1999 Merc 90HP ELPO....surging

[mikeneal]

I have a '99 Merc 90 HP ELPO I bought ~1 year ago. 1st outboard I have ever owned, always had I/Os. Anyhow, right after I bought it I did typical things I usually do when I purchase a used thing. I changed spark plugs, wires (had dry rot), gear oil, water sep filter, decarb, etc.. Anyhow , for past year always ran fine but last weekend was on way back to ramp ~3750 RPM and would surge, feel as if would drop a cylinder, then get it back. Low speed, idle etc. was fine. Was almost back to ramp and trying to get there before rain so didn't mess with it , just headed in. Got it home, runs fine in muffs to flush etc. Local dealer said to replace water sep filter an small in-line filter between carbs (had never done that one) and fuel pump. I drained both and no trash and /or water but changed the both anyhow.

I am wondering if I should rebuild fuel pump, I did a search and I guess sometimes there are ethanol issues with the pump parts. Also might replace spark plugs, but was running at fairly high sped when it acted up and I would expect fouling plugs to be more low speed type situation. Thoughts? Think i should do plugs/compression test. (were all ~110 psi when I bought it, pre decarb) and rebuild pump? Or try it after filter change first? I guess a stator will sometimes act odd at high speeds too.

 

[Barnacle_Bill]

Re: 1999 Merc 90HP ELPO....surging

If that has the square type fuel pump there is an ethanol problem with the check valves in the pump. The kit from Merc come with new style check valves.

 

[mikeneal]

Re: 1999 Merc 90HP ELPO....surging

It is rectangular pump, not sure if that is the "square" one or not you speak of. I think considering the low cost of the pump rebuild kit (dealer quoted like 18-20 doalrs), I will do that too. Can't hurt.

If that has the square type fuel pump there is an ethanol problem with the check valves in the pump. The kit from Merc come with new style check valves.

 

[Silvertip]

Re: 1999 Merc 90HP ELPO....surging

Before shotgunning this engine by throwing parts at it, try some more diagnostics. Running an engine on muffs presents no loading so fuel problems will not generally show up. Besides, you should not run an engine unloaded on muffs at more than 2000 rpm. Run the boat at the point where it begins to surge. Then have someone squeeze the primer bulb several times. If the engine picks up, it is indeed a fuel delivery problem. Next, try running the boat on a separate tank to eliminate the in-boat tank and its plumbing. If the problem goes away, the fuel pickup tube may be broken or the screen clogged, the primer bulb may be malfunctioning, the anti siphon valve may be sticky, etc. If all of those check out ok, then you likely have a bad fuel pump or gummed up high speed jets in the carb. Rebuild kits don't always come with all of the small parts so if the check valves are an issue, I'd replace the pump.

 

[mikeneal]

Re: 1999 Merc 90HP ELPO....surging

Good advice, I see your point about throwing parts @ it. I think the next couple times I have the opportunity to take boat out for sea trail I will be solo, so cannot do the primer bulb test. Maybe I will try to get someone to go w/ me though, as that would be a good clue/bit of info.

Before shotgunning this engine by throwing parts at it, try some more diagnostics. Running an engine on muffs presents no loading so fuel problems will not generally show up. Besides, you should not run an engine unloaded on muffs at more than 2000 rpm. Run the boat at the point where it begins to surge. Then have someone squeeze the primer bulb several times. If the engine picks up, it is indeed a fuel delivery problem. Next, try running the boat on a separate tank to eliminate the in-boat tank and its plumbing. If the problem goes away, the fuel pickup tube may be broken or the screen clogged, the primer bulb may be malfunctioning, the anti siphon valve may be sticky, etc. If all of those check out ok, then you likely have a bad fuel pump or gummed up high speed jets in the carb. Rebuild kits don't always come with all of the small parts so if the check valves are an issue, I'd replace the pump.

 

[mikeneal]

Re: 1999 Merc 90HP ELPO....surging

Sea trialed boat end of the last week and ran fine, WOT for several minutes, idled around caught bait etc. Ran fine for an hour so Saturday loaded it up with people and ran to near shore reef (~5 miles). Boat ran OK for most of the day but on ride back in started surging. I had somebody pump primer bulb and seemed to help slightly. So last night rebuilt the fuel pump. Pump looked fine mechanically, no trash, no swelled parts etc. .

I am going to try to run it again this week after work and take along a secondary fuel container. I wonder if the pickup in the tank has an issue, it is odd to me that it ran OK for most of the day. Side note: tank was full when I sea trailed and left Saturday. I purposely didn't fill tank on way home (still has ~2/3) from weekend) I also will place some clear PVC fuel line (temporally of course) between pump to carb to look for bubbles.

Any other thoughts? BTW, I checked compression and changed plugs (had been a year) and same as always has been 105-110 psi. Feels fuel related (not sharp miss, just slow surge)

 

[mikeneal]

Re: 1999 Merc 90HP ELPO....surging

Update (another one):

Ran boat around last night and ran OK for a bit then eventually started a slight miss. Not real badly, but I could barley tell is was having some starvation issues. Ran it on a secondary tank and ran the same, so pickup/tank is not it. Replaced the fuel line between the pump and carb with some PVC tubing and it looked like it had some air in it, not sure what amount of air is allowable. By that time is was getting dark/cold so just put it in trailer. Think it would be worthwhile to pull carbs apart and check for debris in them?

 

[Chris1956]

Re: 1999 Merc 90HP ELPO....surging

Mike, It would be helpful if you could determine which cylinder was missing. Do the spark plugs all look the same (light to dark brown ash?), or is one a different color? Check to see if any of the spark plug wires are arcing to ground. When you replaced them, you used Merc or aftermarket outboard wires, right? If not, replace them with outboard wires, as they are diffeent than automotive wires.


Check the bolts and nuts on the carbs and crankcase for proper torque. Sometimes surging is caused by an air leak. The fuel recirculation hoses are in place and good?

 

[mikeneal]

Re: 1999 Merc 90HP ELPO....surging

The plugs all looked the same when I changed them. I wouldnt say they looked brown or clean, but I have never changed them where I had not run it on muffs to flush it, so plug color would never look great I assume. I did use outboard wires when I swapped them out a while ago.

I will check for air leak tomorrow in daylight.

Mike, It would be helpful if you could determine which cylinder was missing. Do the spark plugs all look the same (light to dark brown ash?), or is one a different color? Check to see if any of the spark plug wires are arcing to ground. When you replaced them, you used Merc or aftermarket outboard wires, right? If not, replace them with outboard wires, as they are diffeent than automotive wires.


Check the bolts and nuts on the carbs and crankcase for proper torque. Sometimes surging is caused by an air leak. The fuel recirculation hoses are in place and good?

 

[dinikin]

Re: 1999 Merc 90HP ELPO....surging

I have simular problem. TOok it out ran fine, then dropped one cylinder.
Got home, bought spark plug tester and saw no spark from the upper coil to the cylinder.
Did you check for sparks?

 

[mikeneal]

Re: 1999 Merc 90HP ELPO....surging

Yea, spark was good all cylinders. I tested fuel pump pressure and was OK, checked if for vacuum and that too was OK so I was outta ideas and bought carb gasket kits. Took carbs off, cleaned and blew thru with compressed air. No sediment to be seen so was not expecting any changes, but low and behold sea trailed it and runs like new. Gained 300 RPM on top end and most importantly the mid throttle surge is totally gone, runs smooth and clean everywhere. Must have had something in a passage or metering jet I dunno.

I have an occasional sneeze when idling, not sure if the idle low speed mixture is too lean or rich, will have to mess with it next time it is in the water, I have found the idle on muffs is far different then in the water. (Back pressure I suppose)

 

[electricjohn]

Re: 1999 Merc 90HP ELPO....surging

Were your carb bolts nice and tight when you took them off?

 

[hkeiner]

Re: 1999 Merc 90HP ELPO....surging

I have an occasional sneeze when idling, not sure if the idle low speed mixture is too lean or rich

I believe that a "sneeeze" indicates that the mixture is too lean. To adjust, you need to turn the idle screw in until it starts to sneeze...back it out until it stops sneezing...then out another 1/8th turn to prevent a lean mixture.

 

[mikeneal]

Re: 1999 Merc 90HP ELPO....surging

Yes, the carbs were tight and gaskets were in decent shape. I had checked fuel system over pretty good prior to tearing carbs of for leaks. I will turn low speed screws out slightly to try to eliminate sneeze next time boat is in the water. This outboard always runs quite a bit different outta the water on muffs so I try to do fine tuning when it is in the water.