Backfiring 5.0 Merc

[Bondo]

Re: Backfiring 5.0 Merc

I have owned the boat 2 years and never changed or seen fuel filter on any work order as being changed. How often should these be changed (besides now)? What hoses could be delaminating? Should I replace the fuel pump if after replacing filter I still have symptoms?

Ayuh,... The fuel filters should be changed Annually, or Whenever there's a fuel delivery issue....

There's also No reason to just start throwing parts at it....
That's what Diagnosis is for... To see Exactly what's Wrong....


Oh,... 'n as usual,... I completely Agree with Don,....
You need to find a Real Mechainc, instead of a guy who slept at a Holiday Inn last night....

 

[1994 5.0LMercuryAlphaOne]

Re: Backfiring 5.0 Merc

Mech called and referred me to a third mech to try and identify the backfiring issue. I will be picking up my boat and likely bringing it to Atlanta to locate a mercruiser mech.
You guys have taught me several things. I have also been researching other forums,etc... I would like to share what I have learned and get your feedback. I have four categories: mechanical, electrical, fuel and vacuum/air leaks. To rule out items I should look to the most obvious factoring in the cost of diagnosis.
Here is what I have put together:
Rule out Mechanical: Compression test (done 150-160 range). Leak down test to detect valve issues (not done).
Rule out electrical: Distrib cap, rotor, wires, plugs (done) Timing was set to 8 degree from 6 degree last season. Coil, distrib ignition module, shift interrupter switch (can't verify that these have been reviewed/tested). What does a shift interrupter do and how can it effect the engine?
Rule out fuel (lean): Accelerator/fuel pump (not done). Anti-siphon valve, fuel separator (done). I ran the fuel down numerous times last season and added conditioner (Quick silver and seafoam) with non ethynol fuel. Gas tank vent clogged (I'm assuming this is the vent on the side of the boat that spits gas if I over fill. It works). Someone has suggested running with the fuel cap open.
Rule out vacuum/air leaks: Manifold/carb gasket (assume this was replaced when carb rebuilt last year). Leaks in vacuum hoses (unable to verify that all vac. hoses have been ruled out). How do you test vac. hoses?
Misc info:
-Ignition problems get worse under load.
-Backfiring is caused by cross firing in distributor, bad spark plug leads, coil wire, lean fuel, water, incorrect timing, sticky intake valve, overheating causing pre-ignition...
Carburetor backfiring cause(s): ignition/fuel pump/anti-siphon valve
Exhaust backfiring causes: valve or timing
-A fuel pump not pulling enough vacuum will lead to surging @ wot w/ carb engines and lack of wot ability.
-Small pops typically related to ignition. Fuel is typically a hesitation and then a large pop.
First time in the water last year I had small and large backfires. Presently large backfires w/ hesitation are occurring.
Other than finding a qualified mech.(I've got the good women Glenn) what is the next step/test?
Please add to my list or suggest correction. I am not a tech. boat person. Just a boat owner who wants to enjoy his boat before gas hits $5 a gallon.

 

[christoner2002]

Re: Backfiring 5.0 Merc

Does it only do it after the engine has been running for awhile? Alot of your post sounds like deja vu to me. My 97 Merc 5.7 acted alot like that when I bought it. If you question the work of your mechanic, I suggest check compression again. You can rent the tool at Autozone. I had 3 or 4 burnt valves, and mine would diesel, and run terrible when I first bought it. after rebulding my heads, my boat would still go lean, do to a bad reman carb. You could hear it ping at upper rpm. Replaced with a Edelbrock 1409, just like the Weber that came on some Merc's. At that time I changed plugs, wires, cap, rotor, and the pickup in the distributor. It then would only act up after running it for awhile, once it warmed up for like an hour or so. It turns out my ignition coil was getting so hot, it was boiling the fluid out from inside, and causing the ignition to break up at at 3800rpm and above. For the life of me, I thought it was fuel delivery problems. I used a auto parts coil I had laying around. My Mercury dealer told me it was probably the same coil, without the Mercruiser name. Once I found it was the problem, I replaced the part with the Merc coil. It has run like a top since then. I have seen some mechanics that say they replaced everything, but haven't. I would just double check his work.

 

[1994 5.0LMercuryAlphaOne]

Re: Backfiring 5.0 Merc

Mech doing compression I trust. He also rebuilt the carb and I would say has a good reputation. His checklist just ran short. I moved on when the Marina owner tossed a rotor to me and said here try this. I figured I needed a new set of eyes. Second mech is one who claimed I have a resistor wire. Per this forum I do not have a resistor wire. On this note, I believe all these mech are good people.
Boat pops from the get-go if under load (ski or multiple bodies in boat). This all started at the very beginning of the season last year. When put up the previous year it was fine.
Your description of the coil heating up at 3800 is interesting. What causes a coil to heat up? Too much voltage? Should this be my next focus?
What could happen through the winter sitting on a trailer under a roof that would cause this condition?
Is the "pickup" in the distributor the same as the "module" someone has told me could be the issue. It was explained that this is responsible for advancing the timing in demand situations like under load.

Also, if I find out it is valve related, what is the repair called in mech terms and how many deer will I likely be spending? Also, how many deer for coil, fuel pump, dist module or entire distributor?
Just looking for ballpark on what I should be paying.
Thanx
Larry

 

[CharlieB]

Re: Backfiring 5.0 Merc

Boat pops from the get-go if under load (ski or multiple bodies in boat).

Also, if I find out it is valve related, what is the repair called in mech terms

A compression test is a good idea, HOWEVER, a slightly leaking intake valve will seem fine at light loads and then begin back-firing/popping when placed under the heavier loads and higher cylinder pressures.

The BETTER test is a cylinder leak-down test. This will identify any leaking valves.

 

[christoner2002]

Re: Backfiring 5.0 Merc

If you remove the distibutor cap, and the rotor, you will
see a pickup with two wires going to it from the outside of
the distibutor. That is the pickup. I belive the ignition module
is bolted on the outside back of the distributor. You maye also have a
knock sensor back there. Easiest thing I found to check the coil was after
you get the boat out and run for awhile, and get it good and warm,
shut the boat off, and follow the center wire from the distibutor cap,
to the coil. On my boat it was bolted to the back side of the port cylinder
head. Feel the body of the coil, and see how hot it is. You should be able to
grab it with your hand. Mine was crazy hot. I am not sure why it was getting
so hot, but when I removed the plug wire from the top of the coil, you could look
down in it and see a bunch of gunk, that seeped out from the internal parts of the
coil do to boiling. Best of luck to you!

 

[1994 5.0LMercuryAlphaOne]

Re: Backfiring 5.0 Merc

I need a recommendation for a mechanic in the area of Lake Hartwell, GA or Lake Lanier, GA. Someone who knows Mercruiser.

 

[reimelss]

Re: Backfiring 5.0 Merc

Does it only do it after the engine has been running for awhile? Alot of your post sounds like deja vu to me. My 97 Merc 5.7 acted alot like that when I bought it. If you question the work of your mechanic, I suggest check compression again. You can rent the tool at Autozone. I had 3 or 4 burnt valves, and mine would diesel, and run terrible when I first bought it. after rebulding my heads, my boat would still go lean, do to a bad reman carb. You could hear it ping at upper rpm. Replaced with a Edelbrock 1409, just like the Weber that came on some Merc's. At that time I changed plugs, wires, cap, rotor, and the pickup in the distributor. It then would only act up after running it for awhile, once it warmed up for like an hour or so. It turns out my ignition coil was getting so hot, it was boiling the fluid out from inside, and causing the ignition to break up at at 3800rpm and above. For the life of me, I thought it was fuel delivery problems. I used a auto parts coil I had laying around. My Mercury dealer told me it was probably the same coil, without the Mercruiser name. Once I found it was the problem, I replaced the part with the Merc coil. It has run like a top since then. I have seen some mechanics that say they replaced everything, but haven't. I would just double check his work.

I have the same engine..same problems.....hopefully same solution!...thanks

 

[1994 5.0LMercuryAlphaOne]

Re: Backfiring 5.0 Merc

Go to my new posting for follow-up info.
http://forums.iboats.com/showthread.php?t=469986&pagenumber=