I have a 1992 Lesabre with a 3.8L and112,000 miles. I have a rough idle and ormisfire I cant get rid of. I have changed: plugs, wires, O2 sensor, fuel filterand TPS. I have checked resistance of the coils and all three were within the specs. I have also checked: MAF sensor, Idle air control valve. If I disconnect my knock sensor thecheck engine light comes on. The ony time I get codes is when I disconect a sensor and try running it without aparticluar sensor. For those of you who have listened to exhaust, you knowwhat I am talking about when I say I am getting a spit and spudder from the exhaust. For the first few minutes after I start it, itruns good. Then slowly it starts to idle rough andget the spit and spudder from the exhaust. Sitting at a light sometimes the car will idle so rough that the car willseem to jump. When I changed the oxygen sensor I replaced it with a used one just to see if I can get the light to go out. The light did go out. The O2 sensor I put in it was black with carbon, within acouple days of driving it, theO2 sensor was no longer black but a redish brown color. Also sometimes when I am driving down the inerstate I will get a little bit of a studder, if I let off the gas, it goes away. Thiswill usually happen right as the car shifts to overdrive. I also need to mention the car sat for about 7 months before I bought it. I have also added injector cleaner to the tank on more than once and check the injectors with a noid light and the noid light blinks about the same on all injectorsAny ideas of what I should check next?


JeraldEdited by: jerald120374

 

Sounds like it is not figuring out its proper air/fuel mix after it is
in closed loop mode..(after it starts using its O2 sensor). Like it
gradually shifts from one good mix to a worse one.

Here are other ideas of rough idling/running causes.

<ul>[*]Idle control valve dirty. (Checked).[*]Coolant Temp sensor
[*]Intake Temp sensor
[*]Vacuum leak.[*]A Coil can be bad during high demand when hot Ohms good when cold. (usually an occasional or solid hard miss)[*]EGR valve stuck open. [*]Fuel Pump. Fuel flow pressure low during high demand. Gas tanks
collect water when sitting empty. Water and dry tanks ruin Fuel Pumps. Gets warm after running a bit and looses pressure. [/list]You can try disconnecting the battery for 10 seconds and let the engine relearn for a few runs.




Edited by: bassasasin

 

Should I have a gasket between the upper intake manifold and lower intake mainfold? This car did not have a gasket between the upper and lower intake. When I look for an upper intakegasket on the websites of the auto parts stores it shows an upper gasket but I dont think it will fit mine, I think its for the supercharged models. It shows the plenum type, whatever that is.

 

I checked the fuel pressure and when I turned the key on the pressure was 44psi. After the car started the pressure was about 41-42psi. Then after the caridleda few mins the pressure was down to 40psi. When I reved the engine an extra 500 rpms or so the pressure dropped to 38 psi. I shut the car off and waited about 10 mins, restarted the car and the pressure was very erractic. The gauge bounced between 39-41psi very fast. It would not stay on one number. In the first test, it was steady, except for the slight drop in pressure.

 

Jerald:
It seems it's running close to a good pressure, however, under a better load like up hill at 55 mph would help better determine the condition of the fuel pump.

If your still trying to get rid of the idle spit sputter, try looking at the throttle position sensor, they can be a nuisance. The computer sets a range it thinks its going to run in and something changes like dirt in the sensor.

 

It seems to run fine when I have to step on it to make a steep hill. It had power to make the hill today, though it wouldnt downshift when I push it all the way to the floor. I havent had to climb a steep hill while on the highway going 65mph. I live in Central Illinois, most interstates around here are flat. The idle has gotten even more rough. It bucks and jumps at stop lights, as if it's going to die. Edited by: jerald120374

 

OK the fuel pump seems to be faithful. Its often a failure that doesnt throw a code.

Try checking out the Trottle Position Sensor.
Its a simple variable resistor on the throttle body, and very easy to get to.

Try taking off the TPS and measure it with an ohm meter or leave it on and measure it with a volt meter when the engine has voltage, ignition on. If you have a junk yard available you might try one from there, else wise they are usually under $40.

It depends on on how you check it.. it should swing smoothly</span> from a around a half volt to about 5 volts or just under, as I remember.

They are known to get crud in them and do not smoothly change their resistance, or voltage depending on how you look at it.

Often they throw a code, but perhaps not this time.

Edited by: bassasasin

 

Well, I wasnt even thinking about the TPS because it's new. The last time I set the TPS I set it to about .40 volts with key on engine off.

 

I just came across a website researching 02 sensors and your post came to mind...[img]smileys/smiley2.gif[/img]

And re-reading your post, [img]smileys/smiley2.gif[/img]


"Sitting at a light sometimes the car will idle so rough that the car willseem to jump. The O2 sensor I put in it was black with carbon, within acouple days
of driving it, theO2 sensor was no longer black but a redish brown
color. Also sometimes when I am driving down the inerstate I will get
a little bit of a studder, if I let off the gas, it goes away.
Thiswill usually happen right as the car shifts to overdrive. "

This WEBSITE[img]smileys/smiley14.gif[/img]
noted similar problems your having... perhaps the O2 sensor you installed is a bad sensor...

Typically after a cold start, the O2 sensor is warming up and changing the fuel mix. This seems to follow your car's behavior.

While reading the article and thinking of your post, I also thought</span> , REDDISH BROWN
</span>on the sensor may be an indication your getting leaded gas which is bad for sensors.. [img]smileys/smiley11.gif[/img]

P.S. Thanks for your posting.. you have my interest.
</span></span>
Edited by: bassasasin

 

I changed the O2 sensor a few days ago with a new one. When I pulled out the old sensor, it was starting to get black again. Not athick layer of carbon but a grey black color again. I also changed intake gaskets a few days ago too.