350 Mag MPI intermittent rough running.

[c1steve]

This engine is FWC, has 1900 hours, but I have rebuilt the heads and replaced the timing chain set. Recently it will often start out running rough, and the idle will shoot up to 1,300 rpm. If I shut the engine down and re-start, often the engine will run correctly again. However the problem has become more frequent recently. When cold the engine is often running poor, but when fully warmed up there may be no problem for several hours.

This is an older boat where the cockpit drains to the bilge. I have two automatic pumps, and run the manually when out fishing. However there is always some water in the bilge, and so the engine is often damp.

I replaced the IAC valve two years ago, not sure if that could cause this problem. I am considering pulling the two cables off of the ECM, and spraying Corrosion X in there. When I did inspect those connections a year ago, they were all bright and clean.
I connected my laptop with Rinda Tech software today, and the software shows it is not connecting to the ECM. I tried two USB ports, there was no change.

Any suggestions?

 

[alldodge]

What's your serial number?

Re-check the throttle cable
Remove it and then see if the throttle body will go further closed. Also when throttle body is full closed, does the cable just slide on? It should actually have a slight pressure to help keep it closed

Is there any play in the cable?

 

[c1steve]

That is the first thing I did, about a year ago. I also changed the linkage to make sure I was getting full pressure on the throttle body plate.

Another clue: Yesterday I partially warmed up the engine, running rough. Then I restarted and the rpms went to the correct 600 and the motor was smooth. After about two minutes, I stepped off the boat onto the dock. This is a 24 degree deep vee, and so the there is a lot of hull movement with weight shift. IMMEDIATELY the RPMs went up, and the engine ran rough. So I am thinking a loose ground, a shorted wire, or loose connection.

 

[alldodge]

Might be, the EFI harness has small ground wires which usually connect near the ECM.

Connect a fuel pressure gage to rail and a voltmeter on the DLC connector

 

[c1steve]

350 Mag MPI Horizon, s#OM675521

I checked the fuel pressure a few months ago, was 46 psi or whatever the spec is, can check again.
Will test the DLC connector with a meter, good idea, thanks.

 

[alldodge]

Serial number shows 555ECM and HVS distributor

Your unable to get Rinda SW to read so if you put a fuel gauge and voltmeter on it may tell us something when the motor starts to run rough.

The dizzy cap on the motors are known to cause problems and look great. Most keep a spare on board to change because they can go out without warning

Connect meter to pins A and D

 

[c1steve]

Good progress today.
No loose or chafed wires that I could find. However when I compressed one of the IAC hoses, the engine smoothed out and went to idle.
I tested the connector and found with key on there was 13 volts to the IAC valve, so possibly a sticking valve. A year or two ago I changed the IAC valve, trying to solve a problem. Did not use an OEM part. So most likely my replacement part is made in China and is failing. Also for the last year or more, the noise at the air intake has been overly loud. When I clamped down on the IAC hose, the air intake noise went to normal.

The DLC pins tested at 13 volts (battery charger was on). I am thinking that the Rinda Tech component is failing, but that is a guess. Therefore I was not able to determine the IAC numbers.

I may have the original IAC valve, if not will buy another. Any recommendation on brand? An automotive unit I looked at was set up slightly different internally, so I doubt that would work.

 

[alldodge]

Clamping the vacuum hose to the IAC smooths the idle is different. Something is telling the IAC to open more when it shouldn't. Could ring out the other 2 wires going to the DLC. If they are there then you would know its the cable to lap top

 

[c1steve]

So connect the meter to the two center wires on the DLC, and there should be a voltage signal of some sort?

 

[alldodge]

Not sure if its readable with a meter. My thought was use the ohm scale and check the wires from the ECM connector to DLC pins

 

[c1steve]

Okay, will check that.
Also the fuel pressure reading was 44 psi at all times, so that is good.

 

[c1steve]

Also I noticed that the engine appeared to be running very rich, when the problem occurs. Thinking that a temp sender may be faulty so perhaps I may replace the MAP sensor.

 

[c1steve]

I changed the MAP sensor and the engine runs much better, but the problem was not completely solved. I have never been able to troll between 600 and 1,000 rpm. Now I can run at 700 or 800. During a half hour sea trial the high idle issue did appear once, but it was not as dramatic as before. Idle went to 1,000, where before it would go up to as much as 1,500 rpm.

I suspect that the engine is running rich when this happens. After last fishing trip, the last half mile the engine ran extremely poorly, was barely able to get into the slip. I pulled the plugs, and they appeared to have been running in a very rich environment.

So something may be causing excess fuel to be delivered, and perhaps the computer raises the rpm to compensate?

 

[alldodge]

Really need to get a scanner and look at real time data. Can still be a few things, ECT, TPS, vacuum leak, injectors, something else

 

[c1steve]

Will work on the computer scanner Saturday. I had the injectors overhauled 600 hours ago, every time I checked the TPS numbers, they have been normal. Vacuum leak, I agree is a possibility. Looked for one before, could not find one.

For emissions, does this have much? There is no cat converter.

 

[alldodge]

Should not be any issue with emissions

 

[c1steve]

What is ECT?

 

[alldodge]

Engine coolant temp
Your motor has one for the gage and one for the ecm. The ECT is for the ecm

 

[c1steve]

I trolled all day for bluefin tuna, engine ran great at 600 rpm for hours at a time. But after cruising for even a few minutes it would not come down to idle/600 rpm. When it stays at the 800+ rpm, definitely is running rich. Will be replacing the TPS and the ECT temp sensor as soon as I have them.

Would the automotive version of the ECT work?

 

[alldodge]

Yes, if it meets same spec. The isdue us knowing which one