Engine problem - advice please

I would very much appreciate some advice from all you guys who understand Mercruiser 5.7L V8s better than I do. Thsi is the problem:

We are about 15 engine hours into the season and she was fully serviced by a good dealer before the season started.

The last couple of sessions, the boat has started fine off the trailer, run half a dozen sets and then the motor has started to faulter. It will lose power totally and sometimes can be nursed back but sometimes stops altogether. She then won't start without a lot of work of the starter and the gears being disengaged.

When she does start (always with lots of throttle) the chances are she will die again when returned to neutral.

Other times she will faulter and stop when we go to neutral. Again it can be tough getting her going again.

Also, several times today, she idled happily at the dock for 5 minutes and then just stumbled and stopped.

To try and clear things out, I did a couple of 45mph runs and she ran sweetly without problem. Drop back to 24mph and she would hesitate again.

By the way, she was fuelled with clean gas through a filter. And there is a water separating filter installed. I'm wondering if replacing this was missed at the service.

FInal point, the Thunderbolt ignition failedlast season and was replaced with a conventional non-electronic points-type distributor.

Thank you in advance for any advice offered. Some pics of the engine are attached in case it matters exactly which type it its.

It does sound like a fuel issue. But something like that could be most anything.

Is it a carb or tbi?

It's a carb.

There's other possibilities too but you can start here.
Pull your water separator and dump the gas into a can or bowl and look for water.

Inspect your points,rotor and cap for wear. Change out the condenser (it's cheap) and check the dwell. Going to a (back in time) point set up was a really BIG mistake ! If your mechanic suggested a point system over electronic ignition I'd suggest finding a new mechanic.

Your coil might be on the way out too.

Good luck,
Walt

That looks like a Quadrajet carb and there is a filter on it right where the fuel line goes into the carb. Pull it out and check it for crap.


Also there's a module (I'm not sure if that's the proper name) on mercs that acts up sometimes too. They used to mount it on the exhaust manifolds and it would get hot and malfunction. I don't see it on yours though.

Does it only happen when it's warm? If it only starts happening when it gets warmed up, three things that come to mind are, in this order, fuel pump, coil, condenser (next to points under the cap)

The coil and the condenser are easy and relatively cheap to replace. The coil is an easy swap, I like MSD Blaster coils with point ignitions. Do not use any Accel parts, ever. Of course, you could just use a known good coil off an old vehicle for testing purposes. On a side note, have someone who knows how to use a DWELL METER set your points, not a feeler gauge. It will pay for itself in gasoline useage in a couple of tankfuls. A hot coil will help with fuel economy, too.

If it happens all the time, I would run the gas out of it then completely flush the fuel system, you could have a lot of water or trash in your tank or lines. I'd replace the filter and fuel/water separator again and check the fuel pump for adequate pressure and volume delivery. If it is electric, I might even replace it as it may be flaking out when it gets warm. I'd pull the bowls off the carb and dump the gas and check for corrosion. If you want, go nuts and rebuild the carb too. Actually, I have not had great luck with rebuilt Holleys, I prefer to find the nearest fence and throw them over, and buy a new carb. Not the cheap approach, but it works EVERY time.

There are a lot of things that could cause those problems, but it sounds fuel related. The other stuff is less likely but easier and cheaper to try first, troubleshooting by message board

Psyclone ,
I've had the same luck with rebuilt Q-jets as you've had with Holly's.

Sounds like water. Check the seperator for water...

Big thanks for all the help. I'll run the checks and let you know the outcome.

I would guess that it is a problem in the carburetor, low speed circuit.

A typical 4 barrel carburetor uses two "barrels" for low/medium speeds and only opens the "secondaries" when you advance the throttle fairly wide. If the main jet in the primary side was clogged then the boat could start cold (choke on), idle warm (idle jet is independant) and run reasonably well at 2/3 to full throttle when the secondaries open.

If it is the carb then I would suspect that something in the gas fouled it, either water or junk. Sometimes the carburetor has a plug in the bottom of the float bowl that you can remove to drain bowl. If it does, try to capture what comes out and see if there was any water in it. It wouldn't take much in the float bowl to shut it down.

The symptoms don't really seem right for a fuel delivery problem but it is possible. I would check the fuel filter/water seperator closely, looking especially to see if there might be an air leak. If the water separator wasn't put back on correctly or if the gasket was missing it could allow the fuel pump to suck air instead of gas.

One thing that I would do as a diagnostic: When you are having the problem of it not starting, remove the flame arrestor from the top of the carb and look down the carburetor as someone "pumps" the throttle (DO NOT TRY TO START WHILE YOUR FACE IS OVER THE CARB!). As the throttle is pumped you should see a good solid stream of gas being squirted down the two primary barrels by the accelerator pump. If you don't see this, then something is wrong in the fuel system.

If you do get the accelerator squirts then I would expect the engine to at least fire if you pump it a few times. Might not continue running, but it should fire, unless the problem is ignition

does it have a map sensor???

I had some of the same symptoms a few years back. Make sure your oil isn't overfilled, pull off the air cleaner and the black plastic hoses that go to the pvc breathers and clean them with carb cleaner and blow them out good. If they get clogged up with oil, you'll have problems.

This motor does not have a map sensor it is pretty basic set up especially sense he removed the thunderbolt ignition. What type of coil are you using? The points ignition system will draw more amps then an electronic ignition. A pertronix kit runs about 80 bucks and is a great upgrade and a definite way to eliminate the electrical side.

Update: When the repair shop started to work on the engine they found that something had gone mighty wrong internally (which is why she wouldn't turn over at the end). They have removed the engine from the boat and taken the heads off to find the cylinders full of water. The bottom end is coming off next for crack tests and to check the rods.

I guess I suspect a cracked block. Are there any other ways water can get in?

Oh dear. It's all going to be a bit expensive.

Ummmm.....
You said "taken the heads off to find the cylinders full of water"

If they were doing any decent job of diagnostics they should have found the water in the cylinders long before they took the heads off.

In fact, if they didn't check first you can't tell once the heads come off. It is common for water to spill into the cylinders as you take the heads off! There is water in the system, and once the seal is broken between head and cylinder the water flows in.

What they should have done is run a compression check before they removed the heads. That would have given them an indication of which cylinder to be looking at, and perhaps both heads didn't need to come off.

I missed the part that the engine wouldn't turn over towards the end. That is a sign of water in the cylinder, which causes the compression to go through the roof and the starter can't crank it over.

This could all be result of a blown head gasket. It is rare that they fail for no reason but it is fairly common result of overheating. The solution could be as simple as replacing the head gasket although a cracked head is also very likely. A blown head gasket could also explain the water in the cylinder. It is common for the gasket to fail between the cylinder and the water jacket which will allow water to enter the cylinder as it cools.

Rod, thanks for the input - it all makes total sense to me. They would have done a compression test first off but the motor wouldn't turn.

You are probably right about the head gasket. I'd feel happier if there had been some sign of overheating in the period before the engine failed. I'd been keeping a close eye on the instruments all day and the temp gauge stayed solid in the normal area. I'm certainly hoping for the gasket to have failed rather than the block.

In order to run a compression test all of the spark plugs need to be removed. At this point the motor should have turned over, unless it was rusted solid. Even then why wasn't a visual inspection done before tear down? How hard is it to grab a mirror and light and look into each cylinder hole on a boat with the motor in the middle...In the future you might want to find a different shop.

Well a compression test tells you the condition of the rings. What you needed to do is a Leak down test, the correct way is to pressurize each cylinder to 100 pounds and see if it holds.

A leak down tester and a compression tester are two different tools.

Also everyone screams headgasket everytime there is water in a cylinder, but the fact is you have a open cooling system so there is no pressure on the headgaskets. You stated you did not over heat the boat so the likley hood of a Head gasket blow is small.

Did you ever pull the plugs before they took the motor apart?

Nate:

If you have a blown head gasket the compression test will show it. A leak down test is harder to execute, takes longer and is totally wasted effort if you already know the heads need to come off.

As for water in the cylinder, a blown head gasket can definately do that. It doesn't require a pressurised water system although that does help the process along. What happens is that when the engine is shut down any cylinder that was on a compression stroke, and thus has both valves closed will be a "sealed" chamber. Any compression that it had will leak out past the blown head gasket fairly quickly. Now you have a hot cylinder with a fixed volume. As it cools, the change in temperature will create a partial vacuum in the cylinder. That same leak that allowed the compression to go out will now try to pull something back to fill the vacuum. If the gasket was blown into the cooling system then what it will draw back in will be water.

Brutto:

Like Peter said, first step in doing a compression test is to remove all the plugs. If the motor was hydrolocked then removing the plugs would allow it to turn.

Something sounds fishy about your shop! Before they even took the engine out of the boat they should have given you a reasonably accurate analysis of what was wrong.

Sorry guys, it's my explanation that's fishy not the shop. I shortcut the story to avoid boring you all and this has confused things:

First the boat went to the boat shop who took the plugs out to find water in the cylinders and a nasty mechanical noise when they tried to turn the engine over. They suspected a bent valve or broken rod. I wasn't convinced that they were the right people to deal with a serious problem so I moved the boat to a very reputable engine specialist. I know them very well and they are very experienced with V8s.

Everyone concerned has explained to me what is going on and the shop is photographing what they are doing in case the block is cracked and I have to take issue with the company that is supposed to have winterised the boat and stored her inside.

I'll do my best to give you the full story from now on.

Brutto 76 thanks for clearing that up. BTW just turning a motor over, that is hydrolocked, with the starter can bend a rod. Of course it is hard to know until it is too late.

Rod, I always respect what you have to say and consider you a major authority typically with the final word as gospel...except this one time Not that it really matters, but I figured you have an open mind and would like to learn.

Think of this though, as a head gasket can be blown and it will not show up with block check fluid, let alone a compression test. Often we used to pressurize the cylinder with 100+ psi of shop air and put a ballon on the radiator overflow or cap. We would look for an expansion that was so slight it was indetecable by any other means. Rarely will a compression test show a head gasket problem unless it is major. Then the plugs will tell the story.

As to the CLT check it could be very valuable, because if you are going in to do the heads and the rings are bad you are wasting your time. As you know a CLT will reveal all leaks from a cylinder. Of course how often did we ever do a CLT check? Not very often unless we were concerned with bottom end damage. Usually it would be caused by water in the oil and take out the bearings. Most of the time all the accessories needed to come off anyhow along with the intake/exhaust to swap over to a rebuilt motor.

Now I must say your analogy of the expansion and contraction drawing the water in was something I had never thought of but makes perfect sense