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I am having a problem diagnosing an engine run problem with a Top Flight 1970 LS5 w/Auto Trans. This is a low mileage Corvette that has run flawlessly for 20 years since the second owner purchased it.
When he put it into winter storage it was running fine. Then in the spring it came out of storage and runs fine when cold, strong and no unusual noises. It begins running rough when operating temperature is reached, usually after 20-30 minutes. The problem has progressed to the point that it will stall on deceleration or when idling with the trans engaged. Restart will be troublesome when attempted, usually taking several tries. By disconnecting the TCS solenoid, to allow full manifold vacuum, the engine will start slightly better and can be driven using brake and gas to keep from stalling. Once the vehicle speed builds, acceleration is possible with primaries only up to 4000+ RPM. If secondaries are opened the RPMs will max out at 3000.
This is what has been done with little improvement:
Change fuel filter, check plugs and plug wires, check each cyl with KV tester, remove Breakerless module and install points and condenser, reinstall Breakerless module, change rotor, change cap, change orig coil to ACD U505, change to repro 287 coil, block fuel return line from pump, replace fuel pump, blow out tank to pump line and check flow, check fuel pump pressure, check run voltage at coil when idling = 9.5 V, change carb to another same # Q-Jet from another 70 LS5, clean and rebuild orig carb and reinstall, clean out fuel tank and replace strainer, compression and leak down test, check air/fuel ratio at muffler outlets, change to temporary resistor wire jumped from alternator output.
Some improvement noticed with following:
Connect resistor jumper from alternator in parallel with existing circuit. Coil voltage is up to 10.5+ but engine will stall when jumper is removed.
Remove intake - magnaflux - replace intake and new gaskets. Now runs at idle with TCS connected but stumble and stalls when attempt to move in Drive. Will move in Reverse without stalling, just stumble.
Where do I go from here????
When he put it into winter storage it was running fine. Then in the spring it came out of storage and runs fine when cold, strong and no unusual noises. It begins running rough when operating temperature is reached, usually after 20-30 minutes. The problem has progressed to the point that it will stall on deceleration or when idling with the trans engaged. Restart will be troublesome when attempted, usually taking several tries. By disconnecting the TCS solenoid, to allow full manifold vacuum, the engine will start slightly better and can be driven using brake and gas to keep from stalling. Once the vehicle speed builds, acceleration is possible with primaries only up to 4000+ RPM. If secondaries are opened the RPMs will max out at 3000.
This is what has been done with little improvement:
Change fuel filter, check plugs and plug wires, check each cyl with KV tester, remove Breakerless module and install points and condenser, reinstall Breakerless module, change rotor, change cap, change orig coil to ACD U505, change to repro 287 coil, block fuel return line from pump, replace fuel pump, blow out tank to pump line and check flow, check fuel pump pressure, check run voltage at coil when idling = 9.5 V, change carb to another same # Q-Jet from another 70 LS5, clean and rebuild orig carb and reinstall, clean out fuel tank and replace strainer, compression and leak down test, check air/fuel ratio at muffler outlets, change to temporary resistor wire jumped from alternator output.
Some improvement noticed with following:
Connect resistor jumper from alternator in parallel with existing circuit. Coil voltage is up to 10.5+ but engine will stall when jumper is removed.
Remove intake - magnaflux - replace intake and new gaskets. Now runs at idle with TCS connected but stumble and stalls when attempt to move in Drive. Will move in Reverse without stalling, just stumble.
Where do I go from here????
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