unable to idle

[ALAG3]

I am having an idle problem with my 1998 Mercruiser 3.0. I recently began having problems getting my engine to idle. It seems to run fine above idle and at WOT, but when I return throttle to idle position, it usually dies. When in neutral just above idle, the idle is not constant, it gradually goes all over the place, between 700 and around 1700. When it stalls, I have to give it full throttle and it is hard to start. When it does start, there is black smoke from exhaust momentarily.<br /> I also recently had a rise in temperature which began around the same time as the idling problem, so I replaced the water pump, but it did not help. After a 3/4 to full throttle run of a couple of miles, the temp begins to climb to around 170, and when I slow down to just above getting on plane, it climbs above that. If I immediately stop and let it run in neutral just above idle it immediately cools back down, but the temp use to be around 150, and now stays around 160. Can the idle and temp problems be related? Where do I start? Thanks for any help!

 

[dfwyatt]

Re: unable to idle

Well ther black smoke means that it is flooding somehow, thats all I can tell you. This could be anything from certain cylinders not firing properly b/c of bad wires/plugs/rotor/cap, or it could be getting to much gas.

 

[waterone1@aol.com]

Re: unable to idle

Please start by telling us if this is fuel injected or carburated ? also, is the engine raw water cooled, or is there a heat exchanger (fresh water cooled). When you said that you replaced the water pump, was that the raw water pump in the outdrive, or the engine circulating pump on the front of the engine ?

 

[ALAG3]

Re: unable to idle

The engine is carburated, and is also raw water cooled. The water pump I replaced is in the outdrive. Also, along the lines of the carburator, there is a clear plastic tube running from the fuel pump to the carburator which had a crack right where it goes into the fuel pump. I replaced it with new line, which is clear and I can see fuel traveling through it, but it also has air in it. Is this normal? This line does not appear to be the primary fuel line, which is metal.<br /> I just recently purchased the boat and replaced the plugs, but not cap, rotor, wires. The boat was running fine for a few trips, but then developed these problems. As mentioned in the above reply, should I also replace these items?<br /> Thanks

 

[waterone1@aol.com]

Re: unable to idle

As far as I know, the clear plastic line between the fuel pump and the carb should NOT have any fuel in it. If there is fuel in that line, your fuel pump has a ruptured diaphram and needs to be replaced.

 

[ALAG3]

Re: unable to idle

Is this just a vent, or what?

 

[ALAG3]

Re: unable to idle

I was just looking through the online manual and found the following statement about the clear plastic line..."fuel pumps are equipped with a sight tube. Any evidence of fuel in the sight tube indicates a ruptured diaphram and the fuel pump must be replaced immediately". Waterone 1 , you are the man. It sounds like this may be my idling and running rich problem, but I wonder if it could also be my overheating problem? Any other advice would be great!

 

[waterone1@aol.com]

Re: unable to idle

My first reaction would be that the fuel pump problem would not cause the overheat, however it is possible. The only way for the pump to affect operating temperature would be if the bad pump is not supplying enough fuel to the carb when running hard, in that case a lean mixture could be causing it to run hot. I would replace the pump, then see how she does on the lake. Keep us posted.

 

[rbezdon]

Re: unable to idle

One other way is that it is common that if the diaphram ruptures it leaks fuel into the oil degrading the oil severly. Check and change the oil after replacing the fuel pump.

 

[ALAG3]

Re: unable to idle

I have ordered a fuel pump and it will be here tomorrow....hopefully it is the right one. I'll keep you posted.

 

[waterone1@aol.com]

Re: unable to idle

ALA, remember this needs to be a MARINE pump, not an automotive one. By the way, Vatter has an excellant point, some of the leaking fuel may have found it's way into the oil. Great time to change oil and filter.

 

[ALAG3]

Re: unable to idle

Yes, I went to 5 marine stores yesterday looking and nobody had one, then a friend of mine suggested NAPA carries a marine line of parts. I called and they looked it up by "Mercruiser 3.0" and swear it is for marine application. I'll see what the box says when it comes in today. I came across a topic somewhere that says how dangerous fuel components can be if they are non-marine. Thanks! Hopefully, once the boat is running right, it will also calm down the overheating issues!

 

[ron7000]

Re: unable to idle

this seems really stupid maybe it's just me,<br />but the purpose of a "marine" component in this case fuel pump is supposed to not let fuel spill or give off vapors because it would be hazardous since it's in an enclosed area. We all agree on that i think.<br /><br />So why do the marine fuel pumps even have a vent? Especially on the 3.0L, they run a stupid yellow 1/8" hose from the pump vent to the carb, without a hose clamp even. It's just press fit on both ends, and takes very little to pull it off. Where's the safety in that?<br /><br />"marine" component <br />USCG approved <br />ABYC standards <br /><br />Now it's cool because there's no hose clamp and if I need to do carb work that's less I have to do to take it off,<br />and if the fuel pump fails, then I can immediately diagnose the problem by the presence of fuel. But, where's the safety in that? Wouldn't it be better if it's just sealed? It aint to freakin hard to diagnose a bad fuel pump.<br /><br />All automotive mechanical pumps, and even electrical, are sealed. They have no vent tube. So if their diaphram splits, or the vane in an electric pump fails, they just don't pump fuel.<br />buy automotive!

 

[deputydawg]

Re: unable to idle

Here is my opinion. <br />Your carb is causing a problem. It is allowing too much fuel in, probably the float. The erratic idle you first described is because it is building up fuel in the float causing too much to dump into the engine raising the idle. The black smoke is simply ecxess feul burning out. You have to open the throttle wide when starting because too much gas is dumping in. Opening the throttle allows more air in evening the mixture.<br />The overheating may be caused by poor running causing the engine to labor. It could be caused by excess feul not allowing the engine to burn out of it's designed range. <br />The hose should be a vent hose. Automibiles have a vent to get rid of excess gas when the carb floaty overflows. Boats recirculate this overflow. This hose probably means your float is too full. <br />Try a carb kit first. Don't try to fix the heating problem until you get it running properly, I believe you will find this problem solved when you get the furl metered properly.<br />It may simply need a carb cleaning and float adjustment, it may need a complete kit. Either way take the carb apart carefully and see what it looks like.<br />Good luck and let us know.

 

[ALAG3]

Re: unable to idle

Well, I got the new fuel pump and it is exactly like the old one, including being manufactured by Carter. The sight tube now has no fuel in it (like it is supposed to be) but after it warmed up good, still running rough. Also the air/fuel mixture screw has little to no effect on how it runs. I think I have found fuel leaking around the base of the carb on the port side of the engine, but did not diagnose for sure because it started raining. Seemed like the side of the engine was wet just below the carb. This would explain the air/fuel mixture screw having no effect if air was getting in from somewhere else. I'll take another look in the next day or so and keep posting. If my carb needs to be rebuilt, is that something difficult or can I do that with the help of the Mercruiser manual? I feel I am fairly mechanical, but have never taken a carburetor apart other than the one on my lawnmower when it was running rough. Thanks!

 

[rbezdon]

Re: unable to idle

It is highly likely your carb is at least a mojor part of the problem. Rebuild it. I suggest you get a rebuilt one. Changing the gaskets, float etc is easy enough but if yours has a lot of time in it chances are the throttle bushings etc are worn too. Some carbs even have warping issues rebuilders correct. Heck, you usually get a warranty too.

 

[deputydawg]

Re: unable to idle

The carbs are easy to rebuild. There should be instructions with the kit. A handy hint, use a digital camera and photograph as you go along. The linkages are a puzzle to get back together. <br />When setting it, be sure to double check the float specs. <br />The idle mixture screw should only effect operation at low idle. It could be having little to no effect because of your leak, OR because the float is stuck or saturated. The leaking gas around the gasket could be a symptom or a cause of trouble. If the float is allowing too much gas in, the gas will have to have a place to flow to. Either the clear tube back to the pump, out of the top of the carb, or into the engine...probably all 3. This can be a cause of the gasket leaking. OR the gasket could be sucking air causing a lean running condition. First bet would have to be too much fuel flooding it.<br />Before choosing to rebuild, check the gasket surface on the carb to make sure there is no warp. <br />Have some carb cleaner, a few CLEAN lint free rags, and some compressed air handy if you rebuild. Those carbs must be clean inside to run right.

 

[Don S]

Re: unable to idle

The reason for the vent tube on the fuel pump is if the diaphram in the pump ruptures, fuel will go through the tube and into the carb instead of into the bilge. There is no pressure, so clamps on the clear hose are not needed. If fuel is dumped into the carb from the failed fuel pump, it will cause the engine to run extra rich and smoke. Sure indication something is wrong.<br />The carb NEVER send fuel back to the fuel pump. If the float is bad or the needle valve sticks open and too much fuel is in the fuel bowl of the carb, the fuel is vented into the engine not outside of the carb.

 

[waterone1@aol.com]

Re: unable to idle

You had two different problems, you have solved one (replacing the fuel pump) now it's time to fix the second one (the carb). Read the manual and decide if you want to do it yourself or take it to a rebuild shop (only you can make that decision). We already covered why your fuel pump needed to be replaced and Don did an excellant job clarifying it.

 

[ALAG3]

Re: unable to idle

yep, that definitely fixed the problem in the sight tube, and right now, I am too frustrated to even look at it....I'll give it another go in a few days. Thanks for the help.