MEFI-4 Idle Hunt at Start Mystery

[Delorean6855]

Am struggling with a mystery.

I have twin 2004 crusader 6.0s. On any start, hot or cold, the engine hunts for idle with wide swings. It settles eventually or if I give it a bit of throttle, it settles back quicker. It also happens when I throttle down in neutral, it hunts a bit before settling down. (RPMS are stable at anywhere over 1k). I changed the MAP sensors with no effect. I changed the IAC valves, it just made it worse funny enough. I really don't want to spend more on parts or buying a scanner. Are there any other usual suspects I missed? These engines perform flawlessly under load...this issue is more being picky but it drives me crazy.
Any ideas? I was thinking about the coolant temp sensors but I am not getting any CEL and the issue occurs regardless of whether the engine is hot or cold.

Thanks in advance.

[km1125]

That would be very typical of an IAC issue. Does it happen on both engines?

Coolant temp sensors would likely not light a CEL unless they were opened or shorted. You can take some measurements from them at various temps (or just 'cold' and 'hot') and compare them with specs to see if they're OK without having to replace anything.

[bud37]

It could be looking for air, but you would really have to see how the iac is reacting.......is the throttle body itself clean ( iac port as well, do you have the filter) around and near the throttle blades....look in the throttle body throat and see if the throttle blades are sticking or there are marks from binding. Clean the flame arresters as well.

Also is this on both engines and has it been this way or just started ? The other thing to do is make sure all your grounds are clean and the plug wires are away from any electrical wires, grounds included, batteries good. Have a look in the distributors and make sure they are clean in there and the grounds are good.

Is there any engine stalling on return to idle with the throttles ?

Thats enough for now, see what you come up with.

[Delorean6855]

Thanks bud37. Ya, both engines have always behaved this way...though worse after replacing IAC's, especially starboard engine. Starboard engine once fired up and then stalled out. It's also probably worse than the port engine overall. Upon starting, it likes to jump to 2000, then down to 400, then back up to 1500 etc... No stalling on return to idle, just minor hunting swings around 300 RPM.
Batteries are good, replaced a year ago...was hoping that would have helped. Wires replaced too, I'll verify they are clear. I'll check the other items you mentioned this weekend.
Thanks for the input...this has been driving me crazy!

km1125 - I'll check the coolant temp sensor with the multi meter. Thanks!

[bud37]

Source of the post Ya, both engines have always behaved this way...though worse after replacing IAC's, especially starboard engine. Starboard engine once fired up and then stalled out. It's also probably worse than the port engine overall. Upon starting, it likes to jump to 2000, then down to 400, then back up to 1500 etc... No stalling on return to idle, just minor hunting swings around 300 RPM.

Considering this there are a few things to discuss, but check the easy stuff first, work thru it a step at a time......... There was an older thread about Mercruiser Mefi 3 discussion, much of it by me....as I had much the same kind of problems and the work thru and resolution......try a search here and give it a quick read. There are a number of design functions in these ecu's, I am not completely sure but the Crusader version may not be that different to the Mercruiser version of control parameters.You may not completely avoid getting someone to connect up to watch the IAC function and counts at stable fully warm idle...around 40 counts.... among many other parameters.

Let us know what you find.

[Delorean6855]

So… neither of my engines has a PCV valve, despite the parts manual saying otherwise. (Head exploding). Throttle bodies are clean. I should say issue is less prevalent when engines are warm. I had thought otherwise but guess I am insane. Going to bring my multi-meter next week and check the coolant temp sensors… Side note: at 1000 rpms underway, is it normal for a slight flux of +/-20 rpms? Or am I a little to anal wanting perfect stability in RPMs?

[tomschauer]

Do you run them often and do you have good fuel? A bad injector or injectors will cause this.

[bud37]

Source of the post So… neither of my engines has a PCV valve, despite the parts manual saying otherwise.

They need those valves to run properly, are you sure ?.....all engines have that system, needs to be rectified.....are the hoses still there ?

Take a pic of the top of the engines and post it if you can.

[Delorean6855]

Try to run them once a week...the fuel should be good. Still haven't checked the coolant temp sensor with the multi-meter. I'm thinking more that it may the issue. Last weekend, when restarting from hot, it didn't hunt as much, stabilized fairly fast. Still hunts a bit when dropping the throttle back to idle (in neutral).

Regarding the PCV valve, I checked my engines against the schematic where the PCV valve could be expected to be found...it's just a hose plugged into something that looks like its part of the physical valve cover... nothing that looks like a PCV. I'll take a photo. It's really odd.

[Delorean6855]

Finally getting back to this... so this is where the PCV valve should be...I remove the hose and it's just a hose barb.

nopcv.jpg

Super weird... the 6.0 schematic says the engine should have one.

pcv.jpg

Still trying to get the resistance reading from the Coolant temp sensor...try as I might, I couldn't get the connector off...I'll probably have to wait till I change the coolant because I am sure I'll have to destroy it to remove it.