I went to start it cold the other day and the thermostat went straight to hot and she did not want to start. Replaced ECT sensor (p0118), thermostat read cold. Rough cold idle, ran great after, never got warm. Replaced 6 month old thermostat, same results. After driving, the upper radiator hose is warm and the lower is cold, the gauge stays on cold, also the heat works better than it ever has. Replaced ELD(p1298), same results. Just now, to test the needle, I hooked the faulty ECT sensor and the needle reads cold. Any ideas?

 

Have you verified the ground connection on the thermostat housing is good? This is the main grounding point for the engine computer, which controls all engine management. If it's jacked up, it can cause all kinds of unpredictable issues. Great place to start, since it's easy and free to check/clean.

Also, it sounds like either your thermostat is possibly stuck closed, or you may have air in the system. Have you properly bled the cooling system with the blower temperature set to hot? Air in the system can also cause unpredictable readings on the coolant sensor, since it can't actually sense the coolant temperature.

 

I did check the ground, mainly because I noticed it was loose. I do feel like I haven't bled it properly. I followed simple YouTube instructions, is there a better resource?

 

I just did this...https://youtu.be/Dq-ykCWJa_o

Nothing different except the heat might be working even better.

How important is it to clear the engine codes? Also, is it possible to do so without a code scanner?

 

You'd have to disconnect the negative terminal on the battery and let it sit for a little to clear the codes without a scanner.

How many miles on the car? Does the coolant expansion tank fill itself up after a few drives?

 

I'm not sure about the expansion tank, it's been full to the top since I've been doing this. 180000 miles on it. 6 months ago I replaced the water pump, thermostat, radiator, and timing belt. No problems to speak of until this incident. I will let it sit and see what happens.

 

The reason I ask is a blown headgasket is fairly common on this generation. You'll get a bunch of phantom overheating issues that seem like other things. Drain the coolant expansion tank enough where it's back within the range of the hot and cold fill marks, and make note of where it is. If it keeps filling itself up and not sucking the coolant back in after it cools, you could have a blown headgasket.

 

The reservoir doesn't seem to change levels. Are there any other symptoms of a blown head gasket? It's not blowing smoke and it's not dropping fluids. I plan to monitor temps while it's running today, if only for fun. Thank you for all of your help.

 

Perform a cylinder leak-down pressure test on each cylinder to confirm a breached head gasket.
Or test for exhaust gases in the coolant.

 

it turns out the bad ELD killed the coolant temperature sensor I replaced. So.. replace ELD, then replace CTS. Thanks for all the help.