Mercruiser 5.0 MPI intermittent lumpy idle and no throttle response

[shaunsm_uk]

Hi All,

First time poster and long time lurker so be gentle
I have a Bayliner with 5.0 MPI which has been sitting for some time and recently had a full service done, occasionally on startup it will have a surgy/lumpy idle and no throttle response at all as if its in some sort of limp mode, shutting it down and starting up seems to fix it, the shop that serviced it mentioned it might be the IAC, I can understand it messing with idling and perhaps not but it doesnt stall, would this cause the throttle response issues I'm seeing?
When it runs properly I cant fault it.

Thanks in advance.

 

[alldodge]

Howdy

Best way to start a discussion with your motor is let us know your serial number. This lets us know the configuration.

Do the folks that did the work have a scanner to check the ECM?
What was done during the full service work?

My first guess is a loose connection or fuel system issue

 

[kenny nunez]

I had a intermittent rough idle on my Donzi, it turned out to be the throttle position sensor.

 

[shaunsm_uk]

Thanks for the replies, I'll get hold of the serial number this evening and let you know,
Service involved oil, plugs, compression test 150-155 across all 8, manifold and riser check, B3 bellows and shift cable and a few other minor things

Throttle position sensor sounds like an option to change, theyre not expensive and might explain why I cant get it to rev past idle when has a lumpy idle.

Apparently no codes but that was before they did the work, unfotunatley a pain to get the boat to them so might try see if I can borrow a code reader.

 

[alldodge]

Don't know how good your mechanic is, but them mentioning the IAC does not give me a warm fuzzy about them

 

[scoflaw]

Scan tool would tell you wether TPS and IAC are in normal range. It will also tell you about previous faults

 

[QBhoy]

Ah....my specialty!

There are a number of things that could cause this, outwith the obvious fuel quality and delivery etc. Having had the same and other 5.0mpi problems. Below is my opinions !

it could be the IAC....but unlikely because this will throw a code and an audible beeping alarm.
It could be the TPI, but unlikely because this will also throw a fault.
It is most likely your distributor cap and rotor. This will not throw a fault code or alarm at you. It will just do exactly what you describe and worst case not start at all. This happened to me. It is especially likely after winter storage when damp has creeped into it.

Next candidate would be the IAC, it may not have reached the stage of completely being buggered, but on its way. Dead easy to replace too.

Ah...the mpi. Great when they are running right and problems fixed or recognised. The Scottish forever changing climate has tested mine to its fullest. Thankfully I have a hold on things these days.

 

[QBhoy]

Other long shot is that someone has put a performance cam in it like mine. In that case it will seem lumpy at tick over anyway.

 

[shaunsm_uk]

Hi QBhoy,

Thanks for the info, Ive taken off the IAC as its the easiest part to get to, turns out the little valve is stuck solid and didnt even move with the help of a screwdrier so I've replaced it, the new one moves easily by touch as expected, and starts up easier than before but time will tell as the fault was intermittent, if it does misbehave again I'll get a codescanner connected and see what it tells me, definitly not a perfomance cam, its smooth as anything when its running properly, will definitly check the dist cap and rotor too, probably good to keep a spare.

Also found out some bright spark had disabled the audible alarm which I've wired back in

Luckily climate is pretty good here, being in Perth Australia so no need to winterize

Will shout if it happens again!

 

[QBhoy]

Ah. The disconnect alarm would make more sense to it. I’d think you have the problem nailed. Yeah Defo keep a spare Diz cap. Many many stories of people getting stranded with a non starter because of the cap on the MPI.
SW pressure sensor is another weak point. At some point that and the above other favourites will fail.

 

[shaunsm_uk]

Just an update, taken the boat out a good few times now with multiple start/stops and no more issues, starts up quicker, when I first changed the IAC sometimes would idle high and take a while to drop down to a decent idle speed but I gather the ECU needed time to calibrate but all good, thanks for all the input.
Definitly need to get a spares list together!

 

[QBhoy]

Excellent !