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Mercruiser 5.0 Intermittent Surge

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kevinj

Advanced Contributor
2000 Mercruiser 5.0 V8 with 2bbl carburetor.

I have an intermittent surge I'm trying to track down. Sometimes the motor seems slightly sluggish off the start but most often it does fine. While running about 3500RPM it will occasionally lose a couple hundred RPM then pick back up to 3500. It never sounds like a misfire and there is no backfire or sputtering. Oil pressure and water temp are great with no unusual fluctuations.

Could this be a fuel pump not delivering sufficient volume? The filter was new this spring so I think that is fine. If it is the fuel pump, what are the steps to diagnose that issue? I've looked up prices and a new pump is $250, so I'd rather not just throw a new pump on guessing at the issue.

With the lack of sputter or misfire I"m hesitant to think it's an ignition issue. But, if it were an ignition issue, what components should I start tracking down?

Thanks,
kevin
 
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Check fuel pump pressure before replacing it. dump contents of water/fuel separator into a clear container,let settle a few hours to look for contaminants. Are you running ethanol based fuel? Ignition system,I would check dist. cap for cracks,corrosion,pitting, Rotor for same, spark plug wires checked with ohm meter,coil for cracks,very unlikely.inspect plugs for unusual signs of wear&tear When was the last tuneup done? Any modifications done in the last 16 years to motor,carburetor?
 
johnnygjr said:
Check fuel pump pressure before replacing it.
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How? I don't see a schrader valve on the fuel line. Do you disconnect the fuel outlet tube and test there with the key on?

johnnygjr said:
dump contents of water/fuel separator into a clear container,let settle a few hours to look for contaminants. Are you running ethanol based fuel?
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Yes, 10% ethanol fuel but I use Stabil Marine formula with each fill up. I can definitely dump out the filter to check for water or debris. The filter was new this season but that doesn't mean there can't still be an issue.

johnnygjr said:
Ignition system,I would check dist. cap for cracks,corrosion,pitting, Rotor for same, spark plug wires checked with ohm meter,coil for cracks,very unlikely.inspect plugs for unusual signs of wear&tear
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Can do. Do you have a site for references to expected ohm values when I go probing the ignition system?

johnnygjr said:
When was the last tuneup done? Any modifications done in the last 16 years to motor,carburetor?
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I haven't done a tune up in the 5 years I've owned the boat, just oil changes and winterization. In addition to new plugs, what else would you suggest for a tune up? To my knowledge there have been ZERO mods to the boat and everything is bone stock.
 
Distributor cap,rotor,spark plug wires, inline fuel filter. Place an inline pressure tester using a tee fitting to test f/p. A shop manual would have all the specs. you would need to test ignition. I would do tune up first then continue troubleshooting,this way you can eliminate one thing at a time.
 
Five years! Do a full tune up including wires and pick up in distributor, All marine parts, Quick Silver preferably, then see how it runs. Don't waste your time checking old parts, change them.
 
Docksidemarineservices said:
Five years! Do a full tune up including wires and pick up in distributor, All marine parts, Quick Silver preferably, then see how it runs. Don't waste your time checking old parts, change them.
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I didn't get a chance to look further into my issue before I put the boat to bed for the winter. Now I'm setting my sites on spring (hopeful :D ) and I want to start shopping for parts to perform a complete tune up. It's definitely my goof for not having done any of this PM work, so Chris is right....start with a tune up and go from there.

Does anyone have a link to a site with a complete tune-up kit or will I need to get these items piece by piece? I have confirmed it is a Thunderbolt IV ignition but the breather cover does not have an engine serial # on it.

I did some shopping on this site and this is what I came up with. The grand total would be $315 if I got all components. Is it worth starting with all components new to make sure it's all fine? Or would cap & rotor, wires, and plugs for $140 be a safe place to start? For all I know these could all be original parts so I'm kind of leaning toward getting it all. But I'd hate to spend another $170 on on the sensor and coil if I don't need them.

Cap & Rotor kit $46
http://www.marineengine.com/newparts/part_details.php?pnum=MER++++805759T+3
Ignition Coil $89
http://www.marineengine.com/newparts/part_details.php?pnum=MER300-8M0079202
Ignition Sensor $85
http://www.marineengine.com/newparts/part_details.php?pnum=MER87-892150Q02
Plug Wires $73
http://www.marineengine.com/newparts/part_details.php?pnum=MER84-816761Q17
Plugs $22
http://www.marineengine.com/newparts/part_details.php?pnum=NGKBPR6EFS
 
From your location, it sounds as if you run in fresh water. I run in salt water (with salty air!) and have NEVER gotten 3 seasons out of a cap/rotor. The "air" is a factor since the cap is vented to the air and as the engine heats up and cools off it "breathes" local air in and out. End result for me, is severe corrosion inside the cap. You'll probably get more life due to fresh water, however.... still, cap/rotor would be, for me the first thing to change. I've gotten 10 seasons out of a set of OEM wires that were still OK when I changed them. Plugs... IF carb is set up 100% OK, 3 or 4 seasons TOPS. It it were my boat, I'd do cap/rotor, wires/plugs and leave the disti sensor and coil for last ditch replacement. While plugs are out, check compression. Be extremely careful not to misconnect the wires between the cap and the plugs... its easy to do!!!! Double check wiring before you start the engine. Don't forget to set the gap on the plugs to specs for your engine. On my circa 2006 5.7L engine, the S/N was on a sticker on the exhaust elbow. It is also on the block at the rear.
 
Take a picture of distributer cap and wires for reference,Mark each end of plug wires to corresponding cylinder,replace one wire at a time,Be sure to use dielectric grease on all connections,plugs,distributer cap. Keep us posted kevin
 
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Fuel related issues on a running engine usually result in a smooth "surging" which occurs as speeds and loads are increased. This surging is often a fuel starvation issue. Backing off the throttle when this occurs, usually results in the engine temporarily returning to normal operation. This surging could be caused by several factors including old gasoline, a clogged tank filter, water separator, carburetor, carburetor filter, fuel pump, deteriorated fuel lines etc. A defective carburetor could result in fuel starvation or too much fuel. Too much fuel would usually cause black smoke from the exhaust and a constant fuel odor. Ignition issues on a running engine often result in a "Jerky" missing condition as opposed to a smooth drop off in power. Temporarily and carefully substituting a 6 gallon outboard fuel tank for the boats built in tank can eliminate or confirm the problem to be in the boats tank. Pumping the tanks primer bulb as the condition occurs can confirm or eliminate the fuel pump as the source of trouble. Save any old parts you remove and keep the on board in case of emergency.
 
bobbyc said:
Fuel related issues on a running engine usually result in a smooth "surging" ....Ignition issues on a running engine often result in a "Jerky" missing condition as opposed to a smooth drop off in power.
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I'm going to tackle this later in the spring. There is the possibility that my issue is something fuel related. However, I have owned the boat for 6 years now and have not performed a tune-up in that time. Also, I have no information when, or if, the previous owner had done a tune-up. Given that lack of maintenance, I'm going to start with the ignition system because it needs done no matter what. If the problem goes away that's great, if not then I'll know I have to start looking into a fuel issue.

KJ
 
I reread your original post........if you have a water separation filter before fuel pump, remove and pour contents into glass jar and let sit overnight. refill most of the way with fresh gas and re install. Dont install empty........fuel pumps dont like that!!

Take a picture of the results. In some cases a stumble as you describe could be a bit of water in the gas or bad gas.

If that looks good then move on to bigger and better things.............lol

Because you are running a carb, the fuel pump whether electric or mechanical produces 3-8 psi. nominal would be 5-7 psi.

The way to check is to install a fuel pressure test gauge between the pump and carb. The way you do it is up to you. typically remove line from carb, push flair nut back as far as possible and connect rubber hose to that end with a tee to connect gauge in the middle and the other end needs to be connected to the carb. Usually a old fuel line with the flare and nut with a 6-8 inch length of steel tubing left to connect rubber hose to.

Use good quality hose clamps and be sure to monitor for any leaks.
 
kghost said:
I reread your original post........if you have a water separation filter before fuel pump, remove and pour contents into glass jar and let sit overnight. refill most of the way with fresh gas and re install. Dont install empty........fuel pumps dont like that!!
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I have a Mercruiser spin-on water separating filter that I replace each season. Usually the gas coming out looks fine, but I'll empty it into a jar and see what I get. I never thought about pre-filling the fuel filter. I always do for oil but I didn't think about the fuel pump having issues trying to draw a vacuum.

kghost said:
Because you are running a carb, the fuel pump whether electric or mechanical produces 3-8 psi. nominal would be 5-7 psi. The way to check is to install a fuel pressure test gauge between the pump and carb.
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I have an electric fuel pump. I've thought about removing the line and installing a T and schrader valve. It's always nice to have the option for troubleshooting to connect a fuel pressure gauge and watch it during idle and running conditions. Hopefully my fuel pump is fine, a new one is about $250. :eek:

KJ
 
Docksidemarineservices said:
Five years! Do a full tune up including wires and pick up in distributor, All marine parts, Quick Silver preferably, then see how it runs. Don't waste your time checking old parts, change them.
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I thought I'd come back to this and post an update.
I ordered all new ignition components through this site and did a full tune-up on the engine.

Only Quicksilver were used:
Rotor
Cap
Ignition Sensor
Wires
Plugs are NGK BPR6EFS gapped to spec.

The boat has run flawlessly all summer. Take off out of the hole is strong and smooth with a good normal throaty V8 sound out of the engine. When I'm on-plane cruising the engine maintains a good smooth RPM with no surges or fluctuations.

I can't say for certain which component fixed the issue and quite frankly I don't care. The phrase 'don't mess with success' comes to mind. Sure I'd like to know, and if I had to guess I'd say the underside of the cap looked a little corroded and there was moisture evidence on the Ignition sensor so those are probably suspects. But, in the end, all the ignition components were of unknown age...minimum 6 years old.....so it was definitely time for a full tune-up anyway. The fact that it's fixed and I wasn't chasing down other gremlins is just icing on the cake.

KJ
 
kevinj said:
I thought I'd come back to this and post an update.
I ordered all new ignition components through this site and did a full tune-up on the engine.

Only Quicksilver were used:
Rotor
Cap
Ignition Sensor
Wires
Plugs are NGK BPR6EFS gapped to spec.

The boat has run flawlessly all summer. Take off out of the hole is strong and smooth with a good normal throaty V8 sound out of the engine. When I'm on-plane cruising the engine maintains a good smooth RPM with no surges or fluctuations.

I can't say for certain which component fixed the issue and quite frankly I don't care. The phrase 'don't mess with success' comes to mind. Sure I'd like to know, and if I had to guess I'd say the underside of the cap looked a little corroded and there was moisture evidence on the Ignition sensor so those are probably suspects. But, in the end, all the ignition components were of unknown age...minimum 6 years old.....so it was definitely time for a full tune-up anyway. The fact that it's fixed and I wasn't chasing down other gremlins is just icing on the cake.

KJ
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Good to hear....Happy Boating.
 
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