Renix low idle, bad milage, black smoke.

Gold Cobra · Dec 27, 2009

87 MJ 4.0 ax-15 231. Newer plugs, wires, cap and rotor, o2 sensor. Just started doing this a few days ago, idles real low, around 600-700 horrific fuel comsumption, and am getting black smoke from the tailpipe. It started doing this while I was driving.

NW-ZJ-SCOTT · Dec 27, 2009

goblazers_6 said:
87 MJ 4.0 ax-15 231. Newer plugs, wires, cap and rotor, o2 sensor. Just started doing this a few days ago, idles real low, around 600-700 horrific fuel comsumption, and am getting black smoke from the tailpipe. It started doing this while I was driving.

Wow, sounds like a complete pile of crap, probably ready to be scrapped. I'll give you $250 for it. :wave:

bcmaxx · Dec 27, 2009

goblazers_6 said:
87 MJ 4.0 ax-15 231. Newer plugs, wires, cap and rotor, o2 sensor. Just started doing this a few days ago, idles real low, around 600-700 horrific fuel comsumption, and am getting black smoke from the tailpipe. It started doing this while I was driving.

check map sensor, coolant temp sensor, Air intake temp sensor, a few to start

birchlakeXJ · Dec 27, 2009

I agree with bcmaxx. Get your meter and your manual out. Test the coolant temp sensor and map sensor. If they are responsible for faulty input signals to the computer, that will affect your fuel/air ratio.

Other things to check. Verify that your egr valve is working; should have no egr at idle. Cleaning your idle air control and throttle body is never a bad idea. Verify fuel pressure with a gauge.

Gold Cobra · Dec 27, 2009

Coolant and intake temp sensors test good. Map sensor reads ~4.8V cold with ignition on. Engine warm at idle reading is still ~4.8V. http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm - says that enginge warm at idle should read 1.5V-2.1V. Does this mean my MAP sensor is bad?

cruiser54 · Dec 28, 2009

How is the vac line to the sensor. Plugged? Leaking? It does sound like a MAP sensor issue.

birchlakeXJ · Dec 28, 2009

Directly from my Chilton's manual for testing a Renix (87-90) Jeep 4.0 map sensor.

"Test the map sensor output voltage at the MAP sensor connector terminal B. With the ignition switch and the engine OFF. The output voltage should be 4-5 volts. The voltage should drop to 0.5 - 1.5 volts with a hot, neutral, idle speed condition."

This spec is a little bit different than the article you read, but for purposes of this test it doesn't really matter. If your output voltage is not changing, then it is a good bet that your map sensor is bad.

Let us know if you decide to change it out if it resolves your symptoms.

sjx40250 · Dec 28, 2009

Definitialy running rich - a/f ratio problem. Even with the ECU in limp mode, you should not have this kind of performance. MAP sensor is the easies way to vary a/f ratio. Search on MAP Adjuster. Once you get a good one, you may want to try this out.
http://www.go.jeep-xj.info/HowtoMAPsensor.htm

The Lure Washer · Dec 28, 2009

goblazers_6 said:
Coolant and intake temp sensors test good. Map sensor reads ~4.8V cold with ignition on. Engine warm at idle reading is still ~4.8V. http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm - says that enginge warm at idle should read 1.5V-2.1V. Does this mean my MAP sensor is bad?

If it still reads 4.8v even when warm then yes I would say its bad. Its definitely a start. Replace it. If it doesnt fix the problem then there is something else wrong as well.

old_man · Dec 28, 2009

The reason the MAP reads lower when running is a change in pressure. If the hose is bad you will get the higher reading.

Remember that on the throttle body where it plugs in, the rubber "stopper" on the hard line going back to the map can accidentally inverted and plugged in wrong, yielding a non performing MAP.

Gold Cobra · Dec 28, 2009

old_man said:
The reason the MAP reads lower when running is a change in pressure. If the hose is bad you will get the higher reading.

Checked the vacuum line going from the MAP sensor to the throttle body and sure enough there was a spot on it where it had melted closed. Got a new vacuum line from NAPA we're back to normal. Thanks for the help guys.

bcmaxx · Dec 28, 2009

Awesome, gotta love it when you can troubleshoot your own problems and fix em cheap!

old_man · Dec 28, 2009

Cheap???? You haven't come up with the standard case of beer fee for remote diagnosis.

irate1:

birchlakeXJ · Dec 28, 2009

old_man said:
If the hose is bad you will get the higher reading.

Chalk up another one for the Old Man.

The lesson being to look at all things associated with the sensor, not just the sensor itself.

Nice work!

:cheers:

cruiser54 · Dec 29, 2009

Send just one six-pack of Guinness my way.........

stephenspann27 · Dec 29, 2009

Check fuel pressure and injectors.. if its misfiring.. try unplugged each injector at once..

MT Mike · Dec 29, 2009

goblazers_6 said:
Checked the vacuum line going from the MAP sensor to the throttle body and sure enough there was a spot on it where it had melted closed. Got a new vacuum line from NAPA we're back to normal. Thanks for the help guys.

bcmaxx said:
Awesome, gotta love it when you can troubleshoot your own problems and fix em cheap!

stephenspann27 said:
Check fuel pressure and injectors.. if its misfiring.. try unplugged each injector at once..

Good suggestion on checking the injectors, but goblazer already found the problem. :thumbup:

Renix low idle, bad milage, black smoke.

Gold Cobra

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