Mercruiser 5.0L cold start - part duex

[troman9]

I posted awhile back the problem I had with my Mercruiser 5.0L MPI engine stalling when trying to start it cold. As a quick recap: it turned over, but it seemed to rev up and down for awhile until at a low rpm it stalled. This happened three more times. Finally I had to run the throttle up to get it to hold. Once it was warm it seemed OK and idled fine.

I did replace the idle air controller (IAC) as a guess, and at least in my driveway, it seems better, but I'm not convinced that all is well. I still hear some up and down rpm-ing at idle, but it doesn't stall. I have not yet had it out on the water again.

I took a recommendation to get an engine scanner, and I've now pulled some data from the engine, but not being a mechanic, I don't know what "normal" looks like. First, there are no trouble codes. I recorded engine data during engine startup, and I've attached plots of some of the data here.

The first plot shows battery voltage, engine rpm and oil pressure. You can see some oscillation of the engine RPM, but it's not huge. It goes from the high 400's to mid 600's. There is also a huge voltage drop during startup. Of course that's when the current demand is the highest, so I would expect some voltage drop, but here are my questions:
1. Is the rpm oscillation normal, or should it be flatter around 600 rpm?
2. Is the voltage drop to around 8 volts normal, or is that maybe an indication of a weak battery?

I attached the second plot where I replaced oil pressure with spark advance. The spark advance moves around quite a bit during idle - from about 2 degrees to 15 degrees. So I'm wondering if this is normal?

I hope I'm just being overcautious and the engine is OK, but the boat is single engine so I'd hate to have it die on me out on the water.

Thanks in advance for any insight...

Tom

 

[dubs283]

Re: Mercruiser 5.0L cold start - part duex

upon first start-up with the engine cold, yes an MPI is going to have that fluctuation of idle rpm

that is the ecm compensating for cold water temp upon cold start

you say once the engine warms up it idles fine?? perfect

try this, start the engine cold and advance the throttle to about 1000-1200 rpm, let it sit for one minute then return to idle - see if your idle is more stable

just because the engine is MPI dosent mean you don't have to keep a higher idle for warmup

have you done anything else recently besides replacing the IAC?? like tune-up, plugs, wires, etc...??

 

[troman9]

Re: Mercruiser 5.0L cold start - part duex

Dubs - yes I also replaced the plugs, and I had a bad fuel pressure regulator replaced. I haven't done anything else. The engine is 2005 vintage and has 80 hours on it.

 

[nauticool]

Re: Mercruiser 5.0L cold start - part duex

Hi Tom,

Thanks for letting me know about this thread.

Dont worry, after a bit of research there is no filter in our fuel pump just the pre filter/water seperator

I also thought about the low volts issue on start up as the smartcraft/engine guard will cut the engine when it detects low voltage. Im going to try a new battery this week.

I do exactly, and have done for the past year, what dubs283 says. Advance throttle very slightly on startup and run at 1000rpm for 5 mins then she'll be fine all day. surely these engines should start from cold with no advance if they were working properly, just as any efi V8 would in a car?
It would be interesting to see another engine of the same year start from cold. Maybe its a characteristic of this model. Has yours ever been ok from cold as mine has always been this way since i bought it two years ago.

Glenn

 

[troman9]

Re: Mercruiser 5.0L cold start - part duex

Glenn,
I've owned the boat since Feb last year, and it definitely hasn't stalled on me before. There's alot of anecdotal evidence on the internet that cold start problems are associated with this engine, but it does seem like it's fixable. I'm working with the battery theory currently also. I'm going to try and charge them and then collect some more engine data this weekend - see if the idle rpms flatten out any.

Incidently I've got another thread going on marineengines.com, under the mercruiser sterndrive forum. They've also suggested the battery, and said it needs to be 1000 MCA (marine cranking amps). I need to check mine.

We'll get this eventually, I'm sure.

Tom

 

[nauticool]

Re: Mercruiser 5.0L cold start - part duex

Just got my hands on a op manual. Non dts models min is 750cca, 950mca or 180ah. Not sure what my mca is but I know they are only 94ah each.
Will be replacing this week and hit the water next week to test.

Glenn

 

[ENSIGN]

Re: Mercruiser 5.0L cold start - part duex

start with the basics and check the compression

 

[nauticool]

Re: Mercruiser 5.0L cold start - part duex

Tom,

Changed battery at weekend from 94ah 680cca to a brand new fully charged 110ah 1000mca and mine is now so much better. It still hunts around 100rpm, 500 to 600, when cold but never stalls.

Think I'm going to use the new battery for my domestic and get the next one up for engine start.

Hope this helps, how are you getting on?

Glenn

 

[troman9]

Re: Mercruiser 5.0L cold start - part duex

Glenn,

Very glad to hear you're making progress! I also tried a new battery - a 1000 MCA Optima. I'm still seeing the same "hunting" you describe, but I saw that with the old battery. Maybe that's just normal for this engine?

With my engine scanner, I also think I'm seeing an abnormal manifold pressure. I posted a question about that here on the forum but I'm striking out trying to get feedback.

I guess my plan now is to keep using the boat this summer, and unless things get worse, I'll just bring it into a mechanic in the fall for normal maintenance and have them check all of the ECM stuff at that time.

Tom

 

[Pete104]

Re: Mercruiser 5.0L cold start - part duex

Why don't you guys do lunch?!

 

[Irish 5.0 MPI]

Hi Folks,

All of you have described precisely the problems that I am having with my 2005 5.0 MPI with Bravo 3 on a 220 CCR Crownline.

I am literally pulling my hair out and we have very limited resources in Mercruiser specialists here in Ireland.

I have tried ALL of the same things as yourselves except for new batteries although the ones I have are quite powerful (I don't have the specs to hand as I type).

Just wondering if you ever got any final conclusions ? I am getting worried at this point that it is ECM related.

Sincerest thanks for any update possible.

Albert.