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Ron at copper

Cherokee XJ 4.0L Idles Roughly and Misfires with Clutch Engaged

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My 1987 Cherokee 4.0 liter is misfiring when started but when I have clutch depressed it runs fine. Does any one have any ideas? I think a bad TPS but don't want to throw money at something that won't fix it!          

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Hi, Ron at copper!  Misfiring under load from what you describe, somehow related to the depression of the clutch.  Check the crankshaft position sensor at the back of the engine block (driver's side, top of bellhousing around 11 o'clock when looking from the rear of the engine).  Remove the sensor and clean it with electrical contact cleaner.  Check the wiring and connections, too.

 

Does the engine have very high mileage?  I'm curious about the connection between depressing the clutch and releasing it.  This has either to do with engine load or possibly the crankshaft float in the block when you load the clutch and flywheel.  Try the crank position sensor first.  If that does not cure the issue, let us know.  I have other troubleshooting suggestions that will only cost time, not money, if you have the tools.

 

Moses

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I had replaced the crank sensor a few years back because it would run fine for awhile then would not start at all. The mileage is high at 227,212 miles. I checked the fuel pressure at 32 psig but did not check capacity. Replaced plugs,dist cap and rotor as well as the wires, just replaced coil when I was trying to trouble shoot the crank sensor.have not replaced alternator could this be problem?

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Thanks for the update, Ron...You hint about fuel volume, and that could be the issue.  Pressure sounds okay...Unlike later MPI applications, your Renix system has a fuel filter along the chassis rail toward the tank.  (You likely know this, and if not, consider the filter a routine replacement item.) 

 

1987-90 XJ Cherokee 4.0L Fuel Lines.pdf

Fuel Line System for a 1987-90 Jeep XJ Cherokee 4.0L Six

 

A clogged filter restricts fuel volume.  Pleated paper filter matrix likes to swell in the presence of water, and that's a good thing on the one hand, it stops water from reaching the injectors.  On the bad side, you can get nothing more than one tankful of watery fuel, and the filter plugs.

 

If you suspect the filter could be the culprit, change it.  If that's the cure, put some de-icing or alcohol-based fuel additive in the tank to pick up remaining water.  This may or may not clog the filter again, as the water bonds with the alcohol and travels through the fuel system.  Be prepared for more water swelling at the filter. 

 

Let's go from there...

 

Moses

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Yep! Changed filter first. Know you are probably a busy guy, but I think you are right about crank sensor, about how long do those last? Will try cleaning first. Figured out it's been in there about 7 years but my wife only drives 6 miles per day 5 days a week.

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Ron...The crankshaft position sensor on a high mileage 4.0L can pick up oil from a weeping rear main seal.  You may not have a defective CPS but rather a dirty one.  Before tackling the CPS, though, move to the battery and grounds.  Since the engine starts to act up as you release the clutch pedal and load the engine, have you checked the battery condition?  The battery terminals and cables? 

 

A dead or dying cell or poor connection at the battery terminals can create a misfire under load condition.  The engine seems to run okay when not under load because the rpm is high enough for the alternator to step up the voltage and keep the ignition and injectors functioning. 

 

I would run a load test on the battery.  (The safest way is out of the vehicle at a testing station like an Auto Zone or NAPA.)  Clean the terminal ends and also check the battery ground and engine to body and frame (unibody) grounds.  At this mileage, ground contacts and terminals often get corroded.  This is the negative side of a D.C. system, so a voltage drop will result.  This can also cause a misfire under load or stalling. 

 

The PCM/ECU is looking for stable battery voltage, typically in the 12.4V or higher range.  An alternator check would be advised if the voltage output on your dash gauge indicates less than 13.5V as engine speed picks up.  This could approach 14.6V or so with a discharged or defective battery.

 

Pulling on the headlamps with the engine off would also reveal a battery in poor condition, or poor connections, if the lamps go dim quickly.  Slow cranking is of course a symptom.

 

Try the battery and connections before crawling under the Jeep to check the crankshaft position sensor (CPS).  It's helpful to know whether the alternator is functioning well, too.

 

We'll work through this...

 

Moses

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I was looking at the Haynes manual for checking the cps and it said to put an ohm meter on the sensor and " it should read no restiance ( open). We'll I work on commercial and industrial boilers and I know that no restiance is closed! My meter reads 0 which means its a closed circuit. Is this thing a proximity switch? What should it read, infinity or zero?

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Waiting for your update, Ron.  The CPS is a very common problem issue.  The damage is apparent.  Let's see if this is the cure.

 

Moses

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UPDATE! The jeep is back to its same old reliable self. When I installed the last crank sensor I thought I would be smart and tie wraped the excess cable to the fuel line to keep it away from the exhaust pipe, we'll it was too tight and as it curved over the top of the bell housing the movement of the vehicle going down the road caused it to rub a hole in cable exposing the wire to short out intermittently. Thanks so much for you're input!

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Pleased that this worked out well, Ron!  These issues also pop up when installing an exhaust header that takes up space needed for wire clearance.  Wires end up melted on the exhaust header...Routing is important.

 

Glad you nailed this, others will benefit from our exchange!

 

Moses

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Help again, the jeep ran fine for about five hours then just did not want to start. Put on the fuel pressure gauge no pressure had a new fuel pump in garage put that on but before I did that checked voltage to pump was 5vdc, installed pump anyway thinking it was the pump relay. Got a new one and installed that,didn't work either so I started checking voltages at relay using a wiring diagram I got from ondemand5direct.com. And to make a long story short I have only 2vdc across the coil on relay and when I go from ground on diagnostic connection to coil terms I get 12vdc to both sides which really has me baffled. Could it be the ign control mod/coil?

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Sounds like separate issues here.  The 5VDC sounds like the trigger signal from the PCM.  The pump itself runs on 12VDC.  Confirm which circuit or lead you're testing, use the factory wiring schematic to separate wire leads. 

 

Keep in mind that bad grounds can create a voltage drop.  Any time you have a poor ground on a DC system, the voltage drops.   

 

You could have a PCM issue, but given the low voltage readings, I'd begin with the grounds:  battery ground, battery-to-chassis ground, engine-to-body, engine-to-frame, the PCM ground, ground at the fuel pump, etc.  Use a volt-ohmmeter and look at ohms resistance, not simple "continuity tests".  Let's not overlook the obvious, make sure the battery, when loaded, does not produce a voltage drop in a circuit or the entire system.  Test the battery condition.

 

There is another test that helps when checking long wiring circuits including grounds: a lamp load test.  (I talk about this test at the forums.)  You can lamp load test a circuit far from the battery.  I'd consider doing this on the fuel pump circuit if none of the other testing proves fruitful...If you advance to a lamp load testing approach, try a "Search" at these forums, using the keywords:  lamp load.

 

Moses

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Powertrain Control Module, Ron...and there are no "stupid" questions here!  The PCM controls fuel-and-spark management plus several other functions. 

 

As a matter of semantics, I use the term interchangeably with "ECU" (Electronic Control Unit) and "ECM" (Electronic Control Module), as the term "PCM" applies to the majority of Jeep fuel injection systems—except 1987-90 Renix systems like yours!  Yes, my PCM reference is really your ECU on the '87 Renix 4.0L EFI system or the 2.5L TBI system.  Please excuse any miscommunication or confusion.

 

PCM became more relevant when these systems involved both the engine and transmission.  In industry vernacular, the combined engine/transmission is considered the "powertrain"...

 

Moses

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Where exactly is the ECM is it mounted on fire wall? And what kind of $ are we talking about? I checked all the grounds as you described all are 0 ohms and put in new battery still no luck. Please tell me if the attached is the ECM?

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Where exactly is the ECM is it mounted on fire wall? And what kind of $ are we talking about? I checked all the grounds as you described all are 0 ohms and put in new battery still no luck. Please tell me if the attached is the ECM?

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Ron...Your 4.0L MPI engine has an ECU, technically.  The ECU mounts beneath the instrument panel between the steering column and A/C unit.  The ECU should mount on a bracket with three screws.  Make sure you disconnect the battery ground cable before fiddling with the ECU.

 

Here is a PDF of the "microprocessor" (item #15) for the 1987-88 4.0L model XJ Cherokee:  1987 XJ Cherokee Microprocessor.pdf.

 

This should help...

 

Moses

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I got the ECU out of jeep but the part # on it does not match any of the #s in the PDF you sent me? I ordered one from napa and am a little worried that it won't match what I've got herepost-356-0-65633800-1411945803_thumb.jpg

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This is not unusual, Ron...Part numbers do get superceded. In the 2010-2011 Mopar Reman catalog, the OEM part number for a 1987 4.0L Renix (Fenix) Cherokee ECU is 53007842 for automatic transmission models. The manual transmission is not shown for '87 but is for '88 and listed as the 53007843. If the NAPA ECU crosses over to the appropriate Mopar OEM numbers, you should be okay.

There were different OE suppliers. Your unit is a Bendix with another part number...1987 was still an AMC/Jeep year with Renault involved. That changes in '88, and Mopar likely re-cataloged the OE AMC numbers. I looked up your Bendix number, and it shows as a 1987 Jeep 4.0L Cherokee with a manual transmission. This is the same as the 53007843 Mopar, the 1988 listed ECU for manual transmission.

Moses

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Well installed rebuilt PCM/ECU no luck still the same old thing 7vdc to fuel pump. I've checked all the grounds that I know about, from batt to chassis, batt to engine,etc. this thing is driving me crazy. I have the input wiring diagram to the PCM but not the output diagram

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Hey, Ron...I drilled deep on this one and may have a solution for the fuel pump voltage issue.  Your Renix fuel supply system does use a 12V pump, and you're very right in expecting a clean 12V signal at the pump.  There is one item in this electrical supply to the pump that may be the trouble spot.  Your system uses a ballast resistor, mounted at the left inner fender panel in front of the EGR solenoid.  This is a typical ballast resistor with one wire in and one wire out, easy to recognize.  Apparently this is used to clean up the voltage signal to the fuel pump, which has a 12V permanent magnet motor.

 

1987-90 Cherokee Fuel Pump Ballast Resistor.pdf

Location of fuel pump ballast resistor on 1987-90 XJ Cherokee with Renix MPI

 

Check the voltage at each side of the ballast resistor.  If the motor requires 12VDC, and if there's a drop here at the ballast resistor to the 7VDC that you're finding at the pump end, the voltage drop is right here at the ballast resistor.  (The resistor is defective.)  If there is 12V on both sides of the resistor, you have a voltage drop between the resistor and the pump, either the ground side or hot lead.

 

I also want you to read this PDF, Ron, specifically the paragraph with reference to the MPI fuel pump functions and voltage sources, as they vary under different operating modes.  The ballast resistor, starter relay and oxygen sensor relay come into play:

 

Renix Fuel Pump Electrical.pdf

 

Moses

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Fuel pump ballast resistor is not in location described there is a relay there don't know what that is for, could this possibly be caused by a bad ignition switch?post-356-0-05156100-1412369173_thumb.jpg

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Explore the relay's function, Ron...Keep looking for a ballast resistor.  The PDF is 1989 service information.  I would expect this to be the same for '87-'90, but maybe '87 is an exception, or the ballast resistor might be elsewhere—or by-passed?  Perhaps 1988-90 added the resistor after experiencing a problem with the '87 model?  We can explore this further.

 

The ignition switch can always be a source for voltage drop, good thought.  Worth at least probing the contacts and testing the switch's input and output voltages.  Fortunately, the switch and its connector are not that difficult to access on the XJ Cherokee.  This is a possibility.  For the voltage drop you describe, there would have to be a poor connection or high resistance at the switches contacts.  I'd expect this to impact other circuits, but with relays, maybe the voltage is strong enough to close the relay contacts.

 

Here's the relays identification drawing and part numbers for a 1987 Jeep XJ Cherokee:  1987 XJ Cherokee Relays.pdf.  See if this helps, too.

 

Moses

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Does it matter that this is a chief? The relay where the ballast resistor is supposed to be does not match any wire colors on my wiring diagram, the relay coil is black and orange the contacts are red and red with a black stripe? Also after I installed the ECU and with the ignition switch off the left turn signal was on and the column sig switch was off so I pulled the signal timer then this morning when I went out to try and trouble shoot I pluged timer back in and it was not on anymore the turn should not come on with ign switch off, correct?. Thinking more about ign switch!

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post-356-0-26532300-1412438001_thumb.jpg

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Ron...Correct about the turn signal.  Should only work in the KEY ON and possibly the ACCESSORIES position, depending upon the make and model.

 

I drilled down in the 1987-90 Jeep XJ Cherokee Mopar parts listings, and the ballast resistor does date back to 1987.  Here is the 1987-90 Mopar parts catalog showing the ballast resistor, part #24:

 

1987-88 XJ Cherokee Front Engine Wiring Harness with Ballast Resistor.pdf

 

Note from parts coverage that the 1987-90 period is indicated here.  This listing for the ballast resistor does not indicate a particular model year, which means the part is applicable to all years 1987-90.  The Mopar part number for the ballast resistor is 33000682.  When I run this as a Mopar number, the fitment is as follows:

 

This product fits 63 vehicle variants.
JEEP: 3 models, 63 variants between 1987 and 1993

 

Here's a listing for the ballast resistor that clearly indicates 1987 fitment:  http://www.moparpartssource.com/a/42361045__5564270/HARNESS--ENGINE-COMPARTMENT-4--6-CYLINDER-ENGINE-CHEROKEE--WAGONEER--COMANCHE/000018WE.html.  The part is inexpensive (under $7 plus freight from this source).  Likely the ballast resistor is available in the aftermarket as well and from local sources.

 

So, either all of this information about a ballast resistor on a 1987 model (including the wiring harness schematic, the factory parts manual and a factory service manual) is incorrect, or we haven't found the ballast resistor on your 1987 Jeep XJ Cherokee 4.0L MPI model yet...You can zoom-in on the wiring harness illustration to pinpoint the connectors for the ballast resistor.  There should be two spade/female terminals unless someone has eliminated or relocated the ballast resistor.

 

If the resistor has been eliminated with a splice of the two leads, check voltage at this point.  It should read the same as the battery voltage (12.85-12.95 volts for a new and fully charged 12V battery without the engine running).  The reason for the resistor is to lower/regulate voltage in normal fuel pump running mode (not the cranking mode or under wide open throttle, as we've discovered).  The battery and circuits line voltage with the engine running and alternator charging can exceed 14 volts, which apparently is not what the fuel pump should receive in normal operating mode...See if the engine front harness illustration helps, Ron.

 

Here is the 4.0L fuel injection wiring schematic for the 1987-90 Jeep XJ Cherokee.  You asked about pre- and post-ECU wiring, this covers what you wanted to know.  Note that the wires to the ballast resistor are orange with a black stripe, both in and out of the resistor (Illus. #21).  There is a reference note indicating that the ballast resistor is "near the air filter [box]".  This color scheme connects to a pink/black wire that goes to the fuel pump at the tank:

 

1987-90 XJ Cherokee EFI-MPI Wiring Diagrams.pdf 

 

Moses

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