Please help with overcooling issue

[chevy468]

Hey guys, I've got a strange problem. My 04 GTP started throwing a P0128 code which is low coolant temp. So I replaced the temp sensor, which didn't fix the problem so I replaced the thermostat. That worked for about a week. Then I get the same code again. Figured I got a bad thermostat so I replaced it again. Same problem. If I let the car sit and idle, it comes up to temp just fine and the fans cycle on and off as they should. But when I drive it, it never gets even to the quarter mark on the guage. I had it scanned and the coolant temp doesn't get higher than about 174 degrees. I've had it to 2 shops and talked to about 5 more and no one has any idea what's going on. I have great heat in the car, it's just not getting up to normal temp when I'm driving it. Anyone have any suggestions? Thanks

[FoSHO99]

What are the exact thermostats you're replacing it with and do you do any modifications to the t-stat?

[chevy468]

I got one from a Napa store and one from an Autozone, both were 195 degree stats. No mods, just swapped em out.

[Scottydoggs]

check the wires on the plug for the temp sensor, they could be all cracked or missing insulation.

[dagger]

Hmmm how does your coolant look? Also pull your main accessories belt and just grab and wiggle the water pump pulley and check for any play.

[chevy468]

Just checked the wires, coolant and water pump, all look good.

[BillBoost37]

What you want to do is confirm the pcm temp is what you see on dash.

This is an odd problem. Because your tstat keeps the motor temp at 195F when it passes the t stat. If it opens too much, you can overcool. But..the regulating item is the T stat

[chevy468]

How do I confirm the temp is the same? I've hooked it up to a live stream scanner and it looked to correspond pretty well.
It's very odd and it's driving me crazy. There's not much it could be, or so I thought. I suppose it's possible I got two bad thermostats, but it's pretty unlikely

[TheOtherNick]

Point a lazer thermo at the upper rad hose and see what that temp is.

[chevy468]

Well that just sounds too easy. I'll do it tomorrow and post my results. Thanks everyone

[chevy468]

Stopped at the parts store that I got the first stat from to get another one. I figured what the heck, third time's the charm right? I inspected it before I bought it, and it had a nick out of it and didn't seal at all around the whole ring. The other 5 (All Stant) they had in stock were all the same. I passed and went to a Parts Master store and looked at theirs which is a MotoRad, and it sealed perfectly around the whole ring. I'll install it in the morning and see what happens.

[chevy468]

Well I checked the new one on the stove before I installed it and it opened at about 200. Put it in the car and it's still doing the same damn thing. Anyone got any other ideas?

[BillBoost37]

Is the coolant level in the LIM up to snuff or is it low. Might be not taking coolant, which won't give you a good temp reading/fluctuating

[chevy468]

Bill, yes they both show it running cool. there are the 5 "big" marks on the guage..low end, quarter, half, there quarter and high end. There are 3 (I think) smaller lines in between the large ones. It's not getting above the second small line from the large low end mark. Seems to run cooler with the cool weather. I put a Delco stat in it yesterday and it had the gasket already on it so I just popped it in and am still having the same issue. It's turning on my light within minutes of clearing it out.

[CTKGP09]

when my LIM gaskets went bad my coolant temp never read up to the quarter mark either, replace the gaskets and the PCV valve and the fixed the issue

[chevy468]

Just replaced the gaskets, no change. I appreciate all the help guys, thanks

[chevy468]

Badass, that's odd. I have never heard of these issues before, and the fact I can't figure it out is killing me. Especially since it should be so simple. At least I'm not alone here.

[chevy468]

Well I ended up replacing the coolant temp sensor with a Delco unit, fixed the issue. I find it odd that 3 different shops swore the sensor was reading correctly, but at any rate that was the problem. Thanks for all the help everyone.

[Panzer4]

Glad you got it fixed (lets hope its permanent this time!) but, if you don't mind me asking, since you were swapping parts, wouldn't it have been cheaper and easier to replace the sensor first, and then all the other gaskets?

Just wondering - no hate. I guess the lesson to learn is to not trust most shops!

EDIT: Did you notice any difference in performance due to the low coolant temperature? Or was the temperature reaching normal and the sensor just not reading it correctly?

Thanks.

[Badass69]

Welcome to my world Chevy468. I have 2 of these cars, a 2002 GTP and my 04 Comp G. Prior to me tearing my 04 apart it was throwing the same p0128 code you are seeing. Now it was doing exactly as yours....... sortof. Mine refused to exceed 200 degrees without sitting and holding it at about 3000 rpm in park for a couple minutes. Otherwise my fans never cycled, nor did the engine ever actually even get warm. It blew good heat but about 1/4 mark on the gauge, that's it. Upon tearing it down it needed LIM gaskets of course but had a 195 tstat installed. I am not reassembling that car stock so I really will never know with it, and it's getting a 180 tstat.

The 2002 however is stock, does not need LIM gaskets ( replaced and leakage free for about 1.5 years now) and does the same damn thing as the 2004. It never has tossed a code, but verified by my scan tool and so forth it has trouble exceeding around 80 degrees C under normal driving ( or 176 F). Odd that's exactly the same as the 2004.............. And the 2002 has done this from day one of ownership as well.

So do I have 2 seperate GTP's of different ages with bad 195 degree t-stats? Maybe? However if you verified your t-stats operation and still no sucess...... Well I don't know. The 2002 went through last winter just fine and never gave any issue, but the engine never even remotely hits the 195 degree setpoint, just like my other 2004.

It is cool here too, approaching around 32F or less at night and it compounds the issue. I know I am of no help, but I have double your issue!