Logo
FULL SITE MENU

350 MPI needs head job

C

campo

New member
My 350 MPI needs a Head Job af

My 350 MPI needs a Head Job after less than 300 hours. Is this common? What should I look for as a cause?}}
 
By "head job" ...what

By "head job" ...what do you mean? Valve grind and lap? Is there a crack? Is the deck not sqauare resulting in head gasket issues?

water infiltration from the manifolds can be a problem. burnt valves from excessively lean fuel mixture...
 
"If it overheated, expect to n

"If it overheated, expect to need to have the heads shaved. You want to do them symmetrically but normal, run-of-the-mill valve jobs are pretty cheap. OTOH, if you always wanted more juice getting out of the hole, now is the time to do it. It's already being taken apart. A performance intake is less than $200 and won't be a problem for the fuel mapping/injection. Causes- there are many potential ones. Clogged fuel filters, weak fuel pump impeller from running it dry, bad fuel pressure regulator, old gas causing detonation, clogged fuel pickup screen or anti-siphon valve, etc.

Make sure the new gaskets are for marine applications."
 
"This might be a good time to

"This might be a good time to put in a set of roller rockers, should dramaticly improve guide, valve, and seat life. Who ever is doing your work should be able to give you an idea of what caused your problem. Mike"
 
"If it's new enough, it al

"If it's new enough, it already has them (not that the year was mentioned). When GM started putting the Vortec heads on the marine engines, they were using the 'Y' blocks (not in the same sense as a Ford Y block), which were setup for newer parts. The block number would tell more."
 
"Greg: What makes you say you

"Greg: What makes you say you need head work? What tests were performed to make that determination? Minimum testing should inclde the following:
A dry compression test recording the PSI by cylinder and a wet (oil squirted in cylinder) compression test recording PSI by cylinder. These are basic tests to evaluate the valves and rings. If these were not performed, I would insist on them. If they didn't do it and won't; get another mechanic.

If the engine just started running rough, you could just have a build up of carbon on the valves and on top the pistons that can be removed by a competent mechanic and the right chemicals. I would try the carbon removal first if none of the things the other guys talked about previously to this post have not happened."
 
"I would also have a cylinder

"I would also have a cylinder leakdown test done, along with a vacuum test. These, along with the compression test should be a pretty good indication of the engine's health.

Guy- I haven't seen many MPI motors with carbon buildup on the valves although I agree that Greg should include more info when he asks for advice on this kind of thing. Info like how the boat is run, where and how it's stored, fuel quality and source (if performance is the issue that's making him suspect the heads), service history, etc. Who said it needs head work and why?"
 
Jim: Thanks for the info. abo

Jim: Thanks for the info. about NO carbon buildup on MPI engines. I'll keep it in mind when I repower. I have a 10 year old reman. non-Vortec 4.3L that still provides great performance and doesn't use a drop of oil.

I would appreciate add'l info. on the leak down test so I can check my engine this month before winterizing it. Guy
 
"I don't think I would say

"I don't think I would say that there's no carbon buildup with MPI but it is significantly less than on a carb'd motor unless there's a problem.

There's a leakdown gauge made for this by various manufacturers. I don't know where you are but Auto Zone and some other parts stores lend tools- maybe they have one available. It attaches the same as a compression gauge but the cylinder is pressurized and the pressure going in is compared with the pressure in the cylinder, over time. If it drops significantly, there's a problem."
 
"Thanks for all the input. I

"Thanks for all the input. I did a comp test (thats how I know it's the head) The port side had three low readings, all in a row. 30#, 120#, 160#. All other cylinders are 180-190. I don't suspect rings because there is no blue smoke. I do tend to lose a little coolant, though, maybe a pint every 20 hrs. Jim said it might have to do with fuel. I did have to replace the electric fuel pump this year due to it blowing fuses. The heads are coming off in a couple weeks, so I should know more soon. Thanks again for all your comments."
 
"Greg: maybe you'll get b

"Greg: maybe you'll get by with only needing a head gasket; but since you taking them off, have them redone. Guy"
 
"Just my 2 cents worth, but a

"Just my 2 cents worth, but a leakdown test is a great way to pin down the problem before you break out the wrenches.
With the suspect cylinder pressured up, you listen carefully for where air is leaking:
hissing in adjacent cylinder=head gasket leak
hissing in oil filler=bad compression rings
hissing in carb/throttle body=bad intake valve
hissing in exhaust outlet=bad exhaust valve
hissing in cooling system=bad head gasket or block/head crack
You can also determine the degree of the leak by the difference in the supply and cylinder pressure gauges on the leakdown tester.
80 over 80= perfect; no leaks (rarely occurs)
75 over 80=very healthy cylinder
60 over 80=OK cylinder
50 over 80 or lower=bad cylinder that needs work
10 or 20 over 80=big problem
Hope this helps.
Rod"
 
"Greg;
You will need a leakdo

"Greg;
You will need a leakdown tester from NAPA or the like, along with the proper spark plug hole adapter. I have had mine quite a while so not sure of the cost, but likely 50 bucks or so.
Then just follow the instructions.
You also need an air compressor capable of about 100 psig to act as the air supply.
You only want to test the suspect cylinder or cylinders, not the ones that were Ok in the compression test.
Other than this you could get a reputable shop to do it for you. Should not cost much.
Good luck,
Rod"
 
I've ordered a leakdown te

I've ordered a leakdown tester from Amazon. What is the best way to set the engine at top dead center?(compression stroke)
 
"Greg;
You should get some br

"Greg;
You should get some brief instructions with your tester, but you will need to set each cylinder at TDC on compression stroke and HOLD IT THERE, using a long socket bar. 18" or longer is best, using the damper pulley nut to turn it over and hold it.
With the engine warm, and the plugs out, start with #1 using the TDC timing mark on the damper. Go in 1/4 turn increments using 1-8-4-3-etc until you get to the cylinder you need to test. Your 1st test will be either TDC on firing stroke, or just as exhaust is closing and intake is opening. If the latter, the cylinder won't hold air pressure. Turn the crank 1 more full turn to get to TDC firing.
Note that this works best with two people; one dedicated to turning the crankshaft and HOLDING the cylinder to be tested at TDC, while the other applies the air and takes readings.
If the cylinder being tested is even slightly off TDC, it will want to rotate immediately to BDC as soon as you put air to the cylinder, so you need to be ready for this and hold the socket bar tight! Note also that it helps to mark the damper pulley in 1/4 turn increments with chalk or a white marker to make alignment of each subsequent cylinder easier.
Alternatively, you can feel each piston approaching TDC by inserting the eraser end of a pencil into the plug hole, angled down slightly. With your helper turning the crankshaft slowly, when the piston just stops rising, that's TDC.
Leakdown testers were originally used for checking piston aircraft engines, where one person holds the prop blade at TDC, and the other does the test. Holding the end of a prop blade sounds scary but it isn't. It's much easier than holding a V-8 engine crankshaft, because the prop blade is quite long, and provides more moment arm to hold the torque.
Good luck,
Rod"
 
""It's much easier tha

""It's much easier than holding a V-8 engine crankshaft,"

Nothing a long pipe on the wrench handle wouldn't cure."
 
"You can also pull the valve c

"You can also pull the valve covers and visually determine when both valves are completely closed by observing the rocker arms. Valve springs will be all the way up, and you will be able to spin the pushrods with your fingers. Seeing as the heads are most likely coming off anyway, you might find this method easier. Make sure you pull all the plugs first and as jim pointed out a breaker bar with a long piece of pipe works well for keeping things in place. Welcome to the wonderfull world of engine diagnostics.LOL Good luck with your quest, and let us know what you find. Mike"
 
I finally got the boat out of

I finally got the boat out of the water and the heads off. The intake valve on the #6 hole has a chip in the edge. There is water getting in there. My mechanic says its "vortexing" in other words sucking back from the exhaust pipe. There is a reducer from 4" to 3" just aft of the riser. He is suggesting that I move the reducer further aft or install a water lift exhaust system. Has anyone heard of this "vortexing"? It seems strange that a fairly new engine would be prone to that kind of failure. I'm looking forward to your replies.
Thanks.
 
"A good example of a vortex is

"A good example of a vortex is when you dump whatever liquid is in a 2 liter soda bottle and it starts to swirl as it goes out. Tornadoes, hurricanes and cyclones are good examples, too. First of all, if the intake valve has a chip that's large enough to affect what it draws into the cylinder, it's not going to show up as a good vacuum test. It won't draw fuel air in as it should and may idle rough. Water won't get in through the intake valve but it could if it's the exhaust valve with the chip. Either way, you won't have the fuel air charge that you should. The intake valve isn't in the exhaust flow, period. The only way that cylinder could draw exhaust into it is if A) the intake valve seals the way it should, B) the exhaust valve doesn't seal well for whatever reason or C) there's enough overlap in the valve timing that the #6 exhaust valve is open during its intake phase.

The intake port of the head is connected to the intake runner on the intake manifold, which is then connected to the throttle body. The chance of drawing exhaust in through the intake valve is non-existent unless you have a bad head gasket.

I would get a detailed explanation from him, in writing, with a diagram. Have him explain how the intake will draw anything from the exhaust. If the #6 intake valve is chipped to the extent that it affects vacuum, you should hear air hissing back into the intake manifold whenever the piston tries to pressurize the air in it."
 
"Regardless of his theory, the

"Regardless of his theory, the head needs to be repaired. Unless it is spraying in the throttle body (which would effect all cylinders), the water is coming in through either the head gasket or the intake manifold gaslet. Both will be replaced during the repair. I can agree that if there is a disruption in the exhaust flow near the elbow, such as you have, it should be eliminated. If you don't want to reengineer the entire exhaust system, I would at least move that reducer back as close to the muffler as possible."
 
"Thanks for your replies. Jim

"Thanks for your replies. Jim mentioned a hissing sound. I did notice a hissing sound, but at the time I wasn't sure if it was abnormal. On a few occasions, the engine backpedaled on shutdown. This flipped the impeller on the seawater pump. On restart I would get belt squeal until it flipped back over. I also noticed a faint gas smell from time to time. I guess the bad valve was leaking fumes up through the intake. It wasn't until I noticed a loss of power that I discovered the 30# compression on the 6 hole. My mechanic split the manifolds and says the are both fine. I am taking Ricks' advice and moving the exhaust reducers back to the mufflers. I really want to understand exactly what happened to cause this failure on a 300 hour motor. I appreciate all of your responses. It has been very educational. Any further comments/advice would be welcome."
 
""On a few occasions, the

""On a few occasions, the engine backpedaled on shutdown."

You didn't say anything about this and it's probably the whole cause of the valve problem. When it turns barcward, it ingests water and things that are extremely hot cool off way too fast, changing their hardness and temper. Mercruiser has had problems with run-on, although mostly with the 4.3L. If you ran it hard before shutting it down, without any time to cool down, this can cause run-on. I posted a Mercruiser Service Bulletin last spring (bulletin number 97-17) and it has a list of steps that Mercruiser recommends to try to get this problem to stop. They know all about it, they just didn't want to say anything. You obviously have a carb and they have had issues with those, too (Mercarb vs Webber). If your dealer or service tech doesn't know about this bulletin, they either weren't working on/selling boats then or they don't keep up on tehm. I talked to Mercruiser technical and the guy I talked to didn't know about it, either. I had to tell him to talk to one of the old-timers about it. He faxed it to me about 1/2 hour later. I only knew about it because I was working on boats in '98-'02 and had tried their methods. If you're in a hot and dry area, this adds to the likelihood of run-on."
 
"Jim-to reply to your last pos

"Jim-to reply to your last post. The engine on shutdown would just rotate back maybe 1/2 turn. The guy that is doing the heads said it might be "hydraulicing" which he said is caused by the resistance of water in the cylinder. Just to be clear, it wasn't "dieseling." This engine is an EFI with fuel injection. I just talked to the machine shop today. The one head was basically fine. The other one required a new casting and 4 intake and 1 exhaust valves. I'm going to try and find that bulletin you posted and see if there is any relevant info. Thanks again, Jim."
 
"If you can't find it here

"If you can't find it here, I'll see if I can scan it and post it but since you do have EFI, it's not really relevant. I haven't seen a lot of backward rotation on EFI motors.

The principle behind hydraulic lock is that you can't compress a liquid and that's how hydraulic systems can exert as much pressure as they can. When there's a large mechanical advantage, a lot of force can be applied to something. If there's enough water in the cylinder (or any liquid, for that matter) and the valves are closed, you won't be able to crank it over until there's a pressure release, like removing the spark plug."
 
"I was just looking on the BAM

"I was just looking on the BAM Parts site. I shows 2 different exhaust elbows-1 for inline and one for v-drive. I was wondering what the difference was. I'm thinking that might be my problem if because of the angle of the engine to match up with v-drive might cause the water to back up. Maybe the v-drive riser has a sharper down-angle? When I re-powered, the existing v-drive was fitted to the new motor. Perhaps the wrong risers were on the new motor. Does this make any sense?"
 
Back
Top