Changed the heater core and now overheats
 

92 s10 4.3 tbi with 250k miles ... So my heater core was leaking, changed it, no issue there. Now since then it gets too hot on the freeway and warmer than normal around town. Digital gauge starts blinking (around 245 degrees guessing by the marks on the gauge) after about 10 minutes at 60mph or 1500 rpm. always stayed one notch above halfway on the gauge before the heater core never hotter. Flushed and back flushed both radiator and engine separately. Changed stat drilled an air bleed hole in it. Changed gauge sensor and ecm temp sensor. Measured ohms on that when hot and was just above 200 ohms on old and new one . Changed pump and upper radiator hose. That sucks in just slightly when I Rev the engine but not much, when it's too hot and I rev out, it sucks in more Tried running without the stat and of course stays way too cold but no feeling of the hose collapsing. Actually does eventually get up to about 5 lines (one ticket mark under halfway). Had a sticky stat a long time back that didn't close all the way and would only get to 3 lines. Water gushes through the radiator without the stat looking with the cap off (also replaced that 3 times ). With the stat in flow isn't very noticeable and the level seems go up and down watching with the cap off, sometimes gushes over. Tested that in boiling water and it seems to open all the way, old stat opened just as much. Thought maybe head gasket but did block check to see if exhaust going in rad and came out clean. No loss of coolant and none out the tail pipe or in the oil. Coolant clean. Flow is all that makes sense with it getting hot in the freeway. Could I be missing something? Bled air out several times.

Also did pressure check and it would hold solid at 15 lbs.

Have noticed that my oil pressure gauge seems to be reading higher than normal. Will max at 80 when rpms at 2k sometimes and sit at about 40 at idle. That may be a separate issue and haven't had a chance to use an analog gauge to test.

Is all air out of system? Maybe temp and oil guages are both reading high - electrical malfunction? Grounds? Does it really seem like 245°?? Need to verify with external gauges.

Yeah it's getting that high. When the gauge was blinking I was able to pull over really quick and pop the hood, felt the upper hose and could feel the coolant boiling. I swear I heard the thermostat slapping open and shut at one point too. I have an oil gauge I think I can try but will have to pick up one for the temp tomorrow.

Every time I check under the cap when it was cool the coolant was up to the top but I was always able to let some air out while it was warm or hot by cracking open the cap a little.

Are you getting good airflow through the radiator? Bent fins and debris can both cause over heating. Does the fan clutch roar when cold, and quiet down at 2500 engine rpm?

Radiator is maybe 2 years old. Removed it to flush it good and still looks new. No debris or bent fins. Clutch is good too, moves lots of air and the shroud is in place also.

Got the temp and oil gauges hooked up (after breaking the stupid brass fitting the oil sensor was attached to). Oil pressure is normal so my gauge is lying to me. Haven't put the thermostat back in yet to see how hot it's getting yet (been pouring down rain today) think I'll try it tomorrow.

When you replaced the core, you probably broke up some old debris. Maybe it needs a good flush of the block. I use the block drains, but something like this might help. https://www.google.com/shopping/prod...O_Kcule9fGm_gK

I was thinking of trying that flushed it a couple times but didn't use any cleaner. Didn't know about the plugs until a few days ago. Where are they at?

If I'm not wrong the 1992 has the same pickup for the heater core than the 1994.
It flows from the top and rear of the spider into the heater core and back to the water pump or block depending on model and engine.
This is one of the downsides of this heater system, that it always heats up the core. So passenger always has hot feet in summer. (My A/C is gone)
At the same time it is still a bit of cooling to the engine. If this is cut out it heats up more.

When changing the heater core, have you accidentally bent the tube protruding from the block? Has happened before and if bent it blocks the flow not only through the heater core but also through the heads and spider. Hoses from there, bent or squeezed somewhere? Bent one of the copper lines on the heater where it passes the firewall? As the core as it is a PITA to remove and put back in I have bent one of these once.

I've been running on a bypass of the heater core itself in summertime without any problem of temperature rising above normal. Bypassed at the flex hoses to the firewall. But mine comes with the towing kit (larger radiator and heat exchanger for engine oil built into radiator.)

If all is o.k. it comes to my mind:

What is different except for the heater core you have replaced. Any other changes or modifications. Any servicing such as spark plugs, coolant type (antifreeze), driving habits, new place etc.

I'll go and grab my service manual and see what it tells for overheating.

No luck - service manual telling the same as all the posts have come up with already.

Another thought as absurd as it may sound.
What about air duct to air-filter? It has a pre-heat function if that fails it may keep it from turning to get fresh air and instead always sucks in pre-heated air from the exhaust.

BTW: What type of engine? TBI or MFI? VIN W or VIN Z engine?