Our 2000 Monte SS has developed a hard miss under acceleration, and has a rough idle. It is showing codes PO300 and PO131.
Are there any common failures on these 3.8 engines that I should look for?

 

I forgot to add, it started a couple of weeks ago. Pulling away from a stop sign it started missing and bucking pretty hard. Made it to where we were going and shut it off for a few minutes. When we started it again it ran fine, no miss and no check engine light. A few days later it did it again, this time triggering the CEL. It did this off and on for a week or so, now it is missing and running rough all of the time, but misses bad when under a load.

 

Don't know when the last general tune up was or what brand of plugs you're using, but I would start there and replace the upstream O2 sensor. P300 is random multiple misfire and P0131 is the upstream O2 sensor. A faulty O2 sensor will send out bad info throughout the system throwing off the PCM and result in misfires. Replace the plugs with ACDelco Irridium ones, good quality plug wires (ACDelco) and an ACDelco or Denso O2 sensor. Clean the MAF sensor and throttle body. The MAF may need to be replaced.

 

Sounds like a fueling issue to me. Agree by starting with the oxygen sensor pre-cat, and cleaning the MAF (good to get into the habit of cleaning it every oil change). The oxygen sensor might be leaning out the car too much under heavy throttle, sending the misfire code.

If you are still having issues after the oxygen sensor replacement, I would look into fueling issues.

 

The maf sensor has been cleaned. Back in December the intake exploded. SO, now it has a new intake, some new injectors, and a few other new parts. Luckily the resulting fire was isolated to the back drivers portion of the engine and I got it put out before any serious damage.
I will check the O2 sensor first and go from there. Thanks for the information!

 

Ah now I remember, the old exploding manifold trick.

Good luck Steve

 

My 01 intake exploded on me a few years back. I had went to the store, came out tried to start her and KABLAM, fire shot out from under the hood, scared me senseless. I started having issues with mine after that also. After changing O2sensor, MAF,MAP, Thermostat, Coils,ICM, EGR,and cleaning out the EGRTubes, Coolant flush,and full tune up, she ran better, just put a new Fuel Pump and Filter and that seems to really made a difference.
Question did you contact GM about the manifold?
Best of luck

 

No, I did not. I figured my daughter got the car used. The dealership had just replaced the intake gaskets, the insurance paid her enough to pay it off and get a start on a different car. I bought it back from the insurance company for $100, so as of now we have less than $500 in the car.
Today I wanted to see what it was doing, so I cleared the codes and we drove it a few miles out and back on the highway. It started missing again. It started as an intermittent hard miss, then started missing constantly. Really noticeable under a heavy load. I pulled the codes again with my little Bosch code reader. This time I am getting a PO304.
So, when it first started the random miss under load I pulled the codes and got PO301. I cleared that and it ran fine for a few days, then started again. That is when I got three codes, PO300, PO131 and another PO300.
It does run better in colder weather, and when the engine is cooler. In the mornings it is better, but by afternoon on the way home it runs worse.

 

I wonder if heat from the fire damaged the ICM or coil pack(s)? Or did you replace those?

 

The fire was isolated to the left rear of the engine compartment, and was very small. The coils are on the passenger side out in front of the vehicle.
At first I got a code of p0301, and the way it was acting I did replace the coil pack for #1. I think 1 and 4 are shared. It is possible that the ICM is bad giving first a P0301 code, then the P0300 and P0131 code, then P0304. If I am correct the P0300 code shows a misfire, the number at the end shows what cylinder, like 301 is cylinder 1, 304 is cylinder 4?
I cleared the codes again yesterday after driving and getting the 304 code. It has been running very rough with a constant miss. My wife drove to work, 60 mile round trip on the highway. It missed and ran rough all of the way, but is now NOT showing any codes. As much as I hate to do it I might have to take it to a shop. I have not had anyone else work on any of my vehicles in 30 years, but that might be where I have to go to figure this thing out. I could spend a lot of money hanging parts until I find the right one. Or maybe head to the U-pull it junk yard and just get one of every sensor on the engine.