Bizarre IAC reading

badrano · Friday at 5:16 PM

Now that all my fuel pressure and module fun is over, I decided to quick plug in the computer to see what all the parameters are.

Idle is normal but my IAC is at 99.98%. I just realized the one screen shot of the parameters shows IAC at 0%. It was at 99.985... I swear

I also finally did a vacuum check to get a baseline and it was between 10 and 15 inHg. I read something regarding late valve timing with this vacuum reading.

Not sure if I have another problem brewing??

alldodge · Friday at 5:32 PM

Not sure your getting correct readings because the exhaust temps are 32*F and lake sea temp is at same. The 32* is default when no sensor is present

As for vacuum, I will yield to others but I have never found a boat motor in the water find the same Hg readings as a Auto. My assumption has always been the water back pressure causes this issue

badrano · Friday at 6:32 PM

alldodge said:
Not sure your getting correct readings because the exhaust temps are 32*F and lake sea temp is at same. The 32* is default when no sensor is present

As for vacuum, I will yield to others but I have never found a boat motor in the water find the same Hg readings as a Auto. My assumption has always been the water back pressure causes this issue

Yeah, I don't have sensors installed on the exhaust paths. I wonder if I could add them provided the wires/connectors are on the harness.

The engine was running on muffs.

nola mike · Friday at 8:45 PM

Vacuum on my last 2 engines have been comparable to car values. I'm 18 mmhg at idle, but you need to make sure that timing and idle are within spec to get reliable values. if you are at 12 (and it's a steady reading--what the vacuum does as the engine runs is important) that would go along with a high IAC reading, which would be equivalent to a partially open throttle

badrano · Saturday at 7:06 AM

nola mike said:
Vacuum on my last 2 engines have been comparable to car values. I'm 18 mmhg at idle, but you need to make sure that timing and idle are within spec to get reliable values. if you are at 12 (and it's a steady reading--what the vacuum does as the engine runs is important) that would go along with a high IAC reading, which would be equivalent to a partially open throttle

What you say makes sense, but if the IAC is fully open, why is it idling at the normal expected RPM value.... 600 some RPMs? I would expect higher RPMs/high idle.
Also, then, why is the IAC being commanded to be fully open?

nola mike · Saturday at 8:30 AM

badrano said:
What you say makes sense, but if the IAC is fully open, why is it idling at the normal expected RPM value.... 600 some RPMs? I would expect higher RPMs/high idle.
Also, then, why is the IAC being commanded to be fully open?

idk. yeah you'd think the idle would increase or there'd be rough running. then again you say it's fully open but screenshot is fully closed, so not sure what's really going on

alldodge · Saturday at 9:42 AM

Right now I don't think you have an issue to worry about. As for adding sensors, I wouldn't do it even if wires were available, more sensors more problems IMO

badrano · Saturday at 4:54 PM

alldodge said:
Right now I don't think you have an issue to worry about. As for adding sensors, I wouldn't do it even if wires were available, more sensors more problems IMO

I think I would agree. Took the boat out for a shakedown cruise. It ran great.
At least I have a baseline.
Thanks for all the insight.

muc · Sunday at 5:03 AM

You didn’t post a serial number or even a model number, but I see what looks like an ECM 555 in one of the pictures so I’ll take a guess.

The MerCruiser PWM (pulse width modulatio) IAC is a great source of revenue for shops. I’m not sure why they cause so much trouble in marine applications because it’s the same IAC used trouble free in many Ford automobiles. As near as I can tell they are the second leading cause of ECM failures.

Basically they should rarely run at 0% or near 100%. It seems like you have a problem that needs to be addressed.

If you would like to see what I mean. Use Diacom and run a KOEO test on the IAC. Run it up to 90 or 100% and check the IAC temperature with your finger at 30 seconds and 1 minute. Oh oh, burned your finger? Yes those things run to hot at higher duty cycles. With enough time they will melt the electrical connector and sometimes burn out the wimpy driver circuit in the ECM.

Possible causes?
1. Dirty IAC muffler, these things are (insert rant about parts prices nowadays) cheap. Used to cost about $1.00 but someone recently told me they are $3.00. Still cheap insurance and I recommend they be replaced yearly or more often if your belt is shedding. While you’re at it, clean the flame arreestor because that can cause this issue too.
2. someone messed with the “idle” screw on the throttle body. This is a never touch screw, but I see DIY folks and bad techs do this. The good thing is that MerCruiser usually covers this with a bunch of paint so unless someone replaced the throttle body with the wrong part (this setting can be dependent on model and serial number) you can usually see it.
3. engine is down one or two cylinders. You’d be surprised how often I would hear a V8 missing on one cylinder and the customer has no idea, these modern EFI engines will do a good job of compensating for this problem. Easy test, use the cylinder balance test in Diacom. But if you’re not 100% sure how to run this test—- ask. This is a test that I’ve seen many junior techs do wrong.
4. weak ignition, plugged injectors or other fuel issues and a few more.

Either way, a high IAC duty cycle is a “pay me now or pay me much more later“ issue.

badrano · Sunday at 11:35 AM

muc said:
You didn’t post a serial number or even a model number, but I see what looks like an ECM 555 in one of the pictures so I’ll take a guess.

The MerCruiser PWM (pulse width modulatio) IAC is a great source of revenue for shops. I’m not sure why they cause so much trouble in marine applications because it’s the same IAC used trouble free in many Ford automobiles. As near as I can tell they are the second leading cause of ECM failures.

Basically they should rarely run at 0% or near 100%. It seems like you have a problem that needs to be addressed.

If you would like to see what I mean. Use Diacom and run a KOEO test on the IAC. Run it up to 90 or 100% and check the IAC temperature with your finger at 30 seconds and 1 minute. Oh oh, burned your finger? Yes those things run to hot at higher duty cycles. With enough time they will melt the electrical connector and sometimes burn out the wimpy driver circuit in the ECM.

Possible causes?
1. Dirty IAC muffler, these things are (insert rant about parts prices nowadays) cheap. Used to cost about $1.00 but someone recently told me they are $3.00. Still cheap insurance and I recommend they be replaced yearly or more often if your belt is shedding. While you’re at it, clean the flame arreestor because that can cause this issue too.
2. someone messed with the “idle” screw on the throttle body. This is a never touch screw, but I see DIY folks and bad techs do this. The good thing is that MerCruiser usually covers this with a bunch of paint so unless someone replaced the throttle body with the wrong part (this setting can be dependent on model and serial number) you can usually see it.
3. engine is down one or two cylinders. You’d be surprised how often I would hear a V8 missing on one cylinder and the customer has no idea, these modern EFI engines will do a good job of compensating for this problem. Easy test, use the cylinder balance test in Diacom. But if you’re not 100% sure how to run this test—- ask. This is a test that I’ve seen many junior techs do wrong.
4. weak ignition, plugged injectors or other fuel issues and a few more.

Either way, a high IAC duty cycle is a “pay me now or pay me much more later“ issue.

Thanks for the additional insight. It's the ECM-555. All the boat info is in my signature, but I think there have been issues with it displaying....2005 Chaparral 256 SSi - MX 6.2 MPI s/n 0W061713, Bravo 3

With regards to #3, if I were down a cylinder or 2, would I be able to hit max speed? On the shakedown cruise yesterday, I hit top speed with no problems. However, I do want to do a compression check to capture a baseline.

muc · Sunday at 5:04 PM

badrano said:
Thanks for the additional insight. It's the ECM-555. All the boat info is in my signature, but I think there have been issues with it displaying....2005 Chaparral 256 SSi - MX 6.2 MPI s/n 0W061713, Bravo 3

With regards to #3, if I were down a cylinder or 2, would I be able to hit max speed? On the shakedown cruise yesterday, I hit top speed with no problems. However, I do want to do a compression check to capture a baseline.

All the boat info is in my signature,
Yes it is, looks like there is someone who should feel stupid --- me? --- sorry. At my age you would think I should know better. Or maybe that’s the problem?

#3. yes, if it hasn’t been repropped. 25’ SSI w/6.2 should be about 50-55 MPH?

Probably #1. Also, because you see 0% at idle leads me to think there isn’t a restriction. But those are easy maintenance.

Something else to consider, maybe failing IAC? To check this you can remove the IAC and use your trustee Diacom to step from 0 to 100% while watching the plunger move. It should move slightly every 10%. While you have it off, best practice is to also remove the two hoses and make sure a muffler didn’t get sucked in. Before doing this, get a new gasket from a dealer or ask the auto parts store for a 2001 Windstar IAC gasket.

You posted a screenshot of the Diacom, this is just a snapshot of a moment of time. You should try using the record function of Diacom. This will allow you (and others if you post it) to review. The recording is the best part of Diacom. You talked about running a compression test to have a baseline, let me tell you that a good recording will be much more useful in the future. Also you can slow things down and it’s much easier to watch the screen while the boat is still Or your at home.
I’ve posted in the past on the best way to do this. If I can find that file I’ll post it when I have time.

muc · Monday at 2:53 PM

Diacom recording.

Here is my recommendation on how to do the recording.

Start with a cold engine and the boat in the water.

1. Connect Diacom

2. Turn on batt switch, turn on key switch -- but don't start engine

3. Start Diacom recording -- let run for 30 seconds to one minute

4. Start engine

5. Check that Diacom is still recording, it's not uncommon for it to have lost communication during starting. If it did, save that recording and start another one.

6. Run the boat gently at idle to 1500 RPM until up to operating temp

7. Run the boat up to full speed --- take your time, looking to get data here so advance the throttle slowly (spend one to two minutes) now slow back down to minimum wake and idle in neutral for awhile. Use the trim as you normally would.

8. Do a "ski start" -- full throttle all the way to full speed and trimmed out, now back to idle fairly quickly.

9. Drive boat back to the dock/ramp and shut off

10. Stop recording and name the files

badrano · 2025-09-02T08:19:51-0600

muc said:
#3. yes, if it hasn’t been repropped. 25’ SSI w/6.2 should be about 50-55 MPH?

To my knowledge, there's been no reprop done. Original 24P props....I believe.
When I did the shakedown cruise, I did go WOT and hit 40 knots. This is how it's been since I bought the boat back in 2020 with 168 hrs (now 358 hrs). Granted I never took note of RPMs at WOT, but I've always done WOT every year during the check ride after coming out of winterization to make sure I could still hit 40.
Also, to be honest, been really bad lately, have been running without the IAC muffler

At least with the annoying hissing sound, I know the engine is running.

Took the boat out Sunday and the one problem I thought was due to the rich fuel condition occurred a few times. I go to advance the throttle and the engine has a noticeable momentary stumble. More reason to now take the laptop out and do some recordings to see what's going on.....that's probably going to be only way to catch this stumble to see what's going on.

muc · 2025-09-02T19:22:20-0600

Are you sure 40 KNOTS? That boat would have had a speedometer that reads in MPH.
Recordings might show an issue.

badrano · 2025-09-03T06:16:14-0600

muc said:
Are you sure 40 KNOTS? That boat would have had a speedometer that reads in MPH.
Recordings might show an issue.

Yeah, 40 knots per GPS. Speedo doesn't work....some debris got into the pickup hole on the drive and I haven't been able to unclog it.

Kola16 · 2025-09-03T07:06:03-0600

40 knots seems right to me with a Bravo 3. My 22.75' boat that weighs 1,000 lbs less, has the same transom deadrise, is overpropped with a 4-blade stainless steel prop, and my 383 before tuning does 44 knots on the GPS.

Bizarre IAC reading

Petty Officer 1st Class

Moderator

Petty Officer 1st Class

Vice Admiral

Petty Officer 1st Class

Vice Admiral

Moderator

Petty Officer 1st Class

"Retired" Association of Marine Technicians...

Petty Officer 1st Class

"Retired" Association of Marine Technicians...

"Retired" Association of Marine Technicians...

Petty Officer 1st Class

"Retired" Association of Marine Technicians...

Petty Officer 1st Class

Petty Officer 1st Class