Need some help troubleshooting a bouncing idle - JeepForum.com
 
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post #1 of 7 Old 09-17-2017, 10:48 PM Thread Starter
vonericsen
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1995 ZJ 
 
Join Date: Nov 2009
Location: Longmont
Posts: 833
Garage
Need some help troubleshooting a bouncing idle

Quick background: My ZJ was running fine, then I went to the mountains and started overheating. After the trip I started doing some work on it. It was running fine otherwise.
  • New radiator
  • New thermostat
  • Replaced the plenum gasket (used Hughes Engines' aluminum plate to replace stock steel plate). Did this since I already had the parts and had been putting it off for a while.
  • Replaced accessory bracket (wanted one without minor damage to the idler pulley mounting surface)

Since doing this work, when I start my Jeep, the idle is bouncing up and down until it stalls/dies. Bounces up to ~1k, drops all the way to zero (on dash tachometer). On cold starts, it will usually recover quite a few times, sometimes holding at a normal idle RPM for a few seconds before bouncing again.
I thought it was timing related so I tried replacing the cam position sensor (distributor pickup) and the crank position sensor. No luck.
I pulled the spark plugs and saw they needed replacing, so I replaced them with new ones. I also checked the distributor cap and rotor. Didn't seem bad, so I just cleaned it up a little and reassembled everything. Still having the same problem.
I used my friend's fancy timing gun to verify the RPMs on each wire, and they all showed the RPM bouncing up and down in the same range.
The wires were also rerouted according to a Chrysler TSB to reduce the effects of EMI, but that didn't help either.
All the vacuum lines have been checked and I can't find a leak anywhere. I have also tried replacing some of the lines I was moving around when I was pulling the intake manifold.
I have tried reading the codes through the check engine light and I get 12 & 55, so nothing useful to this problem.
The throttle body and IAC are clean. IAC has also been replaced, but didn't help at all.
All electrical connections have been pulled apart and cleaned with electrical contact cleaner.
I have tried 3 different TPS sensors, as well as starting it without the TPS connected and still have the same problem.
The problem doesn't go away when warm. Once I have got it warm, restarting it causes the bouncing idle quicker and it dies much sooner (sometimes it will not even recover from the drop to zero once, other times it will recover once or twice).
The exhaust smells like there is unburnt fuel in it. When it comes close to stalling and recovers (when it recovers), I sometimes see a small puff of black smoke out of the tail pipe.
I am stumped.
Any ideas before I dump more money in this pit or roll it off a cliff?

Summary of new(er) parts:
  • radiator
  • thermostat
  • plenum gasket
  • oil sending unit
  • Crank position sensor
  • Cam position sensor (distributor pickup)
  • spark plugs
  • Idle Air Control
  • Fuel pump (4/2014)
  • distributor cap, rotor, coil (MSD), wires (Magnecor KV85) (2/2014) (30k miles at the most...probably not even that many)
  • water pump (2014) (30k miles at the most...probably not even that many)
  • exhaust (headers, cat, muffler, tailpipe) & O2 sensor (10/2014) (30k miles at the most...probably not even that many)
  • ECU capacitors (2013)



Moss Green 95 5.2l ZJ- 5.5" IRO lift, 255 75 R17 BFG M/T on JK Moabs, Mopar ECU, Mopar Tire Carrier

Last edited by vonericsen; 09-17-2017 at 10:59 PM. Reason: Adding in TPS details
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post #2 of 7 Old 09-17-2017, 11:22 PM
PolkaPower
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1998 ZJ 
 
Join Date: Aug 2011
Location: The Abyss
Posts: 21,227
Kind of sounds like a battery or computer problem. Maybe the alarm is acting up.

Low voltage will do really weird things on a ZJ.
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post #3 of 7 Old 09-17-2017, 11:24 PM
PolkaPower
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1998 ZJ 
 
Join Date: Aug 2011
Location: The Abyss
Posts: 21,227
Kind of sounds like a battery or computer problem. Maybe the alarm is acting up.
When the battery in my 5.9 was acting up it triggered the security no run. It would start up but smelled extremely rich. I acutely had to keep the pedal depressed to keep it running but who knows, it might be a good thing to check.

Low voltage will do really weird things on a ZJ.
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post #4 of 7 Old 09-20-2017, 10:12 PM Thread Starter
vonericsen
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1995 ZJ 
 
Join Date: Nov 2009
Location: Longmont
Posts: 833
Garage
I had the battery tested; tests fine (it was replaced earlier this summer when the other one crapped out). I've also been keeping it hooked up to a battery tender to help maintain it's charge while it sits.

I don't think it's the alarm. All my past alarm issue have been that it will start, run for maybe 3 seconds, then shutdown (foot to the floor or not), and the flash the lights and sound the horn. None of that happens.
After reconnecting the battery, the lights flash like normal and I disable it with the key in the door without any issues.
I can try disconnecting the security, but I don't think it's the problem.

I replaced the injector O-rings since it looked like they may have been bad and leaking a little bit, but that didn't seem to help.

I used a couple coupons online today and bought a new MAP sensor...didn't help. Now it's running even richer and the black smoke out the tail pipe is almost constant.

I tested the resistance on the fuel injectors and they were all as the FSM says. This doesn't rule them out as a problem, but that test doesn't immediately show an issue.

I tested the block ground and it had 1 ohm of resistance...I doubt that's enough to do anything that would cause this problem.

Moss Green 95 5.2l ZJ- 5.5" IRO lift, 255 75 R17 BFG M/T on JK Moabs, Mopar ECU, Mopar Tire Carrier
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post #5 of 7 Old 09-20-2017, 11:26 PM
Uniblurb
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1996 ZJ 
 
Join Date: Jan 2012
Location: Central
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When the idle goes up/down then it dies does it start right back up? If it doesn't start immediately that's when you'd want to do some testing for spark/fuel.

Were all of those sensors aftermarket you installed rather than Mopar? I've seen some get buy with aftermarket on some sensors but not the crank sensor which should be Mopar. You may want to try the crank sensor resistance test in my signature and it should have no resistance between B&C connector pins/cavities.

You have on your list you installed capacitors in your ECU (2013) but in your signature you have "Mopar ECU" listed. Is this the same OE ECU/PCM you worked on or did you install a Mopar reman unit?

Is your upstream o2 sensor Mopar/NTK, in good shape, and giving the readings it should when connected to a scanner with live data?

Check your battery cables to make sure you don't have corrosion going up into them. Also on the passenger side inner fender next to the battery there is a ground with stack of eyelets. One of those eyelets should be your 95 ECU/PCM ground (tan/black wires). Make sure it's good clean as well as the short ground wire going over connecting to your neg battery terminal. Good luck.

Edit/addition: Just happened to think since it overheated you may want to check your engine coolant temp sensor which should be next to the thermostat housing. While you wouldn't think it would do much it does supply info to the PCM which in turn sets injector pulse width and ignition timing. I've seen others have problems with this and be sure and check your connector wires.

Below is some info on the ECT for your 95 5.2.

http://www.jeepforum.com/forum/f13/e.../#post12920925

And here's a resistance test for checking the ETC according to coolant temps.
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post #6 of 7 Old 09-21-2017, 10:43 PM Thread Starter
vonericsen
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Join Date: Nov 2009
Location: Longmont
Posts: 833
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Quote:
Originally Posted by Uniblurb View Post
When the idle goes up/down then it dies does it start right back up? If it doesn't start immediately that's when you'd want to do some testing for spark/fuel.
Yeah, it starts right back up every time

Quote:
Originally Posted by Uniblurb View Post
Were all of those sensors aftermarket you installed rather than Mopar? I've seen some get buy with aftermarket on some sensors but not the crank sensor which should be Mopar. You may want to try the crank sensor resistance test in my signature and it should have no resistance between B&C connector pins/cavities.
No, parts store. I don't think any of these are causing the problem or hiding the problem since the original sensors (all mopar) had the exact same issue as I'm having now. I've seen incremental differences as I've tried different things, but the original problem is still there.
The incremental differences have been seen as running richer or leaner as things get changed and the computer is reset. I tried the MAP sensor for the hell of it, and it had no affect.
I couldn't get a good reading on the resistance from the old or new sensor. I probably need a better multimeter than the super old analog one I have.

Quote:
Originally Posted by Uniblurb View Post
You have on your list you installed capacitors in your ECU (2013) but in your signature you have "Mopar ECU" listed. Is this the same OE ECU/PCM you worked on or did you install a Mopar reman unit?
It's the same one I did work on. Been working fine since then. It's a Mopar performance ECU that was installed in it when it was still new (my parents were the POs and my dad told me he had it installed early on).

Quote:
Originally Posted by Uniblurb View Post
Is your upstream o2 sensor Mopar/NTK, in good shape, and giving the readings it should when connected to a scanner with live data?
It should be fine since it's still relatively new. It's OBD1 and I don't have a DRB tool to get anything fancy like that. My code reader I have, barely even reads codes from it. It's a giant pain to get it to connect and talk.
I don't think it's the O2 sensor since this is happening when it's cold, and will continue happening when hot.

Quote:
Originally Posted by Uniblurb View Post
Check your battery cables to make sure you don't have corrosion going up into them. Also on the passenger side inner fender next to the battery there is a ground with stack of eyelets. One of those eyelets should be your 95 ECU/PCM ground (tan/black wires). Make sure it's good clean as well as the short ground wire going over connecting to your neg battery terminal. Good luck.
They are clean. I also tested that ground point and it doesn't have any resistance.
I also tried running another ground cable from the block grounding point to this one since the ground cable on the block had 1 OHM of resistance, but that didn't change anything either.

Quote:
Originally Posted by Uniblurb View Post
Edit/addition: Just happened to think since it overheated you may want to check your engine coolant temp sensor which should be next to the thermostat housing. While you wouldn't think it would do much it does supply info to the PCM which in turn sets injector pulse width and ignition timing. I've seen others have problems with this and be sure and check your connector wires.
I will need a better multimeter to check this, but I will check it when I get some time to borrow one or buy one.
I may just replace this since I have a coolant leak in that area and since the thermostat gasket was just changed, it looks like this is the next candidate for where it could be coming from.

Could this issue be related to the EVAP system at all? I tried running with the gas cap disconnected and there was no change in how it runs (just a simple test to see if anything changes).
Just wondering if the purge solenoid isn't working right, or there is a vacuum leak in that system, could it cause this issue?

A friend suggested trying to idle in neutral as well to help rule out anything transmission related. There was no change in neutral from park.

I'll be leaving for vacation for a week on Sunday, so it may take me a little while to come back to this.

Moss Green 95 5.2l ZJ- 5.5" IRO lift, 255 75 R17 BFG M/T on JK Moabs, Mopar ECU, Mopar Tire Carrier
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post #7 of 7 Old 09-21-2017, 10:47 PM Thread Starter
vonericsen
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1995 ZJ 
 
Join Date: Nov 2009
Location: Longmont
Posts: 833
Garage
Looks like I may have forgotten to mention it, but the EGR valve was replaced in April '17 when I had high NOx emissions. Replacing the EGR did the trick to get it passing again.

Moss Green 95 5.2l ZJ- 5.5" IRO lift, 255 75 R17 BFG M/T on JK Moabs, Mopar ECU, Mopar Tire Carrier
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