1999 Pontiac Bonnieville 3800 V6 runs good when cold fans operate normally ,operation temp 150 Degree running on highway ,Stop and go traffic temp goes up to 200 Degrees,stall out sometimes.Won't restart after car gets hot 200+degrees.Let car sit and cool down to 150 then starts rough but runs and smothes out .

there are several problem that will give this indication and your information is not comprehensive enough to make a good diagnosis
the time factor is critical as some time could be 5 minutes to 5 hours and driving is too loose as it could be at highway speeds or stop go traffic , hill work or under load
so I will give to certain conditions that will over heat the car or cause a rise in operating temperature
head gasket--check with compression test
low water level-- check at cap
faulty radiator cap--have tested
partially blocked radiator--have a flow test done
fins on cores falling off --replace radiator
overheating transmission oil--check the transmission is fully operational as high speed with out over drive in will cause a rise in temp
if in traffic --check viscous fan and fan operation if electric
if under load -check load and tyre pressures
check for brake operation- adjust hand brake to full off
run the fault codes to check air/fuel ratios and timing
check for failed cat converter-- blocked exhaust system

check you're fuse's for the fan. At highway speed's there is enough air flow for the radiator to cool the motor. If not run the fan from a switch from battery to fan as a last resort.

Engine temperature switch tells the fans when the engine reaches a temperature in which the fans should operate. Change this and you will more than likely solve the problem.

Many things could be your problem
1- coolant temperature or wiring defect
2- Cooling fan not working
3- Need to retrieve any engine faulty code with a scanner.
4- With a scanner look for any data that is not normal.
See a good auto technician

Hope it helps.

the temp seems to be fine. 80degrees celsius or less in stop and go traffic is normal,especially since the cooling fans are working and coming on at the proper time. the what seems to be running rough is another problem that doesn't seem to related to the cooling system temp. may want to check the spark plugs for worn electrodes.

I assume it's the 3.8 V6? They are notorious for intake gasket problems. The fan problem could be a few things, fuse, relay, ECT (electronic coolant temp sensor), ECM, Do the fans come on when you run the A/C? Hope this gives you some ideas. Let me know a little more and I'll try to help. TJ

The are only suposed to come on when the engine temp starts going over operating temp, like standing traffic. if then they don't work check the engine temp sensor

Does it not run at all? Or does it only run after the fiero gets really hot (like sitting in traffic on a 90 degree day)?


But a low-temp fan switch (sensor), which causes the fan to turn on at a much more reasonable level. I think the stock switch doesn't turn on till it reaches 235 degrees or something.

The switch/sensor is located near the top of the engine (on passenger side end of engine area, opposite end of the throttle body).

Buy it here (or other places have them too)
http://www.rodneydickman.com/n20.html

You should be able to see where it installs by matching up the new part with the old, pretty easy operation. A little coolant might squirt out, so only do this when the engine is cold.

Hope this cures it

Check to make sure the radiator fan is coming on watch it for ten minute as it runs it should come on if it doesn't replace the fan relay or the fan coolant temp senson located on engine block upper area of engine.

Don't run the car without a thermostat. It helps the car to reach operating temps quickly. also helps burn off blow by gasses by keeping the temps up and steady. check your radiator level once driving a few miles. Their may have been some air pockets that purged and need to be replaced with additional fluid.