Does this diagnosis sound correct? low oil pressure blue smoke on start

[fumehappy]

Hello,
I was hoping the collective could help me out real quick...
So when I got my truck, (approx 50k original miles) back in the spring, it blew some blue smoke on start-up. about a month ago(after driving all summmer), I noticed my oil pressure had dropped from 30ish down to 10-15. Oil level was fine, and oil color was okay with 1000 miles on it. I was told valve stems seals in the past for the blue smoke on startup. So I brought it in to a mechanic I hadn't used before, but came highly recommended.

They are telling me that the oil rings on the pistons have gone bad. I told them I got a good compression test on all 8 cylinders, and they told me that the top rings were good, bottom are bad. Can anyone verify if this sounds on the level? They say I need a complete rebuild, or a new engine.


The only other "symptom" could be less than optimal gas mileage 11-12ish on 91 nonethanol with 2.65 rear gears and 225 15R/75 tires (up from 205 factory spec). Turbo 350-C trans, 305 engine. I know gas mileage varies widely, but figured I would include it anyways.

Thanks!

 

[Georgeb]

When are you getting this low oil pressure? Does it rise with the RPM? Smoke on start up is usually caused by the valve seals while oil rings would permit a more constant smoke

 

[74 Shortbed]

Smoke on start up only is valve seals, if your burning oil(oil level goes down)then the bottom(oil rings)rings are bad, and you would get constant smoke.

 

[fumehappy]

There is no smoke after the initial exhaust cloud, but there is a bit of slight odor of something burning. Oil pressure goes up slightly under higher RPM. When cold or higher RPM, it goes to about 15, when it's hot it drops to about 10.

 

[CSFJ]

Have you hooked up a mechanical oil pressure gauge to verify that it really is dropping? A faulty sending unit can give a low reading. Like what's been said above, if the oil rings are seized or worn, then you would have a fairly constant oil consumption. If all you're getting is smoke on start up, the rings aren't the problem. They don't have anything to do with the oil pressure.

 

[rich weyand]

Sounds like two different problems. The oil smoke on startup is valve seals. The low oil pressure is probably either the sender to the gauge or the oil pump going south.

 

[fumehappy]

That was kinda what I was figuring... Do you happen to know if the oil pressure sending unit's are pretty homogeneous, or is it like the temp sending units with 4 different varieties?
Thanks!

 

[1987 GMC Jimmy]

That was kinda what I was figuring... Do you happen to know if the oil pressure sending unit's are pretty homogeneous, or is it like the temp sending units with 4 different varieties?
Thanks!

There should just be two. A canister type which is for the gauge and a switch which is for the dummy light.