5.7L mercruiser engines stall out after running for 20 min?

Question

1989 32' Carver with the original merc 350ci engines. have had the boat for 4 years. engines start/run/idle smoothly and powerfully. when first started and warmed up, the engines will run 3000 to 3200rpms (about 14/15 gph) for an hour with no problems. one motor will suddenly stumble, like it's not getting fuel. it will pick up again, and stumble, worse and worse... backing off the throttle all the way and shutting off the engine for 1 minute, then restarting - it will run perfect again for 20 minutes? it happens to both motors randomly - with two completely independent systems. (different fuel tanks/lines/pumps/filters...) have changed the filters (both in-line and fuel water separators) numerous times, and see no visable particles, nor any significant change. pulled the pick up line out of the tank and inspected... nothing notable. ideas?

Thanks for the input! more info -

I forgot to mention that both coils have been replaced with new ones. also we have opened the filler cap on the tanks while the problem has been going on to see if they weren't venting.

both engines do it - and at different times, and at random intervals. they have the original fuel pumps...?

re: the "firing capacitors;" I am not familiar with these. these two engines came stock with thunderbolt ignition in '89... would these engines have them? also, is there a way to test them before replacing them?

thanks again!

interesting on the carbon build up. we do use it for trolling a lot - however, it is almost always preceded and followed by a good 1/2 hour to 2 hour run. also on our way up to the island it's a good 8 hour run, before trolling for the week and running back another 8 hrs.

is that enough running to prevent build up?

while running, the engines run about 160 degrees. while trolling closer to 135 (per the gauges.) I think i will try what you mentioned...

Answer 1

Try swapping coils for the ignition . Sometimes they go bad and it will show up with heat after you have been running. this is a sign of weak coil. If you swap coils and the other motor starts acting up you have found the problem.
Now that you have added some info check for water in your fuel.
Pump tank into a large glass or plastic bottle and see if its cloudy then let it sit for awhile .Tip the bottle to the side and look at the bottom .Water is heavier than gas so it will show at the bottom.If its looks like a brownish or rusty color you have phase separation . Either way you will have to pump tanks out then pour Startron cleaner in tan and fill tanks and change fuel water separators again

Answer 2

You should check for one other thing that has not been mentioned in the above responses: carbon buildup in the cylinders. If the engines have been run at slow or idle speeds often (as is the case with trolling), carbon can build up in the engine. What happens is that for a while, the engine will run fine. After the carbon heats up a condition called "preignition" occurs. This happens when the carbon is hot enough to ignite the fuel before it's supposed to. The result is similar to having really bad timing; no power. The situation your describing could be a result of this.

If this is so, the fix is to fuel up your tanks with the highest octane fuel you can get and add an octane boost additive, if you can get it. then, take the boat on a real joy ride. Really wind it up and keep it's rpms up for an hour or so. You may find that the stumbling comes back during this process. If the engine does stumbles, put it in neutral and run the engines up a bit. You may find that the engines will rev fine while out of gear but will bog down when you put them back into forward. You may need to do this a few times.

You'll know when you've cleared out the carbon because one of those times you'll slowly be bringing the engines back up while in gear and they won't bog down.

After you've gone through this process, always run high quality fuel through your engines and if you do putter a lot, every once in a while take the boat out for a good hard run.

If you can safely replace your thermostats with somewhat hotter ones, this can also help keep the problem down. Check with a Merc mechanic on the best rating.

Answer 3

I had about the same problem with my Merc. It was traced to the firing capacitors. This is a module that contains all the timing capacitors for the firing of the plugs. They change values when the engine gets hot and throws off the timing. At least that was my probelm and those modules are not cheap.

Answer 4

Check the fuel vents as mud dobbers make nests in then you can take the fuel caps lose an see if that helps then you know it's the vents. It's will run longer on low tanks of fuel than full tanks.

Answer 5

sounds like a fuel cotamination problem go to a marine supply store and get a product called "star tron"/ its the lates and best fuel cleaner. a pint will do 300 gallons, we have ethanol in our marine fuel and must put it on each fiiup. it work s.

Answer 6

5.7l Mercruiser

Answer 7

clogged fuel lines from the new e10 gas, replace the fuel lines