502 MPI Irregular running RPM

[boatglms]

I have a 2001 Formula 400ss with twin Mercury 502 mag MPI's. At idle there is evidence of fuel on the water coming out of the port exhaust and at 4000 rpm there is a 200RPM constant variation on the port engine. Does anyone have any suggestions? Im stumped Thanks

 

[Bondo]

Re: 502 MPI Irregular running RPM

Ayuh,........

I'd start with Changing the Fuel Filters,......

If that ain't It,.... It'll probably take a computer diagnostic run to see what's Up....

 

[newport dave]

Re: 502 MPI Irregular running RPM

I would start by diagnosing the (possible) overfueling at idle. It may or may not be related to the unstable RPM at higher speeds, but should be easier to diagnose.

You first have to eliminate the possiblility that a bad plug, wire, cap, or rotor is causing a cylinder(s) to pass unburned fuel. Once (lack of) ignition is eliminated as a cause and the problem continues you would then look at fuel system issues.

Some possibilities are high fuel pressure, overflowing VST, bad mechanical boost pump, or a bad injector.

 

[boatglms]

Re: 502 MPI Irregular running RPM

The engine is completely stock, Im not sure if it is oil or gas coming out but I just tried swapping the ecm's, fuel pressure is a constant 39 psi, I checked all of the plug wires, vacuum lines, cap and rotor, and gauges. I also tried fuel injector cleaner. I think tomorrow I will try and change out the cap, rotor, wires and plugs and see if that helps. So far it doesn't seem like fuel after hooking my computer up and comparing both engines. Thanks for all the help!

 

[newport dave]

Re: 502 MPI Irregular running RPM

So far it doesn't seem like fuel after hooking my computer up and comparing both engines. Thanks for all the help!

What software are you using to connect to your engine? What year is your engine?

 

[boatglms]

Re: 502 MPI Irregular running RPM

I am using Diacom the engine is a 2001. Today we replaced the ignition coil, plugs, cap, rotor, and plug wires. We are still at square one. The only thing i can think of is to test the wiring harness and the injectors.

 

[Fireman431]

Re: 502 MPI Irregular running RPM

Find out if it's oil or gas. Apparently, it's putting a irridescent shhen on the water? Dip your hand in it and smell if it's gas. If no odor (and a little slick), it's probably oil. Check your oil level. Is it down? If it is oil, I'd check the oil cooler first. Probably the only place that raw oil can get into the exhaust.

 

[boatglms]

Re: 502 MPI Irregular running RPM

Now i dont see any sheen on the water after I went through 1/2 a tank of fuel. there is no oil loss or smoke, no O2 sensors either. I checked the PCV as well today and it is clean as a whistle. I am currently trying to use the data I recorded from my Diacom program to look for sensor readings that may be faulty. Today we replaced the ignition coil, plugs, cap, rotor, and plug wires. The only thing i can think of is to test the wiring harness and the injectors.

 

[fixb52s]

Re: 502 MPI Irregular running RPM

So you are saying that the evidence is gone after that 1/2 tank of fuel? Sounds like a sticking fuel injector. Take them out and get them cleaned, or at least run a bottle of Techron in the tank. I say Techron because this is in my opinion the only cleaner (other than Seafoam) that works, and it cleans the valve deposits too.​

For the data readings, you really will not know just what those injectors are doing without the O2 sensor readings. The readings will show the ECM giving the proper outputs to the injectors, but that doesn't mean they are working properly. That is when the O2 sensor feedback to the ECM will tell what the air/fuel ratio is, and try to adjust. If the adjustment doesn't happen, then that is when the ECM will pick up the error. I worked a similar problem last year with a friend's car. I am using a Actron CP 9190 and the A/F map was not there since there was no O2 reading.​

By chance have you done a compression test? Also try to hook up a good old vaccum gauge and se what that tells you.​

 

[newport dave]

Re: 502 MPI Irregular running RPM

I am currently trying to use the data I recorded from my Diacom program to look for sensor readings that may be faulty.

I have Diacom, if you email me your recorded files I might be able to help. Not promising anything, but I do have other 502MPI files I could compare with your data. Also, are you recording both engines at once? I have a data link harness that allows this if you are interested.

Dave

 

[boatglms]

Re: 502 MPI Irregular running RPM

So far I have not done a compression test and can only test one engine at a time. If I can send you the Diacom that would be great!! What is your email address?

 

[newport dave]

Re: 502 MPI Irregular running RPM

I sent you a PM with my email address. I will take a look at the files and post what I find.

Dave

 

[boatglms]

Re: 502 MPI Irregular running RPM

Thanks everyone!