Mercruiser 6.2L IAC sensor

I have had some issues with the IAC sensor on my 6.2L MPI, 0M036015, MEFI 3 ECM. After warm up at idle, the IAC counts should be between 0-40 counts. My scan tool indicates anywhere from 80 to 100. I have installed a new IAC sensor, the operating temp is within specification. I have removed the throttle body and inspected for damage and or any obstructions, and or a clogged muffler. All other sensors are within specification, no DTC's. What I have been able to do is to remove the spark arrestor, and place my thumb over the throat in the throttle body, this chokes the engine raising the IAC counts to 140-147. Upon removal of my thumb the engine gasps for air, the idle rises rapidly and the IAC count drops between 0-40 usually 0-15. Why can I make the sensor work this way and not during normal start up. I have watched the IAC count for 30 minutes at start up and the counts will not drop. I have revved the engine brought the IAC counts up this way but don't get the drop on the counts upon returning to idle. The sensor works, why not the way it should??????

Re: Mercruiser 6.2L IAC sensor

first of all an IAC is not a sensor.... it's a little tiny motor whose only purpose in life is to move a pintle (needle) into or out of its seat... its to meter air to the engine while the engine is at idle...... and thats it.
Secondly... have you checked what newportdave suggested? If your Iac is acting funny you need to check the passage that the IAC draws it's air from... and the passage that it sends it to...

Re: Mercruiser 6.2L IAC sensor

The throttle body was removed, inspected, cleaned, and reinstalled. The IAC is brand new out of the box, and there is no IAC muffler in the throat. The opening that connects the air passage of the throttle body when the butterfly is closed to the plenumn, is clean and unobstructed.

Re: Mercruiser 6.2L IAC sensor

In the previous thread I suggested checking the IAC muffler for restriction, but that would only apply to a ECM555 engine, not a MEFI3 engine.

I'm not sure why the IAC position would change when blocking then releasing the IAC intake, but keep in mind that the ECM uses spark advance to control idle speed as well as IAC position, so you have to watch both when evaluating idle speed control (the engines ability to maintain the Desired Idle Speed).

If the throttle body, throttle plate and IAC pintle and seat are clean, and the throttle cable is not misadjusted (applying too much or not enough pressure against the throttle stop), but the IAC count is just too high, you could adjust the throttle plate stop to lower it. This is not something you normally mess with, but your can if all else fails.

Re: Mercruiser 6.2L IAC sensor

I will check the throttle cable adjustment. When I had the throttle body off, the butterfly and the return spring worked fine with no deposits top or bottom. All the internal passages were clean.

It appears to be a pintel that is sticking and needs just a little help with the additional vaccumn created by choking the IAC throat. This pintel is rather large compared to a 5.7L MPI and the head design is completely different than a 5.7 MPI. The head material appears to be a hard vinyl head with a closed metal sleeve over the drive rod. The 5.7L has a bell shaped head and a open cage with a return spring visible. Same connector.....

Re: Mercruiser 6.2L IAC sensor

Idle on the Rinda scan tool, not the Diacom software, goes 589-615, desired RPM is 600 on the scan readout, spark advance is within spec about 4* @ 600

Re: Mercruiser 6.2L IAC sensor

Pulled the throttle body again and checked everything, cleaned everything again. Readjusted the throttle cable a slight preload on the cable barrel twords the throttle arm.

Ideal RPM "desired" 662 actual between 605-662. I inspected the throttle stop when the body was off, a torx screw from the bottom of the body. Very, very tight, shot some PB penetrating oil on the screw from the bottom still did not want to budge, so I left it alone.

TPS voltage is .88 a little high based upon spec. I also noticed that the throttle body butterfly is not completely closed when on the stop, a very, very small opening between the plate and the body, just enough to see daylight when held up to the light.

I'm stumped, again I can still get the IAC to back down to zero when I choke out the IAC throat. But on start up after warming the IAC base position, and active counts are about 100+

Re: Mercruiser 6.2L IAC sensor

Ok, I'm still confused, but I have a couple of questions/comments. I took some time today and reviewed some Diacom data files. I rarely use a hand held scan tool, I virtually always use Diacom.

First of all, I don't seem to have any MEFI3 6.2 MPI files, I do have a ton of MEFI3 350 Mag MPI data files. All the files I reviewed show that the DIS (Desired Idle Speed) when the engine is at full operating temp is 600 RPM. You said that you had a DIS of 665 RPM. Seems real high and would require a higher IAC position. When you do the finger over the IAC passage thing, and the IAC position drops to 0-40 what is the DIS at that point? What is the actual RPM at that point?

Re: Mercruiser 6.2L IAC sensor

I have a question as well......
Whats your actuall issue with the motor? is it not holding an idle? Not idle after a hard run??????
Are you sure your not chasing a small vacuum leak that the iac is trying to compensate for???

Re: Mercruiser 6.2L IAC sensor

captJason said:

I have a question as well......
Whats your actuall issue with the motor? is it not holding an idle? Not idle after a hard run??????
Are you sure your not chasing a small vacuum leak that the iac is trying to compensate for???

Click to expand...

I was wondering the same thing myself, but hey, I don't mind discussing a specific technical question. BTW, a vacuum leak will cause a lower IAC position, not higher.

Re: Mercruiser 6.2L IAC sensor

newport dave said:

BTW, a vacuum leak will cause a lower IAC position, not higher.

Click to expand...

From his other thread I was gathering that he had at 60rpm swing or so in his idle..... not awefull... but it should be enough to make the iac open and close a bit. The first thing that popped into my head is a vac leak. The second thing that popped into my head is that the rpm may be swinging faster than the techmate can read it.... and that's why he's reporting the numbers he is. He's only getting the peaks.

Other than that... it still sounds like an engine reacting to unmetered air. Of course its not in front of me.

hakukamana said:

TPS voltage is .88 a little high based upon spec. I also noticed that the throttle body butterfly is not completely closed when on the stop, a very, very small opening between the plate and the body, just enough to see daylight when held up to the light.

Click to expand...

From your other thread...... Most throttle plates should close pretty much completely. But in all honesty... i'd probably have to have your's in 1 hand and a new one in the other to tell you if yours was good or bad.
But some things i have seen are misajustment to linkages, throttle cables, throttle stops etc that will cause either the throttle plate to bend slightly, or, start to bore out the inside of the assembley.
You may have to take it back off.... but hold the throttle valve open so the plate is 90 degrees to the bore... does it look perfectly flat? or is it bent? Is the bore nice and smooth... or does it look like the shutter has been boring into the aluminum?
Just for giggles I would check for a vac leak, it's easy enough to do. Just get some 6-1 foaming oil in a spray can and start spraying from the throttle plate gasket and work your way to the manifold. Don't forget to spray the outside of the shaft on the throttle valve. Shaft seals can leak and cause unmetered air. Half the reason I'd do it, is in the event that you do not find a leak... then at least you can rule it out.

Re: Mercruiser 6.2L IAC sensor

Ok, we had a little glitch over the past couple of days being able to reply to your posts. I think Don did something because it appears that I can post reply's now.

I spoke to Stillwater OK, tech service today, the tech believes that the IAC is functioning normally based upon some potential issues. He believes that the ECM is reading engine sensor readings and adjusting to compensate for the other issues which might be going on. If the IAC count was normal he believes that the engine would stall, and the ECM is compensating for this by allowing more air into the TB.

I have been asked to check fuel pressure, I have 32psi peak, drop down and steady hold 26psi. I have been asked to check base timing @8* BTDC. On this motor I need to borrow my buddys advance dial light, to check the timing. He also wants me to do a drop down test, using some carbon fiber hose about the size of wiper spray hose and ground out each cylinder momentarily. Then I need to call him Tuesday with the findings.

Re: Mercruiser 6.2L IAC sensor

a drop down test or a cylinder drop test?


And getting back to the root of the problem here...... and I asked the same thing the other day.... what is your engine doing or not doing??? another thing.... just for giggles... have you done a compression test yet?

Re: Mercruiser 6.2L IAC sensor

I thought he said a drop down test but the way he described the procedure was to get some small diameter carbon fiber hose, attach it to the plug and plug wire and basicly ground out the cylinder to disable it momentarily. His accent was a little think, nice guy, but a little hard to understand.

No compression test yet, I'll do that today.

Base issue that started this whole thing was fuel consumption at 8 knots & idle. That' s how we fish out here not a lot of 30min /1 hour runs to a location. We leave the harbor, and basicly set up and go. I also noticed a exhaust line across the transom, and that rich smell woffing up from the exhaust. I checked the plugs and yes the were black and fouled.

Engine idle has appeared to be good, maybe a tad high 662 @ "desired" but everything else appeared to function fine.

Latest readings are showing TPS voltage at .86 which the tech indicates is high. I have checked everthing you guys have suggested relative to the TB. Checked vacuum leaks (none), checked throttle cable and readjusted, OK.

Re: Mercruiser 6.2L IAC sensor

I had these exact symptoms on my 2002 5.3L Mercruiser. I saw these posts and decided that they were an exact description of what was happening to me. The IAC valve was $104.00 to replace and it fixed the problem perfectly. I wished there was more description about the alarm in the owner's manual. The only description it gives for an intermittent alarm is low stern oil. Would have like to know that the alarm can mean a whole host of things.

Re: Mercruiser 6.2L IAC sensor

Welcome to iboats, If you have questions, start a new thread. Best not to stick your question at the end of a year old post. In no time it will be lost in the middle when people try to answer to original question after not reading all the replies.