2007 Fountain Lightning 42' with 700sci engines at 400 hours. Stock but for very mild computer reflash for a little more boost (8.4psi) and rev limit to 6000.

2019 redid rockers, lifters, pushrods with Teague parts; port engine valvejob /heads gone through as had some valve leak on compression /leakdown on that side. 2019, running fine since, hit best ever 109.5mph on Lake Michigan.


Until: 2022 started getting Guardian Mode IAT warning on STARBORD engine. Checked resistances of all 4 air temp sensors (pre/post intercoolers) all same.

Last year the PORT engine went Guardian on IAT once ALSO...so I figured NOT some "electronic glitch, sensor, capacitor leak in computer" if BOTH starting to do it.


This year I changed seawater impellers (a few years old). They looked "ok" not chunked off or broken; cleaned sea strainers (were clean anyway).

Both intercooler drains out side of boat are flowing about same at idle (not a lot but that is their nature I guess). Seawater strainers are clean and seem flowing normally; engine water pressure /block pressures, engine temps normal and symmetrical. Water pickups under boat are clear of debris.


Got Diacom software to evaluate what's going on and temps at sensors (Diacom only shows 1 IAT and no fuel pressure, wanted more info out of the Diacom but oh well)

This year on first outing, Port goes Guardian around IAT=150. STBD has not (yet) and hits around 127F, when Port =150F.

Lake temp 56F.


What is Guardian Temp setting on these Motorola 555's on 700SCI's? wasn't datalogging but my buddy watching computer when Guardian triggered thought around 150F.

150F on 56F lake water at 8.4psi seems like my IC have really lost efficiency.


Do the stock IC's in freshwater go to heck, corrode, get plugged in normal course of mild to moderate use, just with time?

I may take off fittings and see if I can run my Endoscope into IC, wonder if I can see debris /corrosion there?

I don't have Whipple IC Water Cooler Fitting Upgrade yet (10 or 12 AN sizes). Not sure how involved "putting a new water pickup" under hull for the 12AN size would be for me to do? Whipple site doesn't show that procedure, parts or instructions.

Mercury Racing 600/662/700SCI Intercooler Fitting Upgrade (whipplesuperchargers.com)


I am hoping SOMEONE with experience has "seen this IAT problem 1000 times, it is almost always THIS"...because this is a CHANGE from prior behavior so SOMETHING is altering my IC's efficiency, all else the same.

Thanks for the help, especially from Offshore Sellers, Knowlegeable, Those With A Fix

I'd pull the intercoolers and have them cleaned.they're not doing their job anymore

Did you ever get this figured out? I'm having the same issue at 400hrs with 2008 42 Lightning.

UPDATE: IC's ARE plugged by dead grass, tiny twigs etc when viewed by Endoscope through the fitting port! It's kinda ugly looking in there with leafy/twiggy debris.
This despite the stock Sea Strainers from Mercury, which have holes that apparently let things through.
New Mercury QC4 engines have a "real filter" like oil filter setup for the supercharger water to be clean of this crap, of course they cost over $1000 I recall, though cannot find the part anywhere on Mercury's glitzy but rather USELESS website that won't let you search for actual Mercury Racing parts!
The whole intercooler assembly is $16,000 plus. I haven't found just the IC core separately. Don't know if Whipple or someone has it. It is special copper nickel to avoid corrosion but cost is ridiculous.

I decided to Backflush the IC's with hose water from faucet, using a clear flexible tubes hoseclamped to the outlets from the IC's (I have upgraded #10 AN fittings setup). Ran hose water backward and dump tube over side of boat. Wondered on pressure handling but I used full hose pressure and no problems, though hose was like 200 feet from the faucet so not SUPER high pressure.
Backflushing unfortunately did NOT seem to remove a whole lot of debris; using endoscope didn't "appear much better" at the END of the IC that is VISIBLE...
BUT running the boat afterward, the IC's DID flow more water visibly out side of boat at idle, and at full throttle seem to work better--no more IC Over Temp warnings / Guardian Mode on either engine after the backflush.
Suspect the inner passages had smaller particle plugging that was removed fairly significantly by backflushing, but the larger sized "stuck" pieces visible on the END of IC's were jammed on so stayed.
Was thinking of putting some chemically active solution into the IC's to try to break down the cellulose that leaves and twigs are composed of but would need something that would not eat into copper/ nickel, while dissolving the leaves/twigs cellulose.
Anybody know what chemical to use?

I plan to put a finer stainless steel mesh in the Sea Strainers fitted carefully so nothing sneaks by, since I am NOT spending $2500 on Mercury QC4 filter setup and somehow Jerry-rigging it into my boat. I will use stainless steel screen door mesh from hardware store, probably will cost me all of $20 instead of $2500. It will be almost full flow, as the voids in such mesh are smaller filtering but surface area will still be about as large as the stock setup which will provide support to the thin mesh; this will reduce size of any leaves/twigs to sizes that FIT through the IC tubing.
Summary:
1) YES IC's DO get plugged A LOT despite stock 2007 Mercury 700SCI Sea Strainers, and it DOES cause overheating of charge temps even in coldish Lake Michigan water at only ~8-8.5psi boost.
2) YES Backflushing with hose water DOES help significantly but not completely clean.
3) YES I think stainless door screen mesh in Sea Strainers will reduce size of particles enough to avoid more plugging, having seen what IS in there.
4) YES taking both whole superchargers off and removing the IC's and taking them (where? how? what solution used) to have them cleaned "properly" would be "Best" but holy cow I don't feel like doing it.

I always backflush my 1075 intercoolers every two years with Barnacle buster. I set up a electric pump and a 5 gallon pale with solution and let it run for a few hours. you can set it up in the boat without taking everything apart. Cleans them out like new. You could also send them out to have a place sonic clean them. You should upgrade your 700's to the Whipple -12 AN fittings to get better water flow and cooling out of your inter coolers.