my 2.5L engine is running lean- what's happening? -
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Go Back > Models > Jeep Wrangler Forums > YJ Wrangler Technical Forum > my 2.5L engine is running lean- what's happening?

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Unread 08-09-2010, 03:22 AM   #1
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my 2.5L engine is running lean- what's happening?

A few months ago, I noticed my 1994 2.5L Jeep engine rpms dropping into the 500rpm range, and the engine seemed to stumble occasionally. Changed the plugs, cap and rotor, but no change. Air filter is in good shape. I checked the engine codes and got contradictory codes- 02 SENSOR DETECTS A LEAN CONDITION, AND 02 SENSOR DETECTS NEITHER A RICH NOR LEAN CONDITION. After replacing the 02 Sensor, lo and behold, the codes remained the same- changing the O2 CHANGED NOTHING. The engine still behaves the same, and seems to stumble and run leaner in colder temperatures, and better and smoother when it's warm. What is going on? I know there is no idle adjustment possible for this engine, so now what?

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Unread 08-09-2010, 06:36 AM   #2
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How do you really know it is actually running lean? One way to tell is by the color of the spark plugs. Pull the plugs (keep the wires straight, don't get them mixed up, mark them). Examine the electrodes. a clean running engine with the right mixture ratio should show light tan. Lean will show white or gray. Rich will be black and sooty. You should make this check after you have driven the vehicle for 1/2 hour. Let it cool then pull the plugs.

This is a good place to start. Once you have varified the running mixture you can start to troubleshoot.

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Unread 08-09-2010, 11:28 AM   #3
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1987 YJ Wrangler 
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Check for vacuum leaks. Start with looking at vacuum lines and then check your intake manifold gasket. Sometimes the intake bolts will be loose and you can just tighten them down if the gasket hasn't been ruined.
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Unread 08-09-2010, 04:00 PM   #4
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Crack in the exhaust manifold or leaking behind the gasket would confuse the o2 sensor
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