School me on the 502 MPI

[OldSchoolBoats]

Boat I am most likely going to end up with has an unmolested Merc 502 mpi with around 200 hours on it. I want to know everything I can about the motor (good and bad).

Owner said he has had issues with vapor lock and it has only happened a couple times. Another person told me that if the intake is located on the side, they lean out.

Any information will be greatly appreciated!!

 

[CampbellCarl]

Start a conversation with Brian of B&D Marine...you won't regret it.

 

[GregG]

Boat I am most likely going to end up with has an unmolested Merc 502 mpi with around 200 hours on it. I want to know everything I can about the motor (good and bad).

Owner said he has had issues with vapor lock and it has only happened a couple times. Another person told me that if the intake is located on the side, they lean out.

Any information will be greatly appreciated!!

Is it the Magnum model?

 

[OldSchoolBoats]

Is it the Magnum model?

Yes. 502 Mag MPI.

 

[25Elmn8r]

I put 500 hours on a 1998. Great motor! There were a couple issues along the way though.

The first was an issue where you'd be cruising along hit a wake and it would cut out for a split second. Knew it was electrical, but couldn't find it for a while. Turned out to be a short of one of the wires on top of the motor. The threads on one of the bolts that mount the electrical box on top of the motor had rubbed away the insulation and started shorting. Quick easy fix once it was found.

Second was what we thought was a vapor lock issue. This took a while to figure out as well. Turned out the fuel pump stopped pulling fuel from the tanks. Not sure what the cause of it was, but we added a second pump on the motor side of the tank selector to pull fuel from the tanks that keyed off the ignition switch. Problem solved.

About 400 hours it started burning oil. Sold it with about 500 hours and it wasn't gonna be long before it was ready for a refresh.

 

[OldSchoolBoats]

I put 500 hours on a 1998. Great motor! There were a couple issues along the way though.

The first was an issue where you'd be cruising along hit a wake and it would cut out for a split second. Knew it was electrical, but couldn't find it for a while. Turned out to be a short of one of the wires on top of the motor. The threads on one of the bolts that mount the electrical box on top of the motor had rubbed away the insulation and started shorting. Quick easy fix once it was found.

Second was what we thought was a vapor lock issue. This took a while to figure out as well. Turned out the fuel pump stopped pulling fuel from the tanks. Not sure what the cause of it was, but we added a second pump on the motor side of the tank selector to pull fuel from the tanks that keyed off the ignition switch. Problem solved.

About 400 hours it started burning oil. Sold it with about 500 hours and it wasn't gonna be long before it was ready for a refresh.

Great information, thank you! I always thought that around 350 - 400 hours was time for refresh. If I can get 400-500 before refresh, that is great!

 

[GregG]

Great information, thank you! I always thought that around 350 - 400 hours was time for refresh. If I can get 400-500 before refresh, that is great!

Did you run the SN# to see what year it is? I believe the MPI Mags started around 1996. Depending on the true build year, there were some running changes made to the motors. They are 415hp at the propshaft depending on the year.

 

[Andy B.]

I have a 1999 502 MPI one of the best motors in my opinion. I have 560 hours on mine and still pull a 26 pitch at 4900 rpm in the cooler months. I got water in one of the cylinders motor locked and my mechanic did his magic and runs awesome(that was 2 years ago)!!I do the yearly service and I don't abuse it too much lol! I go thru maybe a half quart ever other trip but I drive it a lot when at the lake!!!!I bought the boat with 200 hours on it and the guy had all the service records to prove it!!!! Hope this helps!!!

 

[Bobby V]

I have a 98' 502 with about 600 hrs on it with no problems so far. Knock on wood.

 

[Bigbore500r]

They seem to be much simpler to maintain than that 496 HO. Less sensors, simpler raw water pump setup, therefore cheaper to maintain also. You dont have the closed cooling, but if you run strictly in lake water do you really need it?
They have forged pistons stock, and use traditional BBC architecture so much easier to upgrade, supercharge...etc

 

[robby dmax]

They seem to be much simpler to maintain than that 496 HO. Less sensors, simpler raw water pump setup, therefore cheaper to maintain also. You dont have the closed cooling, but if you run strictly in lake water do you really need it?
They have forged pistons stock, and use traditional BBC architecture so much easier to upgrade, supercharge...etc

Are you sure on the forged pistons?

 

[Bigbore500r]

Yes

 

[85RiverRAT]

We had just shy of 500 hours before the rebuild. On ours the heads were rebuilt at around 350-400 hours, and then the rings went at about 500 hours. The compression loss was enough that the motor would not start anymore, couldn't hold cylinder pressure.

The other issue was the Vapor Separation Tank, it was crapping out and fuel delivery to the motor was really bad. Under power we would have intermittent losses of power, were the motor would basically cough and re-fire in the middle of the lake. It was never 100% diagnosed, prior to the rebuild, but IMO it was the VST taking a shit.

The new set up has an electric pump supplying directly to the fuel rails. The original fuel system, mechanical pump, VST, etc. was stripped off the motor. VST parts are almost non-existent now, and what is available is really expensive.

Don't get me wrong these issues were mostly at the end though, the 450+ hours prior it as a reliable turn-key 65 mph motor over the course of 13 years. Serviced Regularly.

FYI, in my research for the rebuild, the 502's were built with a "loose" ring set, so oil burn and blow by are common, just keep an eye on the level. We routinely would have to top off the oil, even when it was healthy. Good Luck in your search.

 

[Bigbore500r]

We had just shy of 500 hours before the rebuild. On ours the heads were rebuilt at around 350-400 hours, and then the rings went at about 500 hours. The compression loss was enough that the motor would not start anymore, couldn't hold cylinder pressure.

The other issue was the Vapor Separation Tank, it was crapping out and fuel delivery to the motor was really bad. Under power we would have intermittent losses of power, were the motor would basically cough and re-fire in the middle of the lake. It was never 100% diagnosed, prior to the rebuild, but IMO it was the VST taking a shit.

The new set up has an electric pump supplying directly to the fuel rails. The original fuel system, mechanical pump, VST, etc. was stripped off the motor. VST parts are almost non-existent now, and what is available is really expensive.

Don't get me wrong these issues were mostly at the end though, the 450+ hours prior it as a reliable turn-key 65 mph motor over the course of 13 years. Serviced Regularly.

FYI, in my research for the rebuild, the 502's were built with a "loose" ring set, so oil burn and blow by are common, just keep an eye on the level. We routinely would have to top off the oil, even when it was healthy. Good Luck in your search.

Same rings and pistons as the GM 502 automotive crate motors. Refered to as a "low tension ring" package, ment to reduce friction. Between those and the lack of torque-plate honing (causing the big bores to be out of round)....it is common for them to suck some oil

 

[DaveH]

oil consumption because of the bore/ring package is a known issue. a friend had one in his 24 hallett that leaked down so bad at 220 hours.....he had the engine rebuilt.

the efi system has some issues. I believe the number1 cylinder has a very small passage in the fuel rail compared to other cylinders and is know to lean out because of it. you can bore the passage to compensate.

the fuel pump/VST system in that package....good idea...poorly executed. a good option is to remove the fuel rail, modify it for AN fittings and install your own fuel pumps and regulator. just make sure to have a VST or "swirl pot" in the system as well.

 

[Wombat]

I have 2 x 1999 closed cooling system 502 MPI's in a Fastlane 40.

Both these engines have 250 hours on them with no real problems, except for a leaking heat exchanger on one and both the stock Mercury exhaust manifolds starting to leak water.
The boat is run in a salt water environment so there is extra stress on the engines parts due to corrosion, but they still look and run as good as new.:thumbup:

The exhaust manifolds were replaced with CMI's on both engines which are excellent units and both the heat exchangers were replaced with Mr Cool units which were of excellent construction and substantially cheaper then the stock Mercury units.:thumbup:

My boat is a little on the heavy side, but with Bravo 1 Legs and running Bravo 1 28" props still gives the old girl a top speed of 60 mph when she lifts her skirt up for a run.
This will change when l install a couple of Whipple chargers later this year.:yikes :thumbup:
Old Skool

 

[Wombat]

We had just shy of 500 hours before the rebuild. On ours the heads were rebuilt at around 350-400 hours, and then the rings went at about 500 hours. The compression loss was enough that the motor would not start anymore, couldn't hold cylinder pressure.

The other issue was the Vapor Separation Tank, it was crapping out and fuel delivery to the motor was really bad. Under power we would have intermittent losses of power, were the motor would basically cough and re-fire in the middle of the lake. It was never 100% diagnosed, prior to the rebuild, but IMO it was the VST taking a shit.

The new set up has an electric pump supplying directly to the fuel rails. The original fuel system, mechanical pump, VST, etc. was stripped off the motor. VST parts are almost non-existent now, and what is available is really expensive.

Don't get me wrong these issues were mostly at the end though, the 450+ hours prior it as a reliable turn-key 65 mph motor over the course of 13 years. Serviced Regularly.

FYI, in my research for the rebuild, the 502's were built with a "loose" ring set, so oil burn and blow by are common, just keep an eye on the level. We routinely would have to top off the oil, even when it was healthy. Good Luck in your search.

You say your engines had mechanical fuel pumps, do you know when they changed to electric as both my 99 motors have electric pumps installed.

 

[shueman]

They seem to be much simpler to maintain than that 496 HO. Less sensors, simpler raw water pump setup, therefore cheaper to maintain also. You dont have the closed cooling, but if you run strictly in lake water do you really need it?
They have forged pistons stock, and use traditional BBC architecture so much easier to upgrade, supercharge...etc

There ya go ... good stuff Jer :thumbup:

 

[OldSchoolBoats]

They seem to be much simpler to maintain than that 496 HO. Less sensors, simpler raw water pump setup, therefore cheaper to maintain also. You dont have the closed cooling, but if you run strictly in lake water do you really need it?
They have forged pistons stock, and use traditional BBC architecture so much easier to upgrade, supercharge...etc

Thanks buddy! Great information. I was over in your new neighborhood yesterday when I went to look at the boat.

 

[1540RVRCREW]

I have a 99 with the 502 with about 125 hours. Prior owner hardly used the boat. So far no major issues. I get the power I need and it's relatively easy to maintain

 

[farmo83]

My uncle has a 210 with a 502 Mag and Imco powerflows and he loves it. I believe he has around 450 hours on it and it does use a small bit of oil but not much. The only issue I know of is right after he bought he got water in a cylinder due to his water pressure being to high. We put in a pop off valve from mercruiser and it's run like a top since then.

 

[GregG]

If the labels on the valve cover say "502 MAG MPI" in the lower left the motor is at least a 1996. From 1996 and newer there were MEFI SW changes made over the years that would require a SN check if you wanted to do some upgrades later after you use it for a while.

 

[69 1/2]

The 502s use a siamese cylinder configuration meaning there are no water/cooling passages between the cylinders due to the bore being so large. The result is cylinders that become ever so slightly oval when warm. This configuration requires forged Pistons with a little "extra" piston to cylinder wall clearance so the tend to naturally use a fair bit of oil.

I've seen lots of 502s with well over a thousand hours on them and still running strong. Where I boat 100hrs/year is the norm. I've seen several 502s in Carver houseboats that rarely have the oil checked and they run dry. House boaters don't care for daily engine maintenance.