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TOPIC: Engine problem on my Mercruiser 7.4

Engine problem on my Mercruiser 7.4 31 May 2009 07:00 #1

  • tex'npepper
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On my cruise last week after running 3700 rpms for an hour, the engine changed tone and started sounding flat. I pulled it back to idle and it started sounding better but when I tried to accelerate the engine started missing and would not come up on power. I turned for home with the engine at idle but it would start missing intermitently and then died after about 15 minutes. I am trying to figure out what the problem might be. I checked the fuel sight tube to see if the fuel pump diaphram had broken but it was ok. I then pulled the Racor fuel filter and checked for water. I didn't find water but I did find maybe a 1/4 teaspoon of oily looking sludge in the collection bowl. I don't know what else to check other than to pull the fuel pickup out of the gas tank and see if it has a screen that might have been blocked by more sludge in the tank. I changed the plugs, wires, distributer cap and roter earlier this year so I don't think that is the problem. Any suggestions as to what else I might check before I turn it over to the mechanic. :confused:

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Engine problem on my Mercruiser 7.4 31 May 2009 08:20 #2

  • ekuehn
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You did not mention that you checked and actually have spark to the plugs. If you have no spark you could replace the module at the bottom rear of the distributor held in place with 2 screws that require a 7/32" socket to remove. Your distributor will be a marine one so it may not have this module I don't know.The reason I mention the module is I had one fail on a recently acquired 1996 Chev 1 ton crew cab with a 7.4 L engine. The marine version of your distributor may be completely different and not have a module like this? Check the high tension wire between the coil and the center of the distributor cap. These have been known to fail and although your coil is producing spark the spark is not getting to the cap. Some other things to check make sure your timing chain has not jumped a notch . If this happens sometimes an engine will still run but have no power. Make sure the vent on your fuel tank is not plugged as it could run for a period of time before building a vacuum, slowing the fuel supply down and eventually stopping it. These are a few suggestions, I am sure there are people that have a 7.4 L in there boats will chime in with more suggestions. Ernie

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Engine problem on my Mercruiser 7.4 31 May 2009 11:59 #3

  • mrgoodwrench
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Tex,

Here could be another issue at hand.

Have you checked for fuel flow when crankiing the engine? On my 7.4, the mechanical fuel pump runs off of the Raw Water pump. There is a cam on the raw water pump shaft that moves the fuel pump lever. My cam broke off and the fuel pump would no longer pump fuel.:mad: I ended up being towed in with one of the Jet Skis - how utterly embarrasing - and try docking a 28' in a 4 knot current with a Jet ski as a tugboat...we got it done but it was not easy.

A quick test for this would be to pull the fuel line AFTER the pump and crank the engine - you should get at least a cup full of fuel after cranking a few times. Continue to trace back. I also poured a LITTLE bit of fuel into the carb using a small portable fuel tank and rubber fuel line direct to the carb and tried to crank to see if it would fire up - and it did, and continued to run as long as I let a steady drip of fuel into the carb. THIS IS NOT FOR EVERYONE AND IS EXTREMELY DANGEROUS. I understood the risks and took precautions. It did allow me to trace back to the problem. Please do not 'Flame' me over this as I feel that I have explained the danger.

I purchased a new fuel pump thinking that was the issue - no fuel to the carb and plenty available. Once replaced - same issue. I ended up looking into the water pump in more detail and the cam was just spinning on the shaft. I am not sure if it was an actual cam lobe or a stamped on lobe -either way - it failed.

I ended up replacing the mechanical fuel pump with an electric mariine fuel pump and the required Low Oil Pressure cut off switch to cut the fuel pump power if low oil pressure. This fix cost me approx $160 vs $350 for a new water pump w/ mecanical fuel attactchment. It also happened to solve the issue of hard starting after the boat has been sitting for a week as the fuel pump can be energized for a couple of seconds prior to starting and allowing the fuel line to fill with fuel prior to initial cranking.

Please note - this was MY issue with no starting - there can be several issues and you will need to perform several different tests to narrow down the problem.

Good Luck and report back what was the overall problem and solution.

Here is a list that can be checked out - just a basic initial checklist - there are more detailed checklists and solutions - give us some feedback...

Spark ???- check w/ screwdriver - multiple solutions - distributor - wires - rotor - coil etc.
Distributer Moving ??? - remove cap and crank - moving - Good - not moving - timing belt or cam problem
Fuel ??? - as stated above - check for fuel flow -flow is good - Good - no flow- trace back in fuel system

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Engine problem on my Mercruiser 7.4 02 Jun 2009 17:36 #4

  • tex'npepper
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Thanks for the suggestions. I still haven't found the problem. I took it out and ran it until it failed again and then took it to the mechanic. The mechanic found a loose wiring harness and thought that might be causing it. I took it back out and it still failed so that wasn't the problem. Another mechanic at the shop who works on Bravo engines said he has seen this problem before and traced it to the pickup coil. Again, that may or may not be the problem. I hate throwing parts at it until something works. That can get expensive. He doesn't think that fuel is the problem. He is pretty sure it's electrical.

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Engine problem on my Mercruiser 7.4 02 Jun 2009 19:46 #5

  • redneck joe
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is a 7.4 mercuriser essentially the same as a us marine?

I have had the same issue on mine with no fuel flow. I checked all I could but the strainer in the fuel tank (couldn't get the fitting to budge) so I assumed it was the fuel pump gone bad. Reading goodwrench post it sounds like it could be the waterpump cam?


I also have the hard starting issue. No issues after initial start but that first one was a bitch...


I have my mechanic replacing the fuel pump tomorrow.

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Engine problem on my Mercruiser 7.4 02 Jun 2009 21:19 #6

  • mrgoodwrench
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The hard starting comes after sitting for a while.

Before changing the fuel pump - make sure that the cam in the water pump is not the culprit - do not throw good money down the drain. It could be the fuel pump - or water pump cam. Look at both - I did not realize that the water pump area needed to be oiled every year...

Yes - Mercruiser engines were used by US Marine.

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Engine problem on my Mercruiser 7.4 16 Jun 2009 04:27 #7

  • tex'npepper
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Thank you mrgoodwrench for your suggestion that the problem might be with the cam on the water pump. I showed your comments to the mechanic and suggested that the cam may be pressed onto the shaft and when it heats up it could be slipping on the shaft. The mechanic agreed and installed an electric pump. The problem appears to have been solved.

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Engine problem on my Mercruiser 7.4 16 Jun 2009 11:53 #8

  • mrgoodwrench
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Bingo :arr

Congrats on that issue - I was shocked when I found out the real issue. I am happier with the electric fuel pump as it is easier to start the boat now.

One of the things that I installed when I was having the fuel problem issue was a Gasoline Fume Detector which will automatically sound an audible alarm AND turn on the blower. The unit only cost me $125 or so and I was able to inform the insurance company that Ihad said detector on board for an additional premium savings.

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Engine problem on my Mercruiser 7.4 23 Jun 2009 23:20 #9

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I thought we had fixed the problem but it occured again. I cruised down to St. Helens, OR this weekend and the engine ran fine going down. On the trip back, it took me about and hour traveling upstream and the engine started cutting out just before I got back to my marina. I was able to limp back to my slip but it was a disappointment to end an otherwise great weekend. I am now at a loss as to what it could be. When it quits there is no gas in the carborator. I've:

1. Checked the fuel pickup in the tank-its ok.
2. Checked and replaced the fuel filters-no water.
3. Replaced the fuel pump with an electric pump.

Now I don't know where else to look and my mechanic is just wild guessing. Any other ideas or suggestions would be greatly appreciated. I am taking the boat back to the shop tomorrow but I don't think the mechanic has a plan either.

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Engine problem on my Mercruiser 7.4 25 Jun 2009 19:31 #10

  • Wingnutt
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When it quits there is no gas in the carburetor.


Sounds like you have isolated the problem. Other things to check are the tank vent and the fuel line itself. Don't forget that the alcohol in E10 gas will eat old rubber lines from the inside out causing all kinds of problems. Another problem could be a sticking needle and seat or a fuel soaked float inside of the carb.

Good luck!

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