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Clarkitect

Rough Idle and Smoke

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Clarkitect

Last weekend, after running well all day, my boat started to develop a rough idle and would pour out black smoke but only at idle. We boarded the rest of the day and the only time I have this problem is at idle and when I give it throttle the problem disappears and it will run great from about 1000 rpm's and up. It looks like it is flooding at low rpm's but what would cause this? I replaced the cap, rotor, spark plugs and fuel filter at the start of the season (about 5 trips ago). I called the local dealer and they can't even look at it until July 11! I don't mind running it as is but I don't want to cause any major damage!

Thanks,

Mike

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SacRiverRat

Welcome to the site!

Where are you from?...

I don't know what to tell you about your engine, but wanted to be the first to say HI! ;)

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Baddog

Welcome.

Strong sugestion because it happened to me . . . . .

Check the water temp sensor. If it is bad it will tell the engine that it is always cold and therefore needs to be in "choke" status which will result in smoking hugely at idle and eventually put you in a "vapor lock" like situation where the only way you can restart a warm engine is to put it in neutral and give it wide open throttle. The part is ~$30 and really easy to change but if you take it to your dealer and that is what the problem is, with computer time etc. you are looking at a $150-$200 bill.

Tell me more about what's happening and be specific.

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Clarkitect

SacRiverRat: I am from Modesto, CA and thanks for the welcome!

Baddog: The engine runs like normal above about 1000 rpms, it will pull me up on a ski without missing a beat but as soon as I bring it back down to an idle it will start to smoke and run rough. I can run at full throttle or with a skier on without a problem so there doesn't seem to be a problem under load just at low rpm's. I can still put it in gear and idle around but when I do it acts like it wants to die but as soon as I give it throttle it runs as good as new. The oil pressure, water temp and voltage are all reading normal. When your water temp sensor went out, did your water temp gauge still work? You are right, it does act like it is in choke mode. There are so many different sensors I don't know where to start or how to test them.

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MoonDawg

Perhaps a leaky injector.

Check your plugs and try to isolate the cylinder, then exchange injectors with another cylinder and see if the problem travels.

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DNA'er

Could also be a stuck carb bowl float...I'm guessing that your 99 Response is carbed.

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Clarkitect

My engine is a fuel injected monsoon. My first thought was also a bad injector so I also pulled out the spark plugs when I got back from the lake to see if one cylinder was running rich and they were all tan and very consistent in color.

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gooddog

Did it run like that before you replaced the parts you mentioned? Sounds like the advance may be stuck.

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pup

Have a look at the air filter. If it's dirty the engine would starve for air & run rich.

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SunriseH2OSkier

My money's on baddog. If I'm not mistaken, there is a separate temp sensor that runs your gauge vs. the one your ECM uses to determine how to fuel. If that one is bad (stuck cold), it will enrich the fuel delivery at all speeds. You'll notice it most at idle, because their isn't much airflow happening. When you open the throttle and get the air moving, the engine will still be running rich, but it can handle the extra fuel better.

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Hman442

A fellow might check the small vacuum line from the intake/throttle body to the map sensor (a kinda rectangular 1.5x2.5 or so box with a vacuum hose & a wiring plug in it on top of the engine), along with the fuel pressure regulator vacuum hose(attached to the fuel supply rail). If either of those hoses are ajar or cracked, the engine will run rich like described. Or - one of those sensors could be bad.

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MoonDawg

If the plugs are tan then it is time to check the MEFI for codes...do you have a CodeMate ?

Edited by MoonDawg

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smooth as glass

H20 is correct . There are 2 temp sensors and could be the problem. One on top of block is obvious the other on the front is hidden by hose and bracket, front left looking to stern.

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RTS

The fellows above are correct in that there are two temp sensors, one for the guage and on for the ECM. Once your engine is warm, pull them off one at a time. When your guage goes dead, the other one is for the ECM. BTW, it should be just to the right of your thermostat housing, looking at the engine from the front.

Now that you have identified it, and if you're curious as to whether it is sending a constant "cold" signal to the ECM, here's how you can test it.

You'll need a multimeter, and you won't even need to remove the sender from the block. Here's the readings you should be getting across the terminals:

68 degrees = 3520 Ohms

104 degrees = 1800 Ohms

158 degrees = 467 Ohms

212 degrees = 177 Ohms

By the way, the service manual also says your IAT (Intake air sensor), if defective, can cause suymptoms consistent with lean or rich operating conditions, so you may want to check that, too

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RTS

Here's a little more info...if you can get your hands on a scan tool, code 14 will tell you your ECT (engine coolant temp) sensor voltage is above or below normal values.

It says a sensor producing low output voltage will cause the ECU to deliver too much fuel, causing symptoms consistent with an excessively rich condition.

If I were you, and the dealer couldn't get to me for a while, I'd go pick up the scan tool at a local auto store so at least you'll know if you've got any codes.

If you do this, and find any codes, just post them here or PM me and I'll let you know what the code means.

Good luck fixing the problem.

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SmoothWaterMan

Coolant sensor perhaps, also good chance of IAT or even TPS. An IAT will more likely produce that senario than the coolant sensor.

Peter

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Clarkitect

Thanks for all of the help everyone! Well I tested the water temp sensors and the one on the front left of the engine did not give me a reading at all. It looks like there might be another one on the right side of the engine as well just bellow the intake manifold and it gave me a reading of about 300 ohms. For these readings the boat was cold and sitting in the garage, I will have it out tomorrow to get some more accurate readings. Where is the IAT? Is it the sensor right in front of the TPS? Is there any way for me to test this?

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RTS

Table to check your IAT (intake air temperature) sensor

Temperature Resistance (ohms)

104....... 1459

95........ 1802

86........ 2238

77........ 2296

68........ 3520

Below 68....put the boat away and worry about this when summer gets here

Edited by rts

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