Possible vacuum leak on a V6

[NZ-Kevin]

I recently bought a boat with a V6 mercruiser in it. I haven't got it running well yet. The first time I took it out I could only get the engine to 2000rpm even at full throttle when driving the boat, but it revved fine out of gear. The fuel pump seemed small to me for the size of engine, so I have replaced the pump with a larger marine fuel pump. Tried the boat again today and it was improved, but still not running well (but it was improved on the first run). Lag on acceleration and still only 2500rpm max. I also noticed on the vacuum gauge that at idle it was fine, but as soon as it was under load the gauge fell to nearly nothing. It was fine when reving the engine ut of gear, but as soon as it had to work to power the boat the vacuum fell -and it stayed down low until I stopped the boat and the engine was out of gear. I took this to be a vacuum leak, probably between the inlet manifold and the engine (as there is some silicone selent here it makes me think this may have happened before).

Before I take the intake manifold off, which is a biger job than I realised as it involves taking the distributor off as well, does me diagosis sound reasonable or is there anything else that could cause these problems?

 

[Howard Sterndrive]

Re: Possible vacuum leak on a V6

"if your only tool is a vacuum gauge, all your problems will be vacuum leaks"
I don't know why you are fixated on that aspect, but kerosene and a brush or a propane enrichment test can find a vacuum leak...

check simple stuff like impeded linkage movement, a plugged fuel filter, or tank vent (sometimes people put tape over before washing boat) or dirty flame arrester atop the carb first
then onto checking fuel pressure to the carb and making sure the timing advance weights in the distributor are advancing (if equipped)
then on to the anti-siphon valve to look for debris stuck in it
double check firing order and compression test the motor
also watch for moisture in cylinders during comp test
make sure shift ignition interrupt switch isn't misadjusted or hanging

a great reference here:
http://forums.iboats.com/showthread.php?t=295070

 

[dubs283]

Re: Possible vacuum leak on a V6

ditto to what howard says, would also add:

have you done a full tune-up?? (filters included)

let folks know exactly what you are working on, i.e. engine model/serial numbers - this will aid greatly in helping you with your situation

i will emphasize what howard says again, check your fuel system from fill to carb, taking note on anti-siphon valve and vent, and filters, filters, filters

 

[NZ-Kevin]

Re: Possible vacuum leak on a V6

I'm not fixated with the vacuum, but the gauge was showing a problem, so I thought I should look into it.

Linkage movement is OK, fuel filter is new, tank vent is OK, air filter looks clean (fitted with "pancake" type air filter in palce of the mercruiser item -but it is marine rated). Fuel pressure should now be Ok as I have just fitted a new fuel pump, not sure about balance weights -will remove the cap to see if it has them. But the engine revs to higher revs without load so I don't think it is directly rpm related, it just struggles when under load. Where is the anti-syphon valve? is it inthe fuel tank, as the pipe comes from the fuel tank through the filter to the fuel pump then straight into the carb. Compression was checked by marine engineer before I bought the boat. What is the shift ignition interupt?

I don't have a serial number. I have checked where the manual said it should be, near the starter motor, but there isn't one. Is there any other way to identify the engine, or could the serial number be somewhere else? I believe the V6 was fitted to replace the original straight six in about 2000, but have no real information about the change.

Thanks for the advice so far, I'm workin gthrough the list in the link to make sure I haven't missed anything.

I would be very happy to avoid taking the inlet manifold off, not a five minute job.

 

[achris]

Re: Possible vacuum leak on a V6

I'm not fixated with the vacuum, but the gauge was showing a problem,....

The gauge did not show a problem. The problem is you are applying a 'car' mentality to a boat engine. In a car once you reach cruising speed you back off on the throttle to maintain that speed. In a boat you don't back the throttle off. At cruising speed you have far more throttle open in a boat than you would in a car, so you have less vacuum. Your vacuum gauge is indicating that there is NO vacuum leak.... Look for your problem elsewhere....

Chris........

 

[achris]

Re: Possible vacuum leak on a V6

interesting... it told me I could post, something about waiting 30 seconds, so I did, and now I have 2 posts the same....

 

[NZ-Kevin]

Re: Possible vacuum leak on a V6

The gauge did not show a problem. The problem is you are applying a 'car' mentality to a boat engine. In a car once you reach cruising speed you back off on the throttle to maintain that speed. In a boat you don't back the throttle off. At cruising speed you have far more throttle open in a boat than you would in a car, so you have less vacuum. Your vacuum gauge is indicating that there is NO vacuum leak.... Look for your problem elsewhere....

Chris........

Excelent, thanks for that. So it is normal to have no vacuum (or, virtually no vacuum) once the engine starts to work in a boat. You're right -I was applying car thinking, where this would indicate a problem. I'll work through the list in the link in the previous post. I think there was a fuel issue with the old pump, it just didn't seem big enough. The new pump should have cured that, but there could still be other problems.

When checking the oil level should the engine be level? I was thinking of putting a spirit level on the top of the engine and adjusting the jockey wheel until it was level, in case I got a false reading when I checked it before.

 

[Bondo]

Re: Possible vacuum leak on a V6

Excelent, thanks for that. So it is normal to have no vacuum (or, virtually no vacuum) once the engine starts to work in a boat. You're right -I was applying car thinking, where this would indicate a problem. I'll work through the list in the link in the previous post. I think there was a fuel issue with the old pump, it just didn't seem big enough. The new pump should have cured that, but there could still be other problems.

When checking the oil level should the engine be level? I was thinking of putting a spirit level on the top of the engine and adjusting the jockey wheel until it was level, in case I got a false reading when I checked it before.

Ayuh,... The physical size of the fuel pump is irrelevant,...
It's got Nothing to do with it's volume, nor pressure...

From yer 1st post, I'm thinkin' the Carb is loaded with crap, especially the High speed circuit...

 

[achris]

Re: Possible vacuum leak on a V6

... I was thinking of putting a spirit level on the top of the engine and adjusting the jockey wheel until it was level....

SERIOUS overkill... As long as the boat's not upside down you should fine.

+1 with Bond-o on the carb needing a clean out... I'd also do a full tune as well, new plugs, and leads, cap and rotor, and set the timing... When they're right and you've still got a problem (most likely) then pull the carb. But even with the carb in A1 condition, if the ignition systems not right.......

Also, engine of unknown history, I'd be pulling the elbows and doing a thorough inspect of them and the manifolds.... and while the elbows are off, check the exhaust shutters....

Once you have the engine tickity-boo, start on the leg.... ... And you though owning a boat was all peaches and cream.... ....

 

[NZ-Kevin]

Re: Possible vacuum leak on a V6

Once you have the engine tickity-boo, start on the leg.... ... ....

Closer than you think, the universal joints are getting noisy and will need changing. Plus all new gaskets and seals while they are out, and I have a new bearing so I'll fit that as well.

But, for now, I'm concentrating on the engine.

 

[Howard Sterndrive]

Re: Possible vacuum leak on a V6

I would be very happy to avoid taking the inlet manifold off, not a five minute job.

is the oil milky? I can't make a connection from any of the symptoms to removing the intake manifold.

good tip above about checking Y pipe for old water shutters blocking the exhaust

 

[NZ-Kevin]

Re: Possible vacuum leak on a V6

OK, thanks to some very helpful input I think the thread should really be titled "problem reaching operating rpm on V6" as it seems pretty certain that the problem is not a vacuum leak.

I'm going with my original thoughts that it is a fuel issue, and it was definately improved with the new fuel pump so this seems reasonable.

I checked the carb first, and it all seemed clean. I sprayed carb cleaner through the jets just to check. The air filter looks OK, I can clearly see daylight through the mesh so it doesn't seem clogged. This was before I fitted the new fuel filter (I know size doesn't relate to flow, but the original wasn't a marine filter and was mounted in the wrong place so replacing it was a good thing).

the setup on this boat is a stainless steel under floor fuel tank, with a short length of rubber hose to the fuel filter, which is mounted next to the fuel pump then a rubber hose steraight to the carb. So not many connections to have problems with. The take-off from the tank is near the top art one end, so I presume it must have a pipe inside the tank to get fuel from the bottom of the tank. Would the anti-siphon valve be on this tube? The hole in the tank is quite small, about 1/2", and there is no other access into the tank (apart from the fuel gauge sensor -which is quite a long way from the outlet). Would removing the hose from the outlet and unscrewing it from the tank reveal the anti-siphon valve?

Looking at possible exhaust restrictions does the fact that the engine willl rev without load but not under load mean this is unlikely -I'm thinking the same volume of exhaust gasses will need to be expelled for a certain rpm regardless of engine load.

Thanks for all the advice so far, you've pointed me in a much better direction.

 

[NHGuy]

Re: Possible vacuum leak on a V6

The anti siphon valve will be at the outlet from the tank. I'd remove it & clean it up with some carb cleaner, also put a probe in the anti siphon and exercise the spring loaded ball to be sure there is no sticking. I used the straw shaped plastic of a ball point pen so I wouldn't mar up the ball.
You can test for a blocked vent by running with the fuel cap loose. If you check your new fuel filter and find debris you might have to change it repeatedly til the fuel cleans up.
If all that turns out to be good you might have to go deeper than some carb spray. Might have to rebuild the carburetor, there are small passages that can clog. Some of them can't be cleaned without a teardown.

 

[NZ-Kevin]

Re: Possible vacuum leak on a V6

The anti siphon valve will be at the outlet from the tank. I'd remove it & clean it up with some carb cleaner, also put a probe in the anti siphon and exercise the spring loaded ball to be sure there is no sticking. I used the straw shaped plastic of a ball point pen so I wouldn't mar up the ball.
You can test for a blocked vent by running with the fuel cap loose. If you check your new fuel filter and find debris you might have to change it repeatedly til the fuel cleans up.
If all that turns out to be good you might have to go deeper than some carb spray. Might have to rebuild the carburetor, there are small passages that can clog. Some of them can't be cleaned without a teardown.

Thanks, I need to remove some fuel from the tank to do that, as it's full at the moment. I know the vent is fine as when the tank was filled at the petrol station about two pints of petrol drained from the lower bung hole. When I checked it out the vent tube had been made from clear plastic tube (not fuel resistant) and it had gone hard and become disconnected from the vent -so any excess petrol was just pouring out of the vent. I've now replaced it with the correct grade of fuel resistant hose.

I found a carb service kit amongst the odds & ends from the previous owner -maybe he thought it needed stripping.


Red face time, I'm working on the engine now and the distrubitor cap and rotor were so coroded I'm surprised it ran at all. When the shops open after the holiday I'll replace them, but for now I've cleaned them and will try the boat again. This could explain a lot, or it could just eb another thing to fix onthe way to completely solving the issues -if I get the boat out today I'll let you know how it goes.

 

[NHGuy]

Re: Possible vacuum leak on a V6

oops, I thought I read you had good ignition stuff. Glad you found something obvious.

 

[achris]

Re: Possible vacuum leak on a V6

... -I'm thinking the same volume of exhaust gasses will need to be expelled for a certain rpm regardless of engine load.....

Completely incorrect. With no load on the engine it will rev freely with just a tiny amount of throttle opening. Under normal load half throttle will bring you into the 2,000rpm area, half throttle with no load will have the engine exploding.

I have a fuel flow gauge in my boat. At 3000rpm with no load it flows about 6lph, in gear and moving at 3,000rpm it's using about 25lph. That equals a lot of extra exhaust gas....

 

[NZ-Kevin]

Re: Possible vacuum leak on a V6

I have a fuel flow gauge in my boat. At 3000rpm with no load it flows about 6lph, in gear and moving at 3,000rpm it's using about 25lph. That equals a lot of extra exhaust gas....

Aaah, now I see -I hadn't taken pressure in to consideration. If the engine works hard it requires more force to push the pistons down, so the gas will be at a higher pressure. When this is released into the atmosphere it will be at atmospheric pressure so there will be a larger volume. I had just taken the actual volume of gas the cylinders displace, which remains the same for any given engine speed -but now I see my mistake. thanks.

 

[dubs283]

Re: Possible vacuum leak on a V6

the distrubitor cap and rotor were so coroded I'm surprised it ran at all. When the shops open after the holiday I'll replace them, but for now I've cleaned them and will try the boat again. This could explain a lot, or it could just eb another thing to fix onthe way to completely solving the issues -if I get the boat out today I'll let you know how it goes.

stop...

don't even run the engine until you complete a FULL tune-up, and you haven't even told us what you have!!!

e.g. 4.3L V-6 (common) or the 3.8L merc (rare), what i'm getting at is do you have electronic or conventional ignition??

cleaning igniton parts is a waste of time, if you are tearing it all apart, just replace the components - until you (and we) know what you have its just a waste of time guessing at the main problem(s)

take the time to understand exactly what you have for an engine and drive, model and serial numbers are a huge help!!

taking care of a boat is NOTHING like a car, rarely can you just hop in yer boat turn the key and go, don't think you can dump some seafoam in the tank and rip around

knowledge and experience are your best allies in boating, have patience!!

 

[NZ-Kevin]

Re: Possible vacuum leak on a V6

stop...

you haven't even told us what you have!!!

e.g. 4.3L V-6 (common) or the 3.8L merc (rare), what i'm getting at is do you have electronic or conventional ignition??

cleaning igniton parts is a waste of time, if you are tearing it all apart, just replace the components - until you (and we) know what you have its just a waste of time guessing at the main problem(s)

take the time to understand exactly what you have for an engine and drive, model and serial numbers are a huge help!!

It has been frustrating to me that there does not seem to be a serial number on the engine. The more I look the more I think it was a "custom" installation, probably well done at the time, but makes it difficult to know what anything is. The drive is the original 1976 item, with the V6 replacing a straight six in around 2000. I'm sure I will eventually work out what everything is once I've worked on it enough (and asked enough questions here). It is electronic ignition, well it has a breakerless distributor anyway. I'm used to looking after older engines on boats, the last boat had a 1971 Johnson 60hp outboard on it -which required quite a lot of work in the time I had it.

 

[achris]

Re: Possible vacuum leak on a V6

Sounds like you have a real bastard child there. To help work out what engine you have, go to a forum called 'hotsixes'. (I have the correct URL on the other computer which I can't get to at the moment) Look for a thread that explains all the variants of the V6s. It's well written and will help you identify exactly what you have. Merc were not using the V6 until about 1983, so if you have a 1976 leg, the engine is not original. The leg (if that's still original) will be an MC-1, so that much we can help you with, so far.

Found that URL -> http://hotsixes.com/index.php?option=com_content&view=article&id=14&Itemid=12 Good luck