98 Lumina - Over Heating, Can't Nail Down Issue

Right off, I'd call myself a intermediate DIY when troubleshooting issue's.

In this particular case I'm sure I have probably missed something somewhere, and after reading numerous google searches - I'm not sure 100% in which direction I should go.

3.1 1998 Lumina
114K Miles

First off - these are the symptoms:
1) Engine overheating when idle in stop and go traffic, otherwise normal cooling.
- Troubleshooting already done:
Checked fans, have not jumper cabled yet as they seem to be working properly. ie
a) Does not overheat when heat is on, or more accuarately have not noticed if it has, but cools when heat is turned on or A/C engaged as observed.
a) Unplugged ECT, both fans turned on within a few second and would stay on until ignition turned off (ECT has been replaced as)
b) Hand felt all three relays in engine compartment at ignition and all three would actuate when current applied.
This included observing each individual fan shutting off or staying on according to which relay was removed. All seemed to have an effect on fans being on or off. Also flip flopped relays #1 and #2 with no noticed change from current operation.

Flushed radiator as DexCool sludge was obvious. Originally thought was the issue to begin with. Didn't realize the importance of removing thermostat at the time, and will r&r thermostat and back flush soon.

Low coolant light is on currently, and has flashed numerous times, even when coolant completely full. Checked oil for milkiness, and no water observed in oil. Also have not noticed a considerable loss of coolant, ie LIM gasket blown - but no doubt it still could be as I do not know ALL the symptoms of that issue and that seems to be common.

Also, have observed "softness" of upper radiator hose, and hose is hot after idle, indicating at least flow, but more than likely not enough? Softness as in I can squeeze the hose, have observed coolant shoot out of return side bleeder valve, but feels like there is definitely not enough coolant in hose.

Currently the only code thrown to the CEL is a p0401 which will be tackled next, or if I need to rotate the engine forward.

Next step to isolating and fixing problem?

Unless I missed it, I didn't see "properly bled cooling system"....this could well be the cause of all of it...

Dex shouldn't sludge unless it has oil in it. I'm still wondering about the LIM.

Unless I missed it, I didn't see "properly bled cooling system"....this could well be the cause of all of it...

Dex shouldn't sludge unless it has oil in it. I'm still wondering about the LIM.

I've jacked up the front end of the engine three times and bled the coolant from both the front and rear bleeder valves. Where else would there be air trapped at that I could have missed, then? Hot air is blowing warm. After the second time - hot would be intermittent, bled again, and that's been fine, since.

Ever leave one open slightly and watch it for a while as the motor runs, fully warmed up?

Does it keep blowing bubbles or solid liquid stream? Never-ending air while running would be indicative of a coolant jacket leak.

I've found you can't get a good bleed with a cold motor...I fill, bleed, run to operating temp, shut off, wait ten minutes, bleed while filling, restart, run to operating temp, repeat, etc, until there's no more air. Need the heater on, so air doesn't hide there, too.

Never had to do anything to the motor as far as positioning...

Had the same problem, replace the thermostat and the ECT sensor. Even though this is obvious, make sure you have plenty of coolant in the expansion tank.