Valve tick and miss

[GMC1971]

I have a 79 c10 with a sbc 350 and i drive alot of freeway to work. After about 15-20 minutes of 65-70 mph driving the truck will start ticking and develop a dead miss. Clears up once i get off the freeway and lower speed. I know somethings obv wore out but im not sure where to start. I drive a truck so i dont have alot of time to troubleshoot. Am i looking at a bad lifter? Cam? Oil pressure and temp is good btw but oil pressure needle does begin to "jump" when it misses. Not sure if its relevant. Need some ideas thanks.

 

[1987 GMC Jimmy]

If it were me, I’d start by checking oil pressure. Cold first and see how it progresses through warm up. If nothing there, check cylinder compressions and pull the valve covers to watch for anything abnormal.

 

[HotRodPC]

Lots of questions all related to oil on this one. How clean is this engine inside? Does it have alot of sludge? What is your oil pressure like at idle both cold and hot? What about hot going down the freeway when the rpms are up? When was the last time you did an oil change? What weight of oil are you running? Are you using conventional or synthetic oil?

The noticing the oil pressure "jump" when it starts missing is helpful to know.

So here's a guess. Chances are the cam is fine. Lifter may not be bad YET, but it may be tired. If the ticking lifter is not opening a valve properly, that can cause your miss and it maybe more than 1 lifter and valve too.

Being that you mentioned it happens on the freeway after awhile of driving. If you're engine is gunky and drainback holes to the pan are clogged, maybe your oil is getting pumped up to the top end, and it's not draining back to the pan fast enough so you're actually running out of oil and the lifters aren't getting pumped up and they'll start ticking.

It maybe a well worn enigne and you're using to thin or light weight engine oil for the amount of wear. I would NOT use synthetic oil in a tired engine.

Try an oil change and run a higher weight of oil and also use a high mileage formulation. It might help alot.

 

[GMC1971]

Thank you HotRodPC i havent had a chance to pull valve covers yet so im not sire about returns. Oil pressure is great even when its missing. Just jumps when it misses. 30 pounds at idle 60 at highway speeds. Oil was changed 2800 miles ago sp close but not overdue im not sire what weight as i bought the truck since it oil change. (Mileage pulled from oil change sticker) engine has 139000 miles on it so not super old truck is all stock down to the catalytic converter and am/fm tape player. I will change oil and pull valve covers for inspection here in the next few days when i get home. Thanks for the info ive been googling symptoms for weeks with no luck.

 

[HotRodPC]

If it's only a 139,000 mile engine and the oil pressure is that good, your bearings should be in good shape and able to handle an engine cleansing. The oil is also new enough. Just leave that oil in there for now. Add 1/2 can of SeaFoam to the crankcase. Drive it for about 500-700 miles and let the SeaFoam meltdown some of that gunk and junk that might be in the enigne. Then when you do change the oil, change it HOT. Like after coming home from a 8-10 mile drive or so and you know it's up to full temp. Be careful not to burn yourself, but change that oil hot so that gunk is melted down, oil is thinner and you're likely to get more of the nasty junk out of the engine if it has any.