ive checked all the ignition system and it seems to be working correctly...i took it by my mechanic and he went to look at it with stethascope and temp gun, first thing he does is pull injector wires, when he pulls #7 there is no change in sound oif idle...he keeps going and does it again when he gets to #8, there was a slight change when 8 was pulled but u could tell it must of had a miss also
so with the temp gun he measures all the cylinders and the front 3 on both sides read 500-550
the rear one on both sides were #7=250-270 and #8=260-300
i pulled my egr blockoffs and re gasket and rtv them but no change, so wats next? wat else could cause this

 

Takes fuel, spark and compression to fire a cylinder. Any one of the 3 missing will cause it to missfire. Now if you had a completely dead cylinder and you had a spark problem, you should notice it running rich, even possibly diluting the crankcase. So, if you don't have rich running and/or crankcase contamination, look to a fuel problem.

If the ignition system is functioning(notice I said IF), go for fuel next. If you do go for the fuel system, since other cylinders are firing you can assume fuel pressure is ok and head directly to check for the presence of injector pulses. If injector pulses are all there, try swapping injectors and see if the problem follows the injectors. Or you can try having them cleaned professionally either on or off the rail.

If spark is ok and the fuel system is good, the last thing left is compression. At that point either do a leak down test or compression test.

There is one other oddball explanation but I'm not gona mention it until you have eliminated the more likely ones first.

 

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