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Need Help Diagnosing a Problem

2jeepdrivers

NAXJA Forum User
Location
Rogersville,mo
I have a 92' XJ w/ a 4.0 and auto. If I unplug my TPS and my Map sensor it will start and rev and I can drive it around the field but if I go out on the road it is kind of doggy. So far I have replaced the ECM, Crank Sensor, Map Sensor, Throttle postition sensor, and IAC. This all started the last time I took it wheeling, I was halfway up a ledge and it just died like I had shut the key off. I figured it was just a crank sensor so I replaced it and it didn't do anything, so then I put a TPS in it because mine was getting kind of rough looking after 236,000 miles still no start. So one day I started unplugging things to see if I could get it to start and if I unplugged the TPS it would kind of start and if unplugged the map sensor it would set there and run, does anyone have any ideas what would make it die when I plugged these two in? My fuel pressure is good all of my sensors ohm good, have the right voltage inputs and outputs and they are getting the correct voltages to the computer.
 
I'd check the ignition switch. Not the keyswitch on the column but the switch further down. Did you check battery cables for damage? Check vacuum line to map sensor. The new map sensor or tps may be bad although they're new. Unplug and test CPS with volt meter set on AC, crank engine (no start) and you should get at least .6 volts.
 
have you tried removing and thouroughly cleaning the throttle body, especially around the IAC port?

Have you tested the O2 sensor and its wires?

What did you unplug on the map sensor, the electrical or the vacuum line?

Sounds like a tough one, may be time to start from scratch on the tests again, to see if something else has changed.
 
Given this some thought and sometimes you can have more than one part that has failed. You can run with one sensor buggered but not both. That said you could have bad or failing of the following:

1. CPS weak signal CPS failing.
2. Combination failure of TPS and IAC Unplugging TPS should get you going (engine will run badly) with intermittant rough running engine/high idle/low idle from the IAC.
3 PLUGGED CAT given the scenario of the failure this is certainly possible. Previous to prob did your gas mileage go DOWN? What is was your mileage before this happeded? This would indicate failing 02 sensor which will kill your CAT Converter. It degrades the guts because you're running rich and clog the converter. That day I bet the CAT converter was RED HOT.



Make sure you inspect dist, coil, plug wires for obvious physical damage. You might be surprised at what you may find by taking a close look at everything. Disco the exhaust on the north side of the cross member on the engine side of the cat converter and see how she does.
 
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Back pin your 5 volt supply voltage, the sync sensor is a good place to do this. If something is partially shorting the 5 volt supply circuit this can happen.
A tip off is if you turn the key to run and don't get a CEL light there is often a 5 volt short.
Does the key method show any MIL codes?
 
Alright to help answer some questions. I have checked the crank sensor voltage several times and have tried multiple crank sensors, the map sensor should be good I took it off of a running jeep and the vacuum line on it seems to be in really good shape no cracking around the ends and is still pliable.
As for the potato in the tail pipe I already cut off the cat and the muffler thinking that could be it. I cleaned the throttle body right after I got it back home because when we were messing with it on the trail I noticed it was getting really nasty I am not sure how the IAC was even working before.
Have not checked the O2 sensor how would that affect it as far as making it die when I plugged in the TPS. When I say I unplugged the map sensor I meant the electrical side vacuum line is still hooked up.
I had checked the 5v power supply already and it was good but I am thinking it is something to do with that. That and the purple and white wire are the only things the map sensor and the TPS have in common as far as wiring. Also it had some codes the last time I pulled them but they all had something to do with me unplugging things.
Thanks for anymore help I get hopefully this is enough info to give someone an idea.
 
I would recheck that MAP sensor return signal wire to the ECU, see if it is shorted or partially grounded. Do it with power on, engine off, MAP sensor disconnected electically, and again do a ground test on that wire with power off. See if that wire is shorted to ground, even partially, or shorted to a hot wire. Do the same for the TPS signal return wire to the ECU. Also look for an intermittent signal on those wires indicating a broken or bad connection.

With TPS and MAP disconnected the computer defaults to rich, open loop, so I would see if the fuel system is running too lean in closed loop mode.

Or a leaking FP regulator diaphram perhaps.
 
Alright I have checked every wire wiggled them while I was checking them to see if I could get me reading to change and all of my inputs and outputs are perfect according to my manual and what I have gotten from you guys. The only thing I can find that doesn't seem quite right is that I have .92 ohms of resistance on my ground wire. Is that to much? I am hoping it is if somebody knows how much it can have let me know.:flame: :smsoap: If I don't figure it out pretty soon I am going to scrap my rust free XJ.I have been working on this off and on for three months.
 
NEW THOUGHT Calibrating TPS to spec may not work. Loosen screws and rotate TPS all the way clockways and retighten screws. Start Engine. See what happens.:twak:
 
2jeepdrivers said:
The only thing I can find that doesn't seem quite right is that I have .92 ohms of resistance on my ground wire. Is that to much?

Almost 1 ohm resistance does seem a bit high, particulary given how many sensors probably share this ground. The ECM may also share some if the ground wiring in question. It certainly can't hurt to add a new ground wire from the TPS to the engine block.
 
Sorry didn't realize yours was the HO. So for what its worth look for a loose connector at the STARTER RELAY. On my Jeep its located passenger side near battery. ON the Renix there is a return wire that tells the ECU that the engine is cranking. The HO may be different, but your starter relay may provide power to something besides the starter and fuel pump.
 
Anything less than one ohm is just fine, not a problem, for the grounds. Your meter probably reads 0.5 ohms or higher on a dead short of the test leads which is internal impedance of the test meter and not resistance in your ground wires. Most of the cheap test meters everyone has now days do not have any way to calibrate the ohms to zero before a test. Also a weak battery mades it worse.

The most I read anywhere in mine on a ground wire with a cheap meter is about 1.6 ohms, and about 0.8 ohms of that is from the cheap meter itself. I verified this a while back with my old FET analog meter which has zero calibration knob to correct for the batttery condition.

Is the start, no start consistant with have the MAP sensor connected or not? or is the no start thing inconsistant?

I started to ask this before, but it has come to mind again, could the vacuum hose be bad, clogged, colapsing under vacuum, ?????

Another thought. I had a bad MAT sensor cause a cold engine no start problem. Drove me nuts for weeks. The Manifold Air Temperature sensor was reading hot when it was cold and that was throwing off the spark timing and or A/F ratio estimate the ECU used for timing the injector open time.

Have you ohm tested the CTS and MAT temperature sensors against the FSM to see if one of them is out of calibration, or stuck reading hot when it is cold. That would cause a starting problem, that might, I said might!, allow the engine to start with the MAP sensor disconnected because the ECU would change the timing and or A/F ratio to a start table value and ignor the CTS, MAT and MAP sensors and calculated values because it had no MAP sensor reading.
 
Best advice I can give is to start over, and test one and only one thing at a time. Return to status quo ante after each test, and before starting the next one.

Troubleshooting is usually complicated by having more than one thing change at a time, and you can screw yourself up in a huge hurry if you go messing about with too much without keeping track.

Pay particular attention to running condition - it may be a combination of failures in the system causing the large problem, but I'm willing to bet that making the changes to single components will show up as "smaller" problems that you have to pay better attention to spot.

Also, when it konks out on you, what happens? Does it act like it's not getting any fuel anymore? No spark? Not enough fuel? All these data can be important, and I'm quite sure you know something you're not telling (not from any error of commission on your part - a simple "error of omission" is more accurate. It doesn't seem important to you, but it could break the case once you tell us.)
 
I have checked the cts sensor I don't remember what the readings were now it was a couple of weeks ago but they were dead on for what i found on here that they should be. I have not checked the mat sensor. I have tried several vacuum lines on the map sensor the one that was on it originally had a small crack in it, but the one on it now seems to be fine.
The only time it dies is if I plug in the TPS or the Map sensor, I can go out and start it up it cranks a little while but I figure that is because of the lack of sensor readings and it will set there and run till I shut it off. When it dies it is like you went and killed the key it doesn't stumble or sputter it just shuts off. I have pulled the codes and all I get are no signal from map and TPS.
 
TPS and MAP have a common VIO/WHT supply wire with 5 volts, and the return wire for the MAP sensor is DK GRN/RED. Sure sounds like something is wrong wiith one of them, or wrong with MAP sensor.

Try the old MAP sensor again just for grins, to make sure it did not go bad.

Have you checked the cranking voltage at the battery while cranking? it needs to be at least 10 volts, And the running voltage should be about 13.5 or higher?

5-90, what could go bad, mechanically or electrically that would cause it start with no working sensors (I am assuming the ECU is ignoring everything but the CPS when his MAP sensor is disconnected), and yet not start when the MAP is connected. I am thinking in terms of timing issues!!!!????, guessing the ECU is doing something different fuel or timing wise when that MAP is disconnected.

One other thing to try for diagnostics, try flooring the gas peddle while cranking with the MAP and TPS connected.
 
I have tried the old map sensor a couple of times just in hopes I might get lucky, but I wasn't that fortunate. I am sure it has to be something in the purple/white input wire or the ground, I am thinking I am going to cut it completely out and rewire. My battery was good till I kept cranking it getting readings on things while it was cranking, right now it is setting with the charger on it and I took part of the evening to work on the rear suspension on my Scout to see if a day or two away from the Jeep might give me a new perspective on something.
 
I just thought of something I ommitted earlier like 5-90 said there had to be something. I don't know if there is any connection but about 100 yds. before it died the tach started working and it hadn't worked since I had the Jeep (which was only about two months, then it died).
 
2jeepdrivers said:
I just thought of something I ommitted earlier like 5-90 said there had to be something. I don't know if there is any connection but about 100 yds. before it died the tach started working and it hadn't worked since I had the Jeep (which was only about two months, then it died).

Time to make a tacky joke!

:roflmao:

The tach runs off of the ECU, IIRC, and the ECU gets its tach data from the CPS. What year was this beast again?
 
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