2000 mercrusier 7.4 misfiring

[thegoat86]

2000 reinelle 240 with 7.3 MPI mercruiser/bravo1 drive. I pulled it out for the spring. Noticed heavy misfire under load. Idled rough. Pulled plugs, three with heavy carbon, one brown. Pulled cap rotor, heavy corrosion. Replaced all. Muffed boat, ran strong at startup for about thirty minutes. Then degraded to heavy misfiring. Replacing water fuel filter today. Tank was almost empty when stored, Refueled maybe a month ago. Trying to get some guidance before I pull everything apart to check compression ect. I will however check for fouling plugs as that could be a culprit.

 

[alldodge]

Hey you mention a 7.3 MPI, your sure its a 7.3 and not a 7.4?
Also mentioned 3 plugs black and 1 brown, what holes did they come out of and if this is a V8 what did the other plugs look like?

 

[thegoat86]

Its a GM 454 MPI. 7.4, finger slipped. Two plugs had ash buildup. Caked on. (kinda like the pic) One was a whitish brown. The others looked fine. I bought the new separator filter just now. When it stops raining I will put it in. I will run some water remover as well in the tank.

Side note*** I did put high octane fuel in when I filled it up this spring.91 octane. I usually just run pump gas. 87 octane for over a year.****

 

[alldodge]

Looks like you have something in your tank that shouldn't be there. Your motor should run fine on 87 octane. Whiteish brown doesn't give me concern. What did the plug look like on cylinder 7, last one on the left side in the rear of motor?

 

[thegoat86]

all the plugs on left side were fine. The two offending ashed up plugs came from cylinders 2 & 6. Maybe higher octane fuel messing with the ECM?

 

[alldodge]

all the plugs on left side were fine. The two offending ashed up plugs came from cylinders 2 & 6. Maybe higher octane fuel messing with the ECM?

No I don't see that being an issue. I mentioned 7 because it's an issue with the MPI's, and it can run lean.

You should have a 160 degree thermostat and being a 2000 it should have a MEFI-3 ECM. With it running rough at idle and higher rpm, I would first check fuel pressure, but since you have the build up I would be looking at filters and what's in the gas tank.

When you refilled, about how many gallons went in and was it at a place which sells a lot of gas?

Also was anything added to the tank for the winters nap?

 

[thegoat86]

I filled it, maybe 45 gallons. Yes very high volume chain place around here. I changed the fuel separator, now it wont start period. It revved really high, sounded good doing so... then stalled. I hit the key a few times to make sure I had fuel in the filter. I have fuel spray at the shrader valve.

I ran the tank to half full cruising around the bay a couple weeks ago. I have half a tank now. Feeling I may have sacrificed a filter and need to empty the tank and start over.

Not really a winters nap around here. I used the boat in November. So it really only sat two months. (florida)

 

[alldodge]

If you need to empty the tank and start over there is something really wrong with the fill station or what your adding to the fuel. The thing that gives me concern is the build up on the plugs. Since it won't start need to look into spark, then fuel.

When you crank the motor watch the tach and see if it increases and shows its turning. If the tach does not move then the ECM will not allow the injectors to fire.

When you turn the key to ON do you hear the fuel pump turn on and the two beeps?

 

[thegoat86]

Yes to all. It starts then stalls. I pulled all the plugs to look at them. All but #8 have a black shiny carmelize to them. Not oil. I know what blow by and fouling looks like. I assume I got sugar or something in the fuel. Maybe somebody doesn't like me. No additives other than the "dry gas" I put in after the filter change. Is there an easy way to drain the fuel tank? the fuel line goes uphill so it wont gravity drain.

 

[NHGuy]

If you have access to the tank top you can disconnect the sender and remove it. Then suck out the fuel. Or if you want to get a little fancy get a fuel pump and put a line on the fuel outlet from the tank, or the one that feeds the separator.
We had a guy at my work get sugared, that car would not even run. So perhaps your glaze is some product of incomplete combustion. I hope so for your sake.
Is your boat closed or open cooled? If it's closed maybe you have some coolant glaze.

 

[alldodge]

Yes to all. It starts then stalls.

Put it in throttle only and see if you can start it, try to keep it running at higher rpm. If you can, but it dies when you go back to idle, then its probably your IAC

 

[thegoat86]

Drained fuel tank. doesn't smell sugared. Open loop cooling. (37 gals)

AD, it runs a little (pretty decently) when I crank it then immediately dies. No amount of throttle will keep it running. (RPM gauge works)

This symptom just came about after I changed the fuel separator. It ran before, but was misfiring.

Going to fill it up with fresh fuel tomorrow and flush the rest of the fuel through the lines. then I will get the mercruiser OEM fuel separator filter, and start fresh with fuel off of the table.

AD, IAC?

 

[alldodge]

The IAC is the Idle Air Control valve. It controls engine rpm below 1500 rpms. Its not your IAC if you can rev and it still dies. Don't think its your gas either now, stating to lean toward fuel pump pressure. At the shrader valve you should see 43 psi running

 

[thegoat86]

So as far as draining the tank.... I did get fancy. I took the feed line off of the filter, hooked up a long section of hose enough to reach the behind the drive where I could fill cans, then I made sure the filler cap was tight, then I unscrewed the vent cap near the tank filler cap, stuck an air compressor fitting to it, and blew small puffs of air in to pressurize the tank. Just enough to start a flow. It trickled slow but drained the tank in about an hour. (do not do this at home unless you have common sense enough to not blow your tank apart!!!)

 

[thegoat86]

AD, I will check fuel pressure in the morning after I do all of the fresh fuel stuff.

 

[thegoat86]

So, its been a couple days. We had some bad weather and rain everyday. I was able to get it down to the gas station today for 20 gallons of non-ethanol, 90 octane.

On muffs, it runs good for about thirty minutes. Fuel pressure swings from 45 to 60 when the injectors are firing. I noticed it runs good then lopes for a few minutes, then goes back to running good. Maybe dirty injectors? It wants to stall, but comes back. But it does stall on occasion.

 

[alldodge]

Fuel pressure swings from 45 to 60 when the injectors are firing

Its either the pump of fuel pressure regulator. You should have 34 - 38 psi and holding, not swing back n forth.

To find out if its the pump, the fuel return line is disconnected from the pressure regulator and a hand operated valve is installed. The pump is turned on and the valve is slowly closed. The pump should reach 60 psi but should not be allowed to go over 60 psi, damage can happen to other components. If it will reach 60 psi, the pump is good.

To test the regulator, the motor needs to be running, and pressure gauge is on the fuel rail. When the vacuum line is removed off the regulator and connected to a hand operated vacuum pump. Pump the gauge up to 10 in Hg and pressure should drop 3 to 10 psi.

 

[thegoat86]

What I mean by bouncing, when the injectors open there is a very quick drop in pressure that I can see. It bounces the needle very quickly between 60 and 40. Very fast. At startup, the fuel pressure builds quickly to 60. I will test the regulator today. I am wondering if the RPM gauge may be going bad. As you stated, if the comp doesn't sense RPM it wont run. Today on muffs it starts up fine, runs for 10 minutes then stumbles and stalls. I start it back up, rev the motor a couple times, then it runs fine. There is still the intermittent misfire I can hear.

 

[alldodge]

when the injectors open

With the pressure building to 60 psi, it sounds like either the regulator is not working (you replaced twice) or the return line back to the filter is clogged.

Are you doing the injector balance test with the Merc scan tool? If not what

 

[thegoat86]

Took the boat out on the water today, brought along a mechanic friend of mine. We hooked up a fuel pressure gauge and a vacuum gauge to watch things as we were going along. Boat ran crappy and was dropping RPM and misfiring at lower RPM, but would come out of it and run smooth at higher RPM above 2k. The fuel pressure gauge was around 45 while underway. At all RPMs it was above 35. Vacuum was sporadic. Ruling out everything including fuel, my buddy pulled the computer harness connectors. We waited 15 minutes, plugged it back in, and the motor ran like it was new. We had higher vacuum and higher fuel pressure @ 55 in all cases except when we shut down and just turned the key to on. It would spike to 60 then settle at 45 until started. Then to 55. He believes I needed to reset the comp that it learned some bad parameters while the plugs, cap, rotor were soooo corroded and it had to compensate so much to keep it running. Theory only.

So now it runs better than when I bought it last year. It cant be that easy.... Can it?

We took a 15 mile trip out into Tampa bay and back. All speeds. Even WOT for a good five minutes. 4K RPM @ 53 knots. :noidea: