Air Fuel Ratio Question:

As some of you may know from my posts I have a 95 xj and I have been running down a mileage problem for about a month. My 4.0 is running rich - mechanic says the injector pulse is too long. I have changed the CCV system and temp sensor but no change has been evident.

When he checked the 02 sensor it shows good but I am curious about the open and closed loop thing. My engine runs at around the 190 mark usually - however, I am in the middle of a fan swap and have driven it on short jaunts with just the stock electric fan hooked up and temps get into the 210-220 mark but I still hear the engine ping like it did when I had the stock fan in.

Will an 02 sensor show good and still not work correctly? and there seems to be some debate about my year - is there one or two sensors on a 95 jeep XJ country.


Thanks,

JC95

If your engine's running too rich and the O2 sensor is good, there's a fair chance that the problem's with the MAP sensor. Check the vacuum line going from MAP sensor to intake manifold and make sure it isn't cracked/broken/disconnected. If the MAP sensor isn't seeing enough vacuum, it thinks the engine's under load and tells the ECU to dump more fuel into the A/F mixture, hence the long injector pulse.

Dr. Dyno said:

If your engine's running too rich and the O2 sensor is good, there's a fair chance that the problem's with the MAP sensor. Check the vacuum line going from MAP sensor to intake manifold and make sure it isn't cracked/broken/disconnected. If the MAP sensor isn't seeing enough vacuum, it thinks the engine's under load and tells the ECU to dump more fuel into the A/F mixture, hence the long injector pulse.

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Thanks Dr. Knew you'd have the answer - you seem to be the guru on here. Now if you can just direct me to the map sensor...

The MAP sensor is on the firewall in the center. It has a green connector with three wires attached to it and immediately behind the plug, you'll find the vacuum tube. It looks like this:

I don't know what the cut-off is for the Chrysler programming, but if your engine isn't getting up to temp, or if the computer doesn't think it is, its possible that your engine is staying in cold/warm-up and running rich for that reason too ...

The reason I bring this up despite you saying you changed the temp sensor is, is there only one on the 4.0 ? Some engines (again not sure on Chrysler specifically) there are two temperature sensors, one tells the computer what the temp is, the other tells you/the dash.

On the subject of MAP sensors. I've read about the adjustable sensors, but not sure I understand yet how they work. Do you increase the voltage across to "trick" the computer into thinking you're at a different point on the a/f map ? If so, does making the voltage go up make it higher MAP or lower ?

wish said:

I don't know what the cut-off is for the Chrysler programming, but if your engine isn't getting up to temp, or if the computer doesn't think it is, its possible that your engine is staying in cold/warm-up and running rich for that reason too ...

The reason I bring this up despite you saying you changed the temp sensor is, is there only one on the 4.0 ? Some engines (again not sure on Chrysler specifically) there are two temperature sensors, one tells the computer what the temp is, the other tells you/the dash.

On the subject of MAP sensors. I've read about the adjustable sensors, but not sure I understand yet how they work. Do you increase the voltage across to "trick" the computer into thinking you're at a different point on the a/f map ? If so, does making the voltage go up make it higher MAP or lower ?

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There are two temp sensors on the 4.0 with the guage being at the back of the block and the one for the ECU in the thermostat on the 91+. I believe this one is in the radiator on the early models.
As for the adjustable MAP sensor, the higher you make the reference voltage the richer the fuel mixture but only when in open loop mode when the O2 sensor is not read. This is 80% or greater throttle opening and before operational temp is reached.
Dr. Dyno has a write up on making your own as do I now.

www.go.jeep-xj.info

Just checked the posts again and found the "follow-ups".

Thanks for your responses. I did change th one on the t-stat housing so that was good and I thought that the adjustable map was just for initial warm up but it just looked more complicated than I wanted to get into anyhow.

I will look at the map sensor tomorrow when I get around, thanks for the location and pic!

One last question:

When I did the new CCV plumbing I tried to get the front fitting out of the valve cover and it broke SO, I drilled out the old PVC fitting and tried to install it into the old gasket on the cover. It just would not go in so I cut the gasket and cut out a really small section to allow fitting to go in. Will this have much effect on the question being debated? I will get a new one soon but unfortunately I rarely drive this anymore since I have a company van and I seem to be working all the time; on the up side I have the ability to have it down for days at at time to do mods!

(now if I could only get the money tree to bloom!)

BTW:

Had the problem b-4 and after the CCV debacle (just fyi)


Thanks.

JC95

jeepcherokee95 said:

Will an 02 sensor show good and still not work correctly? and there seems to be some debate about my year - is there one or two sensors on a 95 jeep XJ country.


Thanks,

JC95

Click to expand...

Yes, often an 02 sensor will become sluggish long before it completely fails. I've seen senors that lagged behind by about 5sec specially the Bosch crap.