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Matthew John
Matthew John, Marine Mechanic
Category: Boat
Satisfied Customers: 30
Experience:  Marine Technician, Certified in MERCRUISER AND MERCURY OUTBOARDS (Current), EVINRUDE, SUZUKI, YAMAHA, HONDA.
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Have a 1998 Centurion with a MIE 350 Mag MPI Runs at start

Customer Question

Have a 1998 Centurion with a MIE 350 Mag MPI Runs rich at start up and seems to run a little rich when at temp Smooths right out of the coolant sensor is disconnected so replaced that but didn't solve problem What should I try next?
JA: Sometimes things that you think will be really complicated end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Is there anything else the Marine Mechanic should be aware of?
Customer: Think that sums it up
Submitted: 2 months ago.
Category: Boat
Expert:  Matthew John replied 2 months ago.

Hi my name is***** to JustAnswer ! I will be assisting you.

Just a couple of questions;

What are the Engine Operating Hours?

Do you know the service history?

Have you experienced or come across and other engine related problems or performance related problems since the boat was last serviced?

Have there been any alarms come on since the last service or occurring now?

These are just a couple of things that can really help in troubleshooting your problem that you should be aware of before we start to possibly get into lengthy troubleshooting.

Just going on what you say you might be experiencing with your engine, we can go with that and first look at the IAC. That is the Idle Air Speed Controller. Lets check the filter! It's located inside the Throttle Body or on the Port side of the throttle body. Let me know the Serial Number of the engine and I can let you know which one it is. In the mean time, I can send you a pic of what the filter looks like and the location of the filter in models with filter inside Throttle Body.

Check how bad the filter is blocked, replace it and we can take it from there.

Make sure the spark plugs are replaced at around 300hrs.

Let us know how you go with it!



Customer: replied 2 months ago.
250 hrs Just bought the boat this spring have only put 20 hrs on it Start up has been the main problem the rest of the time it is hardly noticeable Have not changed the IAC yet bu have a new one to put on it
Expert:  Matthew John replied 2 months ago.

Ok! Great! Yeah, as you can probably appreciate, we can be limited with our Fault Finding without the Computer Diagnostic Software to see live data, test IAC and other components. We can just do our best with what we have and you could probably look at getting a service at the 300hr mark or before next season, depending upon when it was last done.



Expert:  Matthew John replied 2 months ago.

Hi there! Just a quick follow up call to see how you went... Feel free to hit us up at any time!

Also, please don't hesitate to let me know of any concerns you may have in the future!

Be sure to hit my Star Rating ! ! ! ! !

All the best!!!


Customer: replied 2 months ago.
I will not have time to try what you have told me till Friday Will let you know how it goes
Customer: replied 2 months ago.
SN 0L003841
Expert:  Matthew John replied 2 months ago.

Ok! Great!

Customer: replied 2 months ago.
Cleaned IAC filter R&R IAC still starts and idles very rich till it warms up Have been disconnecting the temperature sensor which if I understand correctly reverts to a default temperature of 174 degrees then it idled fine Once it's warmed sufficiently reconnect it and it idled fine Any thing else I should check?
Expert:  Matthew John replied 2 months ago.

Hi there mate,

It sounds to me like it needs a good run, get it out there and so how it goes for the day. Maybe it has not been used for a while or maybe it it functioning normal...

Is the boat in the water or on the trailer when you are running it. Sometimes running an engine out of the water can give a false sense of really how it should be running. I hope some of this info is useful for you! Feel free to get back to us with any other information you can think of.



Customer: replied 2 months ago.
Ran it hard for several days pulling tubers and skiers but in morning at cold start still floods until it warms up then seems OK Once it's warmed up it starts with no problem The only to get it warmed up first start of the day is to disconnect temp sensor which tells it it's already warmed up then it idles fine When temp gauge is up to 150-160 plug the sensor back in and it's ok till next cold start
Expert:  Matthew John replied 2 months ago.

Yeah hi,

I would see if you are getting 5 volts at the Coolant Temp Sensor plug without engine running and ignition switch on. If you don't have 5 volts at the plug...Trace the wire back in the loom to through the harness to other connections and wriggle the wiring harness and connections and see if the 5 volts at the plug appears.



Customer: replied 2 months ago.
Have 5 volts at the where you plug it in to the sensor at thermostat housing
Expert:  Matthew John replied 2 months ago.
The other thing I would be able to suggest is for you to try running the Iridium spark plugs. If you have the standard type in at the moment, you can swap them out and the Iridiums should last you over another 300hours.
Customer: replied 2 months ago.
Already replaced plugs when I first got the boat because one was fuel fouled and caused a miss That's when I noticed unplugging the temp sensor would lean it out enough to get it warmed up Then it could be plugged back in and seem to run ok So the problem would seem to be running too rich at cold start Warm start is fine
Expert:  Matthew John replied 2 months ago.

Are you able to check that spark plug again and compare to the rest?? If that one is fouled up again compared to the others, you may need to get that fuel injector checked, serviced or replaced. Aside from this... You would really have to connect the Computer Diagnostic Software to monitor the live engine data and watch the actual ECT Engine Coolant Temp reading from cold to warm and see if the circuit is reading through out the range. The other thing to be checked on the CDS is the Duty cycle % of the IAC valve. The IAC can also be tested through it's opening range with ignition on engine not running.