jeep dies after getting warm.

I have an 88 mj, 4.0, aw4, 242. today i was driving to school, which is about a thirty minuite drive. i was almost to the parking lot, when my mj just died. the battery light came on, then water temp and oil pressure. i have the idiot light dash. it was just like i turned the key on to run. i parked it and tried to start it, it just cranked over and would not even try to start. i came back out at lunch and tried to start it. it started right up. i have no clue what is wrong with it.

anyone have any ideas????,

You are experiencing thermal (heat) failure of some electronic component. My guess would be the crankshaft position sensor. Located on the bellhousing, drivers side. It is a very common sensor fail on the 4.0 When they go south, you no longer have spark or fuel to the injectors. They can be tested but if you go that route, test it quickly when symptomatic as when the sensor cools down, will probably test in spec.

CPS TESTING PROCEDURE 1991 – 2001 4.0L H.O. engines

1. Near the rear of intake manifold, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C (A-B-C looking into connector left to right with the part with the notch in the middle on the right) Ohmmeter should be set to 1K-to-1OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.

CPS TESTING PROCECURE for 1987 – 1990 4.0 L engines

Test # 1

Get a volt/ohm meter and set it to read 0 - 500 ohms. Unplug the CPS and measure across the CPS connector's A & B leads. Your meter should show a CPS resistance of between 125 - 275 ohms. If the CPS is out of that range by much, replace it.


Test # 2

You'll need a helper for this one. Set the volt/ohm meter to read 0 - 5 AC volts or the closest AC Volts scale your meter has to this range. Measure across the CPS leads for voltage generated as your helper cranks the engine. (The engine can't fire up without the CPS connected but watch for moving parts just the same!) The meter should show .5 - .8 VAC when cranking. (That's between 1/2 and 1 volt AC.) If it's below .5vac, replace it.

When it dies let it cool down for about a half hour or so and try to turn it over if it starts its definatly you crankshaft position sensor.
Ask me how i know.....lmao
:dunno:

-PJ-

vortex said:

When it dies let it cool down for about a half hour or so and try to turn it over if it starts its definatly you crankshaft position sensor.
Ask me how i know.....lmao
:dunno:

-PJ-

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X2

well, put a new mopar cps in, and this is what i get.

for test # 1 from birchlakeXJ's post, i get 247.58 ohms

from test #2, i get .21V AC.

Test one is in spec, test two is out of spec. this is the same way my original cps tested. for test 1 i got 237 ohms, and test two i got .3V ac.

Is it possible that i got two bad sensors? if not, what else should i look at. still no spark.

well, third CPS is a known good one, came off of my 2wd trans. Still no start.

so i've gone through the one that was on the trans when i got it, a brand new mopar one, and my known good one off of my 2wd trans i just took out.

Anyone have any other ideas besides the cps?

Have you tested your coil? I'd try that.

birchlakeXJ said:

Have you tested your coil? I'd try that.

Click to expand...

x2

test for spark and fuel.

89xj said:

x2

test for spark and fuel.

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Yup, we can't forget the all important fuel part of the equation as 89xj points out. Do you hear the fuel pump energize and run for a couple of seconds when you turn the key on? I'd put a fuel pressure gauge on the schrader valve on the fuel rail and see how much available pressure you have. Another quick and dirty test is to spray a bit of starting fluid; if your engine starts and runs momentarily, your problem is fuel delivery, not ignition.

I have no clue how to test my coil.

Fuel pump kicks on, and i have fuel pressure. I'll try the starting fluid part when i get home from school today.

88manche said:

I have no clue how to test my coil.

Fuel pump kicks on, and i have fuel pressure. I'll try the starting fluid part when i get home from school today.

Click to expand...

first check for spark at one of the plugs while turning over the ignition, if you have no spark at the plug, you work back to the cap. if no spark to the cap through the coil wire, you test to see if the coil is getting a signal from the icm. could be just a bad connection from the coil to the cap. i would pull the coil wire off the cap and coil to see if there is corrosion

A factory FSM or even a Haynes or Chilton manual is invaluable for the XJ doityourselfer. Lots of specs and testing information in them. I'd score one of those; even if you get past this problem, there will undoubtedly be another. The diagrams in the manuals show you exactly where to put your meter probes and what to look for.

Well, no spark at plugs, or cap, or coil wire going to cap. I had a FSM for my last jeep, but all i have for this one is a Haynes manual.

update -

no spark at all. I have fuel pressure.

checked my coil per instructions in my haynes manual.

i have 11.4 volts going into my coil.
positive to coil body = 12.5 k.ohms
positive to coil output = 6.14 k.ohms.
positive to coil output = 11.4V

is my coil shorted? I checked my ballast resistor on a hunch, it checks out.

This is killin me.

new coil, still no spark.

Cleaned brass contacts on distributor and cap. still no spark.

wiggle/disconnect/reconnect the wiring harness for the cps. maybe a bad harness.

check all ground wires and ground strap.

check wiring harness going into ecu. under the dash near the radio in my 89 xj.

test the ignition control module(icm) or swap one in from the junk yard.