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Chevy 5.7 Stuck inlet valve.

SBC286

New member
Recently became the victim of a stuck inlet valve on the port engine, much to my surprise. It has turned out to be gummed up sticky inlet valves after tearing it down. Upon closer inspection the K&N Intake filters on both engines have a similar ( more than you'd like to see ) amount of oil deposit in the lower cowling of the filter base. Both engines got a rebuild some years ago and have always been strong engines. I've never noted any crankcase breathing at all, but it must be their to get the oil into the filters...?

Last summer the boat went through 3000ltrs fuel and ran fualtlessy on 3 week coastal trip, returned to the boat some 4 days later started it up and straight away it sounded off. Compression test told me zero on cylinder 2.

I find it strange that both engines have oil in the intakes, since rebuild the boat has clocked 600 hrs over 8 years. hours are monitored by port engine which runs a lot more than starboard to charge batteries and give hot water when not on berth as river based. So roughly the Starboard engine has probably ran no more than 400hrs after a rebore and full head overhaul. I could see the port engine possibly glazing up and giving this problem but both engines at the same time???

Im using Oil recommended to me - Quicksilver mineral 25W40, last oil change I added Lucas heavy duty oil stabiliser... Im wondering if this is the culprit with the already thick oil??

So doing some more research I've found that the recommended oil for a 1994 SBC is 10W-30??? not sure if thats mineral or synthetic as yet, can anyone confirm the favourite tipple apart from Gas...

Both engines are 1994 pre Vortec engines on 4bbl carbs.

Has anyone come across a similar problem and found a solution?? any thoughts / help appreciated :)
 
ALL 4 stroke engines have some amount of "blowby" ( crank case breathing) which by standards, is "recycled" into the intake manifold, usually in thru the "flame arrestor"...Marine engines don't have carb "filters/air cleaners" per se. And... no, the blowby is not intended to "oil the filters" ( even though K&Ns require periodic cleaning and oiling). Marine flame arrestors typically have such low flow restrictions, that K&N's are rarely worth it... (I'd like to see the before and after flow data !!)

That said.... re: stuck intake valve. IMHO, nothing to do with visible oil. Question... Is/was the stem of that inlet valve rusted? Likely the oil passage in that pushrod is plugged.
 
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Thanks for your response.

I didnt note any signs of rust on the valve but understand what your saying, I guess if it was a lack of oil from a plugged push rod some signs of wear might be visible around that area possibly...

When I pulled the valve it came straight out not a problem but did note a thick layer of sticky tar like substance on the lower part of the stem and head. Just the same as all other intake valves and piston crowns. This substance has taken some removing!!! Even a soak in the parts washer doesn't help much. The other motor is the same also...??

What sort of Oil do you use in the Merc 5.7L if you dont mind me asking?
 
I use Mercruiser SAE 25W-40 semi synthetic ... What is specified is FC-W Nema Certified grade.

If as you described, the valve was likely not stuck closed ( rather than open...otherwise it would burn) but failed to open... That said, you may have a defective hydraulic lifter or possibly a grossly misadjusted initial lash adjustment... or a wrong length push rod. By now, there is no way to determine if lash was way off.
 
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Build up on the valve stem is from oil coming through the valve guide. The Valves do two jobs. Air movement and heat disbursement. Now if both engines were rebuilt by the same person and they left the old worn guides this would explain why both engines have issues... Worn Guides will cause Positive valve seals to leak.

Slightly sticking valves will cause a lean condition and make a lot of heat. The valve will heat up and burn the oil leaking down the stem. Which in turn will cause more build up to the point of a stuck open valve when it cools after shut down.
 
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Build up on the valve stem is from oil coming through the valve guide. The Valves do two jobs. Air movement and heat disbursement. Now if both engines were rebuilt by the same person and they left the old worn guides this would explain why both engines have issues... Worn Guides will cause Positive valve seals to leak.

Slightly sticking valves will cause a lean condition and make a lot of heat. The valve will heat up and burn the oil leaking down the stem. Which in turn will cause more build up to the point of a stuck open valve when it cools after shut down.

as usual great post Chris.

SBC286 the blow by oil in the flame arrestor is normal. I’ve seen it on both the 3.0 and my newer 5.0. I tend to drive mine pretty hard lots of towing for water sports with several adults in the boat. My 5.0 was a repower as I bought a handyman’s special after previous owner cracked original block. I bought a new GM base engine saw the same blow oil by that you see, which is a red herring to the root cause Chris highlighted. I’d look at having the heads redone while it’s off.
 
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