hi all i have a tpi 5.7 that starts and runs just fine started having problem with low power and knocking under load or hard acceleration. rock solid motor, replaced rotor, cap, wires, plugs, running premium fuel. idles smooth. timing advance is what im going to check this weekend. any suggestions, thanks

 

Have you checked your compression? Sounds exactly what my problem was and it turned out to be a dead cylinder. Good luck.

 

thanks ill test that as well. the car is driven so little nothing would surprise me

 

my 85 has a spun bearing but believe it or not it will sit and idle fine touch the gas n u hear it knocking

 

yikes, niether sounds good, but thanks gives me an idea on where to check. if the problem turns out to be that severe im going to replace the whole motor. ive had this car too long to get rid of it. we never learn-

 

Do you do alot of in town driving? You could have excessive carbon deposits in your combustion chambers. Try GM Top Cylinder Cleaner. We used this product alot when I was a GM drivability tech. These motors were meant to be run a fair amount on the open road where they could be opened up. Extended use in town can cause buildup which can cause pinging or detonation. Its most notable under load.

The standard cure was GM Top Cylinder Cleaner and an injector clean. Today, we use the Motorvac Top cylinder/Injector machine for this. Since you probably dont own a Motorvac, I suggest the GM cleaner. You can purchase it at your local GM dealer. Before performing this service, verify ignition timing is set according to the underhood emissions label. To use the cleaner, just remove your PCV hose and insert a small funnel in the hose leading to the intake manifold. Start the engine and pour the product slowly into the funnel while holding the engine at 2,000RPM.

After the top end service, add a bottle of Chevron Techron fuel system treatment to a 1/2 tank of gas, repeat this twice. If the problem persists after these services, it is likely mechanical and will require disassembly of the engine for diagnosis.

 

Thanks ASE doc- may be the path i take, again the knock "sounds" like detonation, funny thing is it wont do it in park. at a steady speed it is light or unoticeable. its only when i accelerate moderatly or go up an overpass. most likely i'll tackle it after the holidays. got to fix it though, this old bird is in great shape and I see very few offered in good condition and there is none in my area.

 

Dont wait too long to address the detonation. That knocking is your pistons, rings, and rotating assembly being hammered by little explosions of severely high pressure. This is very destructive and if left untreated can destroy your engine. I recommend you get started treating the problem right away or park the car until you have time. Are you running premium fuel? If not, start now. The higher octane rating will reduce detonation. Top tier fuels like Union 76, Chevron, Shell also have great cleaning packages that will help remove the deposits that cause detonation.