1992 C2500 7.4l 454 Auto - No power, blows heater cores

1992 Chevrolet C2500 7.4l TBI 454 Auto

This is for my dad's truck. It has major problems, and after having the local Chevy dealer replace the heater core 4 times without bothering to diagnose the problem, I have taken up the cause. There are two main issues which I do not consider to be related, but I'm an amateur rotary guy (RX-7) myself.

Issue the first:

The truck has no power while in gear. It does not respond linearly to throttle, instead bogging and occasionally making what sounds like a backfire back through the air cleaner. I can use no more than, say, 1/4 throttle to get it to accelerate at all, and even then it accelerates erratically--surging and slowing without having changed throttle. If the throttle is opened aggressively or WOT, the transmission shifts down properly but no acceleration occurs. In Park, the throttle responds smoothly.

In the course of handling the major scheduled maintenance this past weekend, I replaced the plugs, wires, cap, rotor, air filter, fuel filter, and set the timing. The symptoms are the same now as they were before the maintenance, even with having moved the timing from ~12BTDC to the proper 4BTDC. The Check Engine light is not illuminated, although I haven't checked for codes--I assumed the system illuminated a dash warning if a code existed. The warning lights (Check Engine and Check Gages) illuminate properly during the ignition test cycle.

Issue the second:

The truck overheats at the drop of a hat. It has lost at least 4 heater cores that I know of to leaks. The dealer has replaced it 4 times, along with the thermostat and I believe the water pump--unsure on that last, or on the current tstat threshold temp. The coolant reservoir has boiled over so many times the cap neck has melted. The current heater core has lasted awhile, but acts oddly--the dash temp gauge reads 200-210 soon after starting, but the heater does not start blowing warm for 20 minutes or so after that. It also makes loud gurgling and hissing sounds when it operates.

As an illustration of how easily it overheats, I took it out for a test drive after performing the maintenance. I was headed back to the house, about a 1/2 mile away, when I decided to go WOT and hold it to see what would happen. The tranny shifted down aggressively, raising revs, but the truck would not accelerate past 3500 or so revs so the truck just sat in that gear until I let off the throttle to turn into home. In that period (45 secs tops), the temp had gone from 210 to ~270 before I noticed it.

I've been reading the Chilton for the C/K series, but it has no troubleshooting guides. Without an error code, I'm sort've stumped on where to look. Maybe a vacuum leak from the air cleaner base? Clogged injectors? The cooling system woes are even more puzzling to me. Overpressure due to blockage? Any help would be appreciated.

Thanks,

-dave

Here are a few thoughts and sugestions.

Did you remember to unplug the timing advance wire while timing? I think it is located on the passenger side firewall. If you did happen to forget heres how. Unplug battery, disco the wire, connect battery start engine time to zero kill engine DISCO BATTERY plug wire back in connect battery.
On the overheating you can buy fail safe thermostats, If they stick they stick open. A quick way to check a vacuum leak is to use Chemtool and spray around your intake and tbi while idling, if it bogs down you have a leak. Have you checked the transmission linkage? How are the motor mounts and transmission mount?
Has the radiator ever been flushed?

Good luck

In all honesty, It sounds like you have blown head gaskets. Which would cause it to run like shit for one, and cause it to overheat, by pumping superheated exhaust gasses into the cooling system.