I have an 86 z28. I swaped a rebuilt L98 in place of a 5.0 tpi. I live in NY (and the car ran cool all winter!) since it started over heating ive tried

water pump, thermostat, radiator & hoses, factory gauge w/ wire, sender, (insulated from headers), tried water wetter and 20% coolant (green), now I'm back to 50/50 (green) and have a mechanical gauge, 160 thermostat, removed fog lights, ran a fluidyne themostatic fan switch adjustable from 160-220 (set at 160) checked fan voltage and operation. Ive flushed the coolant from system and block 6 times with no contaminates. And i fill it w/ engine hot and running to bleed air out.

My donor car that the 5.7 came from had 2 fans and a heavy duty radiator but one fan had a blown bearing and chewed the fins on the rad. My 5.0 fan has like 8 blades and I heard they pull like 800cfm where each dual fan pulls 350cfm. but my 5.0 rad is definitley smaller. should i play with the timing? or could it be a bad seal in the intake? Or did I goon something up?

 

to get to the bottom of things we need to..

1. Verify that the engine is actually running HOT...infared thermostat or thermometer in radiator neck to check coolant temp.

if it is running hot..

2. Check for debrie between the radiator and A/C condensor ( fuzz, mud, dust all get stuck in the flues)...This will require removing the upper shroud.

3. Check temp at the radiator from top to the bottom while the engine is hot and running, usually a 20-30* drop from the top to the bottom will indiacate an efficent unit. A large drop say 50* or more can mean the lower half of the radiator is plugged with build up that flushing will never remove.

4. Flow, Make sure the T-Stat opens by watching the fluid in the radiator core circulate, when it opens all of the flues should spray while exchanging the cooled coolant with the hot engines coolant, seeing this will verify 1. t-stat operation, 2. correct direction of flow from water pump. (you mentioned you changed the pump...it is possible to get the wrong one on there and reverse the way it flows causing overheating)

5. Fan...this should be running all the time if it is running hot, while it is running hold a sheet of paper in front of the condenser then let it go....the paper should be pulled up against the core of the condenser, this checks that your fan is pulling air, not pushing.

6. observations, the fluid in the radiator should not be bubbling or have bubbles rising to the surface, popping & expelling small amounts of smoke..this is indicating exhaust pressure entering the system. it will give off a fairly strong odor of exhaust when the bubbles pop. there is a dye that can used to check for exhaust in the system, it is good for cars that are just beginning to have HG failures
hoses, watch both for collapsing, the upper is more prone to this but lowers without an internal spring can cause the engine to be starved for coolant.
Radiator size, the 5.7 holds no more coolant volume wise, but it does make more heat, I would make sure to get the proper one.
Lastly the radiator cap...the demise of more engines than you might think....have it pressure checked at a shop or garage, the cap`s purpose aside from keeping the coolant from spilling out is to keep a pressure on the system, yours should hold 16 or more, this raises the boiling point of your coolant..without pressure your coolant 1 . can exit the system into the overflow tank 2. allow the coolant to boil, creating steam which displaces coolant and will "fool" thermosats and temp gauges iof actual coolant temp.

check it out and git er done

 

Wow, I think you just about covered everything there, greezemokey
Scooted, I would also check the ignition timing as you suggested, just to make sure.

 

Would a leaking intake gasket make one (or more) cylinder(s) run lean causing excessive heat? How about mal-adjusted timing?

 

I took it out today and couldnt get it to overheat despite all efforts. Both my mech and wire gauges read no more than 160* (when my fan kicks on) under harsh load. All I did this time is put (another) new 160* thermo in and try to bleed some air out of the system. It couldve been just air and a new bum thermo. I'm still going to check the intake and timimg tonight. My headers also leak even though I have a new multi-layer aluminum gasket set and all new allen key bolts. Maybe leaking headers could throw a wrench in the cooling system somehow. My gaskets are not supposed to blow through and are designed to allow for retightening of any loose bolts thanks

 

Leaking headers can read a lean condition to the O2 sensor, but most likely the ECM would try to compensate by making the mxture richer. (I believe that's how it works...)
but in all honesty, it sounds like you just had an air pocket that was causing your woes.
You could have also gotten a bum thermostat, as you stated. it happens.