Possibly a Faulty o2 or MAP sensor? - JeepForum.com
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post #1 of 3 Old 11-12-2013, 06:52 PM Thread Starter
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1993 YJ Wrangler 
Join Date: Aug 2011
Location: Ridge
Posts: 28
Possibly a Faulty o2 or MAP sensor?

I have a 93 YJ with a 4.0 litre. It has the correct multi-point and pcm engine. For as long as I've owned the Jeep (3 years) it always seemed to have a rough idle. Along with the rough idle it gets terrible gas mileage (even bad for a 4.0 on 33's) and the exhaust is extremely rich. I haven't been able to check any codes because of the OBDI and there is no check engine light. However i believe I have narrowed it down to three possibilities. Either a crack in my exhaust manifold, a bad o2 sensor, a bad MAP sensor, or a combination of the three. I have deleted my CAT but I don't believe my mixture should be this drastically rich, however i could be wrong. I have yet to do any tests and only first started really trying to diagnose my problem recently. I'm just looking for any general insight on if any of my ideas are correct. Thanks.

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post #2 of 3 Old 11-13-2013, 06:43 AM
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1989 YJ Wrangler 
Join Date: Apr 2008
Location: Detroit
Posts: 24,647
Replace the bulb for the check engine light and check your codes. I highly doubt its a bad map. The map input is used for advancing the timing. You would have no power if it was bad. Crack in the manifold/missing loose bolts is easy enough to check. Might be a good time to clean the throttlebody and IAC for a rough idle.

Check your fuel pressure. We have had so many people on the forum this year with fuel pressure issues! 31 psi idle, 39 psi with the vacuum to the regulator off, maintain pressure above 20 psi for longer then 20 minutes.

O2 sensor should be replaced every 60-100k as normal maintanance.
Que89YJ is offline  
post #3 of 3 Old 11-13-2013, 07:02 AM
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1991 YJ Wrangler 
Join Date: Mar 2012
Location: Niagara Falls
Posts: 1,876
Que is correct. One other thing I would make note of the coolant temperature when its running rough. The PCM will not read the O2 sensor until the engine is at 185 deg or above. The temp sensor is in the thermostat housing/upper water neck. The sensor changes resistance (ohms) with temperature so anything that changes resistance (such as a poor cnnection or a short) can affect this reading.

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