I am having a low rpm miss in my car that I have beet trying to fix for a long time now. I have narrowed it down to cylinders 7 and 8. Plugs and wires were changed 200mi ago and I am pulling the motor to put into my TA. I pulled the spark plugs today and found every cylinder looks like new except cylinders 7 and 8 are a little black. So now I know my miss is on those cylinders. I checked the plug wires and they are good; with only 200mi they should be. So what could cause the car only to miss on those cylinders. Will the opti cause this or a couple bad injectors? I did a leek down test a couple weeks ago and everything is fine there. The opti is a msd unit with less than 2,000mi on it.

 

Did you check the plugs to see if they are actually sparking? If so then it's time for a compression check

 

Yep they are sparking, the plugs are not that black just a little on the white part and the electrode. I did a compression and leak down test a couple of weeks ago I had good compression and leakage was within range on all cylinders. Could a bad EGR cause this? I know when I pressed in on the EGR diaphragm the idle did change a little, but not much at all. I don't understand why its only the two rear cylinders; number 7 more so than number 8. My mpg is also bad.

 

Check the length of the plug wires for tell tale dots indicating side firing to the block or other ground.

Lift up the fuel rails and pressurize the fuel system to see if any and particularly those 2 are leaking while they are off.

Then have someone crank it over while you look at the spray pattern for abnormalities.

It also would not be a bad idea to look at the fuel trims.

 

I am going to do that as soon as I get that motor installed into my TA. I hope I can fix this.