Carbureted 7.4 dies after cruise, fine at slow speed

[transamconvert]

Looking for some diagnosis help so that I know what to check in finding this problem.

Boat:
Merc 7.4 carbed / B-2

Issue:
Boat will cruise for a few miles and then lose power, cut out and die. It will then not restart until it has sat for approximately 10 minutes, after which it does restart and run normally. It seems to have normal acceleration and planing power and it runs at 1400rpm slow speed all day. No other starting issues. It simply cannot sustain a ~3500rpm cruise.

Already investigated:
Changed the spin on fuel filter and verified that the gas tank vent line was clear. No fuel visible in the sight tube on the pump.

Out of simple ideas and looking for a direction.

Thanks,
--Mark

 

[achris]

Re: Carbureted 7.4 dies after cruise, fine at slow speed

Hello Mark,

Can we have an engine serial number please... Currently we have no idea which ignition system it has, what generation engine block, style heads or even if it have an electric or mechanical fuel pump... And without knowing all that, any suggestions are just guesses...

Don S would ask for the following information (of the engine)...
Make: (we suspect it's a Mercruiser)
Model: That would be a 454 (7.4l)
Year: ????
Serial number:

Chris.....

 

[transamconvert]

Re: Carbureted 7.4 dies after cruise, fine at slow speed

Ok

The year is 1995. Engine serial OF558258. It is the standard 300hp 454, not a mag. The fuel pump appears to be a manual pump connected to the raw water pump, don't see an electric pump anywhere.

--Mark

 

[armyofdux]

Re: Carbureted 7.4 dies after cruise, fine at slow speed

Ok

The year is 1995. Engine serial OF558258. It is the standard 300hp 454, not a mag. The fuel pump appears to be a manual pump connected to the raw water pump, don't see an electric pump anywhere.

--Mark

Fuel pump connected to the raw water pump? You mean via pulley? Typically a 7.4L mechanical fuel pump will be mounted behind the Raw water pump and be inserted into the engine. The fuel pump would be my primary suspect. But if you also if you have a fuel vibration compensator (silver globe prior to fuel pump) I would check for leaks, cracks or deformities there.

 

[alldodge]

Re: Carbureted 7.4 dies after cruise, fine at slow speed

Ok

The year is 1995. Engine serial OF558258. It is the standard 300hp 454, not a mag. The fuel pump appears to be a manual pump connected to the raw water pump, don't see an electric pump anywhere.

--Mark

According to the serial number it is a 95 454 MAG (821723 92 - 7.4L 454 MAG BRAVO GEN V ) , go figure. That being said you need to test your fuel pump. Also if you haven't done it check the gear lube inside the section between the pulley and sea water pump. The pic below shows one with oil a fill screw, newer ones have a drain and a fill hole. There is a cam lobe inside which accuates the fuel pump and if not checked can fill with fuel over time and wear out the pump arm, cam lobe or both. Your pump should be putting out somewhare between 4 to 8 psi.

 

[slothman]

Re: Carbureted 7.4 dies after cruise, fine at slow speed

the fact that it restarts once the engine is colder makes me think the coil is overheating. Once it cools down you are getting spark again. Test the coil , and RIGHT after your engine dies, feel the coil..if it's hot, my bet is a bad coil....also test the coil resistance, make sure it's not leaking oil, etc

if you are getting spark....well then you can look at the fuel pump and the carb etc

 

[achris]

Re: Carbureted 7.4 dies after cruise, fine at slow speed

Might be a bit of confusion here. My dealer program shows it as just a normal Bravo 7.4 litre engine, Gen V, TB-V, Power steering...

The confusion, I think, is because the Service Parts Catalog includes the Magnum engines.... The description of the engine in question is in the top line....

My suggestion is the same as the slothman, check for spark immediately on engine quitting, if no spark, go through the TB-V diagnostic flowchart....

Will post chart in a second, when I find it...

 

[transamconvert]

Re: Carbureted 7.4 dies after cruise, fine at slow speed

Did some investigating based on this thread so far, though I can't run the engine right now.

I checked out the coil. No oil leaking, primary resistance 0.7 ohms.

I then checked the fuel pump / impeller drive. I first noticed that there was some oiliness on the bottom by the pulley. I located the oil fill plug (there was no drain I could find) and removed it. There was no oil inside but there was dirty black gas which continued to trickle out until I replaced the plug.

 

[achris]

Re: Carbureted 7.4 dies after cruise, fine at slow speed

Did some investigating based on this thread so far, though I can't run the engine right now.

I checked out the coil. No oil leaking, primary resistance 0.7 ohms.

I then checked the fuel pump / impeller drive. I first noticed that there was some oiliness on the bottom by the pulley. I located the oil fill plug (there was no drain I could find) and removed it. There was no oil inside but there was dirty black gas which continued to trickle out until I replaced the plug.

Some of them didn't have a drain screw.. Dirty black fuel would indicate a ruptured pump diaphram. Those pumps are not serviceable, if it fails it needs to be replaced...

Merc part number is -861677T...

Chris......

 

[transamconvert]

Re: Carbureted 7.4 dies after cruise, fine at slow speed

Is a ruptured diaphragm / bad pump something that would cause this? My initial feeling was a fuel issue rather than spark just based on how it acted.

Am I going to be able to replace just the pump or am I looking at the whole assembly with the seawater pump? I can't imagine being out of lubricant did anything good for it.

 

[alldodge]

Re: Carbureted 7.4 dies after cruise, fine at slow speed

Is a ruptured diaphragm / bad pump something that would cause this? My initial feeling was a fuel issue rather than spark just based on how it acted.

Am I going to be able to replace just the pump or am I looking at the whole assembly with the seawater pump? I can't imagine being out of lubricant did anything good for it.

Replace just the pump, reason I mentioned the housing is most folks never check it until something happens. It needs to be checked and replace the gear lube every few years

Edit: reason I din't think it was spark is because you said it would run all day at 1400, that's not a spark issue

 

[achris]

Re: Carbureted 7.4 dies after cruise, fine at slow speed

Is a ruptured diaphragm / bad pump something that would cause this? My initial feeling was a fuel issue rather than spark just based on how it acted.

Definitely...

Am I going to be able to replace just the pump or am I looking at the whole assembly with the seawater pump? I can't imagine being out of lubricant did anything good for it.

Yes, the pump is available as a single separate item. I posted the part number in my previous post... Being out of lubrication is not a good thing, no... What most likely happened is the diaphram leaked and fuel diluted the oil in the housing, and also started to push it out through the bearing. That's why you found oil on the pulley. With that in mind, check the housing thoroughly. There are a couple of oil seals you might need to replace. I would be taking the whole water pump/fuel pump assembly off and servicing it from head to tail....

Chris......

 

[transamconvert]

Re: Carbureted 7.4 dies after cruise, fine at slow speed

Got it all apart and there doesn't seem to be anything obvious wrong with the fuel pump other than it had leaked the gas past the diaphragm. I couldn't find any tears in the diaphragm though and the check valves were good.

I also verified that the fuel tank is clean, the pickup tube is clear and the anti-siphon valve is clean and working properly. So the whole system into the pump is good.

The only thing I haven't examined is the carb itself. Is there something in there that could cause what I've observed if the pump was good all along?