This same thread was posted to another site, but I'm hoping to get as many opinions at this point.

Running a 98 GT, it's been an interesting project. After the job the engine gets up to temp at a normal rate, the temperature peaks just above the 235 mark and never goes below about 215. Before I never saw it above 210.

Here is what I've done:
Flush cooling system with water
Remove and clean throttle body, upper and lower intake, valve covers, tensioner.
Remove heater and coolant hoses
Remove water pump, athough I didn't have to, I did it to check the fins
Reinstalled water pump, however without changing the gasket (Though I don't see an obvious leak)
Installed LIM gaskets, applied rtv to the only the 4 corners, Installed LIM, torqued bolts to 12 ft-lbs, new coolant temperature sensor installed improperly (more later)
Installed new tensioner and coolant elbow (my car has only one plastic elbow), new water outlet, new 195 degree T-stat, new coolant hoses, new intake plenum and everything else per the GM service manual. Filled with Prestone coolant
Fired her up no problem, pulled out of the driveway, temperature gage shot up past 210 before I could reach the end of the block.

Coolant temperature sensor was not properly tightened, nut was stripped due to close proximity to exhaust crossover pipe.
Removed crossover pipe, installed another new coolant temp sensor to torque spec.

Flushed cooling system, now running straight water. Engine heats up at a normal temp, stays up at about 230ish. Outside temp in the 90s.

Flushed cooling system with water again, fill with proper antifreeze mix 5.01 liters pure Prestone, rest water. Still hovering around 230ish. Replaced radiator cap, 195 degree thermostat again, same problem. Fans turn on at 230, turning on AC will allow engine to cool to 215, but never lower. Let sit overnight, morning no coolant on the driveway, coolant reservoir and radiator are at proper levels. I drove over 100 miles today (75 degrees outside), never above 240, never below 215, it likes to sit at about 225-230 while driving.

Here is my parts list:
-GM Lower Intake Manifold Gaskets 89017816
-Gates Lower Radiator Hose 22373
-Gates Upper Radiator Hose 22344
-Gates Heater Hose 19109
-Gates Heater Hose 19110
-Gates Drive Belt Tensioner 38152
-GM Coolant Elbow 24503423
-Temperature Sensor Napa ECH TS4020
-Radiator Cap Murray Caps 7616
-Duralast Thermostat 15849
-Thermostat Gasket Felpro 35666
-Water Outlet Autozone “Compressor Works” 815167
-Dutalast/Felpro Water Outlet Gasket 35595
-Felpro Valve Cover Gaskets VS50080R
-Felpro Valve Cover Grommet Set ES72833
-Purolator PCV Valve PV892
-Intake Plenum ineedparts.com GM Intake “Sure Fix” Manifold Kit 17806
http://www.ineedparts.com/auto-parts/in ... 17806.html

My ideas are:
-Bad guage/pcm/something and the engine is actually running at proper temp
-Clogged radiator, however there are no cold spots
-The new intake plenum, but it advertises to lower engine temps
-Water pump gasket leak. I do not have a pressure tester to make sure cooling system is about 15-16 psi
-I screwed up somewhere and don’t know it.

I know some would say 180 degree T-stat, but I’ve had 2 open at the proper temperature (verified by having the bleeder screw open) and I don’t believe that would fix this.

I do not believe it is a bad temp sensor, this does this with 2 sensors. I also do not believe it is a head gasket or LIM gasket since the oil is not milky. When I pulled off the throttle body when removing the crossover pipe, I did not notice any leaks from the upper plenum.
I have the Scantool.net Bluetooth scanner coming in the mail so I can check coolant temp and other parameters with my Android phone.

Are you positive that you got all the air out of the system?

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Dubs:00 VW GTI VR6, Low and Slow 91 VW GTI 8v, Shave the Bay!! 91 VW Cabriolet 8v, bitch Basket
GP's:04 GP GT, Past now G-Ma's Girlfriend's 97 GP GT, "Project Pink Power"

The temp would fluctuate if it were air

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Dthreap wrote:

Yeah make sure your O2 isn't behind lazy, you want nice up and down.

Planeboy18 wrote:

^^Pro tip for more than just O2 sensor signals

It has done this twice with 2 different bleeding methods. The first time with straight water, without using the bleeder screw and by the gm service manual instructions, 3 heating and cooling cycles while checking the radiator level. The second time with a proper antifreeze mix I waited until the bleeder screw had a steady stream before I tightened it, 3 heating and cooling cycles along with aggressive driving today and the radiator is full.
At the moment I'm beginning to believe it's related to the coolant temperature sensor, but I can't verify until I hook up a scanner. The wire harness is right next to the exhaust crossover pipe so if the scanner reads a normal temp then I will believe the dash is picking up the wrong temp due to a bad wire harness.

The temp guage never moves suddenly, always at a slow rate. Everything else seems fine, 27mpg highway, no hesitation, heat and ac feel normal. The temp gauge moves within a normal range when driving and when the fans turn on, simply about 20 degrees higher than what I'm used to seeing. It never did hit the red zone or turn on the dash dummy light.

Excellent, just wanted to make sure the easy stuff was covered. No one likes spending money on stuff to later find out the fix was free Yeah air in the system would cause temp fluctuations but the clusters sometimes don't respond quickly enough to pick up on those fluctuations sometimes to display them accurately. I just asked since I wasn't sure how much the OP had actually driven the car and if it was a sort of thing like "Oh crap my gauge says I'm running hot, I'll baby it back home" sort of thing, which is usually something I end up doing haha. I know on 2 specific 3800's I've worked on a small air pocket caused the cluster gauge to read crazy high constantly until the system was bleed again (on the Le Sabre the digital display shows the fluctuations but the analog gauge just kept reading high).

A scanner will just tell you what the dash is already trying to tell you (to a higher accuracy though of course) since that harness acts as a signal for both the PCM and cluster, so chances are the sender is defective and just reading too high which would cause the dash and the data to the PCM to both display high temps. I'd also rule in favor of the defective sender since your gauge has read fine before changing it out (rule of thumb that if things don't work after changing something, the problem is caused by what you changed, in this case the radiator being clogged can prolly be ruled out since it was fine before, the UIM don't change the coolant temps, and a leaking WP gasket won't cause temps that high, so temp sender would be the best bet to look into). If I were you I'd try an OEM GM sensor to see if it also is reading too high, try the old sensor you replaced (if you still have it), or take the Napa one back and say that one is also defective and you want to try one more . You can try scanning it but it will more than likely just tell you it's running at the temps you already listed.
Hope it all works out for you.

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Dubs:00 VW GTI VR6, Low and Slow 91 VW GTI 8v, Shave the Bay!! 91 VW Cabriolet 8v, bitch Basket
GP's:04 GP GT, Past now G-Ma's Girlfriend's 97 GP GT, "Project Pink Power"

I appreciate the ideas, turns out my cheap china scanner (ELM327) finally decided to work this morning. The guage reads 230 while the scanner reads 208.

Unfortunately the old coolant temp sensor broke in two. These sensors have 3 pins, from what I understand (I haven't looked at the manual diagram yet) one is an input, two output. The PCM and dash gauge are on a seperate output. I will assume the sender is fine and I will check the wire harness, I wouldn't be surprised if wires rubbed against the exhaust crossover pipe. It's comforting to know that the engine is running absolutely fine. Maybe I'll get a 180 T-stat but the 195 makes winter startup realll nice here.

Hopefully the GM dealer won't screw me over on the price of a small wire harness.

Here's the diagram. Indeed there are 2 outputs.
Try looking at Document ID: 561042 and Document ID: 561314 in the service manual. They have some good info.

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Dubs:00 VW GTI VR6, Low and Slow 91 VW GTI 8v, Shave the Bay!! 91 VW Cabriolet 8v, bitch Basket
GP's:04 GP GT, Past now G-Ma's Girlfriend's 97 GP GT, "Project Pink Power"

Thanks for the diagram, saved me a little time.

The wire harness is $65 at the dealer, so for now I will ignore the dash gauge until I take the time to fix the problem (new trailing arms are ready to be installed). My first thought is to swap the yellow and green wires if I don't notice any obvious fraying. I wrapped all wires in flex tubing 2 years ago after a mouse chewed up some wires, so I wouldn't doubt if this were one of them. If this is the case I may either splice new wire or buy new connectors and make my own harness.