Engine mis-fires only at high load/RPM's

I have a 1997 Mercruiser 5.7 V8. The engine runs fine at idle and at slow exceleration up to cruising speed. When I give it full throttle, I hear a popping (mis-fire) from the engine and it seems to struggle to get up to speed.

Any suggestions, thanks.

Re: Engine mis-fires only at high load/RPM's

Ayuh,... She's Starvin' fer Fuel....

Start with checkin' the contents of the fuel Filters....

Re: Engine mis-fires only at high load/RPM's

The only filter I can find is the fuel/water separator filter. Is there another filter I should change? I didn't see anything in the owners manual.

Re: Engine mis-fires only at high load/RPM's

Does it have a carb? There may be a screen filter in the carb, and sometimes in the throttle body. I thought my 5.0 only had the Mercruiser spin-on filter/sep but I found that it also has the screen in the carb, and it was full of crap. I had similar symptom on by 86 Wellcraft. Once I worked through my fuel issues I had to get on the ignition issues, and a replacement coil was in order.

Chris

Re: Engine mis-fires only at high load/RPM's

what carb you got???
webers have 2 screen type filters behind the needle/seats in the fuel inlet
you will have to remove top half of carb to remove/check/clean them
while the tops off, have a look in the carb base and make sure there isnt any crud down there in the jets

Re: Engine mis-fires only at high load/RPM's

My 1986 3.0L did this last summer when I put E10 through it. Anything over 3/4 throttle and the engine would backfire through the carb. When I went back to non-ethanol gas it ran fine through the whole range of throttle. I haven't had it in the water this summer as I am rebuilding the boat. I figure that I will have to rejet the carb a little richer since E10 is that we can get now.

Re: Engine mis-fires only at high load/RPM's

Thanks all, I ended up replacing plugs and distribitor cap/rotar and she runs fine now.

Re: Engine mis-fires only at high load/RPM's

jsmigielski said:

Thanks all, I ended up replacing plugs and distribitor cap/rotar and she runs fine now.

Click to expand...

I also had a similar problem. I went all through the fuel system hoping that there was some truth in all of the ethanol rumors out there, nope. It turns out that if you are running high test (93 octane) there are different additives that make the ethanol blend very stable and fight against separation.

After looking all through the fuel, I went to the ignition system. I changed the plugs, cap, rotor, and coil: still the same problem. It turns out that I also needed to change the ignition sensor under the sensor pickup wheel, under the rotor. It made all the difference in the world. She runs like crazy now. The new sensor is solid state and very easy to install.

I also need to add that I would have still been chasing this ghost if it wasn't for the help from our boat mechanic. He knew exactly what I needed to do from talking to him over the phone. He is a great guy and made sure that I knew what I was doing for the install before I walked out the door with the part. I truly wish that I had someone this reliable and friendly at the parts counter at my Dodge dealership.