1996 Cadillac Seville SLS -

At about the 101,000 mile mark, my car started running a little rough. I changed out the spark plugs a couple of weeks ago, and the idle would still fluctuate. I knew I needed to replace the wires then also, but i didn't get around to it. In the course of running it for about a week, the timing seemed to adjust a bit and although it idled rough, it wasn't fluctuating anymore. So this weekend I changed the wires. The timing started to fluctuate again. I thought I had heard that the timing would adjust itself a bit, but is that really true? If so, how long?

Also, the service engine soon light came on last week, and my display tells me Check Emissions system. I pulled the codes, and amongst a few other codes that didn't seem relevant, I got these:

P0137 - Heated Oxygen Sensor (HO2S) Circuit Low Voltage Bank 1 Sensor 2
P0300 - Engine Misfire Detected
P1139 - Heated Oxygen Sensor (HO2S) Insuff. Switching Bank 1 Sensor 2

So I think I need to replace this o2 sensor, but where is it located? After I replace this and fix my timing, I think I will be able to at least pass smog.

Thanks for any advice! :thumbsup:

At 101K your O2 sensors are probably marginal in their ability to quickly react to changing exhaust gas conditions. Not sure how many you have, maybe 3 or 4 ? Other controls, such as idle air control (IAC), throttlebody (TB), and perhaps fuel pressure regulation are subject to gradual degrading from deposits and wear over time and miles.

You're in the gray area of a good, comprehensive tune from either yourself or a good tech who gives a damn.

(You won't fix your timing. It's set forever and controlled by 3 or 4 internal sensors.)

Crank sensors would throw a code, right?

Yeah, that ^^^ and he would probably have a low speed/in-gear stall condition to boot. The HEO2 sensors are probably driving fuel trim all over the place.

Thanks for the replies.

I've never stalled, although only one time it dipped into almost stalling before I replaced the spark plugs.

So do you think the o2 sensor is causing my rough idle, or will the timing eventually smooth out? I don't know how many sensors I have, I just read somewhere that they will be along the exhaust, before the catalyst and after it. Where is this one in particular - "Bank 1 Sensor 2?"

Thanks again!

^^^^ 1996 is a different breed of cat (nah, same breed, different fur...) from the post-'99 years. You'll need to hook up with someone who has a Factory Service Manual for your year. Some codes and symptoms are different depending on the management system aboard.

You may have an O2 sensor in each manifold collector, one before the cat, and one after.

The idle condition probably has nothing to do with timing. A lot more than timing controls idle quality.

A lot more than timing controls idle quality.
Like, say, the o2 sensors?

Bank 1 Sensor 2 is the pre Cat sensor.

Timing is controlled by the PCM based on inputs from many different sensors, such as TPS, MAF, MAP, spark knock sensor, and I am sure many more that I am not thinking of at the moment.

Like, say, the o2 sensors?

Even if your o2 sensor was plugged, or even not there, it would not affect your idle (not enough to make it too bad). 02 sensors will affect fuel consumption, AF mixture, acceleration, smog, making car running too rich or too lean.

Rough idle could be caused by; engine being dirty inside, wrong oil for your car, faulty sparkplugs, bad wires, bad rotor/distributor cap, ignition module, fast idle valve (if applicable) MAF, Coolant sensor - can cause it to run too rich ending up with your car not being able to burn the fuel, and run rough, clogged cat or muffler, and tons more.

To get rough idle out of the way, run some seafoam through the oil crankcase or through your vacuum line from brake booster or just put it in your gas tank.

Also when you changed sparkplugs did you put your wires on the right plugs?

Wait a minute. Wrong oil or dirty engine will not cause a rough idle and snake oil will not cure it.

I made sure that when I replaced the wires, I did them one at a time so that I wouldn't mix them up.


Timing is controlled by the PCM based on inputs from many different sensors, such as TPS, MAF, MAP, spark knock sensor, and I am sure many more that I am not thinking of at the moment.


And if I was having an issue with any of these sensors, wouldn't I get an error code?

Also, something I didn't tell you guys. The PCM is definitely not functioning. It wasn't ever since I bought the car because the water pump crapped out at one time and screwed it up (the mechanic thinks). But they wanted like $700 to replace it, and the car ran fine so I said screw it. Anyway, here's a complete list of my codes:

PC
0137 Current
0300 Current
0603 History
1139 Current
1599 History

TC
0033 Current
0063 Current
0083 Current

RF
2560 History

And if I was having an issue with any of these sensors, wouldn't I get an error code?
Yes, they should.

nm....

Thanks for everyone's help.

I am going to replace that o2 sensor, and hopefully the timing will adjust. Since I put the new wires on, I only drove it around for a few minutes. Maybe after some prolonged driving it will straighten out.

Wait a minute. Wrong oil or dirty engine will not cause a rough idle and snake oil will not cure it.

Doesn't heavier oil congest your engine? And as far as I know, if your throttle and inside of it is dirty, it roughs up your idle. BTW not arguing with you or saying your wrong Ranger.

And whats up with you and snake oil? Seafoam is seafoam, not snake oil, and it does help a lot.

kenneth......PCM = Powertrain Control Module. If it were not functioning, even a little bit, your car would do nothing, nada, zilch, ever. Not run, not start, not idle. Nothing.

Download the code list from the N* performance forum and figure out what you have.

Heavy oil does not "congest" an engine, per se. It resists flow through tight tolerance machinery bearing areas more than a multigrade oil recommended for each particular application. Heavy oil does contribute to accelerated wear when used in the wrong applcation, but it ain't like Mucinex and the goobers.

Yes, idle is controlled by a lot of sensors, if one is faulty it will set a code(s), and the entire idle circuit needs to be pretty clean to function properly. I think the P0300 code indicates a near-stall condition.

Ask GM if they recommend running various magic potions in your oil or allowing your engine to inhale them.

Doesn't heavier oil congest your engine? And as far as I know, if your throttle and inside of it is dirty, it roughs up your idle. BTW not arguing with you or saying your wrong Ranger.
Not sure what you mean by "congest your engine". TB is another story, but oil does not directly affect it. If it gets to dirty and carboned up, yes it can affect the idle.

And whats up with you and snake oil? Seafoam is seafoam, not snake oil, and it does help a lot.
I am not a miracle in a bottle fan. Most of it is not needed and does more for your piece of mind and the manufacturers bottom line than anything else. None of it is recommended by GM, but if you are convinced that you need it and it does some good, by all means use it. Consider this. If it was necessary, GM would recommend it and those of us who don't use it would be having problems, wouldn't you think?

I am not a miracle in a bottle fan. Most of it is not needed and does more for your piece of mind and the manufacturers bottom line than anything else. None of it is recommended by GM, but if you are convinced that you need it and it does some good, by all means use it. Consider this. If it was necessary, GM would recommend it and those of us who don't use it would be having problems, wouldn't you think?

Not saying its going to make your car new again, it just smooths it up and cleans it out. Just like fuel injector cleaner, that helps, so this does too, I did it for my mustang and it felt like it ran smoother when I put it in the crankcase.

We can debate this, but I don't want you to take anything I say personal. I have seen these debates get out of hand. So here goes.

How does it smooth it up? What does it clean out and from where? What makes you think that you need it in the first place? Most fuel injector cleaners are strong solvents and can destroy the insulating varnish on the fuel pump and injectors if used in the wrong concentrations or too frequently.

Your comment, "it felt like it ran smoother when I put it in the crankcase" is classic placebo effect. You spend money on something and you want and expect to see an improvement. Otherwise you are admitting that you wasted your money. Been there done that. Conversely, I would bet if I told you that I Seafoamed your engine, but never did it, you would probably still think it "felt like it ran smoother". That's just human nature and the marketers know it.

I am not going to argue with you Ranger, I respect your saying. Lets just end it there.

I do feel like you are correct in some way.

There is a big difference between a debate and an argument. That is why I said not to take anything personal. I did not want it to deteriorate to an argument. Hope you did not take it that way.

There is a big difference between a debate and an argument. That is why I said not to take anything personal. I did not want it to deteriorate to an argument. Hope you did not take it that way.

No offense taken Ranger, like I said, I respect your saying.:)

Fix the misfire FIRST.
An O2 is not likely going to cause misfires, especially if codes are set and the MIL is on.
If the PCM fails an O2 sensor it will use table data for fuel control, not live readings.

Most vehicles have sensor 2 as a post cat, but a few years back GM used them in the y pipe before the cat.
I'd have to see the exhaust layout to tell you exact position. On MOST cars the sensor 2 designation is for a post cat, unless it is one of the early systems with the pre-cat sensor 2. I think Ranger knows more about some of the ones from that time frame than I do.
Any pre cat sensor will affect the way the engine runs, but once a code is set the computer will ignore the data as faulty.

On a car of that time period I'd say if you have a misfire it could be plug/wire/coil related but at any rate it should be diagnosed FIRST. Putting a new O2 sensor in a misfiring engine is like throwing money out the window.

The 603 and 1599 are worthless, those are memory reset codes and engine stall.

Submariner is right, if the ECM was "bad" you'd have a 4000 lb. paperweight.
Getting coolant on it doesn't hurt it.

Ok, here's the verdict:

Took it to one shop, and they said the intake manifold gasket needed to be replaced. Also the front bank ignition coil. They also wanted to replace the o2 sensor reporting the error, but all of this was going to cost me about $650. Well I priced around and I took it to a few other places. Basically here's what happened:

The intake manifold gasket was letting air in. That combined with the one ignition coil being bad caused the misfire. Because of all this air, the o2 sensor was thrown off and giving the errors. I was also getting killed on my gas mileage.

So they replaced the intake manifold gasket and he had a slightly used ignition coil he sold me for $25. All in all it cost me about $300. Now my baby is running like new.

:lildevil: Ain't it wunnerful ???