at 0% and the engine still be running?

 

[alldodge]

Might have a vacuum leak but most times they don't go and come back.

Does the throttle linkage apply any pressure to help close the TB when at idle?
It should apply a slight bit of force to close. Adjustment is normally with engine OFF, remove cable from TB and see if the Throttle is fully against the stop. Next take the cable and see if it just slides back on with no adjustment. If any adjustment is needed do so. Once it just slides back on, then remove and turn barrel nut one more full turn out so it applies just the slightest pressure to hold it closed

 

[badrano]

Might have a vacuum leak but most times they don't go and come back.

Does the throttle linkage apply any pressure to help close the TB when at idle?
It should apply a slight bit of force to close. Adjustment is normally with engine OFF, remove cable from TB and see if the Throttle is fully against the stop. Next take the cable and see if it just slides back on with no adjustment. If any adjustment is needed do so. Once it just slides back on, then remove and turn barrel nut one more full turn out so it applies just the slightest pressure to hold it closed

Got caught up in some business travel....catching up on this issue.
I will take a look at the TB, however, I did go out on the boat over the weekend and it ran fine....no in gear/idle stalling or stinky exhaust smell. I guess I'll just keep an eye on things as the season goes on.