Skipping and misfires

[Tex06]

Hey all, new to the forum, been using this and jeepforum for the past 2 months to keep my MJ up and running. Quick history on the MJ then I'll give ya the dirty dirty:

 

Swapped for the MJ (4x4, 4.0L, Big Ton, with 5-spd) about the end of August down in NOLA.  Exhaust manifold wasn't connected, but otherwise it ran great.  Drove around town like a bandit then left a week later to Alanta with no issues other than gassing myself in slow traffic (also, possible hearing loss).  In Atlanta, bought nuts and reconnected exhaust.  Much quieter, ran good for another tank of gas (300 miles maybe less) started idling rough.  Pulled plugs (they were shot, maybe even original to the jeep).  Replaced with Autolite 985 copper core (gapped to .035" I'm pretty sure).  Fixed idle issue. Loaded up and headed back to NOLA.  Left mid afternoon, got a hesitation or two along the way, could have been a bump in the road.  I get about to the La. state line (about 11pm) and the truck starts misfiring and surging erratically.  Hold the throttle and try to cruise at any speed/gear/rpm and it bucks and fights.  Under hard acceleration the problem is there but much less pronounced.  Let off the throttle and the issue seems to go away (truck isn't shaking anymore, hard to tell over hwy noise).  No CEL at any time.  Limp into NOLA and over next 3 weeks:

 

Replaced Fuel filter- no change

Tested fuel rail pressure and flow rate- ~31psi and 39psi with regulator connected/disconnected.  With return pinched off flowing close to 2l/min (filled 500ml bottle in 13 seconds).

Pulled spark plug wires and checked for resistance...all were similar with longer wires slightly higher than shorter wires.  Cleaned ends and reattached (all done one at a time then double checked for order) - no change in symptoms

Pulled old O2 sensor and bench tested.  Readings very erratic.  Order new Standard SG1829, bench tested it to compare readings.  New one cycles as expected from .1-1.0A in flame.  Installed new O2 sensor and problem appears to be fixed.  Test drives, no bucking, no skipping.

 

Next day leave NOLA around 4pm, drives great for first 150 miles.  Pouring rain, temps drop and from around Lake Charles to Victoria, Texas the misfires are as bad as ever.  Pulled over several times and disconnected/reconnected O2 sensor trying to get the ECU to run the motor rich, used ether to locate at least one vacuum leak (repaired with electrical tape), tried plugging vacuum lines, looked for spark leaks, wiggled wires and lines, dumped in a bottle of B12 Chemtool and 89 Octane out of desperation, nothing.  If driven at high rpm the misfire is dealable but anything below 2000 rpm I'm afraid I'm going to blow the bottom end of the motor out unless I'm accelerating WOT.  Then, after taking a nap in Victoria I wake up this morning, fight the misfire for about an hour and after the sun comes out and it's 84 outside, the truck drives with no issues.  I can't get it to misfire, even by lugging the engine under 1200 rpm. 

 

I've got a cap/rotor/wire set on order, bought PCV(CCV, little plastic elbow thingys) to fix my blow by, and some new vacuum lines just as maintenance items on this thing.  If these don't fix it, does anyone else have any other ideas?

 

I've read about a coolant temp sensor maybe? The fact that it seems to drive ok in 80*F and hotter has me a little confused.

 

Posting this on two separate forums....I'm not looking to parts swap and guess.  If you can give me an idea and a possible troubleshooting (or hell steps even!) I would rather do that than throw money away.

[87MJTIM]

What year is the MJ?  I ask because Renix (87 - 90) and HO (91 - 92) have different engine systems.  This will help other diaguess (diagnose and guess) the problems.

[gogmorgo]

From the sounds of it, your misfiring seems to coincide with humid, rainy weather. Usually that indicates worn ignition components. Hopefully your new cap, rotor & wires do the trick.

[cruiser54]

Also, click on the link in my signature below and do Tips 1 through 5 if 1987 to 1988 or 1,3,4, and 5 if 89 or 90. 

 

Keep this in mind. The plugs, wires, cap and rotor were 30,000 mile maintenance items.

[Tex06]

Need to turn on email notifications for replys....hence the late response. Thanks to those who have chimed in so far, here's where I'm at now.

 

MJ is a 92 Eliminator (so not a Renix).

 

Changed cap/rotor/wires with MOPAR parts.   No change.

Tested TPS.  Had definate flat spot in center of range (~2.4V) where more throttle actually decreases the output voltage.  Full range was .64V to 4.99V.  Replaced with 2 BWD units from O'Really, first one was defective and made the Jeep undriveable.  Second one has lowered my idle RPMs to under 900 (previsously above 1200), output is .84V to 3.8V.  Have MOPAR unit on order but will be a week before it gets here.  Also, no change.

 

Ohmed out engine coolant sensor.  Was showing a little out of spec from the chart I found.  Replaced with Autozone special.  No Change, but if you leave it disconnected it gets worse. Put in a 195*F t'stat and new belt while I was there (old one was a 160*F).

 

Numbers from the old Coolant Sensor:

Temp *F               Ohms

119                       3220

161                       1330

115                       3800

103                       4990

 

Disconnected the exhaust to rule out a plugged Cat as well.  When I first bought the jeep the manifold was disconnected from the rest of the exhaust and I drove it on a 500+ mile trip like that with no issues.  It's louder now (can hear the misses much easier at low rpm) but again, no change.

 

Next step I'm thinking is take off the hood and drive it with fuel pressure gauge attached to see if pressure is dropping when it misfires (something in the tank/plugged sock/faulty fuel pump) or possibly ignition coil or module.  I have read that the test done at the local auto parts store doesn't always catch faulty coil/modules on these.  Is there any other testing I can do before just swapping in a new one?

[gogmorgo]

Since it's an HO, check your codes. Its a simple process of rocking the key back and forth.

http://comancheclub.com/topic/28111-reading-obdi-91-and-92-mj-fault-codes/

[Tex06]

Cleared out yesterday (forgot to add that).  I removed and replaced the leads on the TPS and water coolant sensor with the key on (whoops....there was a lot going on in my head) and afterwards the CEL popped these (had not been lit previously):

 

12

22

24

17

55

 

I know 12 is battery disconnected, 22 was bad H20 sensor, and 55 is end of codes.  Since I've replaced the H20 sensor and cleared the codes I only get 12 and 55.

[HOrnbrod]

Skipping and misfire can be caused by a bad CPS. It's happened to me on my 91 - twice. It did not trigger a code either because it was intermittent. There's really no way you can test it on an HO except by replacement, preferably with a Mopar unit. Also make sure the CPS 3-pin harness connector near the rear fuel rail is clean and tight. 

 

Ohmed out engine coolant sensor.  Was showing a little out of spec from the chart I found.  Replaced with Autozone special.  No Change, but if you leave it disconnected it gets worse. Put in a 195*F t'stat and new belt while I was there (old one was a 160*F).

 

You are referring to the temp sensor on the thermostat housing, correct?

[Tex06]

Skipping and misfire can be caused by a bad CPS. It's happened to me on my 91 - twice. It did not trigger a code either because it was intermittent. There's really no way you can test it on an HO except by replacement, preferably with a Mopar unit. Also make sure the CPS 3-pin harness connector near the rear fuel rail is clean and tight. 

 

Ohmed out engine coolant sensor.  Was showing a little out of spec from the chart I found.  Replaced with Autozone special.  No Change, but if you leave it disconnected it gets worse. Put in a 195*F t'stat and new belt while I was there (old one was a 160*F).

 

You are referring to the temp sensor on the thermostat housing, correct?

 

Yep, the one in the t'stat housing.  The gauge sending unit isn't working, never reads above 120* but that's lower on the list for now. 

 

Thanks for the tip on the CPS.  I'm calling and trying to see if there's one in my area now......

[Tex06]

Ohmed Coil. Primary is 1.1 Ohm, secondary is 1243 Ohm. Well within specs from what I found (The Chilton manual specifies ranges of: Primary: 0.95 - 1.2 ohms Secondary: 11.3 -15.3k ohms).
Coil is cracked around mounting bolts and one on the back side, I think I will replace for good measure. NAPA loves me...

Fuel test showed no drop in pressure from stuttering while driving. if anything I got a small spike when it stuttered from 32-35 psi up to 40 psi.

I took apart the dizzy and found the magnetic pickup, I don't think this jeep has a Ignition Control Module since it's like an HEI.....is that correct?​

​

[HOrnbrod]

That's about all you can do to check the HO coils. I've had several check good through the primary and secondary windings but didn't work at all. And they have never caused an intermittent ignition problem; when bad they all caused a no-run condition.

 

There's no ignition module for the HOs. The pickup, or sync sensor, or camshaft position sensor, on the 91-93 distributors is internal. In order to change it you have to pull out the distributor, remove the drive gear from the shaft, and disassemble it completely to get the sync sensor out. Read major PITA.

 

 

A nice upgrade is to swap in a 94-99 4.0 distributor. The sync sensor is a black disk and it sits right on top under the rotor, so to change it you don't have to pull the dizzy out. The 94+ distributor swap is pure PnP.

 

 

http://www.xjtalk.com/showthread.php?t=9111

[Tex06]

OK, about ready to light this thing on fire. Pulled the crankshaft sensor plug and cleaned it and found I was missing a bolt on the sensor itself. Reconnected, tapped on sensor to make sure it was seated and drove 20 minutes to Lowes to get a bolt with no issues (a first. It had gotten so bad I couldn't leave the driveway). Drove home and it hesitated once or twice on the interstate but nothing bad so I swapped in the BWD sensor I bought. Drove 20 minutes to Walmart, bucking the whole way. Come out of Walmart and driving in town, no issues.

 

WTF, mate? Wiring harness issues? I've wiggled all I can find and nothing...

[HOrnbrod]

Be advised that the 4.0 CPS mounting bolts are unique shoulder bolts and an all-thread bolt may or may not work. I wouldn't use them. These bolts hold the CPS at a precise distance from the flywheel or flexplate to read the pulses. You can get the correct mounting bolts here as well as other places: 

 

http://www.morris4x4center.com/mopar-screw-qty-1-6503520.html

[Tex06]

Yep, ordered a set. Unfortunately they'll be delivered to my place in New Orleans and I'm stuck in Texas.

[Tex06]

Annnnddd stranded now. Cleaned the IAC and checked grounds, runs worse now than it ever has. I'm afraid the bottom end isn't going to hold up if I drive out, I can't even limp it home. Calling the Dad tow service, lol.

 

 

It's idle is erratic, goes from normal to over 2000 now. Think my IAC is toast, still don't know why it stutters.

[Tex06]

Went to Pull-a-part today and bought a ton of sensors and pieces. Will be swapping them in tomorrow and going at it in a shotgun style attack now.

 

3 Tps

1 CPS

1 O2 sensor

2 throttle body assemblies

2 IAC

1 '95 dizzy (only 165k on it!)

2 window cranks

12 Bosch 703 Injectors (pricey but I wanted to upgrade anyway)

2 MAP sensors

1 Coolant temp sending unit (forward)

2 MAT sensors

1 '92 Cherokee 4.0 ECU/PCU/computer-thingy

Enough Pigtails to test the whole thing twice over

2 Cool 4.0 litre high output emblems

4 CPS Bolts (guess I shouldn't have ordered the others...oh well)

 

 

 

 

And a full set of performance aftermarket gauges!

 

 

Tomorrow will be an interesting day, that's for sure!

[MJ_Milam]

I would suspect possible MAP sensor. Mine would occasionally start up and idle at 2000 rpm. I'd pop the hood and tap it with a screw driver a few times and it would smooth out.

[Tex06]

I would suspect possible MAP sensor. Mine would occasionally start up and idle at 2000 rpm. I'd pop the hood and tap it with a screw driver a few times and it would smooth out.

Well folks we have a winner.  I disconnected the MAP before a test drive and walla!  All hesitation issues were gone.  Plugged it back in and they came right back (although the idle still was pretty smooth).  I had swapped in the JY sensor, guess it was bad from the get-go.  Ran rich all the way into town but got a new NAPA unit (only $73 with tax) and she's purring again.  Should have know that was the issue when I saw the "GM" on the back of the original MAP sensor, lol.

 

So, if you have hesitation issues at all RPMs and want to rule out the MAP early, just disconnect it then test drive.  If that clears it up, leave it running and reconnect, the problems should return right away.  Easy troubleshooting solution.

 

On a good note though, my 4.0 has a lot of new parts so I shouldn't have to do maintenance for like, 20 years now  :D

 

Thanks to MJ and Hornbrod for helping me out with this, you guys rock!

[Tex06]

That's about all you can do to check the HO coils. I've had several check good through the primary and secondary windings but didn't work at all. And they have never caused an intermittent ignition problem; when bad they all caused a no-run condition.

 

There's no ignition module for the HOs. The pickup, or sync sensor, or camshaft position sensor, on the 91-93 distributors is internal. In order to change it you have to pull out the distributor, remove the drive gear from the shaft, and disassemble it completely to get the sync sensor out. Read major PITA.

 

 

A nice upgrade is to swap in a 94-99 4.0 distributor. The sync sensor is a black disk and it sits right on top under the rotor, so to change it you don't have to pull the dizzy out. The 94+ distributor swap is pure PnP.

 

 

http://www.xjtalk.com/showthread.php?t=91

 

Installed the New Dizzy from the '94 - works great!  Also picked up 12 Bosch 703 Injectors that I'm going to rebuild and install for a little better throttle response.  There were probably 50 of them available in the JY, I figure I'll get at least 6 good ones.

[Tex06]

Spoke too soon it looks like. The new MAP only fixed the problem for about 50 miles worth of driving. Exchanged for another from NAPA, the second didn't help at all. Starting to suspect the connector to the MAP, have a Mopar MAP I ordered at the dealership but there's no GM markings on it like the OEM one. Should this worry me?

[HOrnbrod]

Don't worry about the GM markings on the MAP body. I've seen OEM sensors with and without.

 

Your MAP connector has three pins, A, B, and C. Pin A is sensor ground (BLK/BLU wire), pin B is the MAP output to the ECU (RED/GRN wire), and pin C (VIOLET wire) is the 5VDC reference input voltage from the ECU.

To test the connector inputs, unplug it from the MAP sensor, turn IGN ON, and measure across pins A and C. You should see very close to the 5VDC reference voltage. To test the MAP output, plug the connector back on, start the engine, and measure the voltage across pin B by back-probing the pin B connector and a good ground. With the vacuum line hooked up to the sensor, this voltage will vary between ~ 0.02V to 4.94VDC depending on engine RPM and intake manifold air pressure if the sensor is working. An analog meter is best to use if you have one for this test. A steady or no voltage indicates a bad MAP sensor.

[Tex06]

Don't worry about the GM markings on the MAP body. I've seen OEM sensors with and without.

 

Your MAP connector has three pins, A, B, and C. Pin A is sensor ground (BLK/BLU wire), pin B is the MAP output to the ECU (RED/GRN wire), and pin C (VIOLET wire) is the 5VDC reference input voltage from the ECU.

 

To test the connector inputs, unplug it from the MAP sensor, turn IGN ON, and measure across pins A and C. You should see very close to the 5VDC reference voltage. To test the MAP output, plug the connector back on, start the engine, and measure the voltage across pin B by back-probing the pin B connector and a good ground. With the vacuum line hooked up to the sensor, this voltage will vary between ~ 0.02V to 4.94VDC depending on engine RPM and intake manifold air pressure if the sensor is working. An analog meter is best to use if you have one for this test. A steady or no voltage indicates a bad MAP sensor.

Ok, tested for voltage at connector before hooking up the Mopar MAP.  A-C shows 4.93 VDC, and B-C shows 4.87VDC.  Checked both to ground just to verify, 4.99 V at C (closest to center of truck) and 4.87 VDC at B (middle pin on connector).  

 

Is this normal or am I frying MAP sensors because of a voltage leak somewhere?

[HOrnbrod]

Maybe it's time to rent a vacuum pump and test the pin B MAP output properly.......

 

Ignore all the BS at the end.

 

[Tex06]

Didn't have my vacuum pump (they're in storage out of state....this not having a shop sucks) but was able to see the changes in vac as described by sucking on the line, running at idle, creating a vacuum leak (pulling off booster), and at various rpms.  Readings kicked out like you would expect, idle was ~1.60 V (close to video), WOT was below 1V (momentarily, no load on engine then would spike as throttle closed).

 

Still can't drive with the MAP connected although idle is nice and smooth.  Autozone, Advance, and O'reilly don't seem to have a vacuum pump or brake bleeder for rent.  Any other suggestions?

[HOrnbrod]

Ok, tested for voltage at connector before hooking up the Mopar MAP.  A-C shows 4.93 VDC, and B-C shows 4.87VDC.  Checked both to ground just to verify, 4.99 V at C (closest to center of truck) and 4.87 VDC at B (middle pin on connector).  

 

Is this normal or am I frying MAP sensors because of a voltage leak somewhere?

I got 5.03 (A-C) and 4.96 (B-C) disconnected, so yes, normal. How's the MAP vacuum line? Still have the original hard plastic one? Slap a piece of rubber tubing on in place of it and try it.

 

I got rid of all that MAP vacuum crap and mounted my MAP sensor on the throttle body like the later models. That vacuum line is too easy to break and/or knock off when working in the area.