1994 XJ running issues, Long read sorry! - JeepForum.com
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post #1 of 6 Old 07-02-2020, 04:27 PM Thread Starter
nothingbutdarts
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1990 XJ Cherokee 
 
Join Date: Sep 2009
Location: Bailey, Co.
Posts: 29
1994 XJ running issues, Long read sorry!

I have a 1996 XJ that I rear ended somebody in in 2017 & thought I might have bent the L. front frame because the fender got bowed a bit so, I left the 1996 sit & proceeded to find another XJ, what I finally came up with was a 1994 from a guy at work, (109,505 miles) pretty clean inside. I test drove it very gently because of course the bolts were sheered on the R. motor mount (I have done two already) It seemed to drive OK, it started hard because I also knew the fuel pump was not holding pressure when you turn it off. So I knew I had to deal with: motor mount bolts, fuel pump & of course the cracked exh. manifold. I buy it for a bit more than I should have, gently drive it home, replace motor mount bolts, change the oil & all. Repl. the Dist. because the shaft was very warn & moved back & forth a bit, but not the fuel pump yet. Think, I'm ready to drive it for a daily driver, I take it on a good test drive, nail it it takes off OK for a non High Output & then, all of a sudden it looses all power but does not die, only a mile from home I limp it home which means about 3 MPH, give it any mort throttle than that I it wants to die. Kind of frustrated with it & all the work it needs I just decide to take the front header panel, grill & what not off & put it on the 1996 again, that was in 2017, fast forward to a month ago & the 1996 died on the way home (long story on it & after replacing the computer it was the crank sensor I was an idiot)
I feel like I need a back up now so, on the 1994 I just replaced the fuel tank, pump, screen & header, I also pulled the muffler & converter to check for a blockage because the dyeing I believed to be from the loose rattling honeycomb in the converter, sure enough, it appeared some of the honeycomb had turned & looked like it was blocking the outlet (NICE, I found my sudden loss of power) Put it all back together, before I drive it I check fuel pump pressure, 40# not running & drops to 35# after I unhook the pump bypass wiring I had rigged up. I get it started, it sounds fine, take it around the block for a bit to warm it up, seems to run OK light to medium throttle runs, then go for it with wide open throttle, it takes off for about 5 seconds than seems to backfire & AGAIN, looses all power but, does not die, I limp it home about 3 MPH, it idles a bit rough & wants to die when giving it any throttle. Once home I turn it off & try to restart it, it cranks over very slow & kind of hard cranking for a few seconds & then spins pretty fast, it will not start, did a couple attempts & gave up. I did check blink codes this time, 12, 51 & 55. Yes, I had the battery disconnected quite a lot while I was replacing all ther listed pieces so that takes care of codes 12 & 55. #51 says: A lean Air/Fuel mixture has been indicated by an abnormally rich correction factor (02 sensor). Could this code #51 really cause the engine to not have ANY power & want to die when you give it any kind of throttle???? After letting it sit a few hours, it did finally start & would take throttle (just goosing the pedal, I did NOT try to drive it) I am looking for any help here!!!! Thank You!!

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post #2 of 6 Old 07-03-2020, 03:32 AM
BagusJeep
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51** O2S Signal Stays Below Center (Lean) Oxygen sensor signal input indicates lean air/
fuel ratio condition during engine operation.

52** O2S Signal Stays Above Center (Rich) Oxygen sensor signal input indicates rich air/
fuel ratio condition during engine operation.

** Check Engine Lamp will illuminate during engine operation if this Diagnostic Trouble Code was recorded.

Code says O2 sensor is staying at lean, do you doubt it?

The O2 sensor outputs from 0 to 1 Volts. If you have 0 Volts or it is grounded, it will appear to be very lean.

The code is tripped when the engine has been running lean. I understand, not certain, that this code is only thrown after 11 minutes, 55 seconds, and if it continues for 48 minutes, the PCM will change from closed loop to open loop, ignoring the O2 sensor and forcing rich running.

If I am right you could reset the codes with a DRB tool and run for 20 minutes and see if it runs OK but you have a code. If the engine stops after 48 minutes, you know what is happening.

Of course this info could be wrong but you will not get this code instantly when it is lean, the engine is lean and then rich and lean again in normal operation. It must be thrown after a period of leanness. if it runs fine until the code is thrown, it is the code causing the loss of power beacus ethe O2 sensor is not being read correctly.

It may be a faulty O2 sensor but it could also be a wiring issue to the sensor. If the wire was grounded by cable chaffing, it would tell the PCM that the engine was was running lean. A Bad connection also.

Running lean could also be low fuel pressure but you seem to have satisfied yourself this is not the case. low fuel pressure was the first thing I thought early in your story. fuel pressure is not monitored.
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1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
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post #3 of 6 Old 07-03-2020, 12:28 PM Thread Starter
nothingbutdarts
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No DRB tool. If the battery is left disconnected for say a day, will it reset the codes?

Please correct me if I am wrong, the way I read code #51 is that, the computer saw an "abnormally rich" condition & then told the ECM to lean out the fuel mixture???????

I did not have a fuel pressure gauge on the engine when I was out on the road doing the hard acceleration, I only did a fuel pressure test after installing the new fuel pump & new tank, I just pulled the pump relay & put 12 volts to the pump and watched both the pressure 40# and also the pressure after disconnecting the power to make sure the fuel pressure held & did not bleed off like it did with the old fuel pump.
There are 4 wires going to the 02 sensor, I believe two are for the internal heater & the other two are what the ECM is reading to see what the A/F ratio is? Anybody know what the pinout is for the 02 sensor where it goes into the ECM so I could ohms check each wire? Does the 02 internal heater get it's power from the ECM? Is there a fuse for the heater?
I really thought the engine losing all it's power was from the honeycomb in the converter plugging the outlet, I was quite disappointed that it is still doing it after all that I recently did to it!!! By the way, I forgot to mention the EPA will not be very happy with me because of what I did to the catalytic converter after I got it pulled off & then reinstalled it! ;^ ) This 1994 year Jeep only has one 02 sensor located in the head pipe that connects to the exhaust manifold/header, no 02 sensors are in the catallactic converter.
Thank You for the help!!!!!

I also just read thru 3 different threads on this sight where people were asking about code 51 issues, NONE of them mentioned that the engine looses all power, seriously, I had to drive home @ like 3 MPH , if I gave it any more throttle it would die out.
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post #4 of 6 Old 07-04-2020, 03:20 AM
BagusJeep
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An O2 sensor gets its energy from the difference in oxygen levels between exhaust and outside but it only happens when hot, really hot. This small voltage is read by the PCM.

Heating element disconnect and check resistance across white wire terminals of 5 to 7 Ohms. Power comes from the fuel pump relay and is grounded by the PCM.

The black terminal is the signal, grey is a ground. The black goes to the PCM. This is true for a 4 wire NGK/NTK sensor, you need to check what you have.

Testing the sensor on the bench is more tricky. I took this off the Internet. the alternative is t take it out and heat it with a blowtorch.

How do I Test an Oxygen Sensor?

On Car Test - Before you can test the operation of the sensor, you will need an oscilloscope. You should first check that the basic engine set up is to the manufacturers’ specification, then thoroughly warm up the engine - remember that the sensor will only function once it has reached its operating temperature.

Two methods of testing an oxygen sensor are using an oscilloscope or a multimeter. An oscilloscope is the best method for testing. This will give you the exact output of the sensor along with its response times. A multimeter can also be used but this will only give an indication if there is an output or no output. The sensor will be switching too quickly for any response times to be measured.

OSCILLOSCOPE - Using an appropriate connecting device, connect the sensor output to your oscilloscope; do not disconnect the sensor from the ECU. Run the engine at approximately 2000 rpm. A properly functioning oxygen sensor will show a rapidly fluctuating output voltage between approximately 0.1 and 1.0 volts. The time taken for the voltage to change from 0.1V to 1.0V (referred to as the lean to rich response time) should be about 300 milliseconds. A similar time should be measured when the voltage changes from 1.0V to 0.1V (rich to lean response time).

MULTIMETER - For testing with a digital multimeter you will also need to connect the sensor output to the multimeter using an appropriate connecting device. Run the engine at approx. 2000-2500 rpm. The output will be a DC voltage, oscillating between approximately 0.1V and 1.0V. Although the sensor output is technically an oscillating DC voltage, some multimeters may require to be set on AC voltage measurement to correctly read the sensor output. Also, the response time of the multimeter must be better than the response time of the sensor. If the multimeter is too slow, then a constant output will result, even though the sensor is actually switching.

If the sensor output is constant or the response time is too slow, the sensor should be changed. It is a good idea to check the oxygen sensor function at every tune up and before submitting cars for emission tests. A slow sensor will affect fuel economy. A new sensor will pay for itself by cutting fuel bills.

IF you decide to replace then only use Mopar or NGK/NTK. Teh others like Bosch will not work properly in a Cherokee.

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1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1951 Willys CJ3A/MB/M38 - Little Willy
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post #5 of 6 Old 07-04-2020, 11:40 AM Thread Starter
nothingbutdarts
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Thank You Bagusjeep!
I will ohms test the heating side to see if there is continuity & the voltage/ground to the heater. I also know I have a vacuum leak where the little plastic pipe goes up to the canister inside the front bumper, battery acid kills that line! However I don't think the line would make that much of a difference to the engine running & setting a code, but a leak is a leak!
Then I plan on rechecking to make sure the new distributor is installed 100% correctly & not off a bit. In the factory manual I have for a 1992 it says to use acetylene & test for vacuum leaks @ the intake, I might try that because of the fact I put a new gasket on it, I did however torque it in the listed sequence, just want to check!
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post #6 of 6 Old 07-16-2020, 08:35 PM Thread Starter
nothingbutdarts
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Would the engine run if the distributor was installed one tooth off? It is really sluggish & at times pops/stumbles with hard acceleration right at take off.
Driving fuel pressure was between 33 & 40 PSI depending on vacuum. I ohms check the heater side of the O2 sensor & checked for power & ground to it. It was all good.
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