500 efi needs AIM and Thermotat Housing
 

I am praying there is some help here; I have 2006 formula 400ss with twin 500 EFI's only 250 hours. The port will not stay running. Starts but stalls unless you manually massage the throttle.... shipyards through troubleshooting felt it was thermostat. Ordered new thermostat and then proceeded to take Aluminum Intake Manifold and Thermostat Housing to see a bad thermostat. However due to salt water damage putting them back housing back on the AIM does not seem feasible. Shipyard can not find the parts. Mercury says they stop making them 10 years ago; Does anybody have any information on this; I have checked 4 or 5 distributors and none of them have the parts;

<!--td {border: 1px solid #cccccc;}br {mso-data-placement:same-cell;}--> Mercury part number 849900t3 Aluminum intake Manifold
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Mercury part number 849927t1 Thermostat housing.

Both engines run so well but now I can not keep the port running it just stalls, sometimes hard to start.

Thank you for any advice....

There is an upper and lower on eBay

https://www.ebay.com/itm/31473992000...mis&media=COPY


There are threads on here to modify a T-stat housing or just pay Mercury ransom if nearly $1k. Mercury has become ridiculous on pricing for parts.

Unfortunately Mercury discontinued this part 10 years ago... :(
Thank you for your help!

Not that its gonna help you with the parts, put the odds that your thermostat has anything to do with your engine staying running is about zero. I'm not going to guess sensors but one of them has a problem most likely

As far as the problem with corrosion, without seeing a picture, its hard to give you a fix easier/cheaper than a new part. The thermostat housing is available all over the place, just at a stupid $1000 and going up from there.

This one ?


https://cimg6.ibsrv.net/gimg/www.off...c37f8b98fc.png

Best price i found doing 30 second search for OE part is $733 here:
https://nuwavemarine.com/special-ord...ermo-849927t1/

I also just remembered Speedy buying some aftermarket replacements for his 500EFI’s. Maybe he’ll chime in. If not, send’m a pm.

exactly why you need to buy a used one

Same issues
 

Something weird going on here. Put thermostat back in, boat would not even turn over. Turn Key and then move shifter slowly until engines turns over and it starts right up and run great like the starboard... not loud, but then about 60 seconds into, it runs harder and starts stuter unless manually controlling, then I hear the IAC kick on, and just runs rough and when more gas is applied it stutters until more gas then it jumps up to 2200 rpms and runs rough... I have had mechnic out with computer, they do not know.... replaced IAC last year tons of sensors... spark plugs, distributor cap, etc..

Compression test, fuel pressure test, and a look at the plugs....may have something going on in the valve train or a bad hole causing havoc in the computer trying to compensate for it?

What about the spark module inside the distributer, those can cause all kinds of issues for starting/running as it can throw spark timing off. They get hot and fail over time

Had issues with mine going bad 3x, replaced the original distributors and ran perfect, then one started acting up and replaced it - ran perfect again. You need to buy a Merc module or distributor as automotive versions don’t work right ( a few perf companies offer them and work but it’s hard to know which are good)

I agree on checking fuel psi and compression. Most diagnostics should start this way.

Also, yes, mag pick ups and modules can have issues in those dist’s.

Also, injectors can be issues on those motors too. Search Arcticfriends posts.

If salt damaged/ate thru the tstat housing, other components (intake/block/heads/exhaust) could be compromised too. Have we ruled out any of that ?

Thank you guys,

Here is the update;

Found 1 problem was the shifter mechanism had apparently got tons of salt water on it and under the 4 arm shifter / gear was the neutral switch going bad.... Had to turn key and move gear shift back and forth until it engine ignition barely engaged and then the engine would turn over.. Fixed that, replaced all the plugs, rotor and cap.... engine start then fail as it has since summer of 2022.

Decided to swap the entire fuel pump, cooler and regulator from starboard to port. Sure Enough the Port started and ran great. But no Starboard did not start. Swapped out the fuel pumps and starboard does not start but port does... So got it narrowed down to the regulator or cooler. So it is the cooler but most likely the regulator part #15 - - which is 43 psi..... if it has gone bad, alot makes sense.... we smell gas, feels like it floods out.. and the pump sounds like it has too much pressure


Thoughts welcomed.... wish I could find someone in SO CAL to do all this.... I have a decent mechanic but not a big troubleshooter, called everyone and the shipyard charged me over $2000 and did not find it, wanted to replace the entire manifold and thermostat housing.... I love this boat and the layout but the lack of service for the racing engines coupled with salt water I might have to sell after I get it running....

most likely regulator. Did you check fuel pressure? We had same issue a few years ago. Same symptoms.

good luck