It seems they are all GM style 1 bar sensors but I can't validate that. Are the Mercury "marinized" somewhow???
8.1 idling 10 rpm too high
- Thread starter boatrboy
- Start date
boatrboy
Well-Known Member
It shows further down the page it fits my 2006 as well… seems to the same as the Chevy truck with an 8.1. Any one else use an auto one?
I have personally not, but know several people who have used them and not reported any issues at all.
boatrboy
Well-Known Member
Well I picked up the NAPA version so when I get to the boat I’ll have it. I’ll check the voltage as you suggest. If this is in fact the issue I’m hoping this brings the IAC back to controlling the idle speed.
boatrboy
Well-Known Member
It’s a three wire so in my research of the wiring show one wire 5 volts in, the middle ground, the other end something less than 5 volts out.
What is bothering me is, as @ttmott brought to my attention, is the 1 psi difference at idle. Hoping to get back to boat Saturday to dig deeper.
Tri County Trail
Active Member
- Aug 29, 2021
- 129
- Boat Info
- 2005 Rinker 360
- Engines
- Mercruiser 8.1l Bravo 3s
I replaced the foam mid season last year and have a couple of hours on it this year so i will assume that's clean enough. It was doing this before I changed that.
Here is the port snapshot from yesterday if this helps
View attachment 147239
And here is the starboard
View attachment 147240
I did not try the IAC test on the good motor.
S/N's Port OW393606, Stb OW393610
... actually 20 rpm higher
Here are some random screen shots of my 496s. Idle is dependent on lots of variables. Try it in gear and see if throtte position and other parameters change
Ericinmich
Active Member
- Dec 12, 2021
- 213
- Boat Info
- 48 Sundancer, 2007... formally PapaG's
- Engines
- Cummins
Map should be grounded back at ecm, anything else will cause issues.
I would get a snapshot of the data while engine off right after key up. Iac should step full in and out and stop at a number. Then after idle flare it should start to move to control idle rpm to the target.
The other thing I'd suggest is when starting the engine be sure to pause with ign on before cranking. The init login needs to establish a good baro from map and if engine starts cranking before that happens you can get wonky numbers.
I still think there is an air leak and the proof will be if the iac starts at a normal number then keeps stepping down as it tries to pull rpm down. It could be throttle blade not closed as well.
I initially thought a vacuum leak also until he showed the fault data and the Baro. at 14.91. No way it could really be higher than 14.7 PSIA.
Ericinmich
Active Member
- Dec 12, 2021
- 213
- Boat Info
- 48 Sundancer, 2007... formally PapaG's
- Engines
- Cummins
My 48da had f'd up baro and boost readings which I thought was a huge problem. I figured out that it was because I would hit the ignition and start switch without a pause and the ECU would come up with odd limits for the pressure readings. I actually repeated it, surprised the local Cummins service guys with what I found. I later found a post on some site stating the same issue found by someone else.
Iac will only keep counting down if rpm is high during idle state. An incorrect map will just add a bit more fuel which could make the idle a bit higher (although they all run a bit rich of stoic), the iac and spark control should be able to pull the rpm down to target.
boatrboy
Well-Known Member
I will start recording before I start engine next time I’m there, hoping Saturday. My normal routine is to flip switch just to power gages and read them first, then after a few seconds push to start.
I will look at what is happening before I start the motor or replace the map and see.
Since I bought the MAP anyway, my plan was to install it and see if the psi comes down to where the port motor is. If not I will start removing the flame arrester for plate inspection and look for vacuum leaks as well.
Thanks for all the input on this.
boatrboy
Well-Known Member
Update
I checked the map pressure before starting the motor. I then connected the new one I bought and checked the readings. The readings came down to 14.72 while it was just sitting on top of the engine. Just to be sure I started the motor with the old one and recorded the numbers. I installed the new map sensor and restarted, and the running pressure came down about the one psi I was expecting. At that point the idle came down to 650. When I looked at the iac duty cycle, it was close to 99%. I thought this was strange. But after a few minutes, I heard a click (or at least I thought I did), and the idle went back up to 670. The iac duty cycle went to 0. Since I had the original Mercruiser IAC valve, I changed it. I checked all the connections to the PCM and the iac and map. Reconnected it and started it. It now idles exactly at 650 RPM and the iac duty cycle matches the port side closely.
I now wish I had only changed the iac before touching any wires. But I’ll take this one as a win for now.
Thank you guys for the help.
I checked the map pressure before starting the motor. I then connected the new one I bought and checked the readings. The readings came down to 14.72 while it was just sitting on top of the engine. Just to be sure I started the motor with the old one and recorded the numbers. I installed the new map sensor and restarted, and the running pressure came down about the one psi I was expecting. At that point the idle came down to 650. When I looked at the iac duty cycle, it was close to 99%. I thought this was strange. But after a few minutes, I heard a click (or at least I thought I did), and the idle went back up to 670. The iac duty cycle went to 0. Since I had the original Mercruiser IAC valve, I changed it. I checked all the connections to the PCM and the iac and map. Reconnected it and started it. It now idles exactly at 650 RPM and the iac duty cycle matches the port side closely.
I now wish I had only changed the iac before touching any wires. But I’ll take this one as a win for now.
Thank you guys for the help.
Tri County Trail
Active Member
- Aug 29, 2021
- 129
- Boat Info
- 2005 Rinker 360
- Engines
- Mercruiser 8.1l Bravo 3s
Would you mind sharing the Mercruiser IAC valve and map pressure sensor part numbers you used. I would like to add to my spare parts list. The software is great for trouble shooting. Did you do the fuel and ignition checks yet using the software?
boatrboy
Well-Known Member
The map was the Napa one I showed earlier but the Iac was the original Mercruiser one. I had only replaced it because I had a bad one on the port side and I decided to replace both at that same time. The one I replaced it with was the AutoZone part that scofaw suggested (tv278).
boatrboy
Well-Known Member
Tri - sorry I missed your other questions.
I did try the iac test again but for some reason it did not affect the rpm in either direction. That was the only check I did and it was too hot out to keep playing so I called it a day.
Jimmy Buoy
Well-Known Member
- Dec 3, 2008
- 2,468
- Boat Info
- 2003 Cobalt 293
- Engines
- Twin Volvo-Penta 5.0 270hp & DuoProps
I just checked the IAC test with my boat yesterday using my Rinda. Selecting to close the IAC does begin to lower the idle RPM starting at 50%. Closing beyond that will drop the RPM's enough that the engine begins to show signs of stalling. Increasing it back to zero, the RPM returns to the normal 600. However, increasing the IAC to 100% open has no effect on RPM. Both engines are the same. Thought it was just peculiar to my engines.
One thing about these readings that is a bit confusing is regarding the IAC number. Running above 1500RPM the IAC should be closed since its only active for controlling the idle RPM. The IAC reading at that speed is 99% (meaning 99% closed).
Ericinmich
Active Member
- Dec 12, 2021
- 213
- Boat Info
- 48 Sundancer, 2007... formally PapaG's
- Engines
- Cummins
Good news OP. Here are some random thoughts on IAC based on implementing various packges into production.
- Just because the IAC shows 50% that doesn't mean it's actually there. There is no feedback on pintle position, it just moves in or out and ECU keeps track of position.
- Bad or dirty IACs can get lost. On init the ECU will move the IAC in and out to "clean it" then move it to target position for starting.
- If the ECU detects a lost IAC position then it will do a fast reset trying to get it to a know position.
- IAC motors can get lost when moving one way then the other too fast. Later versions of SW put a pause in the control to let it settle before changing direction.
- Just because the IAC shows 50% that doesn't mean it's actually there. There is no feedback on pintle position, it just moves in or out and ECU keeps track of position.
- Bad or dirty IACs can get lost. On init the ECU will move the IAC in and out to "clean it" then move it to target position for starting.
- If the ECU detects a lost IAC position then it will do a fast reset trying to get it to a know position.
- IAC motors can get lost when moving one way then the other too fast. Later versions of SW put a pause in the control to let it settle before changing direction.
Pirate Lady
Well-Known Member
Idling 10 rpm too high? Tell me this is a typo.
So @ttmott does this seem better and rule out a vacuum leak at this point in your mind?
Even thought the snapshot shows 657 is is really averaging a rock steady 650rpm. running for 1/2 hour.
And the Florida folks are whining about 90 degree water at the beach?
