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Ask Noel Schembri Your Own Question
Noel Schembri
Noel Schembri, Marine Technician
Category: Boat
Satisfied Customers: 1944
Experience:  Advanced technical diploma
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Problem with 5.0 MPI engine, doesn't idle only start with neutral gas and runs really

Customer Question

Problem with 5.0 MPI engine, doesn't idle only start with neutral gas and runs really rough. The problem started after top speed run when coming to neutral the engine died. The heads were replaced this spring with brand new ones after the old ones overheated and cracked.
Submitted: 1 year ago.
Category: Boat
Customer: replied 1 year ago.
I put only about 4-8 hours on the new heads since they were installed. The boat was running perfect all this time but now something has failed.
Expert:  Noel Schembri replied 1 year ago.
Hello my name is ***** ***** I will assist you. Are you hearing any alarms? Will the engine run at high speed and die only at idle or it will not run at high speed? Noel
Customer: replied 1 year ago.
No alarms. Scanned the engine too and no codes. Engine runs very rough and only keep running at minimum speed around 1500 RPMs. I have not attempted to run the boat with the engine just idling on neutral. I have attached video of the engine running if that would be helpful. It seems like some exhaust fumes gas smell comes out of the intake. Or something is wrong with the timing... I have since replaced the distributor and rotor no help at all. Oil is good. Suspecting valves. The engine runs fine and distributor is secure so I don't see how the timing would have changed all the sudden.http://www.youtube.com/watch?v=Zc1TgORPENc
Customer: replied 1 year ago.
I've also checked that there is spark on each wire at the distributor end. I unplugged them one at the time while engine was running. To my surprise unplugging them didn't seem to affect the RPMs. I did this test to figure if couple cylinders were not firing for some reason. I've also checked that all wires are secure and tried to move them around when engine was running. I've also checked that all wires are still properly plugged into the spark plugs. Next step I was planning on doing cylinder compression test and inspect the plugs. Plugs were put in brand new with the heads. Also replaced all the plugs in the other engine at the same time and it has no issues running.
Expert:  Noel Schembri replied 1 year ago.
Sorry for the phone call as i live in Malta and it will be expensive for you to call overseas. If you still want to call its no problem. From what i am seeing in the video there is a big miss that means some of the cylinders are not firing and they are more then one. At this stage I suggest you perform a compression test on all cylinders. I am thinking you may have some cylinders without compression. You also mentioned that you have replaced the heads lately, did you tighten the valves too much? as i am thinking that some of the valves may remain open during the firing stage. Noel
Customer: replied 1 year ago.
Ok no need to call noticed they charge me extra for that too. That's how it sounds like for me too the engine either has an ignition issues or something wrong with the compression as you stated. Again the engine was running fine and strong no issues. Something failed after several hours of running. I will do the compression test later today and see if I can find anything. This gives me an opportunity to check on the spark plugs as well.
Expert:  Noel Schembri replied 1 year ago.
Ok keep me updated Noel
Customer: replied 1 year ago.
So I did the compression test and one cylinder had no compression at all. Pulled the valve covers and found out that one valve is stuck open and that the push rod had fallen into the intake. Pulled the intake and got the push rod which was bent. Now my question what the heck could have caused this. Did my heads come with some defect or I am dealing with something else like hydro lock issue. The engine only has 300 hours on it.
Expert:  Noel Schembri replied 1 year ago.
Ok so for a valve to get stuck in marine engines normally is due to water, was the valve rusty? When was the last time you replaced the exhaust manifolds / risers? A worn manifold / riser will leak water into your exhaust and damage your engine. If the risers are ok then a partially blown head gasket may leak water into the cylinder too with the same consequences. Noel
Customer: replied 1 year ago.
Somebody suggestion online that the engine may have hydro locked from the water that was pushed in thru the exhaust. The boat has flappers that should prevent this from happening. And there was probably contact with the piston. I haven't pulled the head yet to see if there is damage to the piston or not. Could hydro lock bend push rod? I've seen this in cars and usually you will bent the piston connecting rod instead....
Customer: replied 1 year ago.
That was a quick reply. The heads are brand new and only been in the engine less than a month. Risers manifolds have 300 hours on them only so I doubt there is an issue. No rust at all under the valve covers.
Expert:  Noel Schembri replied 1 year ago.
A hydrolock could bend the pushrod but it would also fill all your cylinders with water and the engine would not turn or start as the amount of water would be a lot. In your case i doubt that it hydro locked.
Expert:  Noel Schembri replied 1 year ago.
At this stage the next step is to remove the head and inspect the gaskets of both the exhaust and the cylinder head for any water coming in.If the exhuast of that cylinder is rusty then most probably it is a failed exhaust riser - manifold gasket or any part of the exhaust. If the cylinder head it may be due to a little over heat. Also since the head you said it was new were the valves new as well? Or used from another head? Were the guides checked for clearances as it may have just blocked mechanically. Noel
Customer: replied 1 year ago.
Maybe the valves were not installed correctly or there was some problem with the valve guide that lead into it getting stuck. The heads should be under warranty I am going to call the seller in the morning. Again this happened after high speed run that lasted about 10 to 15 minutes. The heads are made by Sierra not by Mercruiser. I am worried this could happen to the other valves too not sure if I should just pull them both take them to machine shop and have them inspect redo anything that is not using quality parts...
Expert:  Noel Schembri replied 1 year ago.
Since you will have the intake manifold out i suggest you take out the other too for inspection, to be 100% sure. Noel
Customer: replied 1 year ago.
The heads came with brands new valves springs and guides etc... I just needed to bolt on the rocker arms and pushrods from the old heads. Is it possible who ever machined the heads and installed the valves screwed up?
Expert:  Noel Schembri replied 1 year ago.
Yes everything is possible at this stage, as for the rockers if they were overtight it would have the same effect but it would have happened on all cylinders as i guess you tightened them all the same. Best thing to do is to get them checkedNoel
Customer: replied 1 year ago.
I installed the rockers myself. Removed all the lash to the point where I couldn't rotate the pushrods with my fingers. Then added another 1/2 turn and left them there. All were done in the right sequence too. I got some 5 hours out of them till this happened. I am leaning towards bad clearance or bad valve guide myself.
Expert:  Noel Schembri replied 1 year ago.
Seems to me too I would contact the seller and check the clearances of both heads at this stage. If i can be of any further assistance let me know and kindly rate my service. Thanks, Noel
Customer: replied 1 year ago.
Thanks will do. I will keep you posted till I figure the cause of the failure.
Expert:  Noel Schembri replied 1 year ago.
Ok Great, if you need further assistance let me know. Noel

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