: 01 STS running rich

04-07-08, 08:56 PM
Hear goes,

Have had the car about 2 months, love it. After about 2 weeks code P0420 tripped. Looked it up and found it indicated Catalyst System Low Efficiency. Car has some black exhaust under med to hard acceleration, sooty deposits, fuel smell and poor mileage. Changed plugs and filters. No other codes, can reset but P0420 will reappear after about a week of normal driving or 10 minutes if I get on it. Have read hear that it is likly FPR, sticky injector or o2 sensors. As it seems to be an overall rich condition as noted by the plugs and not specific to any one cylinder should I look at anything else. i will be replacing the FPR this weekend and will attempt to test the O2 sensors.
82k miles runs great other then above.

Great site


04-07-08, 11:13 PM
You said that you have read here, so am wondering if you have done a Wide Open Throttle yet, WOT ? Got this from the black banner at the top. I clicked on Technical Archive.

There are many advantages to occasional full throttle accelerations with a Northstar and any engine.

It keeps the carbon cleaned out of the combustion chamber. This is maybe a little more important with the Northstar than some other engines due to the tight squish volumes between the piston and the cylinder head. It's designed this way to promote good in-cylinder mixture motion (good combustion) but it has the down side of providing a ready place for carbon build-up to touch the piston - causing noise. Ever heard of the Northstar "cold carbon rap" problem?? Simply put you'll hear a rythmic, piston slap-like noise when the engine is cold. Very prominent and very annoying. Cause: excessive carbon build up causing the the piston to contact the carbon on the head - causing it to rock in the bore and "slap" Much more evident when the engine is cold and the pistons haven't expanded to full diameter yet. Simplest and easiest "fix" for this: A few good WOT (wide open throttle) accelerations to clear the carbon out. That is all it takes to eliminate the problem and prevent it from re-occurring.

Occasional WOT accelerations also help seat the rings to the ring lands and exercise the rings and keep them mobile and from becoming stuck in carbon in the ring lands. At high RPM and WOT the rings move around on the piston - they actually rotate on the piston and will polish away any carbon and seat themselves to the sides of the ring grooves. This is especially important on the 2000 and later Northstars which had hard anodized top ring lands on the pistons. Very hard and wear resistant - also harder to break-in and seat the rings to the sides of the ring-lands to promote the best possible seal. Many oil consumption complaints on the 2000 and later engines are related, to some extent, with the rings never seating to the sides of the ring-grooves due to lack of load as the engine was babied around forever. Even engines with rings stuck in the ring-grooves due to carbon build up can eventually be freed up with enough high RPM operation.

WOTs warm up the engine thoroughly and clean out the exhaust due to temperature in the exhaust and high flow rates blasting particles, rust and such out of the system.

Frequent WOT operation will not hurt the engine or the transmission. They're designed for that. The healthiest engines that I have seen at high miles are always the ones that are run the hardest. Rings are free on the pistons and sealing; no carbon buildup.

The exercise that I think works best for many things is to select manual 2nd gear on an isolated stretch of expressway. This takes the transmission shifting out of the question if you are worried about hurting it. Start at 55 MPH or so and go to WOT in 2nd gear and hold it until the RPM reaches near the normal shift point - i.e. 6500 for an L37 and 6000 for an LD8. Hold the throttle wide open until the engine reaches, say, 6200 for an STS and then just let completely off the throttle. Leave the transmission in 2nd so that the engine brakes the car and creates some pretty heavy over-run conditions at high vacuum levels. Let it slow until it is about 55 or so and then go to WOT again and repeat. This exercise really loads the rings, allows variable RPM operation at WOT for several seconds continuously, creates heavy over-run which tends to unload the rings and make them move and thus exercise them in the ring grooves and it will blow-out carbon and the exhaust - all without creating a spectical of yourself and attracting the attention of cops. You can do it on most any freeway and stay within the 70-75 MPH range allowable. Once a week like this will keep the engine cleaned out and healthy and is DEFINITELY recommended for the Northstar in particular.

The Northstar engine was designed/developed/validated to be run hard. It was expected that people would use the performance of the engine - which few people seem to do. The biggest single problem that many issues stem from is lack of use at full throttle by the owners. It just doesn't like to be babied around. The rings are low-tension by design for good high RPM operating characteristics and low friction/good power. They work best if "used" and kept free.

In every conversation with owners I have had, once the owner started doing the WOTs and using the power of the engine they report no more carbon rap, better oil economy, no "smoke" when they do light it up (keep the exhaust cleaned out. If you notice a "cloud" at WOT then you are not doing enough WOTs...) etc... A bit of judicious use of the other end of the throttle travel is a GOOD thing...

04-08-08, 05:30 AM
I have run it a bit now and then but not like you are suggesting. I will give it a few runs and see what happens. I do have the "cold carbon rap" you refer to and have not heard of that term before. I am looking forward to seeing if I can make that go away. It is very annoying but completly stops after the cars warms up. I know just the road for this am. Thanks for the time and the info.

04-08-08, 01:38 PM
#03-06-04-028: Service Engine Soon (SES) Light On, Reduced Vehicle Power/Acceleration/Speed, Diagnostic Trouble Code (DTC) P0420 (Diagnosis and Replace Catalytic Converter) - (May 15, 2003)

Subject: Service Engine Soon (SES) Light on, Reduced Vehicle Power/Acceleration/Speed, Diagnostic Trouble Code (DTC) P0420 (Diagnosis and Replace Catalytic Converter)

Models: 2002-2003 Cadillac DeVille, Seville

04-09-08, 05:34 AM
It could also be a faulty coolant temperature sensor. If the sensor never tells the ECM that the coolant is warm/hot the engine will remain in open-loop condition and run rich. It sounds as your engine doesn't go to closed-loop, which leans the mixture for normal running.

04-09-08, 06:37 PM
I have done about 10 WOT runs as suggested in 2nd gear and no longer have black smoke or the cold carbon rap. Still have a rich smelling exhaust but it is no way near what it was. Gas mileage is up about 3mpg on a stretch of highway I used to test the lattest. How would I check the temp sensor? is it just a temp switch that closes at temp or is it a variable output type device? Also would I not get a code of some kind if the ECM never switched into closed loop operation.

Thanks to all for the suggestions

04-10-08, 08:51 AM
The coolant temp sensor is just to the right of and under the throttlebody, in the coolant crossover. It is a temperature variable resistor (thermistor) which "talks" to the PCM and temperature gauge, supplying a variable voltage as temp rises and falls. The PCM queries the circuit at (don't quote me) 1 kHz data rate and uses the temp signal to control other functions.

Where does your dash temp gauge run all the time?? If it's right around 12:00, then it's safe to assume the sensor is OK because the gauge gets the same info as the PCM (temp-wise).

If you pull the O2 sensors you risk getting oil or grease on the tip...no-no. At 82k, the sensors should still be OK. If you can, disconnect and reconnect each one (3) twice. The sensor after the cat has a connector up in a heat shield which is pop-riveted to the floor pan on the right side, about under the passenger seat. PITA. You can get to that one when you install your CORSA exhaust......

Pursue the fuel pressure idea, and dump a 20 oz. jug of TECHRON on top of a half tank of 93. (TECHRON is cheaper at West Marine.......) and then run the devil out of it again.

When everything's right, your road mileage should be 23 - 25 mpg at 65-70. Rural, about 21-22. City, low as 13-15. Car will run just fine on 87.