Hey. I have a 98 xj that's been running rough and stumbling on idle. It's fine driving, it's just bad at idle. I've been checking sensors, ignition and fuel and I can't find anything else wrong. I've been getting bad MPG, it's been running rich and fouling my plugs, and there's gas in my oil. I assume it's washing the cylinders and it's seeping down into the oil. I couldn't find anything wrong except when I unplugged my MAP nothing changed with the idle. It idles the same without the MAP plugged in. I've shown no codes for the MAP though. Do you think it is bad?

 

Check fuel pressure and leak down test? Scan tool? Vacuum leak?

 

No fuel pressure test but injectors aren't leaky. No vacuum leak I can find. Scanner found no MAP codes. Just 02 sensors which I replaced and a code for "lean in bank 1"

 

Fuel pressure test also tests the fuel pump, and regulator which has the filter. Lean means not enough fuel.

 

It's not running lean though. It's running way to rich. So much so it's washing the cylinders and fouling my plugs.

 

It's running rich because when it senses a lean condition it increases fuel therefore causing it to run rich. Mine is doing the same only once in a while throws a code for misfire cylinder one. My problem is something with a valve. Had a leak down test done and that's how I know. Also I believe that a leak in your exhaust somewhere upstream of your O2 will fool the O2 into thinking the engine is running lean, therefore increasing fuel and causing it to run rich.

 

You can rent a loaner fuel pressure gauge from AZ or somewhere for free. It's step #1.

 

I was just going through the easy stuff first. That's why I asked about the MAP sensor. I checked all of my sensors and the MAP plugged or unplugged didnt make a difference. I know when they go out they can cause an engine to run rich and lean. They usually trigger a 02 sensor code because of this as well. Ill give the fuel pressure a shot. I know my check valve is out in my pump.

 

ok. Doesn't get much easier than a FP test. I got one at harbor freight for about $20. Just screw it on. In my experience when the check valve goes, the pump is shot and the filters plugged too. MAP sensors are quite durable, I won't say they never go but I've never seen one.

 

Fuel pressure stays steady at 49psi when the jeep first starts. 5 seconds later it jumps between 39psi-50psi back and forth.

 

Well obviously that is not goot. How is the leakdown test?

 

What he said.

Also, check your O2 heater fuse. Doesn't always throw a code.

 

No leak down test yet