2001 mercury optimax 135 issues

[Muskyhunter08]

Hi,

I recently purchased a 2001 ranger 681VS w/ a 2001 Mercury 135hp Optimax serial number OT343374 205 engine hours. It ran bad at lower RPM's say 800 to 1200 and would idle rough and spudder and die. So I replaced the fuel line and bulb to the pump and external canister water fuel separator filter. Bulb line was badly degraded. I also rebuilt the diaphragm pump found 2 small particles lodged in diaphragm nothing much. Drained and filled fresh fuel with quickleen and quickcare additive. new set of NGK IZFR5G plugs gapped to .040 (was told by mercury dealer in town to gap them to that) I've heard different gaps from various sources.. Ran great for 2 outings then started acting up again having intermittent spudder and miss (not near as bad as original rough idle and miss) at low speeds idle is still okay but not perfect. checked coils and wires they are good. Checked tracker valve no tears and looks good. Anyone have any advice on what else to do? I'm at a loss here. Maybe replace fuel line from tank to bulb? Check VST internals? Is there a filter inside VST? Maybe regap plugs? Any help would be great. What size fuel line do I need from tank to bulb and where can I order it. Thanks

[SilverFox579]

Great resources in the FAQs at top of forum. Correct spark plug gap for your 2.5L engine is .032 - .033. Look below your VST, there should be a spin-on filter which is all the filtration your engine needs and includes a water in fuel sensor. I'd remove the external filter assembly completely, plumb new lines from tank to bulb and from bulb direct to engine at VST, 3/8" from tank to bulb, 5/16 bulb to engine. Did you use Mercury gray line with blue lettering? Read the fuel line link below and ensure all recommendations are followed start to finish skipping nothing or you may chase problems for some time.

Suggest you contact European Marine for any parts and, if you don't already have one, a genuine Mercury service manual for your engine by serial number.

If you want to order or price Mercury Engine Parts or Accessories...

FUEL LINES AND PERFORMANCE PROBLEMS

Oil, Battery, Spark Plugs, Fuel and Fuel Additive Recommendations

[Muskyhunter08]

Yes I used new bulb and fuel line with blue lettering. Replaced fuel water separator filter, and rebuilt diaphragm pump. All new mercury parts were used. Will to much of a spark plug gap make the engine run bad I gapped them to .040 and not .032 I was told by local merc dealer to do .040.. Also is there any other internal filters Inside the vst? I'm in the process of trying to pull new fuel line from tank to bulb royal pita.

[SilverFox579]

Most current specifications are in the FAQ, so you need to re-gap to .032 - .033. Using a wider gap might be fine in the short run but you will experience increased plug wear and running issues if you don't re-set to correct gap.

When you mentioned external canister fuel filter, were you referring to a boat mounted filter or the spin on filter below the VST?

I searched European Marine's parts website with your serial number and your VST shows a spin on fuel filter on the bottom of the VST. If you haven't replaced this it could be contributing to your problem. The filter looks just like a small oil filter and is the only fuel/water separator your engine needs.

Mercury does not recommend additional external filters for your engine (I have a 2.5L Optimax) if you have one it can induce fuel restrictions and should be removed.

There may be filters inside the VST but I can't confirm that so someone else will need to verify.

[Muskyhunter08]

Yes external spin on filter I already replaced. I found a loose plug last night had oil dripping out of it. Was not even hand tight. Would that be the cause of my rough idle? I'm going to regap and tighten all plugs down tonight

[SilverFox579]

Loose plug could be. Make sure you torqued plugs to 20lb ft.

[Muskyhunter08]

Tightened plugs with torque wrench 20ftlb no change. Also put in new fully charged battery Optima D31M Blue top no change. New Fuel lines, fuel bulb, pulse pump, dons cocktail, New plugs gapped to spec and wires, new vst screw on filter, fresh fuel, cleaned compressor strainer, checked and tightened all grounds on engine, checked all plugs to injectors made sure they were connected. ran engine and pulled each coil off to see it drop a cylinder and it did, couldn't find one that didn't change the rpms. checked pressures at idle 90 fuel and 80 air. What else should I check, I changed the plugs the first time ran great for a 1-2 days then started running terrible again? Was told to try power tune in the cylinders??

[EuropeanAM]

Let's try to keep your engine in this ONE thread for now. Lots of others... most of which don't have the required info.

THANK YOU for providing the serial number here!

What do the plugs LOOK like when you pull them to inspect?

[Muskyhunter08]

Plugs looked real dirty after power tune. New replacements gapped to .032-.033 installed. The issue seems to be intermittent when I mess with the throttle and move from neutral to forward randomly then it will run rough just off of idle. Also noticed that when I restart the engine after rough idle/miss it doesn't have in issue. then it will come back randomly while trolling slow again.

[Muskyhunter08]

Talked to merc tech that is working on engine. Says he could not find any issues with tps. He said at random times the fuel pressure would run 90-94psi and all of a sudden will jump up to 108 110. Says he filled fuel filter with quickleen and ran engine until it ran rough and then let it sit for 4-6hours started it back up and ran the rest of the quickleen out and he said he could not get it to act up. Ran it for an hour and did numerous restarts to try and get it to idle rough again but wouldn't. He seems to think that it was a injector that was possibly sticking. He also opened vst and cleaned that out. And also replaced a check valve on the return fuel line. Im thinking I had a stuck injector that the quickleen possibly loosened up and now is operating again to spec.

[EuropeanAM]

My guess is that it was actually starving for fuel (intermittently), hence the sudden drop in fuel pressure at the rail.

Hopefully it's behind you now.