3.0L mercruiser running cool

sti1471

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I have a 1984 Mercruiser 3.0L. I hooked it up to the garden hose today and tried to adjust the idle and mixture screw. It seemed that no matter what I did, it never ran right. once the choke warmed up and the plate was wide open it still was acting like it was getting too much air no matter where the mixture screw was. so if I took my finger and closed the choke plate so that it was only open about 1/4" then the engine idled much smoother.

I then noticed that on my temp gauge it was only reading about 110-120 degrees after about 30 minutes of idling.

Here is where I things might go downhill!!!. I was making some adjustments, and then started the engine. I was making adjustments for about 2-5 minutes and all the sudden the idle smoothed out and seemed just where I wanted it. I then looked at the hose, and there was no water moving through it. The hose has a kink in it on the grass. I jumped down and unkinked it and water started flowing.

I am pretty sure I will need a new impeller, I will need to remove the outdrive to make sure. I did not smell any burning rubber or any odd smells at all really. Right before I noticed there was no water, my temp gauge was saying 175. SO at least the engine did not get too hot.

Here is my question... Once the engine reached 160-170 it ran much better idle and quick flips of the throttle seemed much better and it no longer acted as it was getting to much air and wanted to die. So could my idle issues be caused by the engine never getting above 115 degrees and that was why it was getting too much air and needed the choke plate to be closed.

I will need to take the lower off and look at the impeller (that I just replaced a couple months ago) and see if it did any damage.

Could my 2 month long battle with my idle issue, and removing and cleaning the carb twice all have been caused by the thermostat not letting the engine get above 115 degrees?
 
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achris

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Are you sure there is a thermostat in there? But, yes, if the thermostat is stuck open and the engine's not getting to operating temperature it will want to run rich... Time to check the thermostat. Also, if you have one, use an infra-red 'gun' and make sure the temp gauge is telling the truth....

Chris.....
 

sti1471

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Yeah there is one in there. I think it might be getting stuck open. I will replace it out. When the boat was running with no water I saw white smoke coming from the prop. However once it got 2 feet from the prop it was gone so it looked more like steam. Also I did not smell anything abnormal so I don't think it was smoke. I will check the impeller but I think it might be OK. Smelling the water inlet fins I don't smell any burnt rubber or plastic so it might be fine.
 

Rick Stephens

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I did pretty much the same thing. A few minutes without water doing exactly the same problem, trying to get a smooth idle on a 3L. I can't blame a kink in the hose - kinked brain. Anyway, the impeller still pumped, but when I took it out it was hard and brittle. Just waiting to fail. The cup had rubber streaks on it too.

Best,

Rick
 

Watermann

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It's a pretty cheap fix to put a water pump kit in it for piece of mind. How long ago was it replaced? if more than a year ago you really should consider it anyway. You may want to check your fuel delivery too especially the fuel filter.
 

sti1471

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Yeah I will replace the impeller for sure. It was replaced last year but the boat has not really went out since then. But I will replace it anyway. I was afraid I might have ruined something in the engine or something worse. But I think I might just need the thermostat and impeller. Fuel filters are all clean and and I just reset the floats and all. It was weird as soon as it actually started running good it was the tamp gauge read 160. Plus if I close the choke plate it runs much better. Only thing I can think of is it is running rich with them temp running low I will replace the impeller, and then check the temp with an ir thrmo then replace tstat if needed.
 

airshot

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I have the same cool running condition on my 1983 3.0, however my engine idles and runs fine. I have purchased 3 thermostats over the past three years
and checked with a temp gun and can only get 120-125 degrees. My engine will not get any hotter than that, don't know what else to do but it runs well and choke opens up so I guess I just run it that way. Checked the gage and it seems fine and tested the 140 degree thermostats in a water pan with a candy thermometer and yep the all open at 125 degrees and yes they are Mercruiser thermostats, only one was another brand.
 

sti1471

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OK so I pulled the thermostat and put it in a lot of water on the stove. Sure enough at 140 degrees it opened and around 120 it closed. So I assume the thermostat is working. So it could be the temp sending unit. However if that is the case then why did the engine run way better when my gauge reached 170 degrees on my dash, and why does the engine run much better when I close the choke plate 85% when the engine is at "normal temp" which my gauge says 115. Even though the thermostat is working is there another reason it is not getting up to 140? Could a bad gasket cause water to pass by? I noticed there is a small hole that allows water to pass through even with the tstat closed that is built into the housing. Could this hole be passing through water and not allowing it to heat up?
 

airshot

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If you can close the choke plate 85% and it improves the engine performance when warmed up then I believe you have a fuel flow issue. Even though you may have rebuilt it something in there is restricting fuel flow, now check fuel pump and be sure you are getting fuel to the carb. If I closed my plate to 85% my engine would load up and stall within a few seconds. If I read your story correctly I am not sure warming up is your problem.
 

sti1471

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I have a fuel pressure gauge and when the engine is running it is saying around 5 psi. I checked the fuel filter in the fuel pump and it is in good clean shape. If I turn the air mixture screw in from 1 1/4 turns out the engine starts to stumble. It seems to run better when the mixture screw is about 3 turns out from seated but still not right. Even when the idle seems pretty good, if I give it gas, it wants to stall unless I increase the throttle slowly. I sprayed carb cleaner around the carb and the idle does not change so I don't think a vacuum leak is it. With the engine off, if I press the lever I can see 2 good streams of gas being shot into the carb, so I have ruled out accelerator pump. I am making all the adjustments with the flame arrestor off and the boat on the water muffs in Neutral but I don't think that makes a difference. I just replaced the points, condenser, cap, rotor. Set the dwell to 31*. Cleaned the spark plugs, confirmed cap was .035 set the timing to 6* at 750 rpm. I can get the idle right but the main issue is when I give it throttle at the carb I can hear the carb sucking air and it has severe hesitation with the choke plate all the way open. What the heck am I missing!!!!
 
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sti1471

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I took the carb apart and took every piece and cleaned them all. I put new gaskets in, check balls. I checked the spring rod and made sure it was not sticking. I set the float levels to spec. When I looked at the power valve it was free and looked new. I can check the jets to make sure. They are not dirty or clogged. But I triple checked everything during tear down.
 

sti1471

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I was looking at the carb and noticed something. All of the screws on the outer edge of the carb are tight. But there are 2 screwsscrews in the center of the lid that seem to be stripped and are not tightening down. They are the screws thatthat are close to the venturi valve. Could these 2 screws cause an issue like this?
 

sti1471

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No Title

These are the 2 screws that are not tightening down on the carb lid. If every other screw is tightening down to spec and these 2 are not, will this cause the issues I am describing? Maybe it is not allowing the vacuum so the power valve rod is not working or something?

If this would cause the issues, then I need to drill and re-tap the holes to get them to tighten down. However if this would not likely cause the issue, then I will look elsewhere and re-tap them later.

Thanks guys for all your knowledge. I am dying here as my season here in Utah is running thin!
 

Rick Stephens

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One of those screws is supposed to about a 1/4 inch longer.

I don't know if those particular screws would cause your problem, never had that exact problem. I'd be tempted to put a kit in the carb though. Not that your cleaning might not be enough, but if one part is not to spec, you'd get the performance issues you see.
 

sti1471

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if one what is not to spec. Besides setting the float levels what else is something that I need to adjust to spec. everything else seems like it is a fixes position / design on the carb. With my season running down and also my "boat play money" I can't really afford ot throw money at parts that might not fix the issue.

Is there some sort of test(s) I can do to kind of narrow some of this stuff down?

I noticed that 1 of the 3 screws that hold the venturi cluster down is on the verge of stripping and is not tightened down as tight as the other 2. But I would think if anything that would cause too much fuel, not a fuel restriction problem. I might need to take it to a shop cause I am running out of ideas.
 

alldodge

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From your post number 6

Yeah I will replace the impeller for sure. It was replaced last year but the boat has not really went out since then. But I will replace it anyway. I was afraid I might have ruined something in the engine or something worse. But I think I might just need the thermostat and impeller. Fuel filters are all clean and and I just reset the floats and all. It was weird as soon as it actually started running good it was the tamp gauge read 160. Plus if I close the choke plate it runs much better. Only thing I can think of is it is running rich with them temp running low I will replace the impeller, and then check the temp with an ir thrmo then replace tstat if needed.

My 2-bits worth --- Your having problems with it idling and also an issue that it may be running cool. Running cool will have little effect on overall idle. Your gauge may just be the issue as you previously mentioned. You can adjust the mixer screws and have little effect and I'll will guess that unless they are turned all the way in. My suggestion is to get another carb, it is money but everything I'm reading leads me to the indication that the carb is damaged.
 

sti1471

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Alldodge,

The idle mixture screw has a great affect on the carb idle. If I set the idle mixture screw 1 1/4 turns out from seated the boat runs, if I turn the mixture screw more than 1/2 turn in from that, then the engine starts to stumble. If I back it out to the original 1 1/4 turns, it will recover but the engine is still shaking a bit and rpm needle jumping a bit. If I continue to turn the screw out, the RPMs go up and the engine seems to be pretty good around 2-3 turns out from seated any more out than that and the engine starts to stumble. However when I am happy with the idle, if I give it some throttle, it still has bad hesitation. If I close the choke plate 85% of the way and blip the throttle, the engine is much more responsive.

I can get the idle to work, that's not the main issue, the main issue is the hesitation when giving it throttle.

I noticed my anti-siphon valve is not working, cause after the boat is turned off, 20 minutes later the fuel line from the pump to the carb is dry, but the carb bowl is still full with gas. But I doubt an anti-siphon valve will cause this, but I am just throwing out everything I have noticed in case it helps diagnose this.
 
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sti1471

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P.S. I pulled the lower unit and removed the water pump. The impeller had a broken fin. I removed the impeller and the rubber still felt soft and responsive. There was no burnt rubber on the side of the housing, so I guess it had some water in there enough to lubricate the impeller. I will replace the impeller and fire the boat up again and see if I can see anything I am missing.

If I have valve rockers that are adjust incorrectly, could that cause an issue like this? None of them are chattering, but I wonder if maybe I over tightened them when i did them last fall.
 

alldodge

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Alldodge,

The idle mixture screw has a great affect on the carb idle. If I set the idle mixture screw 1 1/4 turns out from seated the boat runs, if I turn the mixture screw more than 1/2 turn in from that, then the engine starts to stumble. If I back it out to the original 1 1/4 turns, it will recover but the engine is still shaking a bit and rpm needle jumping a bit. If I continue to turn the screw out, the RPMs go up and the engine seems to be pretty good around 2-3 turns out from seated any more out than that and the engine starts to stumble. However when I am happy with the idle, if I give it some throttle, it still has bad hesitation. If I close the choke plate 85% of the way and blip the throttle, the engine is much more responsive.

I can get the idle to work, that's not the main issue, the main issue is the hesitation when giving it throttle.

I noticed my anti-siphon valve is not working, cause after the boat is turned off, 20 minutes later the fuel line from the pump to the carb is dry, but the carb bowl is still full with gas. But I doubt an anti-siphon valve will cause this, but I am just throwing out everything I have noticed in case it helps diagnose this.

I'll still say it's the carb, and as mention by another previously (Rick) power valve and/or accelerator pump.

Agree, don't see your issue as with the anti-siphon valve

Edit: ever put a vacuum gauge on the engine?
 
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