Ok, 496 Merc Mechanics : Fuel pressure?

[Marty Gras]

Oh, I get your question about the "0 to 10 thing". What I was trying to say is that a normal IAC 'reading' at idle (no load) would stay within 0 to 3 increments of change. (Example; a 5.7 idles around 30%(counts), (if the throttle plate setting is as from the factory) and it would be idling (no load) at normal temp between 27 and 33. If the numbers got out to the 26- to 34+, you would be feeling the motor idle going up and down, and read the spark timing going all over the place. If the numbers went way out to 20- to 40+, you would have a very unstable idle speed. (surging like a too rich blown motor). Yes, no IAC motors have such a small (ten count) range, 100 (of whatever measurement) is a normal starting point for travel increments. PS most all of the "old style GM" stepper- motor IAC's are still working fine, however, the "new and improved Ford" triple-throw down IAC motors have been nothing but problems!

[Dave_N]

Marty/BigKat,
Thanks for the clarification. I too work with (OEM marine) EFI every day. I understand what you are saying about normal IAC position/duty cyle being pretty stable at idle, and it may be true on the small block (ECM555) applications, but not on PCM555/496 engines.
Normal IAC activity on a 496 at idle will be very different. It will rapidly change up and down by ten or more percent each way (20% swing). Maybe Beer30 can tell us what his new IAC is doing at idle.
I use Diacom software, not the Merc DDT, and I don't have a seperate "counts" data field, just the one called "IAC Duty Cycle". Maybe I am missing something.
The last thing I will still disagree on is that a stuck IAC will not trigger a fault code. Personal expierience and the Merc service manual say otherwise. Maybe not in all cases, but in every one I have seen.
Dave

[kevnmcd]

Dave N - I see you are from NB, where do you work? I just did a rebuild on my 502 EFI and need someone to help me with reducing my idle down a little. Currently it idles at 800 and I would like to get it down to around 600 or so but I can't do it without getting into the ECU. Just wondering if you could help me out or know who could. Thanks.

[Dave_N]

Kevin,
I work for myself. I would be more than happy to help you figure out what to do about your problem. Where is the boat located?
Dave

[kevnmcd]

Kevin,
I work for myself. I would be more than happy to help you figure out what to do about your problem. Where is the boat located?
Dave
Dave - The boat is stored down south but I can bring it up here (Mission Viejo area) anytime. I will be bringing up this way in a couple of weeks to get it ready for a lake trip.
Do you have a shop or work out of your house? I believe all I need to do is hook it up to a computer and reset the idle rpm setting. Can I do this any other way?

[Beer-30]

Ok, for anyone that experiences the surging idle or other idle problem related to the IAC valve, here are the part numbers that I used:
I looked the parts up on Mercury Marine, but after a Google search of the part number(s), I found Marine Power. They had the items at a significant discount.
Idle Air Valve: 862998
Gasket for same: 27-863112
Merc listed them for $88.25 and $1.10 - respectively.
Marine Power had them for $62.88 and $0.68. I see, however, that the prices have gone up on Marine Power. They are now similar to Merc's.
http://www.marinepowerservice.com/

[Beer-30]

Ok, scratch that last post.
After a few outings, in started surging again. Still sooty on the transom. Decided to take it to pro's.
Dropped it off yesterday AM. Told service writer what it was doing and what I had replaced. He said it sounded like TPS. I told him I had checked that BEFORE it had started surging and it was showing perfect voltage ranges.
The service writer calls me today, and said his thought was confirmed. TPS has gotten "sticky", probably a broken internal spring. At idle, it was showing in the high 3 volt range. So, the ECM was sending fuel and spark for 60-70% throttle. I had told him it was fine after I had changed the IAC, but started again. We figure it was because I had removed the clean air hose to the IAC from the throttle body - all the commotion had shaken it to return to idle position. After a few more runs, it stuck again. New one ordered. Now it's on them if it happens again.
Had them service the sea-pump while waiting for the part.
We'll see if new TPS / adjustment lowers the soot output.
If I had bought an ECM reader at $350, I would have found it. New TPS is about $90. The writer told me the TPS portion - including labor was $320. PLUS, I put the new IAC on at $65. I would have been money ahead by now.