Merc 250 efi - gutless till 3000 rpm

[rvaha]

s#0T445638. Efi. This has single motorola computer

This is a powerhead that Chris Carson built for me 3 years ago. Started developing an issue last year occasionally when accelerating with load. Last year had some trouble pulling thru 2500-3000 rpms when warm. No issues when cold. Above 3000 pulls hard to 6000rpm

Problem is there constant now and has gotten worse over summer
I’ve purchased some parts from Don hoping I would find the intermittent part.
So far, replaced CPS (last year - definitely bad)
Map sensor,
new ignition coil
new pulse pump.
new spark plugs. Champions like recommended

Thought I had a bad coil but was wrong. Installed new one and then moved it to all
6 cylinders. No luck
compression around 110 each cylinder
checked fuel pressure in garage - 42lbs

once past 3000 rpms it runs perfect. If i slow down to 3000 or below it starts to struggle bad and comes off plane. Full throttle it and it stays around 2300 rpm and maybe after a few minutes it starts to pull to 3000. Once it hits there it’s great

not sure if this is fuel or ignition. Perhaps TPS? It’s not adjustable but I’m sure it could go bad.

Thoughts??

[JOE54]

very possible TPS , could be viewed on CDS to verify

[rvaha]

Thanks Joe. I left out 1 issue that i forgot to include last night. Getting some unburnt oil on the floor. No oil leaks inside engine so think its fuel that has the gasoline evaporated. I spoke to Chris Carson about it he thinks it could be reeds. I haven’t checked them yet as boat is on a slip at lake. This seems to make as much sense as anything. Reeds on the way. I’ll update my findings. Thx

[JOE54]

wheres it comming from reeds dont leak fuel

[rvaha]

Yes, you are of course right. But if there was bad reeds on 1 cylinder and it didn’t run below 3000 rpm where does the fuel go? Chris’s opinion was that it would leak down thru the exhaust plate. He didn’t think it was tps and i tend to agree as it struggles below 3000 rpm no matter where I have throttle. The tester you spoke of would be nice but of course I don’t have one. So, if it’s not reeds or tps maybe computer or injectors or something else? I’m certainly open to yours and the forums experience. I can’t find any leaks in engine. All bleed lines look fine. I unplugged the overflow from the vapor separator that I guess dumps fuel into intake if there’s a float issue but it’s dry as a bone. I’ll keep looking. Many thanks Joe!

[JOE54]

so its comming from exhaust /prop then yes it could be a reed broke has it ever ran right , sure it has if he did it , did it happen suddenly

[rvaha]

Ran great for about year and half after rebuild. Very strong. Crankshaft position sensor went out last year. Replaced and did fine for a while last year and then this quirk started happening. Ran fine when cold. Got worse as day went on. Now bad no matter cold or warm. Very strong now - above 3000 rpm.

[JOE54]

inspect and report back

[EuropeanAM]

I agree that it's wise to inspect reeds as a possible culprit. Make sure to check the injector harness for any chafed areas (inspect every wire) while apart.

[rvaha]

Thank you Don and Joe. I have some new reeds to install today. I’ll check the wiring first. I’ll report back. Thx

[rvaha]

Update - got new reeds from Chris 10 days ago. Took off reed box - his original reeds looked perfect but put new ones in since needed to replace reed and airbox gaskets. Ran on muff. Fuel pressure 46 and hold fine when off. No leaks. Checked spark with timing light on muffs. Idle fine - all port cylinders had steady flash. All starboard cylinders were more intermittent on flash. ??

Took to lake. Motor maybe still a little warm. Started and idled fine. But gutless hole
shot and bad surge again at 3000rpm. Wouldn’t run there. Let it sit for a 3 hrs. Ran it - holeshot not great - but surge gone. Ran it for 15 miles at 3000rpm - 30mph

now yesterday. Started fine, idle fine - holeshot immediately jumped to 3500 rpm and with 4 people. Ran 15 miles. No surge. Sat for half hour. Holeshot now bad - but it finally pulled past 3000rpms then took off. Ran 25 miles rest of day - surge back again.

Seems temp related. The somewhat intermittent flash on the timing light on muffs may be a clue or possibly just my error.

Not sure if what else. Tps, injectors, something common to starboard coils and plugs, computer??? Any ideas? Thx as always.

[Gamblerman65]

pm sent..

[JOE54]

was afraid of that bad reed runs bad at all ranges not heat related or intermittent , id be looking at TPS and grnds on mtr test all cdms as outlined in serv manuel

[rvaha]

Pm sent

[EuropeanAM]

Can you verify that fuel pressure with another (known-accurate) gauge?

[rvaha]

Thx Don, I’ll check and see about another gauge. Are you thinking it’s reading high or low? I think next step is to get a helper and check fuel pressure and spark on all cylinders at lake under load when this is happening. Can’t duplicate it at home. I’m starting to wonder if it’s the computer.

[ChrisCarsonMarine]

Although not common,it might be fuel pump or screen in the VST.We recently had a 2.5 EFI that held 38 lbs on the trailer,at idle and revved up,but would loose power after 4000 rpm.Pressure test under load(much higher fuel needed)started dropping around 3000,was 25 lbs by 4000.New pump and screen...All better,Chris

[JOE54]

Although not common,it might be fuel pump or screen in the VST.We recently had a 2.5 EFI that held 38 lbs on the trailer,at idle and revved up,but would loose power after 4000 rpm.Pressure test under load(much higher fuel needed)started dropping around 3000,was 25 lbs by 4000.New pump and screen...All better,Chris

had it been sitting up for a bit ,seen a 115 that the screen was restricted on did same thing

[EuropeanAM]

Thx Don, I’ll check and see about another gauge. Are you thinking it’s reading high or low? I think next step is to get a helper and check fuel pressure and spark on all cylinders at lake under load when this is happening. Can’t duplicate it at home. I’m starting to wonder if it’s the computer.

If you can strap the gauge to the side of the engine- take someone out with you and have them video the gauge/engine while you drive. You can then watch to see what (if any) pressure changes are occurring when the engine is acting up.

Specifically looking for any higher (or lower) fuel pressures.

[rvaha]

If you can strap the gauge to the side of the engine- take someone out with you and have them video the gauge/engine while you drive. You can then watch to see what (if any) pressure changes are occurring when the engine is acting up.

Specifically looking for any higher (or lower) fuel pressures.

Just got around to working on problem this season. I bought a tps from Don last
week and just put it on. I didn’t think it would help and it did not.

Surged a bit at lake when cold yesterday and then when warm was gutless below 3000rpm. Also got Rinda software but it hasn’t identified anything I can understand yet. Fuel pressure is 47-48 with key on and holds well with key off. Going to take apart vst tomorrow. Looks like I need a lake test.