outrunner

I've been searching the threads, troubleshooting different systems and can't seem to isolate this issue.

I have a 2008, 170 hours 525 efi that will start missing or stumbling at idle. Sometimes it will just come back to life on its own after 5-10 seconds. Sometimes an increase in throttle will assist in clearing it up. No issues whatsoever at higher RPMs. It feels/sounds like a cyclinder or two stops firing.

Last time putting it on the trailer, I couldn't get it to clear up. Ran it in the yard with the water hook up and it ran fine until about 10 minutes, then started its stumble. Wiggled wires, ran codes, tested IAC with diacom, watch fuel pressure on the rail (increased to 43 as idle bounced around 500)...nothing would clear it up. Then all of a sudden it comes back to life. Couldn't get it to repeat.

Here's what I have tried so far: ran codes with the diacom - nothing. Check fuel pressure on the rail - 40-42 at idle. Pull off the vacuum and increases to 43-44. All fuel filters changed. Compression all cylinders 150. Fuel tanks vents checked. Ohm reading of plug wires 8-12. Plugs replaced.

Going to hook up some spark testers next to verify if its a spark issue when its starts its stumble. Other than that, I'm running out of ideas.

Anyone have any thoughts?

C.I.M.

You might have a leaking header. It fills up with water at idle and puts the cylinder out, but will clear itself out once the rpms start increasing. Try pressure checking the headers.

outrunner

[QUOTE=C.I.M.;4105802]You might have a leaking header. It fills up with water at idle and puts the cylinder out, but will clear itself out once the rpms start increasing. Try pressure checking the headers.[/QUOT
Thanks for the response. I recently did leak check the headers with no issues.
Fired it up this weekend again, had 4 spark checkers on all the odd cylinders waiting for it to stumble, couldn't get it to duplicate. I guess I'll take it out the the lake again and have all the test equipment available to hook up if/when it comes back.

HyFive578

iTrader: (3)

Just curious... how did you test the IAC? Seems to me like the most likely culprit based on what you described... I have one that's going bad at 125 hours...

TooLateVTEC

iTrader: (1)

Seems like youve narrowed a lot of things down, I would look into removing the IAC and clean it out.

Might be a piece of trash in there blocking the pintle from opening/closing correctly causing the idle to screw up or theres build up of crud in there.

outrunner

I tested the iac with the diacom unit. It appears to just command it open and closed to raise and lower the idle.

What are the symptoms with your iac going bad? Does it act/sound like its missing? Last time I was at the lake and was bringing it back off plane, I could hear it start to miss around 1500 rpm. I'm guessing that may be when the iac starts to come into play?
My other thought was i have an injector going bad, but i would think that would show itself at high rpm too, not just idle. Fuel rail pressure stays after shutdown, so that seems to rule out a leaking injector.

HyFive578

iTrader: (3)

Noticeably rougher and jumpy idle than the other two motors and you can hear a whistle coming from it when I advance the throttle slightly. I haven't tested it, but I've ordered a replacement anyway.

FIXX

iTrader: (2)

i wouls pull the headers and check them..again..the crack will open after they get hot and fill the cylinder with water and water wont fire...those headers are known for cracks..

outrunner

Pulled the IAC and cleaned out tonight. Some build up of crud in there. Hopefully, get it out to the water this weekend and see how it goes.

HyFive578

iTrader: (3)

Curious to know how you made out? I changed the IAC on my center engine this weekend and the idle is now noticeably smoother. Starts cold at around 1000 RPM and as it warms up, drops down around 700-800 but nice and steady sound...