i have a 350 tbi and ive noticed in the last 2 weeks that i was getting spark knock at a certain load range. then i noticed a very large lack of power. and i guess it would be worth noting that i do go through some coolant but i will say its been doing that for the last 13,000 miles i put on it...... i will also note that when i start going down the road on a colderish engine my oil is at 70psi.... anyways theres no white smoke coming out the exhaust. i had a compression check and 2 cylinders are at 60pis when they all should be 140psi. so some thoughts before i get it back home and tear the top end apart.....
Take the radiator cap off and look for bubbles in the tank, also check the oil and see if it looks like chocolate milk. Then tear the top off anyway.
Does your temp guage go way up to 220-230 before the thermostat opens from a cold start but just the first time? My tbi 350 did that about 10 years ago head gasket had failed but I was getting no white smoke or oil an water mix but was losing a lil coolant .Changed out driver side head gasket an drove it another 100k...
One of the easiest ways I've found to see if the coolant is getting past the head gasket is to first look for obvious leaks, then check the oil. If the oil is milky looking, it's got coolant in it. Between your coolant loss and the results from the compression test, I'd feel pretty safe to assume at least one of your head gaskets is toast.
I had a 88 Vette with aluminum heads and TPI, dead stock, bought new and never had a wrench on it for anything. Noticed at about 40K mile that it would lose some coolant. Maybe a quart a month. No evidence of any leak when parked and no evidence of any water in the oil. Figured it wasn't worth worrying about. At about 70K miles the passenger side head gasket let go big time and blew out all the coolant. When I tore it down, all the head bolts on the passenger side head were so loose I took them out with a 3/8 ratchet and little effort. Maybe 30 ft-lb max. The blown out area of the gasket was from the cylinder to a water passage. How it had ever held that long with the head bolts that loose is a mystery to me. All the head bolts on the opposite side had full torque on them. Replaced both head gaskets just to be safe, and it was still running fine when I sold it about 30K miles later.
Head gaskets blow in different ways. If it blows between cylinders, there will be compression loss but no mix of coolant and oil. Sometimes they will only leak coolant, sometimes oil, sometimes both.
just changed the oil and the old stuff was black as it should be. and no temp gauge always reads normal. if it indeed is head gasket. i will change them suckers myself rather than get raped buy the shop.....there already raping me $130 to tell me something that should of taken 10 minutes to figure out with all the equipment they have....
With the compression that far off on two cylinders you are going to have to get in there anyway and pulling the heads is a good starting place. you may get lucky and a quick facing and a pair of gaskets is all it will take.
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