06' 496 Mag HO Duty Cycle

[Bootkie2]

Recently I could feel our Formula 260 (w/496 mag ho) vibrating and idling rough going through No Wake Zones. Almost feeling like it wanted to stall. No issues while underway.

I have the DIACOM software on my laptop and it is a handy tool since I hate losing water time and paying $$ to have it scanned when silly sensor issues and things like this pop up. Though I am no mechanic, I can interpret basic parameters and can always reach out to people smarter than me (as I am doing now).

There were no active faults but while I was driving through the no wake zone but I could see the duty cycle stuck at 99.9% flat line. It stayed there until I advanced the throttle to 1400 RPM then it went down to 20% and and at 3100 RPM cruise speed it stayed between 42-43%.

I replaced the IAC as well as the Foam muffler last year (75 hours ago), but none the less my first inclination was to start small. Let's see what IAC muffler looks like today. So I pulled the flame arrestor and sure enough the foam muffler was in bad shape. Surely it was restricting some air flow. They are only a few bucks so I got a new one. I also cleaned the Throttle body and intake. Neither were too grimy but good to give them a quick clean.

Fired the boat back up and I could feel it idling much better overall. So I plugged it back into the scan tool and the 99.9% flat line was gone. Cool! But now at no wake speed I'm getting a series of up, down, up, down duty readings. Its like up to 45% then down to 5% then up to 49% and down to 3% and so on. See the time lapse graph? (Image attached)

Are these readings normal? Or should they be more consistent on a warm engine at low RPM in forward gear rather than swinging up and down so much by 40 percentage points?

 

[alldodge]

I'm thinking a bad plug, plug wire or injector

Run
fuel injector output test
ignition coil output test
misfire test

 

[Bootkie2]

Thanks! Ill give it a try this week and report back!

 

[tpenfield]

Wow . . . I think this is a first. Diagnosing and MPI engine problem with the proper tool (computer S/W) instead of running out and buying all kinds of unnecessary parts to replace. :thumb:

 

[Bootkie2]

The tests came back inconclusive (also a high possibility I didn't do them right). Engine has 795 hours and I am the second owner. I did plugs and coils last year for preventative measures so I think they would have been unlikely culprits.

The IAC flattens to 43-45 % constant line at 3000 RPM. Still possible maybe injector starting to go, maybe just not gone enough to register in ECM or tests?

My new plan is to keep driving it until I get an active fault or something shows in the Freeze frame (or worst case I break down).

-Kyle

 

[Bondo]

But now at no wake speed I'm getting a series of up, down, up, down duty readings. Its like up to 45% then down to 5% then up to 49% and down to 3% and so on.

Ayuh,...... Here's My 1st thought, when I read yer post,......

At no wake speeds, the driveline is constantly loadin',..... 'n unloadin',....
The prop pushes, then glides,..... pushes, then glides,.....

It's why trollin' is tough on motor couplers, especially if not greased regularly,....
The constant load, 'n unload grinds the splines outa the coupler,.....

That what yer 'puter was seein',......

 

[Bootkie2]

Alright so the initial problem has returned. The duty cycle is flat line 99.9% going through no wake zones on a warmed up engine. Once I hit 1,300 rpm it returns down to 20% and stays there. Once i get up on plane at 3,000 it hits 37-38% and stays there too. When I come off plane the boat briefly tries to idle normally, you see it goes down to 15% but then it just creeps back up to 99% and stays there for the entire duration of the no wake zome.

I just replaced the iac muffler this summer and I replaced the iac itself last summer as we were having a similar issue last year. I have heard some stories about PCM555 and how running on a bad iac for too long can fry the pcm.

Where to begin?

 

[dubs283]

there are no "normal" numbers for an iac, there is no spec because all the iac does is help keep the engine running as close to the pre determined idle spec as possible. the ecm controls how much the pintle moves based on engine load, speed temperature, outside parameters, etc...(i.e. using the sensors to gather information and do its best to keep the engine running smoothly)

basically all your looking for with the iac information is that it is moving, which it appears yours is, or was and now may not be. replacing the muffler is a good idea, they do get gummed up but its job is to just quiet down the loud hiss the air makes when the throttle plate is closed and the iac is acting as the air control

like bondo says the issue with iacs failing is due to low speed cruising and idling when the prop is constantly loaded and unloaded from wave action against it. the iac has to constantly react in order to keep the engine rpm as consistent as possible at idle/low speeds. a car with an automatic transmission has a torque converter that can absorb the low speed loads to keep the engine running consistently, manual transmission would be the clutch.

higher rpm does not matter because load is much more consistent and the throttle plate is open much more. merc does not use an iac motor anymore, hasnt for about ten years now because they fail so much, the ecm controls the throttle plate opening at idle and lower speed to compensate.

you say you replaced the plugs and coils, did you use the right plugs? 496 is picky about that and they destroy spark plugs pretty quickly especially if you idle and cruise at low rpm a lot.

 

[Bootkie2]

Interesting. Thanks for the input.

Yes correct. The iac does not budge at no wake idle now. No variance. It's 99.9 % flatline straight through up until we hit 1400 RPM.

The plugs are AC 41-983 which many have recommended for the 496. If the plugs are bad after only 70 hrs of use id certainly be surprised but i guess it could happen?

 

[dubs283]

If the plugs are bad after only 70 hrs of use id certainly be surprised

have seen plugs with < 50 hrs in a 496 be bad, overall age does affect as well but at 70 hrs you are better off with new plugs especially if you are motoring < 1200 rpm consistently

983 is fine, i think the 993 superseded to that plug, can't say for sure but id guess they are different temp range, no idea which is the "hotter" plug

as far as the iac, i'd try plugs first see if it helps or anything changes and go from there

 

[alldodge]

First and foremost, post here are opinions (including mine). Statements of exactness without info to back it up is opinion

Yes there is a normal for the IAC, its 40 counts or 40%, and can be found in manuals up to 2000 or so (below is 454/502). Later manuals many of the values, even resistance values of sensors were removed for some reason, but often show up in bulletins.

If its above 40 then the IAC can have trouble compensating.

Vacuum leak, incorrect MAP, temp or weak motor, low compression

 

[alldodge]

If you want some expert advise, PM MUC and if he is not to busy he may come look

 

[Bootkie2]

Thanks Alldodge! Really neat to see that chart. Interestingly in neutral with the throttle closed I am meeting all of those parameters. Even the IAC <40 in neutral. But When I a add just a slight amount of throttle up to 800-900 rpm, that's when the IAC starts to flatline at 100℅. The IAC stays at 100℅ only with the throttle slightly open 800-1200 in forward gear. Above 1200-3100 rpm it stays below 50.

Who is MUC he's in New England?

 

[alldodge]

MUC is a member here on iboats. He is a Merc and Volvo technician and helps out with questions when he has time. If you send him a private message he may come and provide some insight

 

[Bootkie2]

I had some help and was able to test a new good running IAC just to be sure. But got the same result at No Wake speeds (stays at 100% duty). Got some new AC 41-983 plugs on sale for 50$. I will try that next. Nothing to lose for 50$.

 

[alldodge]

What does the IAC do when placed in base timing mode?

Gives me the thought of cylinder issue(s) but the misfire test should have showed something

 

[muc]

dubs283 I think is getting your duty cycle IAC confused with a stepper motor IAC.
A stepper motor IAC only uses electricity to move, so let’s say that IAC is at 50 counts or 200 counts (it doesn’t matter what it’s running at) it only uses electricity (generates heat) when it moves. Your duty cycle IAC is using electricity anytime it is at any duty cycle above 0%. The higher the duty cycle, the more electricity (and heat created) is used. So when your IAC is running at 99.9%, it is generating a lot of heat (way too much) this is what burns out the IAC, electrical connector and the weak IAC driver circuit in the PCM. If you want to see what I mean. Hook up the Diacom to your cold engine, KOEO and select the IAC control in the tests area. Run the IAC at 95% duty cycle and listen to the god awful buzzing it’s making. See how quick it gets hot enough to burn your finger? That’s not good.

The duty cycle numbers you see above about 1000 RPM are numbers that are programmed into the PCM. At speeds above 1000 RPM the IAC is no longer trying to control engine speed, it’s function is to now be ready to supply extra air in case you close the throttle rapidly and keep the engine from stalling.

My my guess is your problem should have shown up during a cylinder misfire test. Is it possible you didn’t do the test the best possible way?
To do this test properly. Engine at normal operating temperature. In gear at 1200 to 1500 rpm. Drop one cylinder at a time for about 30 seconds and listen for how much the RPMs drop. Let the engine run for about one minute before doing the next cylinder.

This is something that needs to be corrected. If you keep running this engine at duty cycles above 70% for prolonged times it will (if it hasn’t already) burn out the IAC driver circuit in the PCM.

 

[Bootkie2]

Thanks Muc. When I did the test it was mid afternoon on a busy Saturday. There was quite a bit of boat activity, wind, and wave action (we are in a busy area on weekends).

It's possible I could have missed something for sure. I'm going to try again on a quiet weekday morning so I can try to be a little more in tune with the subtleties in the RPM drops.

Agreed. I don't want to fry the PCM (or myself). I am trying to make this a priority to resolve it.

 

[Bootkie2]

So I have not ran the misfire test yet. (More to come on that in a second)

HOWEVER I started by pulling all the spark plugs. These plugs were changed last winter. They have about 65-70 hours on them. Pictured from left to right are cylinders 1-8. #7 was the worst of the bunch. It was a little wet when I pulled it too and you can see all the carbon build up compared to the others.

So I installed 8 new plugs yesterday and then I connected my laptop to the PCM. In a stroke of bad luck as I was booting up my laptop I realized there was a giant crack in the screen. My 5 year old must have dropped it somehow. The screen was flickering and half it was not visible at all. Somehow I was able to get the Diacom software up and running enough to get a reading on the IAC which of course was still at 100% in forward gear on a warm engine. Then finally my laptop screen went dark completely. But clearly the new plugs did nothing.

Given how bad #7 was compared to the other plugs I have a sense the misfire test may have caught something there if my laptop had not died.

What do you think?

 

[alldodge]

No 7 looks good, No 2 is a bit lean, and the rest are real lean.