Cool Fuel plumbing question
 

502 Mag MPI with cool fuel (non VST) system. Intuitively, I would think that the "out" arrow from the fuel filter head should be plumbed to the inlet of the fuel pump, and the line from the pressure regulator goes back to the "in" arrow port on the fuel filter head. Is this correct?

The boat I'm working on is reversed. The line from the fuel tank goes to an "in" arrow port on the fuel filter head, but then the "out" port is connected to the regulator, and the other "in" port is connected to the fuel pump.

At best, I'm thinking that this is running the filter in the wrong direction, potentially decreasing its efficiency? At worst, more load on the fuel pump or even reduced fuel delivery under load?

Your correct the Reg and pump is swapped

Doubtful, in’s/out same size and paper flows same both ways. but the fuel pick up in the water separator is the threaded nipple the filters spins on to attach. I would guess prming woukd be difficult and lot more chances of vapor lock without having a lower fuel pick up in the separator. Think of separator as a fuel reservoir.

Now, if system has low psi pump feeding separator, this would not be much of an issue as it’s keeping separator (reservoir to high psi pump) full.

hopefully explained that okay ?

No pump feeding separator.

As I think about it further, chances are the fuel isn't even being filtered. The pump is drawing from the same side of the filter as the tank is supplying it. it's basically bypassing the media entirely.

Going to put new filter elements on and then swap the lines.

Thanks for the input.

yeh, if from tank line is on same side as element to hpressure pump, you are right.

and if that’s the case i’d be checking injectors screens and the screen in cool fuel just under regulator. Look at microfuce, have to temove reg from cool fuel and will be right there,

inlet arrow: from tank
other inlet arrow: return fuel
outlet arrow: to high ores sure pump
other outlet arrow: pipe plugged

I thought in factory form the to and from lines to the pump had different size fittings at the fuel filter head ?? goof proof.......thats the way I remember my 502 mpi's were....IDK been a few boats a go........

Maybe. I'll take a closer look... they appeared interchangeable at a casual glance.

In addition, these 502s have MEFI 1 PCMs and the early MPI Keihin style injectors, but have the later cool fuel system instead of a VST. So it's possible someone bastardized the whole thing at some point over the years.

They are

mine were gen 6 mefi 3's

Just posting pic snd link: #16 is screen filtrr under reg /top of cooler i was referring to.
https://www.mercruiserparts.com/bam/...31904/2690/100


https://cimg5.ibsrv.net/gimg/www.off...871a602fc6.gif

Yup thanks. Gonna disassemble that after work and check it out.

I believe you are correct. Pretty sure the MEFI 1's were only on VST engines

MEFI 1 with VST and mechanical pump mounted on raw water pump. MEFI 3 no mechanical fuel pump, and both used the standard fuel filter head

No mechanical pump. Filter heads on the forward bulkhead, plumbed directly (albeit backwards) to Cool Fuel under port side of motor. PCM has connectors on the ends not on the top/side

Sounds like a 7.4 L29 MEFI-2 and put on a 502
https://www.mercruiserparts.com/7-4l...-454-v-8-199-1

Look on the back of the MEFI for the numbers

Another thought, you sure its a 502?
Might check casting numbers

Lucky for you the half your seat lifts up, those are a PITA to get to on most.

Yeah. I pulled the PS pump and that gave me pretty good access laying face down on the back seat.

The screen under the regulator had just a tiny bit of debris in it. Nothing all that significant.

Block has 8.2 near motor mount on port side . So 502 confirmed.

I was giving to say that I have a nice set of injectors for your MPIs, but as mentioned it looks like you have cool fuel added to an older set up.

I’d definitely have the injectors serviced.

New pump installed, showing 36 psi with vacuum hose disconnected from plenum. What a PITA it was getting that cool fuel module lined up with the fasteners to the block. I spent more time on that than any other step in the whole job.

Also, I did not see anyone mention this in other threads -- the factory procedure for this job calls for using heat sink compound / thermal grease to facilitate heat transfer where the pump body contacts the retainer plate, and where the retainer plate contacts the cooler body. Likely key to pump longevity.

Should be 43 psi p/n 860349A1

Port motor doesn't hit 43 psi either. Really think both regulators are bad?

Need to first figure out how the motor is configured. Is it a 502 MEFI 3 which was converted to a Mefi 1. A Mefi 2 for a 7.4 mpi installed on a 8.2. Some kind of EFI dystem tied into a mpi or something other.

Right now I dont know what should or should not be there

Well, following the Merc diagnostic charts in manual #23, it states that 34 psi is the minimum allowable pressure with key on. I'm seeing 36. When the pump stops running after 2 seconds, it drops to about 30. If I pinch off the return lines, the pressure spikes up to 60+ psi (I was using a power probe for this test so I immediately let off when I saw the spike).

So, this would indicate that the pumps are both supplying adequate pumping ability, and that the regulators seem to be limiting pressure to 36 psi, which per the manual, is acceptable. In the mish-mash of parts that I seem to have, perhaps a different regulator was used when the cool fuel was adapted to the early injectors?

Ran the boat to see if there is a performance issue or not. Very smooth, accelerated well all the way to 65 @ ~4000 rpm, where I let off because this is a new boat to me and didn't want to overstep my comfort zone. No hesitation, temps all normal right at 160.