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2001 350 mag ski mpi Gen rough idle running rich

C

centurioneclipse

New member
"I have been chasing this prob

"I have been chasing this problem for awhile. The boat runs fine above 1500rpm(planed out). Although it is really using way more fuel. I obtained a service manual. It says it should Idle at 600rpm. It idles a little below and rough. If it is run hard(say 36-3800rpm thru the slalom course) the idle is rougher and slower when you go to idle. I checked for error codes and there were none(only code 12). It has the MEFI3 ECM. I have verified timing(in service mode). Replaced plugs. They verified it is running rich. I went through the IAC functional test. (that is the 1st step after OBD and visual inspection in the "rough, Unstable, incorrect idle fuel injection troubleshooting chart. This test has you record RPM at warmed up idle, then turn off engine, disconnect IAC connector, restart engine and record RPM. If RPM is NOT > 200rpm MORE with IAC disconnected and all 4 iac harness terminals pass light blink light is says to replance faulty IAC. Now what bothers me is how rich the boat is running. can a bad IAC cause it to run rich? I thought it just controlled idle speed? Can any of you experts concur that a bad IAC can cause this? Since it failed the test should I just buy the damn thing like it says?"
 
"try checking your coolant tem

"try checking your coolant temp sensor reading.
it may be off but within expected threshold as to not trip a code. while in live data mode
also monitor intake air temp. if equiped.
what are your fuel trims reading? (long and short term)? are your o2 sensors switching? should be about 700 ms or so.
an IAC only cotrols idle, not mixture. Check its duty cycle or percentage in live data mode, and get back to me with all your data."
 
Rich at cruise or under power

Rich at cruise or under power sounds like dirty injectors or too high pressure in the fuel rail to me. Your fuel rail should run 40 psig +/- 2 psig at all load conditions.
I would also run some injector cleaner mixed in the gas through it if you haven't done so recently. Can't hurt.

Rod
 
"Another thing; is your intake

"Another thing; is your intake air restricted in any way? Is the flame arrestor clean? Is the exhaust restricted in any way.
Any restriction to air intake or exhaust exit will cause an overly rich mixture, since there are no oxygen sensors in the exhaust and therefor no feedback for the ECM to fine tune the mixture.

Rod"
 
"thanks for your responses guy

"thanks for your responses guys. Kurt, I do not have equip to run diags you want. I see where if I disconnect temp sensor the ecm will supply a temp of 175 after 2 minutes. I try it and see what it does. I have been runnning injector cleaner in it for some time now sea foam and now mercury quickleen. My ecm is not throwing any error codes. I have to assume if fuel pressure was high or low enough for what I am seeing it would throw the appropiate code. checked hose going from ThrottleBody to cool fuel regulator and no fuel in the hose. This indicates it is working properly. I have removed the flame arrestor and same symptom. if it was exhaust restriction it would effect my top speed and it runs great at high speed. I have already ordered the IAC. I hope you are wrong and the IAC not working properly can cause a rich fuel mixture. Another symptom is that in service mode it should idle at 1200rpm and it idles just under 1000. Any other thoughts?"
 
"unfortunatly, the ecm does no

"unfortunatly, the ecm does not monitor or regulate fuel pressure.
hook up a guage to the test fitting and monitor
pressure, then unhook vacume supply to regulator
and pressure should rise some.(simulates wide open throttle), as stated you should see around 40 psi.+/-.
are all your plugs showing rich?"
 
"the iac only regulates intake

"the iac only regulates intake air bypassing throttle body for idle control. all it is, is a solenoid valve, controled by ecm."
 
"An IAC that is stuck partiall

"An IAC that is stuck partially closed can cause a rich idle condition. The ECM provides fuel based on the assumed IAC position, but it won't know if there is less air being provided than it thinks, resulting in a rich mixture. But if your getting 1000 RPM in base timing mode, that would seem to indicate that your IAC is working.

A couple of other things, you ECM does monitor fuel pressure, and should set a code 61 if it's too high, and a code 62 if it's too low. I would check it anyway, the cool fuel systems have been known to clog return line screens resulting in high fuel pressure and rich condition. Also, your ECT could have shifted values which will not set a code."
 
Correct me if I am wrong but a

Correct me if I am wrong but an IAC is just an Idle Air Control motor. I have removed a number of them from car & truck engines not idling smooth and cleaned the gunk out of the throttle body passages w/ a Qtip dipped in TB cleaner to get a smooth idle. Can't the same be done on the boat engine? I also swab the throat of the throttle and both sides of the throttle w/a heavy shop paper towel and TB cleaner to get it clean. It doesn't take much gunk around the throttle plate to create a rough idle. It's part of my annual winter prep service on my trucks and works every time.
 
"Dave - thanks for posting. R

"Dave - thanks for posting. Remember, the IAC failed the functional test big time. my idle was 150 rpm lower with the IAC connector disconnected and was suppose to be over 200 rpm higher.
Guy - Good common sense stuff. I will do that. what is a good throttle body cleaner? Never seen anything advertised as a TB cleaner.
Kurt - Thanks. the mefi3 does monitor both low and high pressure and throw the 60/61 codes. If IAC doesn't fix I will purchase/rent a gauge and check it. If it is high what do you do to fix that???"
 
"The ECM controls idle speed i

"The ECM controls idle speed in two ways, ignition timing and IAC movement. In base timing mode the ignition timing is locked, so the ONLY means the ECM would have to accelerate the engine to 1000 RPM is with the IAC valve. I don't think the IAC is your problem.

At this point I would try replacing the ECT or MAP sensor (input from these two sensors is critical to proper fuel/air mixture) rather than the IAC, or invest some money in a scan tool."
 
tim;
If all else fails you ma

tim;
If all else fails you may have to get your ECM reflashed by a Merc dealer. It seems odd that there are no fault codes stored (as you said in your first post) with it running that poorly.

Rod
 
"Tim:

There are a number of

"Tim:

There are a number of name brands available at auto parts stores. Just make sure it says THROTTLE BODY CLEANER and not CARB cleaner. TBs in autos are coated with something to reduce dirt buildup that carb cleaner will eat. I will assume the same for boat engines.

There is also one that comes in a jar that has small sponges and an applicator. You just run the soaked sponge in/out of the TB which is easier than a paper towel. Use blue paper shop towels because they are very durable. I haven't found it yet locally but it is advertized on a car repair show called "two Guys Garage".

SEA FOAM also makes a spray product that works very well because it is thick and does not evaporate much. I've used it on my 2 cycle lawn engines to clean carbs, pistons and cylinders of the brown sludge buildup."
 
"In going thru the fuel system

"In going thru the fuel system diagnostics section of the service manual, "the TP signal is one of the most important inputs used by the ECM for fuel control and for most of the ECM controlled outputs". Yet none of you has mentioned it as a possible failure point. Why?
I think I have pointed to idle to strongly. The larger symptom is running very rich at idle. Also, passenger side exhaust is cool while driver side gets hot when run above 34mph@3600rpm. I have an IAC and pcv valve arriving today. I am going to replace plugs with Mercruiser recommended NGK's. I am not at all confident this is going to fix the problem. But functional test said to replace it so I am. If it doesn't fix the prob I will re-run that test just to see if that did in fact fix something. I am going to focus on TP and then if necessary MAP sensors next. I will let you know results as they become available."
 
"Tim, please do keep us inform

"Tim, please do keep us informed if you get it resolved. It is difficult, as you realize, to even first-hand diagnose some problems. Even more difficult via internet where we may misinterpret what is reported - but we all do give it our best based on previous experiences. What you report back helps us all learn.
Believe me, none of us know it all (as reported by my wife)!"
 
"Maybe the TPS wasn't ment

"Maybe the TPS wasn't mentioned because you didn't mention a hesitation or flat spot in acceleration that would be consistant with TPS failure.

You really should consider investing in a scan tool rather than just replacing stuff, but if your going to replace something, you should replace the ECT sensor first. Remember that the temp you see on your gauge may not be what the engine is seeing because the gauge uses a seperate sensor."
 
""passenger side exhaust i

""passenger side exhaust is cool while driver side gets hot when run above 34mph@3600rpm."

A plugged manifold/riser or water feed outlet can cause one side to run hotter. Start w/a laser temp. gun, $35, and read the temps. along the cooling hoses and then the mans. and risers. Compare one side to the other. You'll find it."
 
"The IAC did not fix the probl

"The IAC did not fix the problem. I don't have any error codes. So what can cause a rich fuel mixture? From easiest to functionally test to hardest along with a functional test description.

Dave- a test for ect senseor is to disconnect it and after 2 minutes the ecm will substitute 175%F. It didn't make a difference. sometimes when crusing in the 2900rpm range I will notice some mild surging, more of it dropping off and then coming back to orig rpm, maybe 100 rpm or so. could this be a TP symptom? Also, the harder the boat is run the more it seems to "load up". I begin to lose some of the hole shot pulling up the skiier.

Thanks again for all your input!"
 
"You can test the Throttle Pos

"You can test the Throttle Position Sensor in two ways; for resistance and voltage.

Disconnect the sensor and hook up a digital ohmmeter to the two pins. Slowly turn the throttle from min. to max. and watch the reading change smoothly. If it jumps to "0" or "out of range" for a micro second, it should be retested and if it reoccurs then replace it.

If resistance seems OK the plug it in w/ign. on and probe both leads on a 1 volt scale. Same procedure as before. A quick change in voltage (-/+ spikes) in either directions; replace."
 
"Guy, There are 3 connectors

"Guy, There are 3 connectors for the TP. Black, Blue, and white or very light color. I'm assuming with ignition on we check ohm's and then voltage on 2nd test??? Pls provide more detailed instructions."
 
"All. The troubleshooting in

"All. The troubleshooting in the service manual just doesn't work. I tried the voltage tests for the TP, map, ECT, and IAT. All were high(over 4 volts) which according to the manual pointed to that particular circuit being bad. They can't all be bad!!! My engine is extremely clean. No corrosion etc. The test for the ECT was particularly vexing. The test did not involve the sensor itself at all. And yet it said to repair circuit or replace sensor!(when the voltage was high). I'm just going to have to take it in and bend over."
 
"Let us know. <[img]"http://ww

"Let us know.
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"One of my 350 mag mercruisers

"One of my 350 mag mercruisers is running hot, 200 degrees at 2000 rpm, 190 at 1600 and 180 at 1400. I changed the impellar and put in a new thermostat kit from mercruiser. What temp. should the engine be running at? It is a fresh water cooled system, i was thinking of opening up the heat exchanger to look for clogs, any thoughts or suggestions."
 
Bob;
Please start your own to

Bob;
Please start your own topic.
Piggybacking on someone elses wil not get you the best responses.

Rod
 
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