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Having an issue with a Marine Power 454, fuel injected engine. The engine runs great, at all speeds other then idle to maybe 1200 rpm. In this rpm range the motor speed fluctuates 50-100 rpm constantly.

Last year a mechanic looked at the engine, sprayed some carb or injector cleaner and it ran good for a short while.

Then we where told it was the Idle Air flow Valve, it took forever to find the replacements and once installed the problem was eliminated for the day. The second use of the boat the engine was back to the usual fluctuation game.

Any ideas of what to look for? I'm thinking to starte swapping parts from one engine to the next.

Aluminum tank or fiberglass? Ethanol problems sometimes start like that.

It's an Alum tank in my Buddies 32 Luhrs

Sim, I know on automotive inj. systems the IAC valve is just that, controlling air flow around the throttle body for the engine to have controlled idle speeds. Usually it is carbon that makes them fail. I would try to see if there is a vacuum leak in the intake area, but when the tech sprayed inj cleaner before this time and it made it better, that does not seem related other than forcing some cleaner into the IAC's passageways, clearing it out for some time. If you can pull the valve easily, check for carbon and clean the passageways out.
Rocky

Thanks Rocky...
Passageway look real clean, not a whole lot of hours on motors.

He found new valves at Napa Automotive which was able to cross reference with the newly aquired valves from Arizona Speed, which is where Marine Power got theirs. Not that that means anything, other then the 2nd set of new valves did not help either, same thing. They did not pull easy, but then I really didn't try all that hard, all three seemed similar.

I had thought the cleaner either cleaned or lubed the valve stem making it work for awhile, but when the same thing happen with the new valves I realize I was wrong.

I'll look for vacuum leaks and recheck passageways.

Maybe a sensor screwing up?

It's times like these I really like the old non sensor, computerized motors...

Is it setting any codes in the computer (assuming it has that capability)?

Good point Rawleigh...

I was thinking of going down the path of swapping parts from good to bad engine to see which component will cause the other motor give the same responce, but not being familar with system and not being my boat I hesitated to go that route.

But hell your right it's a computerized motor and I know the surveyour checked the engine with a puter so maybe something has come up since then. Apparently the motor was doing this on sea trails and everyone pointed to that valve, but maybe...

Could it be plugs or wires?

Do you have the Smart Pack gauges? They can be made to read codes. I had a friend with a 32 Luhrs that had the same problem. he said it was a vacuum leake between the plenum and the intake. He sprayed Carb Clean around it to find it. I think WD 40 would be safer though.

It is an IAC motor or idle control motor w/ a carbon build up. It is electricly controled and should be mounted on the intacke manafold. Works like a egr valve. Has the owner got a scan tool to hook up. My best guess is the gasket to either the egr or iac is bad and you have vac leak. Scanner or a Vac gauge should show the prob.
Chris
ps spray WD 40 around the intake and trottle body and the rpm should rise Vac Leak

No my buddy does not have a scan tool, although the mechanic should. It's just a matter of getting him to the boat, hence why I'm involved.

I'll look into vacuum leaks, I have a vacuum guage. Although it can be readily heard as it wanders up and down.

If you can tap into ecu through a scan tool, you can see live data. That should tell you if any one sensor is out of spec along with a code check as long as you know what to look for.
Rocky

Sim; Do you have a egr valve on those engines? Or if you have vac leak spry some WD 40 along the intake carb etc and see in the idle come up and then fall back down.
Chris

Sounds like you checked IAC pretty good. Those motors do not have mass air meters, should be speed sensity motor but highly unlikely the cause of your idle problem. If you have exhausted all checks for vacuum leaks I would go straight for the ground wires. Poor ground on the battery side or the engine side might cause it at low RPM but it sure sounds like vacuum problem. At speed it would still run fine because the computer would add more fuel to compensate for the lean condition. of a vac. leak. I am better at Ford computer stuff but that is where I would be checking if it were mine.

Great info...thanks.

I hope to get down to the boat by the weekend, it would be great if we could get a handle on this. Funny how I just bypasssed all the simple stuff like Vacuum leaks and faulty grounds...even the complex computerized motors can be faulted by a vacuum leak and a ground... not saying that is what it is, but it does make sence.

Appreciate the direction, better to look for something then just wait on the mechanic another weekend.

CArl

They would probably fail under a load as well as idle speeds, but look at your plug wires in the dark. You might see arching in places the charge is finding an easier path to.
Rocky.

Rocky I won't dismiss bad plugs and wires and will add that to the things to look for...but it does not act like a miss, it does not sound like a miss. It is smoothly going from 750rpm to 900rpm and back down again when not in gear, at idle. Put it in gear and she'll wander a bit less but still wanders. This happens up to about 1250-1500rpm then seems to run fine.

I'll take that back, she may not be running perfectly smooth at cruise, we get a slight vibration which I believed was some misalignment on the Port Shaft, however when he changed the valves the engine did not wander dockside and we took her out for a spin and the vibration was gone, ran good when we got back to the dock too. On the next run the vibration was back and when we got back to the dock the engine was wandering again. maybe differnet engine loads as we had several people with us, differnet weather as we had a heavy chop... not sure but could be because of that..."that" being whatever is wrong.

Boats a real pain in the "arse"

I have marine power carb motors--just installed new tachs that are programable---there are 3 dip switches on the back of the guage---if signal comes off the alternator--need to know how many impulses per revolution--options are

8-12

12-17

17-25

called marine power--they are no help--anybody have a clue?

Sim I still feel a vac leak but! You more than likey have a computer stuck in a open loop. This will cause the engine to run rich and keep hunting for the proper idle. Yes a scan tool is what you need. Question is do you know of a surveir that will hook one up for you as a favor and it may cost you a dinner and a clear
Chris