Mercruiser 5.0 bogs and surges at high RPM

[jeremy holbrook]

Hello all,

New to boating. I have a 1995 Mercruiser 5.0. The engine starts, idles, and revs perfectly in "sittting position". I can cruise at low RPM's just fine. I can also plane out with full throttle for a few seconds. A few seconds after planing, though, the engine bogs down and surges. It will eventually stall if I don't back off. If I back off enough, it cruises OK at low speed. What should I look into and what can I rule out?

Thanks for the help.

 

[Reel-Broke]

Re: Mercruiser 5.0 bogs and surges at high RPM

Filters are a nice start, just make sure you fill them with fuel before you install them or your fuel pump will over heat and then that you will need a new one of those...I would also check the a anti siphon valve on the top of your tank which by USGC rule is required to be there...sadly the stupid little buggers have an old habit of not wanting to open to allow enough fuel through.

Ironically enough, I am actually having the same exact problem you are...i have replaced and redone basically the entire fuel system up to the pump

My mechanic is convinced that my issue is the pump cutting out intermittently seeing as my carb was rebuilt last season when the boat was purchased, typically clogged needles is an issue with idle and initial run, the higher end running jets are not usually the first thing clogged by bad/dirty fuel.

I would start with the cheap fixes first...sadly could end up being the pump or the float in the carb which would require a rebuild

Things to Check:
1. Filters are clear
2. Fuel delivery lines are clear (including tank pick-up tube)
3. Tank vent line is clear (a blocked line can cause a vacuum which will cut fuel off)
4. Anti-siphon valve is not stopping fuel
5. Pump is working correctly (sadly I dont really know a way to actually test this, an electrical reading at idle would be insignificant to a pump cutting out under load intermittently because clearly it works at idle if the boat is still running)

I am picking my new fuel pump up Tuesday morning and will install and run, I will let you know how I make out!
Tyler

 

[6meter]

Re: Mercruiser 5.0 bogs and surges at high RPM

And on the electrical side, if it has electronic ignition, in a car a bad module will act like a fuel problem. If it has a pick up coil, I've seem the wires going to it break because of the timing advance movement.

 

[Reel-Broke]

Re: Mercruiser 5.0 bogs and surges at high RPM

Ok so put the new fuel pump in and of course, not the issue...so $238 later and I still can't get the old gal running properly... can't say for sure that you don't have a bad fuel pump, but like I said before, start with the cheap areas first because they can just as easily, if not more often, be the culprit. I think I am going to take some of meters advice and look into the ignition system...

Its funny though, that when you talk to different mechanics you get a whole array of what the problem is: I was told deffinitely fuel pump, deffinitely carb, and deffinitely ignition just by listing the symptoms.

Sadly, there never seems to be an easy fix...I will let you know how I make out, I have tomorrow off so I plan on digging into it some more

 

[bcons]

Re: Mercruiser 5.0 bogs and surges at high RPM

I'm having the same prob. 1989 mercruiser 5.0 thunderbolt ignition
Mine is electrical/ignition.
1st thing for you to check would be spark vs fuel. Take it out, duplicate the problem, make it stall. If you have a meter great if not another cheat is to bring a can of starting fluid (ether). Make the engine die, spray fluid into carb if you have a fuel problem, the engine will fire right up and run for a few seconds on the ether. If it doesn't start immeadiately, you have a spark problem.

Spark problem could be coil, distrib cap/rotor, igni sensor, or ECM comp chip.
Coil is the cheapest one, I replaced mine $45, & cap rotor & wires. Still nothing, the ignition sensor is in the distributor under the rotor they're $70 and fairly easy fix. I did mine still having the problem. Last one is the module, yours is prob mounted to the port side exhaust riser. Most expensive part $300-$400.

Basically I can cruise around (and I did all wkend) below 2500 rpms no trouble, get on plane & I have 3-5 mins then boats acts like it's running out of gas & dies. Most of the time it won't start again for 10-30 mins, then once it does start it runs fine just below 2500.

 

[jeremy holbrook]

Re: Mercruiser 5.0 bogs and surges at high RPM

Thanks for the suggestions. I haven't had time to try anything yet. picked up a new filter coming into the carb and fuel/water separator today, along with new plugs. I will start there and keep everyone updated. Good luck to all of you.

 

[Reel-Broke]

Re: Mercruiser 5.0 bogs and surges at high RPM

I think I am going to try bcons idea. Sadly I really can't duplicate the problem, or else I would know what it is and be able to fix it. I'm going to dig into ignition this afternoon once this heat wears off. Almost 100 degrees up here on Cape Cod! I will let you know how I make out.

 

[smarks]

Re: Mercruiser 5.0 bogs and surges at high RPM

I had similar problems, take a look at the thread if you like:

http://forums.iboats.com/showthread.php?t=401539

Steve