Mercruiser Alpha One 3.0 V8 Bogs won't go over 2000 rpm

[dmillerss]

My 2003 Glastron starts fine, idles smoothly, but won't hit high speed. .... when I try to accelerate it just bogs. The boat cruises along at idle for hours just fine, but when you try to go past 2000 rpm's it just bogs. Mechanic checked the fuel/water filter and it was fine; he was wondering if there is water in the fuel tank? The tank is full right now; is there an additive I should put in? Could it be the fuel pump? Should I have the tank drained? Or is there something else going on?

thx DMiller

 

[Dave^5]

Is it an mpi or what? Could be the tps giving out if it is an mpi. It tells the computer to add fuel as it senses the throttle open if it's out...to much air not enough fuel makes a bog a scan tool will tell you for sure

 

[Watermann]

Sounds fuel related, some more info on your motor would be helpful. DId the filter get replaced or just dumped and put back on?

 

[achris]

As I have always said, start with the basics, use the results of some simple checks to move in the right direction. Just guessing is a complete waste of time and tell you nothing. And can actually cost you a lot of money in unnecessary parts.

Checking simple things can tell you a lot about what's wrong or, just as important, what's not. The tests are easy, quick and very cheap (mostly free)...

BTW, Dave, TPS is not used in the fuel calculation beyond about 5% throttle. After that, it's done on engine speed, manifold pressure and coolant temperature.

So, the list is as follows, in this order.

1. Compression tests.
2. Check spark plugs, replace if needed.
3. If points, replace, along with condenser and rotor. Check and adjust, if needed, points cam dwell angle (far more accurate than a feeler gauge. Will also tell you if the shaft or cam in the distributor is worn).
4. Inspect distributor cap, rotor and leads, replace if necessary.
5. Check spark quality (needs to be a nice BLUE spark drawn over 1/2").
6. Set timing. Check timing advance is working correctly.
7. Check/change fuel filters and check that the fuel system is not sucking air.
8. Check fuel pump pressure.
9. Check for full opening of the carb. Also ensure choke is opening fully when the engine is warm.

Once you've done all that you'll have a much better idea of where you should (or shouldn't) be looking.

Chris........
(It would also help if you tell us what engine you've got. There was no 3.0 V8. Can we please have an engine serial number?)

 

[dmillerss]

Thank you for the information ....I've been asked for the Engine Serial Number before; it's unreadable on the air filter and was never noted on the documentation. although I do have a Hull ID on the title The engine is a Mercruiser Alpha One 5.0L. Items 1 through 6 were performed just before winterization. The filter was just dumped and put back on; it was checked last year and this year too. I do think it's fuel-related; my thought was that a fuel pump either works or it doesn't, and that it's unlikely that the fuel pump could deliver gas sufficient for 2,000 RPM, but not more than that.... Thx for the advice and information all you experts provide ....

 

[philipp10]

As I have always said, start with the basics, use the results of some simple checks to move in the right direction. Just guessing is a complete waste of time and tell you nothing. And can actually cost you a lot of money in unnecessary parts.

Checking simple things can tell you a lot about what's wrong or, just as important, what's not. The tests are easy, quick and very cheap (mostly free)...

BTW, Dave, TPS is not used in the fuel calculation beyond about 5% throttle. After that, it's done on engine speed, manifold pressure and coolant temperature.

So, the list is as follows, in this order.

1. Compression tests.
2. Check spark plugs, replace if needed.
3. If points, replace, along with condenser and rotor. Check and adjust, if needed, points cam dwell angle (far more accurate that a feeler gauge. Will also tell you if the shaft or cam in the distributor is worn).
4. Inspect distributor cap, rotor and leads, replace if necessary.
5. Check spark quality (needs to be a nice BLUE spark drawn over 1/2").
6. Set timing. Check timing advance is working correctly.
7. Check/change fuel filters and check that the fuel system is not sucking air.
8. Check fuel pump pressure.
9. Check for full opening of the carb. Also ensure choke is opening fully when the engine is warm.

Once you've done all that you'll have a much better idea of where you should (or shouldn't) be looking.

Chris........
(It would also help if you tell us what engine you've got. There was no 3.0 V8. Can we please have an engine serial number?)

You list does NOT sound like the basics at all. More than likely, he's not getting enough gas to run above 2000 rpm. Look at your fuel circuit and see if you can find the cause. Unless you have some reason to check compression, THAT should be at the bottom of your list. And spark plugs seldom go bad nor would the motor idle if they were bad. Same for points and timing (but they would be third after looking at fuel issues). Look for vacuum leaks too.

 

[achris]

You list does NOT sound like the basics at all. More than likely, he's not getting enough gas to run above 2000 rpm. Look at your fuel circuit and see if you can find the cause. Unless you have some reason to check compression, THAT should be at the bottom of your list. And spark plugs seldom go bad nor would the motor idle if they were bad. Same for points and timing (but they would be third after looking at fuel issues). Look for vacuum leaks too.

After almost 40 years as a mechanic, I can assure you, these ARE just the basics. Now that we know this engine is a 5 litre V8, we can lose #3 as it has electronic ignition. If that engine came into my shop I may skip a few steps on hearing it run, but over the internet we don't have that luxury. All we can go on is what the poster writes, we can't verify that in person, so I ask for all the tests that cost nothing (or very little and are nothing more than basic maintenance anyway), and yield usable results. It may sound like a fuel problem to you, and there is a chance it is, but we had just recently a thread involving an engine running for about 30 seconds and then dying. The OP and most subsequent posters jumped to the conclusion it was a spark issue. I asked the OP to check if spark disappeared as the engine shut down. For S&G he 'humoured' me.... Lo and behold, not a spark issue after all. Turned out to be the oil pressure switch that powers the fuel pump. Point is, without seeing, hearing, feeling the engine, no conclusion jumping! Do the tests, post the results, move to the next set of tests.

Now, your list. Compression test should ALWAYS be the very first thing, no exceptions. Spark plugs do go bad, seen plenty in my time, or they could just be old and in need of replacing anyway. I have seen plenty of V8s idle perfectly smoothly with one cylinder completely dead, or a couple of plug wires crossed (do the search here, we have at least a couple every year), I've seen a Merc outboard V6 run smooth as silk with #2 and #5 plug leads swapped, it just wouldn't run up passed about 3,000rpm.. Remember, we can't see, feel or hear this engine. What the OP says is a smooth idle we might see as a rough idle. Do the tests.....

Thank you for the information ....I've been asked for the Engine Serial Number before; it's unreadable on the air filter and was never noted on the documentation. although I do have a Hull ID on the title The engine is a Mercruiser Alpha One 5.0L. Items 1 through 6 were performed just before winterization. The filter was just dumped and put back on; it was checked last year and this year too. I do think it's fuel-related; my thought was that a fuel pump either works or it doesn't, and that it's unlikely that the fuel pump could deliver gas sufficient for 2,000 RPM, but not more than that.... Thx for the advice and information all you experts provide ....

Sorry for the above diversion.

Serial number. Can also be found on an aluminium tag on the block down on the starboard side, just above the starter motor. Read HERE if you're unsure what I'm referring to.

Even though those items were performed 6 months ago, doesn't mean something hasn't changed. If it's not too difficult it's worth verifying, just for peace of mind that it isn't one of those. Not difficult for an engine to drop a valve after coming out of winter. When I pull filters, I dump the contents and look at what they have trapped, but they ALWAYS get replaced. As far as I'm concerned, they are an expendable item, and never get put back on unless there are extenuating circumstances. Put a new one on, you might be surprised. If there was water in the fuel tank, there would be water in the filter, so unless there is, there isn't. If you do fear some water in the tank, put about a litre of metho in for every 100 litre of fuel in the tank. The metho will pick up the water and carry it through to the engine and allow it to be burnt and expelled.

One test worth doing is to put a vacuum gauge on the incoming fuel line. Make sure the pump isn't pulling against something. You need to see less than 2" of vacuum on the gauge in this position. Another test is to put a section of clear hose in the line between the pump and the carb. Make sure you see no bubbles...

Chris......

 

[southkogs]

So, the list is as follows, in this order.

1. Compression tests.
2. Check spark plugs, replace if needed.
3. If points, replace, along with condenser and rotor. Check and adjust, if needed, points cam dwell angle (far more accurate that a feeler gauge. Will also tell you if the shaft or cam in the distributor is worn).
4. Inspect distributor cap, rotor and leads, replace if necessary.
5. Check spark quality (needs to be a nice BLUE spark drawn over 1/2").
6. Set timing. Check timing advance is working correctly.
7. Check/change fuel filters and check that the fuel system is not sucking air.
8. Check fuel pump pressure.
9. Check for full opening of the carb. Also ensure choke is opening fully when the engine is warm.

Once you've done all that you'll have a much better idea of where you should (or shouldn't) be looking.

You list does NOT sound like the basics at all ...

Actually it fits the line of work I wound up doing on my boat over last year and this year ... I almost (inadvertently) followed this exactly.

Chris may be on to something ... even from that far away

 

[Bondo]

The filter was just dumped and put back on; it was checked last year and this year too. I do think it's fuel-related;

Ayuh,..... Fuel filters aren't a reusable item,....
When ya do, the junk on the dirty side gets to the clean side, 'n goes into the fuel pump, 'n carb,....

I use a freezer baggie to catch spills, 'n then to inspect the Contents of the ole filters,....
But I Always screw on a New filter,....

 

[philipp10]

After almost 40 years as a mechanic, I can assure you, these ARE just the basics. Now that we know this engine is a 5 litre V8, we can lose #3 as it has electronic ignition. If that engine came into my shop I may skip a few steps on hearing it run, but over the internet we don't have that luxury. All we can go on is what the poster writes, we can't verify that in person, so I ask for all the tests that cost nothing (or very little and are nothing more than basic maintenance anyway), and yield usable results. It may sound like a fuel problem to you, and there is a chance it is, but we had just recently a thread involving an engine running for about 30 seconds and then dying. The OP and most subsequent posters jumped to the conclusion it was a spark issue. I asked the OP to check if spark disappeared as the engine shut down. For S&G he 'humoured' me.... Lo and behold, not a spark issue after all. Turned out to be the oil pressure switch that powers the fuel pump. Point is, without seeing, hearing, feeling the engine, no conclusion jumping! Do the tests, post the results, move to the next set of tests.

Now, your list. Compression test should ALWAYS be the very first thing, no exceptions. Spark plugs do go bad, seen plenty in my time, or they could just be old and in need of replacing anyway. I have seen plenty of V8s idle perfectly smoothly with one cylinder completely dead, or a couple of plug wires crossed (do the search here, we have at least a couple every year), I've seen a Merc outboard V6 run smooth as silk with #2 and #5 plug leads swapped, it just wouldn't run up passed about 3,000rpm.. Remember, we can't see, feel or hear this engine. What the OP says is a smooth idle we might see as a rough idle. Do the tests.....



Sorry for the above diversion.

Serial number. Can also be found on an aluminium tag on the block down on the starboard side, just above the starter motor. Read HERE if you're unsure what I'm referring to.

Even though those items were performed 6 months ago, doesn't mean something hasn't changed. If it's not too difficult it's worth verifying, just for peace of mind that it isn't one of those. Not difficult for an engine to drop a valve after coming out of winter. When I pull filters, I dump the contents and look at what they have trapped, but they ALWAYS get replaced. As far as I'm concerned, they are an expendable item, and never get put back on unless there are extenuating circumstances. Put a new one on, you might be surprised. If there was water in the fuel tank, there would be water in the filter, so unless there is, there isn't. If you do fear some water in the tank, put about a litre of metho in for every 100 litre of fuel in the tank. The metho will pick up the water and carry it through to the engine and allow it to be burnt and expelled.

One test worth doing is to put a vacuum gauge on the incoming fuel line. Make sure the pump isn't pulling against something. You need to see less than 2" of vacuum on the gauge in this position. Another test is to put a section of clear hose in the line between the pump and the carb. Make sure you see no bubbles...

Chris......

I guess my main point was compression almost never goes bad. This would be major engine rebuild. Sure it's free but the odds we lost compression is probably 1 in a 1000. I too have been a mechanic for a long time and I just prefer to start with the simple, most likely stuff and then work deeper. Lots of people on these boards seem to jump around all over the place replacing this part, then that part with no real diagnostics first (not referring to you). I personally don't agree compression should ALWAYS come first. It is a rare event to have a cylinder go bad.

 

[Bondo]

I guess my main point was compression almost never goes bad. This would be major engine rebuild. Sure it's free but the odds we lost compression is probably 1 in a 1000. I too have been a mechanic for a long time and I just prefer to start with the simple, most likely stuff and then work deeper. Lots of people on these boards seem to jump around all over the place replacing this part, then that part with no real diagnostics first (not referring to you). I personally don't agree compression should ALWAYS come first. It is a rare event to have a cylinder go bad.

Ayuh,...... Most of the posters here at iboats are Not mechanics, 'n never played one on TV,....
Now, We know you are, so We can speak the same lingo,....

Compression can be lost for many reasons, 'n a compression test oughta be done, 'bout any time the spark plugs are comin' out,......
A compression test is also a very valuable diagnostic tool,...

Diagnostics is the drivin' force that makes iboats forums what they are,.....

It's what makes it tough dealin' with a non-mechanic, that's hired out another Mechanic,.....
I'd rather talk a non-mechanic through a repair myself, 'n himself,.... I Hate bein' 2nd fiddle behind another Mechanic's back,....

Something you'd be hard pressed to find here at iboats forums is,....
Advice to change the coil, because ya ain't got no spark,..... common sense Right, the spark comes from the coil,..??

Personally, I'd bet 99.999% of the coils found in dumpsters at boat launches, are perfectly Good coils,....

 

[philipp10]

Ayuh,...... Most of the posters here at iboats are Not mechanics, 'n never played one on TV,....
Now, We know you are, so We can speak the same lingo,....

Compression can be lost for many reasons, 'n a compression test oughta be done, 'bout any time the spark plugs are comin' out,......
A compression test is also a very valuable diagnostic tool,...

Diagnostics is the drivin' force that makes iboats forums what they are,.....

It's what makes it tough dealin' with a non-mechanic, that's hired out another Mechanic,.....
I'd rather talk a non-mechanic through a repair myself, 'n himself,.... I Hate bein' 2nd fiddle behind another Mechanic's back,....

Something you'd be hard pressed to find here at iboats forums is,....
Advice to change the coil, because ya ain't got no spark,..... common sense Right, the spark comes from the coil,..??

Personally, I'd bet 99.999% of the coils found in dumpsters at boat launches, are perfectly Good coils,....

So you test compression every time plugs are pulled? That's just a waste of time. Tell me, in all the years working on cars, boats etc, do you ever see low compression. It just doesn't happen anymore. Now on something 20 years old? Sure, I understand. But even on a 20 year old motor, if the compression was checked just last year, the odds are its still good. Telling a non- mechanic to check the compression when he has a minor fuel delivery problem just mucks up the discussion and gets them off in a tangent to areas they have no business being in. The boat is a 2003 Glastron so the motor probably has less than 200 hours on it. Then the advice to change plugs? Nothing to do with only able to reach 2000 rpm. The OP would be better off hooking up an inductive timing light to each plug and watching for the flash at 2000 rpm to see if it is consistent. Throwing in new plugs will accomplish nothing unless poor spark is detected. Just my opinion but prefer a simple and direct route.

 

[Bondo]

Ayuh,.... There's a recent thread on here that bad compression WAS the Problem,.....

No power, Can't plane off,.... almost 2 full pages of posts before the Op stated he'd adjusted the valves,....
Compression test, 'n Bingo,....
The Op had over-tightened the valves, 'n little to no compression,....

If you feel a compression test has no value,... Don't do one,....
Tell the world,.....

We here at iboats, will continue teachin' the Non-mechanic's how to play one, when far from the dock,.....

 

[Scott Danforth]

from the adults only section, http://forums.iboats.com/forum/engi...6-list-of-possible-causes-of-low-wot?t=295070

 

[philipp10]

Ayuh,.... There's a recent thread on here that bad compression WAS the Problem,.....

No power, Can't plane off,.... almost 2 full pages of posts before the Op stated he'd adjusted the valves,....
Compression test, 'n Bingo,....
The Op had over-tightened the valves, 'n little to no compression,....

If you feel a compression test has no value,... Don't do one,....
Tell the world,.....

We here at iboats, will continue teachin' the Non-mechanic's how to play one, when far from the dock,.....

Ok, point taken. In that particular case the OP should have told you he was adjusting his valves....which BTW, he should NOT have been messing with as he clearly didn't know what he was doing if he over thightened them to the point of leaking. Lucky if he didn't burn the exhaust valves.

 

[achris]

Ok, point taken. In that particular case the OP should have told you he was adjusting his valves....which BTW, he should NOT have been messing with as he clearly didn't know what he was doing if he over tightened them to the point of leaking. Lucky if he didn't burn the exhaust valves.

Welcome to forums. That's the way life is here. Now you might understand why we ask what we do.