Idleing problem

[venetrex]

I have a 2002 21v with the 350 mag mpi towsports engine. I had an issue with the cap and rotor, I replaced and engine is running great until later today. The engine at idle cut out for a second, beep and then go back to idle. I actually have the scan tool, I get no error codes. However when the engine is running I am getting a communication error. Which makes me think the ecu is possibly on its way out. At $1600, thats not good. Could I be missing something? WOT is okay and it was okay at cruise.

[dogbert]

I have a 2002 21v with the 350 mag mpi towsports engine. I had an issue with the cap and rotor, I replaced and engine is running great until later today. The engine at idle cut out for a second, beep and then go back to idle. I actually have the scan tool, I get no error codes. However when the engine is running I am getting a communication error. Which makes me think the ecu is possibly on its way out. At $1600, thats not good. Could I be missing something? WOT is okay and it was okay at cruise.

Before replacing, you may want to disconnect and reconnect it from the wiring harness.

[chpthril]

I'd start with a good cleaning of the IAC and throttle plate and see.

What Kinda data will the scan tool give you? CTS, IAC counts, INJ pulse, fuel trim, TPS voltage?

[R&T Babich]

... However when the engine is running I am getting a communication error....

Also, try reseating the main harness connector near the transmission.
The way it was routed in our boat there was tension on it trying to pull it apart.
I added large wire ties on either side to support it better.

[venetrex]

Still doing it

It will actually give me a beep, cuts out and comes back. It is random in how often it occurs. Sometimes it will go 20 min or more at a good idle, and others it will do it again and again. My scan tool will tell me iac and fuel flo, rpm, reset codes. There are NO codes except for when it happens a communication error. It is like it resets. Has anyone had this trouble?? I need to fix this.

[dogbert]

How's your battery and alternator? Others on here have reported that the Merc is very sensitive to voltage fluctuations. I experienced this when my alternator went out and I ran down my battery. It would beep once and die.

[venetrex]

If it were only that simple

I had a squeeling belt this year and replaced the alternator, welded the bracket, I also replace the sea and circulating water pumps, unrelated, but I changed. My voltage moves around between 12.5-13.9 volts depending on rpm, wondering if this could be part of it. My battery is new too. I am working on this right now. I have checked every harness connector on the engine and under the dash. Damn, there are a lot of them. The beep really seems like the computer is resetting, like when you start it. My idle speed is low, 600rpm. But that is what it is supposed to be. Iac motor is new last year, That is definately not the prob. I think it is electrical. Explain about the sensitive voltage issue if you can. It runs totally fine at cruise and makes full power at WOT. No cutting out then. Only between idle and 1800rpm. Give or take. When it cuts out with the engine revving up a little above idle, when it comes back, the speed drops, more thing acting like the computer is resetting.

[R&T Babich]

... My voltage moves around between 12.5-13.9 volts depending on rpm, wondering if this could be part of it. ...

The voltage at idle would be battery voltage (around 12.5 v) and when the RPM starts to pick up the alternator will take it up to the 13.9 v.
Even with the newer solid state voltage regulators the alternator doesn't really output much until you get above idle.

[dogbert]

Ok, so it's not the alternater or battery.

If your motor has more than 400 hours on it, it may need new plug wires.

[venetrex]

Problem solved

My uncle hastled me and kept saying my new cap and rotor could be the culprit. I hesitated but reinstalled. And I for once was happy he was right. I bought mercruiser original for 85.00 + 29.00 for the rotor. Funny thing stamped on box made i CHINA. Chinese CRAP. I want my money back from the dealer. I wonder if aftermarket is in USA? I really hate mercruiser not tige. This issue made my go and buy a new boat. Why mine is for sale. I almost think I should keep it. I'll add another thread on my comparo.

[dogbert]

I think I've said this before on this site, Mercruiser distributor caps and rotors need to be replaced regularly and can cause the boat to either not start or stop running at odd times. I don't think I've experienced it causing an alarm.

Glad you got it figured out.

[venetrex]

Worst PArt

What made it so difficult is it is the new part that failed. I was told it has to do with at lower rpm the voltages are much higher and its easier for the spark to jump in the cap. Once it jusmps once it burns a microscopic hole in the insulation rendering the cap useles. I only had the cap one day before it failed.

[mneal]

I have a Volvo Penta I/O and last year I replaced cap and rotor as maintenance (boat was running fine, just maintenance) and 1st trip out after about 30 minutes and boat was @ operating temperature for a while it started to miss and run badly. Reinstalled one cap and rotor and problem went away. There can be hairline cracks from manufacturing defects, mis-handling prior to you buying it etc. Stuff happens, cant really blame any manufacture about it, it is not like Mercruiser, Volvo, Marine Power etc. make their own parts. I always take old parts with me after replacing anything for 1st couple trips. Distributor caps, wires and rotors are ancient way to distribute ignition to combustion engine, especially in a humid marine environment. Soon marine stuff will joining the 90's and have coil packs and all this goes away. Why cant we trade newer ignition technology for the ridiculous catalytic converters? Next my 2 stroke weed whacker will be outlawed, yay that will solve the global warming issues.