This is my 1999 Bonneville SE with 114,000 miles. I've had it about 3 weeks now and have replaced the thermostat because it was stuck open and only would hit about 150 on the temp gauge. Replaced the air filter and the fuel filter which was a replacement and not the original. I have a new Gator belt, AC Delco plugs and wires to replace still. Also fixed the nasty rust holes in the right rear wheel well. Also found that my rear bumper reinforcement tube is really rusted out. I was going to replace it with a straight tube, but decided to just drive the car for the cheap transportation that it was bought for.

So now that it has gotten cold out, and I replaced my thermostat, I expected a nice hot ride in to work. But that isn't the case. It only blows mildly warm air and it seriously takes 30 minutes of driving before it feels warm in the cabin. Plus my temp gauge on the dash seems like it fluctuates alot between about 195 and down to like 160? I don't think it gets down to like 150 where the needle stayed before I replaced the thermostat. It does seem to rise back up to 195 fairly quickly after it was down to about 160. I'm guessing the heater core is bad? When I replaced the thermostat, I syphined out about a gallon and a half of coolant and it was fairly pink/orange still, so I'm guessing that it has had the coolant replaced because it isn't a nasty brown color. I thought about flushing the radiator, but would like to just flush it after/while replacing the intake manifold gaskets which I know I should have done pretty soon..... just finding the money to do it though.... The previous owner who had the car for 6 1/2 yrs and had it since it had 32,000 miles did not have the UIM and LIM gaskets replaced, so I'm assuming they are still originals.

It still feels like the motor bogs/hesitates when I am making a turn, either while moving or from a stop sign. Maybe a bad wire grounding out while turning the car? Sometimes I think that the tranny just isn't kicking down? From a stop going straight it runs fine and shifts fine.

Thanks for any input.

So everyone thinks that it is normal for the temp gauge on the dash to flucuate between 160 and 195?

It should not fluctuate that much. What temp thermostat did you install? A 180* or a 195* The 195* is stock and that'* where the PCM is programed. If you installed a 180* then the PCM is not turning on the fans at the correct time and could be why it'* jumping around.

As for the hesitation. Give a look for a vac leak. Do you have any check engine lights?

Two things I'd check first.

Are you sure the radiator itself is full after filling with new coolant. Check the level in the radiator and besure to clean the underside of the cap where the little plate closes off the flow to the reservoir and opens to let coolant back in when the radiator cools. I use an old toothbrush.

I'd check to be sure the thermostat is in with the heat sensor down toward the hot water from the motor? And did the thermostat go into the middle of the rubber seal ring in the groove for it to make a good seal.

I installed a stock 195 t-stat from NAPA, with it in between the rubber seal and correct side down. I sealed the t-stat housing down to the block with blue silicone permatex. Then I filled the radiator with the coolant until it was full and didn't go down anymore, then dumped the rest into the reservior. I checked it a couple of times and it was still full. I looked at the radiator cap and it wasn't too nasty. Maybe I'll scrub it tonight just to clean it.

There are no check engine lights on. I don't know how I'd check for a vac leak that happens while the car is going around a corner?? haha because it accelerates fine in a straight line.... I hoped the new fuel filter would take care of that problem but it didn't. If I get time tonight I'll get under the hood and wiggle the vac lines while the car is running and see if the idle changes.

Thanks for the suggestions.

Make sure the engine is not to hot and no pressure in radiator, pull the cap off and start engine. Then wait for it to get up to temp. Carefully look in the radiator, is the fluid moving. You will see it moving rather fast. It should look rather still until the thermostat opens, and then move fast. If it'* moving all the time, the thermostat is stuck open. If it'* not moving at all, it'* stuck closed or the water pump is not working.

To check for vac leaks you need to have engine running. Then use carb cleaner or better yet, TB cleaner. Spray it on each vac line and connection. If there'* a leak the engine RPM will go up.

Wouldn't the engine overheat if the thermostat was stuck closed or if the water pump wasn't working? I have a 35 mile commute to work and a 47 mile commute home and this has been like this for 2 weeks now, without overheating at all, never gets over 200.

I will try and check the coolant flow tonight if the car cools down enough after running my errands.

I forgot about the carb cleaner testing trick. Thanks

https://www.gmforum.com/t199069/

Maybe I'll just try a different T-stat....

I didn't get a chance to mess with the car last night.

Just found this on a website:

If you own one of these vehicles…

1995 - 1997 Buick Riviera 1995 - 1998 Oldsmobile Eighty-Eight
1996 - 1998 Buick Regal 1998 Oldsmobile Intrigue
1995 - 1998 Buick LeSabre 1995 - 1996 Oldsmobile Ninety-Eight
1995 - 1998 Buick Park Avenue 1995 - 1998 Pontiac Bonneville
1998 Chevrolet Lumina 1997 - 1998 Pontiac Grand Prix
1998 Chevrolet Monte Carlo

…and are experiencing any one or more of these complaints:

• Smell of Coolant Burning • Poor Heat
• Coolant Leaks • Temperature Gauge Fluctuation
• Low Coolant Light On • Engine Uses Coolant with No Visible Leaks

What these vehicles have in common is the 3.8 liter engine (VIN Code K). These complaints will likely be caused by a bad Upper Intake Manifold. This part is made of a composite plastic material that degrades over time. An updated intake manifold is available for these vehicles. This should also be a very easy checkout and can be done while you wait. Please give us a call to make an appointment.

LIM and UIM gasket failures are some what common. A fairly easy fix.