I have a 2000 Buick LeSabre 3.8 V6 (of course) with less than 140,000 miles. which has popped a DTC P0300 multiple cylinder misfire code. It is running rough but not excessively, and the check engine light is flashing, informing me that I have a malfunction condition that may be injurious to the catalytic converter. There are no other codes besides the P0300.


I first replaced the spark plugs and plug wires as they appeared to be original tot he car, but no joy.

I then monitored the oxygen sensor data with a scan tool. The upstream sensor was active but never peaked above .4 volts and mostly much lower indicating a lean condition.

The downstream sensor however was almost flat .8 volts indicating a ultra rich condition. However there was no black smoke coming from the exhaust nor any type of smell from a catalytic converter indicating a high concentration of unburned fuel is passing through. There also was no white vapor coming from the exhaust.


I replaced the downstream sensor and cleared the codes. Same symptoms. I then disconnected the downstream sensor. The output from the upstream sensor then trended up and varied around the .450 volt mark but still did not seem to be switching properly and the check engine light started flashing again with the engine running roughly. I reconnected the downstream sensor and disconnected the upstream sensor. When the car was started the downstream sensor showed activity for a couple of minutes then flatlined at .8 volts.


I have not taken a fuel pressure test as of yet, but pressure remains in the fuel rail for quite some time after stopping the engine, indicating to me that there is not a significant fuel injector leak.

There is one other thing. When I turn the key to the full on position but do not start the engine, the fuel pump will cycle on then off as per normal. But after a few minutes it will cycle on again for several seconds, maybe as many as thirty. I do not remember any of my other GM vehicles doing this.


I am pretty much stumped at this time. I have never seen anything like this and signed onto this forum hoping that there will be a gru or two or three or four that can point me in the right direction. I am trending towards the idea that it might be a grounding problem or maybe even a PCM going flaky on me.


I really think that it is my stepdaughter. I had been driving the car for several weeks after buying it at an insurance auction and getting the kinks out of it. never had any driveability problems though. It got good gas mileage, ran great, smooth, with good power. Then the radiator went out in my stepdaughter'* car and she wanted to buy the LeSabre. Not even two days later and now this. She is a car jinx.


If anyone has any ideas, please let me know. I know just enough about cars and PCM'* to be dangerous.


Thanks,

Glenn

We'll get you through this. Get the fuel pressure and let us know what the fuel trims are doing. With the pre cat O2 only going to .45 it may not be going into closed loop.

Yeah, fuel pressure is important here. If pressure is getting low but not too low it will run lean.

Also, have you replaced the fuel pressure regulator?

Just a couple of quick replies. Am too sick right now to get back on the car. Had to work on it in the weather yesterday to be able to get it to to the shop and now am paying for it.
Carfixer007, every (good) downstream O2 sensor that I have monitored when the system is working normally settled down below the .450 level, else I would get a catalyst below threshold efficiency type of DTC.
The P0300 is the only code that has been thrown. I would be happy if I could get a specific cylinder or even a couple of them.
I will check the fuel pressure to morrow and get back to you. Thanks for your quick responses.

Glenn

Totally understand how that goes. Take it easy!

What scan tool did you use to get codes? It'* strange the tool could not read all codes.

I used Autoenginuity Version 11 and a Bluedriver scantool. There are have been no other codes or pending codes reported.

Glenn

I would be doing a coil output test.....

Also, since you can monitor upstream O2 data(should be varying above and below 0.455 volts), disconnect the MAF sensor and see if the O2 data changes....

And as the guys have mentioned, check fuel pressure....

If spark on all cylinders is ok, and fuel pressure is ok, then you may have to check compression....

Okay guys. Thanks for all of the suggestions. I am much better now, after fighting a sinus infection most of the week.
I finally got my brain engaged and did some more diagnostics with Autoenginuity. It has the ability to read specific cylinder misfires so I set it up to monitor all six cylinders. Only cylinders 3 and 6 were misfiring. I have no idea why codes P0303 and P0306 were not being set. That would have clued me in much quicker.

There is a coil pack common to cylinders 3 and 6 but swapping it with another coil pack for a different set of cylinders did not change anything. The misfires stayed on # 3 and # 6. So I trudged down to my local pull-it-yourself treasure yard and relieved one of the junkers of the ignition control modules, and oila! the problem is fixed? (By the way, I did check the fuel pressure. It stays at 49-50 PSI, which is good.)

That answers my question about the voltage output from the downstream oxygen sensor being so high. It was rich. Don't know why the upstream sensor was so low. Once I installed the "new" ignition control module the outputs of both sensors are switching properly. Just hope the catalytic converter was not harmed.

Thanks again everyone.

Glenn

Glad you found the problem.