[99 Mariah 258]

I'm still having problems with my 99 Z258... I'm pretty sure it's vapor lock . It can run perfect for 5+ hours with no problem but when she's hot and I turn it off she won't fire up without cooling off for about 1 1/2 hrs. I had 2 previous breakdowns that I turn the boat off and when I try to fire it back up the motor turns over and fires up but stumbles off. After being towed in and getting home she started right up on the hose.... Yesterday I take her out and was anchored at a local sandbar and after about 45 min tried to fire it up and it wouldn't start...So I decided to wait her out..opened the hatch and pulled the seat forward for about 1 1/2hrs and she started right up. Then I ran her hard for a couple hours to burn off the 93 octane I had put in with no problems. Took her home within 30 min and she wouldn't start on the house. I left her alone and came out in the morning and she fired up a few times with no problems.. A few observations I made was that at each breakdown I had more than 3/4 tank of gas and I filled up with 93 octane instead of my normal fill up of Rec 90 non-ethanol ..Question is has anybody had this issue and/or does anyone know where my fuelpump is and if I have the "cool fuel" system.... Thanks everybody...
The boats been running great otherwise.. I have a fresh tune up , new coil , new raw water pump and thermostat 160*

[Hudson River Steve]

I don't think it is possible for 7.4MPI to experience vapor lock since it is fuel injected. Vapor lock is usually caused by the heat soaking of the intake manifold/carburator and boiling of the fuel in the carb bowl. The electric fuel pump should push fuel through the lines even if they fuel in them were heated and re-prime the system
My guess is you have either an intermittent fuel pump, intermittent fuel pump relay, intermittent ECM, an intermittent sensor or an intermittent wiring issue somewhere in between those components. Have you checked your fuel pressure when the boat won't start? The pressure will tell you a lot. Here is how the fuel system is supposed to operate.

When the ignition switch is turned to the RUN position, the ECM will turn ON the fuel pump
relay for two (2) seconds. You should be able to hear the fuel pump buzz.
When the ignition switch is turned to the crank position, the ECM turns the fuel pump relay
ON causing the fuel pump to start.
If the ECM does not receive ignition reference pulses (engine cranking or running), it shuts
OFF the fuel pump relay, causing the fuel pump to stop.

You may want to pay a mechanic with a scan tool to come for a ride along and catch it in the act.

[99 Mariah 258]

Thanks for the response Steve.... I'm going to monitor the fuel pressure the next time I take her out and talk to the mechanic.. I googled "7.4 MPI won't start when hot" and there is a ton of people with the same problem..
Seems to be vaporizing in the fuel lines... Something to do with shutdown temps and engine compartment ventilation. Also says to open the hatch and let her idle for 5-10 minutes before shutdown...something I haven't been doing.... Time to save up money for the mechanic...

[Motoxxx43]

I have the 7.4 on a 2000 250. I have had a really similar issue two or three times. The first time threw me for a loop. As did the second seeing as the weather moved in and I needed power ASAP. Have not found the culprit but it eventually fired up after pausing a few min between cranking. Nothing more than a few min.
I have a hunch it might be the crappy E10 fuel I have to run. I plan on pulling fuel/water separator an inspecting the contents when it happens next.

[Hudson River Steve]

The E10 fuel angle is interesting. Alcohol does have a very low boil point.

One other point of interest is that our fuel systems are somewhat dissimilar from standard automotive fuel systems. Standard auto systems usually have the fuel pump submersed in the tank as well as a return line from the fuel pressure regulator back to the tank. These two features makes the auto system self priming. Our systems have the fuel pump mounted on the lower port side of the engine and a fuel return back to the fuel filter which is mounted on the upper starboard side of the engine. The fuel pump creates a vacuum and draws fuel from the tank into the filter and eventually into the input of the pump. As long as the pump is getting a steady supply of fuel it should be able to overcome any vapor in the system by pushing liquid fuel out of the pump, through the fuel rail and ultimately through the injectors. Any air after the pump should be ables to be pushed out of the injectors. I wonder if your pump input or fuel filter is being starved for fuel due to vapor lock?

I know that when I change my fuel filter each year....it takes some time before the system is purged of air. I wonder if your fuel tanks are lower than the tank in my Z280 and the heat is causing a vapor lock in the feed hose to the filter or if there is a vapor lock issue between the filter and the pump? The top of the tank in my Z280 is abut the same height as the fuel filter. Are your tanks below the filter level?

In any case, when the motor won't start, monitoring the fuel pressure on the fuel rail will tell you if you are having a fuel issue.

[99 Mariah 258]

Here's the service bulletin for vapor locking issues....

http://www.boatfix.com/merc/Bullet/99/99_07.pdf

[99 Mariah 258]

My fuel filter looks like center to center with the fuel tank.... I just picked up a fuel pressure gauge and a temp gun and I'll start a log with engine temps and fuel pressure .. Also. I'm changing my blower and adding another and I'm going to try the cool down steps described in the bulletin.. Plus I'm only filling with Rec 90 non ethanol fuel from now on. It's about the same price as 93 @ 2.99 gallon.... Just have to trailer her a extra 10 miles to the 1 gas station that sells it around me. These are the cheap steps to try to remedy the problem before a trip to Mercruiser...

[Cory]

I hope this will help. I had same issue with 5.7 mercruiser bought inline spring loaded check valve and all problems fixed. I bet I could run all day now and not use exhaust fan and never vapor lock. I posted pics of check valve. You can buy a check valve for $80 to $100. Or you can make your own for about $25. Go to amazon and look up Campbell Mfg SLC-125 Acetal Polymer Spring Loaded Check Valve $13.09. All you will have to add are connectors for hose and drain plug.. I still have pics in my phone if you need them. What is happening is water in block is flowing backward toward fuel pump and turning fuel cooler into fuel boiler.. If you hold your hand on fresh water intake hose after you shut engine off you will feel hot water flowing back toward fuel pump and cooler.. Hope this helps. Had this same problem last summer. After check valve install not even a hickup!!!!!! The reason I know this is I bought the Quick Silver version for $100 and looked up part # and found on amazon for $13. I wish I would have known before I bought $100 version. Any questions let me know. I will send pics of installed valved.

[iqraceworks]

Originally Posted by Cory

I hope this will help. I had same issue with 5.7 mercruiser bought inline spring loaded check valve and all problems fixed. I bet I could run all day now and not use exhaust fan and never vapor lock. I posted pics of check valve. You can buy a check valve for $80 to $100. Or you can make your own for about $25. Go to amazon and look up Campbell Mfg SLC-125 Acetal Polymer Spring Loaded Check Valve $13.09. All you will have to add are connectors for hose and drain plug.. I still have pics in my phone if you need them. What is happening is water in block is flowing backward toward fuel pump and turning fuel cooler into fuel boiler.. If you hold your hand on fresh water intake hose after you shut engine off you will feel hot water flowing back toward fuel pump and cooler.. Hope this helps. Had this same problem last summer. After check valve install not even a hickup!!!!!! The reason I know this is I bought the Quick Silver version for $100 and looked up part # and found on amazon for $13. I wish I would have known before I bought $100 version. Any questions let me know. I will send pics of installed valved.

I have the same vapor locking issue with my 7.4 MPI. I bought the check valve off of Amazon like you suggested. Do you have a picture of the other fittings you used, and also the drain you installed about the check valve? I'm hoping this will cure my issue. Any pics of the install would be great also!

Thanks!

[theeasynine]

I'll chime in and say if you can, buy straight gasoline. Boat engines don't like ethanol whatsoever.