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Cold start/idle problems with Mercruiser 5.0L MPI

T

troman9

Member
A few weeks ago I tried to start my 2005 Mercruiser 5L MPI engine after three days docked, and it turned over but stalled. I tried 3 more times, and finally had to run up the throttle to get it to keep running. Once it warmed up, I could bring the throttle back down and it would not stall. I'd been noticing a fluctuation in RPM when first starting up, but this is the first time it stalled. Since then I've been able to start it (in my driveway) and I still hear the up and down RPMs, but it doesn't stall.

I recently bought an engine scanner, and recorded some data while starting the engine. I'm not a mechanic so I don't know what "normal" should look like, but the things I'm seeing don't seem right.

I've attached two plots of the data here. The first plot shows battery voltage, engine rpm and oil pressure. You can see some fluctuation 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?

Thanks in advance for any help,
Tom

enginescan_rpm.jpg

enginescan_spkadv.jpg
 
Forgot to mention, that was the only thing I tried, as it seems to be a common problem. I replaced it after the stalling episode, but the charts above are with the new IAC.

I should mention also, there were no fault codes reported by the ECM.
 
Your timing is normal, computer is always looking for the bert idle. At no time can you drop below 11.2 volts, computer will do dumb things.
I hope you are using MCA 1000 for your battey.If you have temp senor out of range, computer will not set code.When started next time put scanner on and see what your computer puts your temp at....
Steve
 
IAC and filter are common problems for this sympton. Steve touched on a commonly overlooked cause; battery condition. ECMs don't like voltage drops. MPIs love to hunt when voltage issues are present. Which ECM are you running? As Steve stated, Merc's battery recommendation is 1000 MCA or 800 CCA. Good luck and keep us posted.
 
OK, lots of good thoughts. So the voltage clearly dropped below 11.2 Volts in the beginning, even though though it was brief. I can believe that would do strange things to the computer. Would it affect things after the voltage returned to normal?

To answer some questions:

I did replace the IAC filter the same time as the IAC. Also replaced the gasket.

The ECM is a PCM555.

I'll have to check the batteries - I'm not sure about their rating. Is MCA "marine cranking amps" or something like that? I'm familiar with CCA from working on cars.

I still have the data I collected from the engine which includes the full telemetry. I can only plot 3 things at a time though. The two temperature readouts are "coolant temperature" (it's raw water cooled, BTW) and "manifold air temperature". Looking back at the data, the "coolant temp" started at 51.3F and after starting, gradually increased to 100.4F before I turned it off. This included throttling up to 1600 RPM for a short time. "Manifold air temp" sat at 55.35F for a long time, then gradually went up to 57.18F. I ran it for about 4 minutes. I can post plots of the temp data if you think that would help.

Tom
 
Bump to add some battery info.

"As Steve stated, Merc's battery recommendation is 1000 MCA or 800 CCA. "

I have two batteries. They are Nautilus NC-27, which are rated at 730 MCA. With the battery switch I can select: 1, 2 or 1+2. It sounds like to start the engine, I need to always select 1+2 to get > 1000 MCA, correct? I don't remember what position the switch was in when I had the scanner recording.

The batteries are at least 2 years old. Is it possible they need to be replaced this soon?
 
The fact that there was a voltage drop to 8 volts indicates there's a problem. It could be battery, wiring or hardware related. Check main battery cable connections at batteries and engine. 2 year old batteries or new batteries for that matter can have issues. Improperly maintained batteries will die in no time. One bad cell will cause problems. Have the batteries properly tested. This will indicate a battery issue or illiminate it as a possible culprit so you can move on to another area.
 
Woodie: I appreciate the feedback. I think the cables etc are OK. I don't see any corrosion or loose connections. I did another test: I topped off the batteries with distilled water, and then charged them last weekend. Today I ran the engine and took some more data. The plots are attached. The voltage dropped again down to 9V while the engine was cranking. I still see RPM and voltage fluctuation, but then I ran the throttle up and back down to idle. The RPM and voltage seemed much more well-behaved in the latter two cases.

Could the batteries be weak enough that they're a drain on the alternator so when the engine is cold there's not enough power to go around? Then when the engine warms up, or maybe the batteries have had a chance to charge a little, things smooth out?

Trying to decide whether to try and replace them. I know you recommend getting them tested, but I'm not sure where to go to do that.

start.jpgthrottleup.jpgwarmidle.jpg
 
You could try taking the battery to a local auto supply or garage for a proper load test. For a small fee (some do it free), you'll know for sure.
 
what i see you have a temperature senor problem...that why so hard to start....it's tilling ECM motor doesn't need a rich mixture to start... and yes this doesn't show up as code most times... unplug try:eek::eek:
If you had a vacuum leak, your IAC would be near zero


Steve
 
Last edited:
Steve,
Sorry, I didn't follow your suggestion about what to try... are you saying unplug the temp sensor? I think there are two: the coolant temperature (which seems to be OK based on the plots above) and the manifold air temperature. If I unplug one of these, how can I tell if they need to be replaced?

Tom
 
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