!996 Deville, Northstar 4.6. idles at 1700 to 1800rpm and won't slow down. If I hit the accelerator the rpm will remain at the highest speed. I have cleaned the throttle body (twice), changed the

SOURCE: Intermittent High Idle, Dies on cold starts, bad mileage

These cars are notorious for intake manifold leaks. This could also be a throttle position sensor. If you have a MIL (check engine light) I would scan it with an OBD II scanner and pull the relevent code off the scanner it will give us a much more accurate direction to proceed

SOURCE: codes po174/171 cadillac deville northstar engine

Check for vacuum leaks around the intake. This is the most common cause for these codes.

SOURCE: 1999 cadillac deville overheating

my husband did everything that everyone else did that had this overheating problem. It was so weird that my engine cooled at a light and heated up going down the road. I have a north star 4.6 engine and this is natorious for happening. my check engine light also came on and so I had a diagnostics done which told me my #1 injector wasn't firing , car ran like **** as well. After putting in a new water pump and plugs,ect. and realized injector was good a friend told him we could have a blown head gasket. So just to see if this was the problem I went to buy a bottle of glassbeads and the auto parts guy told us to purchase a bottle of BARS STOP LEAK BLOCK SEALER NOT RADIATOR BUT BLOCK SEALER $31.99 . He followed the directions and we waited 24 hrs. before driving it. What ever the problem was has stopped my car runs like brand new and I am loving it again. no over heating no injector problem, runs great.......I hope this helps. I know its probly a temperary fix but its a fix and now we know what the problem was. SASSY

SOURCE: 1991 deville idling very ruff

Hi you should change the coolant temp sensor this is a white sensor on top of the intake and is part ot the computer system.

Mark Carrington

• 192 Answers
• Posted on Jul 01, 2010

SOURCE: I have a 1999 Cadilac Deville that I replaced a

Are you sure the thermostat was in the wrong way ?.

The thermostat is located in the bottom hose unlike most conventional cooling systems.

The thermostat in the Northstar engine is a dual action device.
One end of the stem is the Thermostat(obviously) and at the other end is a spring loaded pressure valve(the thinner end).

Make sure the thermostat end is nearest the rubber hose and the pressure valve end is in towards the engine.

Also note that the cooling fans don't come on until the coolant temperature hits around 225 degrees.
During normal highway driving the fans will very rarely come on(if at all) due to constant air flow through the radiator.They will only usually come on in heavy traffic conditions on a hot day.
But......if you have the Air Con switched on, the fans will run all of the time.

For overheating problems consider the following......

The most common overheating problem with the Northstar engine is a blocked purge line(small hose).The purge line is designed to purge air out of the cooling system.
There is a small diameter hose coming from near the top of the coolant tank (not the cap) ,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.
If the hose is clear, check for blockage at the nipple on the tank.Or trace the hose back until you find the blockage.
The hose goes in to the engine to a crossover and comes out the other side close to the thermostat housing,the blockage may be there.

Testimonial: "Thank you very much for your help. The spring of the thermostat was towards the rubber hose and it was the wrong one. I did have a blockage in the purge line. I also found a small leak somewhere under the crosover pipe. This is my first experience with a northstar 4.6L engine. I realy like the car, I just got to learn how to take care of it. Thank you so much again for your help and if you can tell me any thing else I should know I would be forever greatful. Richard "