OMC 5.7 COBRA Valve tapping after W.O.T

[pecothern]

Puzzling new symptom from my 5.7 1993 OMC. After about 4-5 minutes at W.O.T. I slow to an Idle and hear a distinct valve tapping and dead miss. This will drift away over a minute or two and miss will too. I only have experience with lifter being flat at idle when first started Any body seen this before? It's not spitting through the carb so I geuss it's prob an exhaust valve sounds like its on the port side all the way back to the back cyl near the transom. Do you think the valve is sticking,or the lifter is failing at high speeds?

 

[tylerin]

Re: OMC 5.7 COBRA Valve tapping after W.O.T

do you have oil pressure?

 

[magster65]

Re: OMC 5.7 COBRA Valve tapping after W.O.T

If the answer is yes for oil pressure then the lifter is bleeding down. An oil change might help.

 

[pecothern]

Re: OMC 5.7 COBRA Valve tapping after W.O.T

Yes it seems to have good oil pressure There is a gauge on the dash. I noticed the pressure goes up a bit when you rev the motor so it is good and high when at W.O.T. I have noticed looks like light white smoke coming out of those hoses attached to the flame arrestor from the valve covers after running it real hard. But no smoke coming out the thru hull exhaust ports only steam. I just changed the oil last month 10w/40

 

[hondon]

Re: OMC 5.7 COBRA Valve tapping after W.O.T

You are running too light an oil.Recommendations for this engine would be SAE 30, or a good blended SAE 20/40or50. Those lighter weight ,fuel saving oils work great in your car that has a transmission to get you up and down the hills.Your boat however is always going uphill and never coasting.When is the last time you ran you car or truck consistantly at 5 grand.Your boat engine is made to take that day in and day out but it needs the oil to do it and 10w 40 ai'nt it.

 

[Northern Eclipse]

Re: OMC 5.7 COBRA Valve tapping after W.O.T

Are you sure the noise is not a exhaust shutter in the "Y" pipe, that has been a common cause of sounds that sound like a bad valve or a missfire, but go ahead and at least get the oil changed.

 

[Lou C]

Re: OMC 5.7 COBRA Valve tapping after W.O.T

Yep OMC recommends a straight 30 weight, it specifically says NOT to use multiweight oil in their marine engines. I use Chevron Delo 400 in mine, other brands that make a heavy duty straight weight are Shell Rotella and Pennzoil Marine.

 

[pecothern]

Re: OMC 5.7 COBRA Valve tapping after W.O.T

Well that certainly makes sense to me I will change to sae 30 right away thanks for the advise.