4.3L MerCruiser possible fuel problems!!!

[martys]

Here's some back ground info with my problem. I have a 1992, 4.3L, V6. The engine has been running great until last week. Its starts up fine, I can run it without any problems for about 45 minutes (anywhere between idle speed to wot.). After this time, starting from a hole shot, once the rpm gets around 3600, the engine starts to stumble and loose power. If I pull back the thottle to 3200 rpm, she seems to run ok. Also, there's no problems running the motor at idle speed or going 6 mph.
After reading my shop manual, I changed both fuel filters, fuel is good, the vent line is clear, and I can't find any ignition problems.
Should I start looking at the carb or the electric fuel pump next? What is the best way of installing a (temporary) fuel pressure gauge on the fuel line?

 

[enforcer30]

Re: 4.3L MerCruiser possible fuel problems!!!

Quick question, after the engine cools down or the boat has been sitting for some time will it run good again for 45min.? If so it sounds like a ignition module or maybe plug wires. How old is the cap/rotor, spark plugs?

 

[martys]

Re: 4.3L MerCruiser possible fuel problems!!!

I tried letting the boat sit for about 20 minutes the last time and had the same results. If I try again the next day, the problem re-occurs after running the boat around 45 minutes.

Rotor, cap, coil, ignition module were replaced in May of last year. Spark plugs were changed this spring. Spark plug wires are two years old. The ignition amplifier is original.

 

[Don S]

Re: 4.3L MerCruiser possible fuel problems!!!

Give this a try.

http://forums.iboats.com/showthread.php?t=167561

 

[martys]

Re: 4.3L MerCruiser possible fuel problems!!!

Thanks for the link. I'm getting the material this week. I'll let you know whats going on.

 

[martys]

Re: 4.3L MerCruiser possible fuel problems!!!

I installed a fuel pressure gauge right before the carb, just like the diagram in my shop manual. At idle, the pressure was around 2.3 psi. Still in neutral, the pressure stayed about the same when I increase the rpm's, between 1400 to 2000. In gear, at idle, pressure was again around 2.3 psi. As I increase throttle, the pressure started to go lower, and again the motor sputtered until I decreased throttle. In the manual, the pressure should be 3 - 7 psi. Before the test, I changed both filters and I have been treating my gas all summer. The fuel pump is 13 yrs. old.
Time for a new pump or should I try a different test?

 

[Don S]

Re: 4.3L MerCruiser possible fuel problems!!!

Before you buy a fuel pump, re-read my previous link.

If you can't get fuel TO the pump, you're not going to get anything OUT of it.

 

[martys]

Re: 4.3L MerCruiser possible fuel problems!!!

I removed the hose barb and 90 degree fitting, on top of fuel tank, and found the problem. There was plenty of junk between the two fittings blocking the fuel line. After cleaning, I took a ride, and the motor ran great. I will add this check to my spring start up list.

Thanks again for your help and insight.
I love this site.