Help with 350 engine low compression

Can anyone help? I have a 1994 GMC Suburban with 350 engine and 140k miles. I have been having problems with it idling rough. I did a tune up but it did not help. Checked the cylinder compression and the #3 and 5 cylinder cylinders are at 30-40 lbs all other at 140-150. I have pretty much determined it is not a head gasket however these cylinders are next to each other. No antifreeze coming out exhaust, and none in oil. I get a intermittent knock when the engine is under stress at acceleration. Any idea what is next? Look forward to some advise.

Welcome to AF. Do a leak down test of these cylinders, remove the adjacent plug when doing the test, air coming out of the non tested cylinder would indicatte a head gasket issue, if air escapes thru the intake or exhaust this is a valve problem.

start out with what max said. if no luck and nothing appears to be out of the ordinary, chances are you've got some bad rings. check the oil for gasoline contamination, if that is the case you are better off just replacing all of the rings. there is also the chance you have some scored cylinders in which case depending on the severity you may need to bore it over and get larger pistons and rings. as for the knock.... you may be tempted to automaticly either go for the cheapest fuel, or the best of the best plus some octane boost. this can sometimes cause a knocking situation, bad timing can cause this as well. use the fuel that is rated for the vehicle. most 350 engines ask for good ole regular unleaded. if thats what your using right this second step up a notch and try out a couple tanks of super unleaded. while it will not harm the vehicle irrepairably to just keep using super unleaded, you will notice over time the mileage can drop.
hope this helps out!

Thanks for the info all,
Fairly sure it is not the head gasket no air leaking from cylinder. Could not determine if it was a valve issue for sure think it may be a ring or both.
I have a local well respected mechanic that will remove re-install and rebuild the engine for $1829, as long as the block is OK. Will find out what was broke when he tears down motor. Since it has 140,000 mi and many oil leaks it is ready to be rebuilt and drive for another 140,000. Hope you all have a great Thanksgiving

in terms of overhauling an engine, the 350 is by far one of the easyest to rebuild. you can save a load of cash by doing it yourself. i taught my brother how to do it in a few hours and before that he had only changed oil on cars!

I suppose in the long run it comes down to what your willing to do, however if you do choose to go the Do-It-Yourself route there are manuals dedicated to rebuilding the 350, just go to your favorite auto parts store and ask them to order one....

also you may be thinking tools may be an issue... while you would need some basic tools you can get by without taking the engine out.

im personaly thinking that price is a little steep so thats why i bring up the DIY route.
also remember once you do have a rebuilt engine it would be a prime time to keep up on maintinance!
Things even as minute as an oil change every 3000 and a new set of spark plugs and wires every now and then can drasticly improve the length of time your ole 5.7 is around!

1989 GMC Suburban
350 CID 5.7 379043 original miles on entire vehicle
19 MPG Highway 16 City
No Major Repairs all preventative maintinance...
Greased fittings every month
Oil Change EVERY 3,000 miles
Tire rotate every 6 months or 7,500 miles
Fuel filter every Year or 15,000 miles
Coolant Flush every 50,000 miles
Brake check or replace every 60,000 miles
New Plugs and Wires every 75,000 to 100,000 miles I personaly use E3 plugs but standard ACdelcos Are second best!
and last but not least i replace the belts hoses and battery whenever they look even close to worn... 3 cracks per inch on the belt, soft spots or cracks on the hose, 2 or more dead starts or after 4 years.
Battery life is around 5 years in my climate.

Preventative maintinance is WORTH IT in the long run. my baby is as prime as when she rolled off the assembaly line!