502s weird problem
 

Ok so my friends boat just started with this weird issue yesterday. while the engines are above 3500 rpms the starboard side engine will drop out after about 10 min. it loses rpms and then just quits but restarts and drives normally immediately. It happens faster the higher the rpms you go over 3500 and doesnt do it at all under 3500. I thought initially it was a cap and rotor, so i replaced them both, they needed it anyway. Problem is still present so i think it might be the coil. I figure i should swap the coils from engine to engine to see if problem moves. beyond that not sure, plugs are fine and wires are good. Guess i have to take it one step at a time. I don't think its a fuel pump issue because he has mechanical fuel pumps and it runs for a good while before it presents itself. Anything else I should be looking at?

another thing the wires are not in the same position on the caps, but the firing order is identical on the spec plate attached to each engine. All the wires are labelled and the starboard wires are 1 position counter clockwise different then the port. I thought i was crazy or did something wrong when i put the new caps on but i verified all the wires each side cylinder to caps and they all are correct. I ran both engines and they both run fine and start normal. I even switched the wires on the starboard side to match the port side and it wouldn't run at all. Why would identical engines with same firing order have different wire locations on the caps, like i side its only one position off counter clock wise all the way around the cap.

example if cylinder 1 started at the 12 o'clock position on the port side then his cylinder 1 on starboard is at 11 o'clock and so on all the way around the cap, cylinder 2 at 1 o'clock port then cylinder 2 at 12 o'clock starboard

i cant figure that out at all.

the distributors are just clocked in different positions, no big deal.............502 what ? carb or efi.........sound like it may be a fuel issue

502 MAGS EFI.

I thought it could be fuel but once it starts happening even if you back off on the throttle it still drops rpms and stalls. so i thought it may just be the coil getting hot and on its way out.

have the engine scanned

Does it store info of that nature, i have never had any experience with that.

The fact that your one engine quits at a certain rpm and quits sooner at higher rpm is indicative of a fuel supply issue. In other words the fuel pump(s), both mechanical and electric can only supply enough fuel up to XXXX rpm, in your case 3500.

Before BUP chimes in I'd recommend posting the serial number of the engine(s) in question so we know exactly what we're dealing with.

Has there been any work done on the motors recently?

Check fuel pressure. My guess is one of your pumps on the suspect motor is going south OR there is a restriction in the fuel supply lines/filters.

It would also be wise to have the injectors pulled, cleaned and flow tested. The filter baskets in the top of the injector can get clogged. This issue would not manifest itself with the systems you are currently experiencing but none the less you should address injector cleaning.

502mpi with VST. The VST is always suspect on these engine's.

I will have to get the serial numbers off the engines tonight or tomorrow. The boat is a 96 wellcraft 45 excalibur engines were replaced 5 years ago and this issue is new. I know that the engine in question had a fuel pump replaced 2 seasons ago, he has mechanical pumps on his engines. I guess I better do a fuel pressure test also. I not sure if he has the vst units never paid much attention to whats on it, but will check.

It just seems weird that it takes like 10 min or more for this issue to present itself, I would think if it was a fuel issue it would almost be instant. Like i said i will do some basic checks and see how it goes.

Unfortunately I am t boat now and can not read serial numbers it does have vst on it and two belts for the drive pulley system if that helps I brought a fuel pressure gauge but realized mine is made for a Schrader valve set up this does not have one I can see.
can vsts be removed completely or do they need replacing? I am still gonna do a coil swap

Schrader valve should be just behind t-stat housing on frt of fuel rail. I believe it has a brass cap on it.

Yes but not for his particular ailment. He's not experiencing vapor lock from what he's described.

I will have to check for that behind the tstat housing i didnt see it earlier. what i did find on the problem engine is on the coil it had green build up on post and on the wire also. the other engine coil was clean. i cleaned them both cause i didnt have time to buy wires but i want to test it first.

It can have more issues than just vapor lock. Remember, the VST is a big float bowl complete with a float/needle/seat type deal
+ with an electric pump in it....

http://www.mercruiserparts.com/Merc/...MMON/39670.gif

There is a real fine screen in the VST that can get plugged. Had a 96 Scarab that had that issue.

My vote is with SB and the VST having issues.

I won't know till I get it out on the water for a good run. If it is the vst can they be removed or rebuilt or what do you do. I will have to keep a fuel pressure gauge on it I guess to see i take it?

So you saw my post and got the fuel psi tester on it ?

Another issue some have run into is the fuel pump 'lobe' in the sea water pump having issues.

i saw the post but haven't yet I was already home by then running down today to locate the Schrader valve.

my bet is vst issue,that is one of mercury marine,s biggest failures.

Can I open the vst unit and look at screens and stuff with out buying parts or once you open them do you need new gaskets and such?

Both engines reading 32 psi at idle and drop to 28 at 1800 rpm seems low for both gonna run it now and see what happens changing water separators now on both

Mechanical fuel pump is not pumping anything and engine will not run at all so I guess we have our answer I removed the screw and it's got gas in it. Anyone have a link on replacement​ looks super easy but how much oil and what type to fill it with and do I replace just the pump or the pump and housing.

If engine is not running. how do you know the mechanical pump is not pumping anything ? IE: what was your test method ?

0 fuel pressure removed output line right off pump cranked engine no fuel at all engine had fuel pressure earlier in day we got on it under load and engine quit I watched fuel pressure drop right off and engine quit after that 0 fuel psi then checked lower housing where pump sits on and filled with oil and gas

waiting on mechanical pump, thank you guys for all your help so far. I was thinking in my head that the original issue may be different then just this mechanical pump but will have to wait and see.

Here is what doesnt add up to me before the engine quit i had a fuel gauge on both engines running at idle they were both identical 33 psi. that seems low to begin with. When i brought the rpms up the psi went down to about 28 ish maybe 29 but never below 28.

what is your guys opinion about this fuel pressure?

both engines were identical before the one died completely mind you then as that engine died i was watching the fuel pressure go right down to 0. Like i said i removed the output line on the mechanical pump and cranked engine and 0 fuel output not even a dribble. I told my buddy i still think there may be an underlying issue but hopefully it is just this dumb pump.

Just before i left today i pulled the pump off lobe covering is ok no drastic wear on pump arm or lobe cover but loaded with gas and oil. Cleaned it all out prepped it with new oil and waiting.

PSI will go down when bringing up the rpm on the trailer. No load and higher rpm = more intake vacuum. Vacuum is attached to regulator. Higher vacuum lowers fuel psi.

When you put boat in the water and increase rpm in gear intake vacuum will go down which makes the fuel psi raise.

As far as the base fuel psi at idle, depends on your motor. Some 502's where near yours, other where higher psi.

thank you so much for all the assistance. that made total sense cause i did see a rise in fuel pressure under load in the water exactly how you stated above.

so i am hoping it is just the mechanical pump then.

the other one died last year, i was not around to help him with it. the water housing was leaking into the oil for the pump and they replaced the whole housing they went to install the old pump back on it and it was dead between housing replacement and the pump they charged him 1300 i was like wtf. I think they ripped him off

Compared to doing it yourself it will sound like a ripoff, but it is not for a shop to do it. Those sea water pumps are expensive.
Discounted at $710.00 http://www.mercruiserparts.com/bam/s...31902/1614/330
Fuel pump discounted at $223.50 http://www.mercruiserparts.com/bam/s...31902/1614/130

Put it this way, how much do you think the VST units where from Merc when they where available ?

Hint, it rhymes with $2,000. :)

http://www.mercruiserparts.com/bam/s...31902/1614/150

Oh I know all about expensive parts I work for Mercedes-Benz
i guy I know just replaced his brakes on his s63 AMG at the tune of 7800 dollars parts were 7200 of it.

Yup, those dumb rotors are mega bucks !

See, your buddy with the boat got off easy at the dealer. :evilb:

I am going to be the VST contrarian here. I've run a VST for 22 years and am still running it. I've replaced one rubber diaphragm in it that aged out. Thats it. The VST problems people have had (specifically vapor lock) is predominant primarily due to it's location on the big blocks due to heat soak. The symptoms he has here have never been attributed to VST.

What i would like to know is can i open up the vsts and clean out the screens for preventative maintenance and not have to replace the seals? Or do the seals need replacement once you start messing with them. The boat is 22 years old and i assume its always had the vst system on it and my friend said he has had them cleaned out a few times since he has owned it. I just worry about opening them up cause the dealer is like they dont make parts for them anymore and you have to upgrade to the newer style once they break. He said there is a service bulletin on how to update them to the newer style at 1200 per unit.

update boat fixed mechanical pump and lower unit to the pump was the issue pump seized and the lower part was worn in to much so replaced both parts. Only thing that stumped me for a bit was the fuel line must have gotten to much air in it because it wouldn't prime on its own. I had to disconnect fuel line before water separator on the fuel tank and at the inlet of the vst blow into the line before water separator till some fuel came out the hose going into the vst hook it all back up and boom we have lift off. We ran it up to 4200 rpms for awhile and all is good.

thank you for help on this.

BUP just sold me new o-rings for the VST lids on mine. Previous I had taken them apart to clean them and the O-rings expanded so much they wouldn't fit once I tried to reinstall them. I searched back then on here and found a thread where someone cut a small portion out of the o-ring and super glued the ends together to make it fit properly back in the lid, it worked for me and it didn't leak. Obviously not the best solution but those o-rings are damn expensive...

In case anyone is looking for the large top o-ring by itself it is part# 25-803831

$10.00 or so. A decent amt of places keep them in stock,

http://www.boatfix.com/merc/PBullet/98/pb98_06.pdf

Full o-ring kit is part# 808500 and just over $100.00

Edit: I purchased the complete VST o-ring kit that's why I mentioned it being expensive... just wanted to clarify. :)

well it seems this problem has come back again, friend called me yesterday said problem started again. I cant imagine the mechanical pump went again, but i will have to check oil in lower part to see if it has any fuel in it. other than that i think it may be a vst issue and gonna have to tear into it i guess.

So I swapped vsts side to side and problem stay with same motor so I know it's not a vst problem but when the problem occurred I had the pressure gauge on it it and the pressure was dropping and bouncing all over. I have a new mechanical pump on and a new lower part for the that pump I have swapped vsts I know 100 percent fuel supply issue so I am thinking it's either the new mechanical pump went bad or pressure regulator only two things I have left really I guess any ideas.

i think i have to bring it in to get scanner hooked up to it. i have swapped vst, mechanical fuel pump, coils side to side and it always stays with the same engine. it takes a few minutes for the engine to start losing fuel pressure but its only above 3400 rpms. the pressure is good then it just drops off and bounces around till engine stalls

really need help

i was thinking its the fuel pressure regulator or ecm or map sensor maybe i suppose i could swap those side to side to see if problem moves to the other motor.