The cars idle has gotten rougher and rougher lately and I am barely getting 150 miles per tank! I also have the occasional miss on the big end. Ideas or suggestions?

 

Misfires could be Electrical, Fuel, or Mechanical.

Electrical could be anything from sensors to Ignition.
Fuel could be clogged injectors to a fuel pump going out.
Mechanical could be any vacuum leak, rings, rocker arms, etc....

This is a generalization but essentially, it could be a number of things. Give us some detail... when it happens, car temp, acceleration or deceleration, noises, etc....

Give us all of the details.

 

I would replace the fuel filter first it is really cheap and easy to do.

 

Here are the details...

When the car is cold (but not over night cold), sometimes it does not run rough. Once in awhile it will have a couple of misses above 5000 rpm's. I am getting ~160 per tank. The plugs look like very rich conditions. I do also have a low end stumble. According to data master I am running very rich 3100 rpms and under (histogram). I am not throwing any codes. O2's are still functioning.

The car is a m6 355 at 11:1 with a cc305 cam and stock fuel system.

Hope that helps.

 

check under the hood somewhere very dark no lights. in a garage maybe or a dark parking lot.. Look at your coil and wires..stick ur head in really close. See if u can see any sparks arching from the coil or the wires. rent a fuel presure gauge from a auto parts store like autozone and check ur fuel presure while driving. might have to tape the gauge to the window i think. that should at least get u started.

 

O2 sensors wont throw a code till they are dead. so you could have a bad O2 sensor that is still barely working and as time goes by it just gets worse and worse till one day it dies. this is what happened to one of mine. i monitored it on my scanmaster and could see that the sensor would not function from the 0-900mV range like it should but only from 400-600 and never got a code till it would no longer function at all. my gas mileage was also suffering too and my idle was noticably rough. its just something to check if you have a scanner.