Merc 8.1S IAC & PCM questions

Z

Zephyr

New Member
Mar 7, 2013
15
Perth, Western Australia
Boat Info
2001 400DB
Engines
Cummins 450's
Hi Everyone. A few questions for anyone with Mercruiser 496 expertise.
Does the IAC open due to TPS or MAP readings or a combination of the two?

I have recently burnt one out (the connector block on the top of the IAC looked like it had blown up/melted), replaced it, checked the vacuum tubing & cleaned the filter on the throttle body - yet the beeping remains and the engine idles poorly. Swapped the IAC with one on the ‘working engine’ and still it runs like a dog.
Can anyone point me in the right direction?

Symptoms:
Engine: Mercruiser 8.1S (496) circa 2004
Poor idle @ around 600-620rpm, the other engine idles rock steady at 650 (they both used to)
Engine is very quiet compared to working engine (ie you can hear the working engine breathing away – the problematic engine is relatively quiet)
Fuel pressure has been checked by Merc Service and is fine (40psi)
Guardian beeping twice every minute with no fault indicated on Smartcraft gauges – the beeping stops at around 1500rpm & above.
Will no longer start unless I give it a little throttle (1000 rpm ish) and slowly reduce back to the mechanical stop.
Throttling back smoothly from higher revs the engine gets to around 1800 shoots up a little to 2000 then drops away quickly (catching up to throttle lever position)
Runs well once away from idle, say 1000 rpm.
Plugs look fine, mixtures fine, no water etc, oil looks fine, oil & water pressures/temps & volts all look fine.
It has always used slightly less fuel than the working engine – around 12% less when I fuel up (both engines have the same duty placed on them, starts are shared ie Port first then stbd and vice versa, hours were identical – also can discount any generator consumption)

Thanks in advance all.
Z.
 
You mentioned that you cleaned the filter in the throttle body. Might make a difference if you replace it. I've been told that that filter getting cruddy is the cause of IAC's getting fried. It has become part of my annual fall maintenance to replace the filter, which they call the IAC Muffler, Merc part# 35-863829. If that doesn't work you're going to have to pull code from the computer to see what's wrong. MUFFLER is #5 below.


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I would start with the muffler. It's literally a couple of dollar fix. If the muffler is clogged the IAC cannot do it's thing.

As Ron said, there are folks who say that a dirty muffler causes the IAC to work harder, overheat and burn up as you described. Make sure you put the gasket back on the IAC so there is no air leakage.
 
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And buy your spare IAC from an auto parts store. Same thing (just not painted black) for about $40 less. Got mine at NAPA. Keep it on board just in case. Rick taught me everything I know about this topic!
 
That ain't saying much...

Somewhere on here is a thread that has the NAPA parts numbers for the muffler and IAC.
 
I posted the information below on another boating forum he (the OP) asked on then i realized he asked the same question here as well. So not to leave sea ray owners in the dark, here is another option to take into consideration if your IAC gets badly damaged shorting out your 8.1/496L PCM.

Unfortunately this is a common problem with the 8.1/496 Mercruiser engines and probably other engine brands as well I can only assume unless it's specific to Merc's 555 PCM's. I'm not sure of that. Typically the IAC gets stuck and eventually melts and in some cases the iac connector will catch fire if not noticed in time. Also the iac driver inside the PCM will short out as well. If the wire harness connector gets damaged, without true confirmation, the new connector harness is rumored to be motorcraft Part # WPT357 seen in post #8, http://www.offshoreonly.com/forums/general-q/257741-496-ho-iac.html

Now there is an option to repair vs buying a new PCM. Dennis @ Whipple Industries (AKA) Whipple Superchargers out of Fresno, CA is able to repair the PCM for less money then new. Information below.

Whipple Superchargers, the performance technology people, can now repair many of the 2001 to 2009 model year 555 ECM and PCM’s used on the Mercury Marine and Mercury Racing engines. Common issues such as the IAC failures and coil driver failures can all be repaired saving you thousands of dollars. No need to spend $2000+ on a new computer, just let the experts at Whipple fix if for you.

This is as simple as removing the ECM/PCM from your engine (3-bolts and 3-connectors), shipping to Whipple Superchargers and within a couple of days, you can have your ECM/PCM back ready to bolt on and go boating. No tuning, no reworking components, no head aches, just bolt on and go.

ECM/PCM repairs for some of the following: 200, 225, 250, 275, 300 XS engines. 135, 150, 175, 200, 225, 250, 275, 300, 350 Verado engines. 4.3L MPI, 5.0L MPI, 350 Magnum MPI (and Horizon), 350 Scorpion, 6.2 MX MPI, 377 Magnum, 377 Scorpion, 496 Magnum Base, 496 Magnum HO, 525HP EFI, 600SCI, 662SCI, 700SCI, 748SCI, 850SCI, 1025SCI, 1075SCI, 1200SCI.

Prices start at $500.
http://www.whipplesuperchargers.com/product.asp?ProdID=1270

Also there is one other place in CA that does ECM repairs but I'm not sure if he does 8.1's or not, he is mainly an outboard repair shop.
http://www.simonmotorsports.com/

Hope this helps, Good luck.:smt001
 
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I've got the NAPA box containing the spare in my ER. I think it was $63. Might have to wait til the spring thaw for the part #.
 
Thanks so much for all the comments guys, I have new mufflers ready to install and a spare IAC at the ready. I'm running diagnostics on Friday to ascertain the extent of the PCM damage - if at all. The guys at Whipple replied to my email almost immediately and said it is common to have the IAC valve fail and if the connector burnt out it probably damaged the chip that runs it in the computer. Around $500 to repair is much better than the $2350 replacement. I'll keep you posted ont he findings,
Z.
 
Z...
please stay in touch on this. Certainly is an issue that could apply to many of us one day. THANKS!
 
Just want to point out that the IAC filter issue does not apply to boats with DTS. In those cases the computer regulates the air flow thru the throttle assembly...no filter needed.


Peter
 
Last season I was having idle and stalling issues on my two 496's. Both at the same time. I replaced the two IAC's and all was well. After lots of research I found Rock Auto. I bought a bunch of spare sensors among other frequently replaced parts. They were the best deal around. Do research on the parts you choose though to make sure they have a good reputation. Most are quality replacement parts but some of the manufacturers are garbage.
 
Carbo said:
Just want to point out that the IAC filter issue does not apply to boats with DTS. In those cases the computer regulates the air flow thru the throttle assembly...no filter needed.


Peter
Click to expand...

I looked at mine the last time I was on the boat, and couldn't find the muffler everyone is talking about. I do have DTS, so I guess I fall into this category?

Kevin
 
Kevin
if you take off the flame arrestor and look inside, you'll see it's a round tube, almost like a 5" PVC pipe. Inside the tube there is a cavity, I think it's around the 12 o'clock position. The muffler (little 1" square of foam) is inside this cavity. It is tricky to find if you don't know what exactly you're looking for.
 
But for you, being a dentist, finding a cavity inside a confined space is easy, right?

Dale
 
Without an xray, no!
 
Thanks Ron,

I did see the large tube, but no cavity. The tube curves down into the intake, and appeared solid. I'll take another look next time.

Kevin
 
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