I need some help. We have a 2005 Ford Ranger 3.0. A few weeks ago we had to have the engine replaced. We replaced it with a used 2005 engine with 93k miles on it from LKQ.

A few days after picking the truck up from the mechanic the check engine light came on and the truck started idling very roughly and misfiring. The problems were somewhat intermittent and the CEL would turn on and off. We checked for vaccum leaks, replaced spark plugs and wires, and ran some fuel injector cleaner through the tank but none of that seemed to fix the problem. A few days ago the CEL started flashing and the misfire was so severe I had to turn the truck off immediately and had it towed to a shop.

The CEL codes were P0136, PO306 and PO304. Mechanic did a compression test and compression was low in cylinder 6 so they were pretty sure it was something wrong with the head. I gave them the green light to take the head apart.

Today they called and said they couldn't find anything wrong with the head when they got into it. They think it is a problem with the engine and said I should contact LKQ for a replacement under their warranty...

What else could cause this low compression and misfiring? How would LKQ agree to send a replacement without knowing what the cause of the problem is? Any advice would be greatly appreciated...thank you.

 

P0136 - Faulty Rear Heated Oxygen Sensor Bank 1
- Rear Heated Oxygen Sensor Bank 1 harness is open or shorted
- Rear Heated Oxygen Sensor Bank 1 circuit poor electrical connection
- Inappropriate fuel pressure
- Faulty fuel injectors
- Intake air leaks may be faulty
- Exhaust gas leaks

Tech solution: Replacing the Rear Heated Oxygen Sensor Bank 1 usually takes care of the problem.


P0306 - Cylinder #6 Misfire

Tech solution: Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector

P0304 - Cylinder #4 Misfire

Tech solution: Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector

Do you see a "common" denominator to these three codes?

 

The P0136 code is for the O2 sensor that checks if Cat Converter is working, Bank 1 sensor 2, computer compares that O2 data with Upstream O2 data(Bank 1 Sensor 1) to make sure Cat is cleaning the exhaust, working, it is most likely a wiring fault or water in connector at the sensor.
This sensor doesn't control fuel mix(lean/rich), just detects if Cat is working or not.

You probably don't have the actual numbers from compression test but we will assume mechanics know what they are doing.

Low compression can be caused by a few things(assuming it is just one cylinder out of the six)
Valve or valve seat problem(3.0l heads in the early 2000's had a recessed exhaust valve seat issue)
Head cracked in cylinder(if seen on compression test it would have caused overheating engine)
Head gasket(if seen on compression test it would have caused overheating engine)
Rocker or cam problem, valve not closing all the way or cam lobe worn down and intake valve isn't opening all the way.

Rings/cylinder wall(rings are a longshot)

If a compression test does show one cylinder is more than 25% less than the others then the next step is a leak down test.
You pump known air pressure into cylinder, like 100psi, then measure how much pressure you are losing in cylinder, say cylinder is holding at 70psi, so you have "30% leak down"
You then put in a tablespoon of oil into cylinder, rotate crank a few revolutions then redo the test.
If cylinder now holds at 90psi you know rings were leaking 20psi, if you are still near 70psi then valves, head or head gasket is the issue.

Most places that sell used engines spin the crank and check compression, not always but if they offer a warranty they often do.