CEL on with PO128 code. Recently replaced radiator and thermostat. No engine cooling issues, had a leaking rad. Checked coolant flow; hot and cold, normal. Was going to replace ECT sensor, but want to check reading first...

If your 1998 Cutlass is overheating and the heater is blowing out cold air, and you are also noticing that coolant is leaking out, there are a few possible causes. Here are some things to check:

Check for a coolant leak: Start by checking the engine and radiator for any visible signs of a coolant leak. It's possible that a hose or other part of the cooling system may be damaged or loose, causing coolant to leak out.

Check the radiator: If the radiator is not working properly, it can cause the engine to overheat. Check to make sure the radiator is not clogged or damaged.

Check the water pump: The water pump circulates the coolant through the engine. If it's not working properly, coolant may not be flowing through the system correctly, causing the engine to overheat.

Check the cooling fan: The cooling fan helps to keep the engine cool by pulling air through the radiator. If it's not working properly, the engine may overheat.

Check the thermostat: Even though you recently replaced the thermostat, it's possible that it's not working correctly. A faulty thermostat can cause the engine to overheat.

It's important to address any issues with the cooling system as soon as possible, as overheating can cause serious damage to the engine. If you are not comfortable diagnosing and repairing the issue yourself, it's recommended to take your car to a qualified mechanic.

Ensure thermostat is installed right side up or it wont allow proper flow. If thermostat is correct other problems could be Air in cooling system, low coolant level, plugged rad internal, water pump failure,coolant flow valve stuck,internal engine combustion leak to cooling system.
Auto Repair Diagnostics How to Diagnose Cooling System Problem

Possible trapped in system. Bleed system, should have two small screw bleeders, one in front of left valve cover on pipe behind water pump and the other on coolant pipe outlet as you follow upper rad. hose towards engine. Engine cool, remove rad cap, open both bleed screws and fill rad until coolant flows free out both bleed ports. close both bleed screws, rad cap off, start engine and let idle until upper hose is hot to touch , shut off, let it cool down for 20 minutes, top up rad and install cap. restart car ,drive and check.

There may be a thermostat problem, but your symtoms indicate low coolant level. When coolant is too low, it will not circulate, so you get no heat in car, and the coolant remaining in engine block overheats because no cool fluid coming in.
You need to fill the system up properly then look for leaks.
With cold engine, open radiator cap. Fill with antifreeze directly in radiator. Start engine, top up rad if level drops. Put heat control on Hot. Fan on low, heat on high. Gently rev engine and top up rad as needed. IF as engine heats up, it starts kicking fuild violently out of radiator, then the thermostat is bad. if not, then recap radiator, top up overflow bottle and you're done. Look under car for any sign of leaks. Steady dripping indicates a leak, then address that.

Costs nothing to try this, just in case coolant level was low when or after bottle changed, and you got an airlock. Start car when cold, rad cap off and heater on. Now let in run until it comes up to normal operating temperatures and rad fan(s) kick on and off, 2 or 3 times, and add coolant to proper level in rad if it goes down during process. Then after that, put rad cap back on, fill coolant return bottle to proper level and try it. Quite often when re-filling an engine cooling system, people forget to turn on heater to get coolant circulating through the heater core.

It really seems like you'll need a radiator. The rad can appear to have good flow because even if it's plugged, coolant will bypass the cooling fins. If you can get at it, try putting your hand on the surface of the radiator when the engine is hot but not running. The rad will fell relatively cool across the surface if it's bad...hot if coolant is flowing through the core. Those hand-held laser thermometers are handy for doing this as well.

well it might overheat because air got into system, replace the thermostant and then fill system, run van keepin an eye on the temp, it will start to run hot because coolant has not filled the motor just the radiator, when it starts running hot shut it down, wait 3 minutes and start, now this is done with cap off,after restart coolant should get sucked in get filling radiator as needed,may get messy but got to bleed cooling system. hope thius helps.

when i had the problem it was my themostat. easy fix, may also be your radiator cap needs to be replaced.

That's probably coming out of your coolant return bottle. With rad cap off & engine cool, look at your level in the rad, if necessary top it up to an inch or so from top. Start engine, turn heater(not defrost) on and let it run until up to normal operating temperature. Keep an eye on temperature gauge & rad level. When it's at normal, watch for good flow going through rad when thermostat opens. after flow slows down,(thermostat closes) top it up if needed. If you watch 2 or 3 times for flow, and gauge doesn't go over normal, then put cap back on, then fill bottle only to mark for warm or hot engine, with coolant. If gauge goes up over normal from there, replace thermostat, and I'd get a new rad cap too. If that has electric cooling fan, make sure it comes on shortly after you see thermostat open & coolant flowing through rad.

Solve? Although I clicked the link to solve the problem, I have no answers but am tempted to go after honda.

Same problems still occuring on my 2003 honda civic (103,000)!

I have:

-Changed thermostat's twice (180 degree; Yes, it is correctly positoned with the jiggle tab to the top and the correct side facing the engine)
-Installed new water pump
-Checked coolant level
-Changed the timing belt
-Checked for head gasket symptoms (white exhaust, corrosion in cylinders/plugs, loss of fluid, checked cylinder compression (all check out)
-Had the entire system presure tested 3 times with no visible leaks
-Run the car for 45 minutes with no visible drips/leaks
-proped the front of the car up to get air out of the system

This weekend I had spare time on my hands so I replaced the thermostat (checked the other, which according to bioling water at 180 worked as designed) I flushed the entire system, took out the radiator and had it blown out, flushed the entire system, installed new upper and lower hoses, checked the fan switch, I will be replacing the coolant with new, installing a new radiator (since they cost only 100 online and I have the hole front end off anyway, I'll be reinstalling the fans and switches, and trying it out again.

I drive about 100-450 miles between overheating, but it is very sporadic.

Still have the same heater issues- cold air when stopped and not accelerating at times. I am tempted to check the heater core next- but there is no loss of fluid from that ethier?

When I get the system back together, I'll try running the car for a few weeks without the heat on to see if that corrects the problem.

All of the steps to repairing the cooling system seem easy and cost 1/10 of getting them checked at a shop if you have the time!

I'll update again next week!