Okay, I am new to this and have never posted a thread so if I goof up don't laugh at me. I am a car dealer in Portland Oregon that has been chasing a cyl 4 misfire for some time. It has been at my mechanic (I have used for 20 years and reputable) for this problem. Starts in the am every so often with a rough idle, then a check engine light that is ALWAYS cyl 4 misfire. Have done head gaskets, swapped injectors from one to another, new plugs, new wires, done pressure test (over 3 day weekend and held pressure), borescoped cyl and no signs of coolant, checked cam lobes and shown to be fine along with valve travel. Lifters seem okay as well. Heads show no signs of carbon build up. Driven 100 miles with no failure, UNTIL TODAY when it threw code for....you guessed it,....cyl 4 misfire. Always cyl 4! Also replaced ECU before the 100 mile drive. Vehicle has 60K and has had LR dealer service. Now we are at wits end and don't know where to turn. Tech says bad O2 would throw it's own code along with fuel pressure, cam sensor and crank. Only does it once in a while. Sold this LR to a client and want to fix it for him and make it right! What to do, what to do?? I have an appt at local LR dealer but don't need the colonoscopy just yet.

 

Your tech is way off.
If you want some help send me your number and I'll be glad to walk you thru what to check/do.

 

I'd ask tech to start at the injector and work back on the harness toward the ECU. All injectors get +12 volts from a common point, and the ECU grounds them to squirt fuel at the right moment and for the correct duration. So a skinned wire can cause strange things. Wires can get pinched in HG work. On the spark side, the knock sensors need to be tightly mounted, not flopping around on the block. The wires that operate the coils again run back to the ECU, check for any skinned places. The ECU uses the knock sensor and Crank shaft position sensor, other things, plus a lot of math to determine a spark advance individually for each cylinder on the fly. Sometimes the misfire is a breakdown of insulation, you can look under the hood in the dark and check for arcs around wires, boots, and coil packs. A failing crank position sensor has a mind of its own.

If you are a local independent auto retailer that is bending over backwards to make it right for a customer kudos to you. Not many of your competitors do that.

 

Mike, My ph # is 503 784 2822 Thanks again!