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Unread 03-20-2013, 06:17 PM   #1
Dennis88
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Jeep fire after swap, Rough Idle Help

Hey yal, new to the forum.

Need help diagnosing this rough idle.

So i got an 88 jeep Cherokee Auto tranny I6 4.0 ( my trail rig ) that i recently put a used motor in it. Unfortunately it sprung a fuel leak at the fuel rail and then it back fired, throwing a flame out of intake and ignited the gas Was a good 10 second fire. Only noticeable damage done was the wiring and vacuum lines. Had an extra wiring harness. As far as i know i got the vacuum lines hooked up correctly.

Got her cranked up today and she was running rough with a constant intake backfire along with exhaust back firing.. When i push pedal nothing happens.

This is what i have done so far. re stabbed dist, Found TDC on cy #1. Firing order is correct. Replaced TPS and the other sensor near it on the TB. New plugs (used wires). A picture old engine bay would help me track my vacuum lines Any other info would greatly be appreciated.

The motor was pulled from a friends jeep and sat in a shop for some months. I pulled valve cover off and oil pan inspected what i could. When she cranks there is no smoke from exhaust. Motor seems good.

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Unread 03-21-2013, 06:11 AM   #2
cruiser54
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Just in case you need it.

Setting Your 4.0 at #1 TDC


With #1 spark plug removed, turn the engine over clockwise using the 3/4" front crankshaft bolt. You will see the degree marks on your front cover. Mark the 0 with chalk or white-out.You will see the timing mark on the balancer. Mark it with chalk or white-out. While turning the engine over, put your finger/thumb over #1 spark plug hole. As the mark on the balancer approaches the marks on the front cover, if you are coming up on #1 TDC, pressure in the cylinder will push on your finger. If there's no pressure, you're at #6 TDC and need to crank the engine over until you see the marks coming together and you get pressure. Set the mark on the balancer to the 0 mark on the front cover.


Low fuel pressure will also cause an intake backfire.
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Originally Posted by chris87xj View Post
If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

Cruiser's Mostly Renix Tips
My Jeep Technical Photos

http://www.cherokeeforum.com/g/album/1725214
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Unread 03-21-2013, 06:40 AM   #3
Dennis88
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Yea thats the method i used for setting my tdc.

Would vacuum being incorrect cause severe idling issues?
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Unread 03-21-2013, 06:45 AM   #4
cruiser54
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Yes. Especially if the hose/pipe to the MAP sensor from the throttle body is trashed.
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If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

Cruiser's Mostly Renix Tips
My Jeep Technical Photos

http://www.cherokeeforum.com/g/album/1725214
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Unread 03-24-2013, 07:14 AM   #5
Dennis88
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Ok so now i have replaced the MAP sensor with all new hoses and vacuum lines. Bought the vacuum housing from auto zone. No change, Still runs rough And when i press throttle nothing happens. Extreme backfiring from intake and exhaust. Horrible idle. Cant seem too figure out what the problem is. Any help is appreciated
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Unread 03-24-2013, 07:18 AM   #6
cruiser54
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Double check the dizzy installation.

Did you get those 3 or 4 little ground wires hooked back up to the clean and shiny dipstick tube stud?
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Originally Posted by chris87xj View Post
If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

Cruiser's Mostly Renix Tips
My Jeep Technical Photos

http://www.cherokeeforum.com/g/album/1725214
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Unread 03-24-2013, 07:29 AM   #7
Dennis88
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I got the ground wires hooked up, Not clean and shiny. I thought it was clean enough for them too ground.

The dizzy im gonna re do today, Got a known working extra dizzy that im gonna replace mine with.

When its idling, it sounds like it has a aggressive cam in it, Loping, missing, backfiring. Could bad injectors be the cause of this? what about EGR valve or solenoid.

Replaced TPS and the other sensor next too it. ALSO... The gound wire that grounds too the firewall from the motor. It hooks onto the motor neat the valve cover and bolts too firewall. I noticed that ground is broke. Is that a crucial ground? Being that the motor is already grounded twice.
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Unread 03-24-2013, 08:36 AM   #8
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Dennis: The grounds need to be clean and shiny. They're super critical. Changing dizzies isn't gonna help if the present one is outta place. You can run a Renix with the dizzy wires unplugged. All that deal inside the dizzy does is help the eCU fire the injectors sequentially. Has nothing to do with ignition/spark/timing.
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Originally Posted by chris87xj View Post
If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

Cruiser's Mostly Renix Tips
My Jeep Technical Photos

http://www.cherokeeforum.com/g/album/1725214
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Unread 03-24-2013, 08:41 AM   #9
Dennis88
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Ok. I am about too clean all grounds and fix the ground on firewall. Will update in just a bit. If this fails my next step is replace injectors. (I have extras). Thanks Anymore tips is appreciated
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Unread 03-24-2013, 08:51 AM   #10
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Renix Ground Refreshing

The Renix era XJs and MJs were built with an under-engineered grounding system for the engine/transmission electronics. One problem in particular involves the multiple ground connection at the engine dipstick tube stud. A poor ground here can cause a multitude of driveabililty issues, wasted time, failed emission tests, and wasted money replacing components unnecessarily.

The components grounding at the dipstick tube stud are:

Distributor Sync Sensor, TCU main ground, TCU “Shift Point Logic”, Ignition control Module, Injectors, ECU main ground which other engine sensors ground through, Oxygen sensor, Knock Sensor, Cruise Control, and Transmission Sync signal. All extremely important stuff.

The factory was aware of the issues with this ground point and addressed it by suggesting the following:

Remove the nut holding the wire terminals to the stud. Verify that the stud is indeed tightened securely into the block. Scrape any and all paint from the stud’s mounting surface where the wires will attach. Must be clean, shiny and free of any oil, grease, or paint.

Inspect the wire terminals. Check to see that none of the terminals are crimped over wire insulation instead of bare wire. Be sure the crimps are tight. It wouldn’t hurt to re-crimp them just as a matter of course. Sand and polish the wire terminals until clean and shiny on both sides. Reinstall all the wires to the stud and tighten the nut down securely.

While you’re in that general area, locate the battery negative cable which is fastened to the engine block just forward of the dipstick stud. Remove the bolt, scrape the block to bare metal, clean and polish the cable terminal, and reattach securely.

Another area where the grounding system on Renix era Jeeps was lacking is the engine to chassis ground. There is a braided cable from the back of the cylinder head that also attaches to the driver’s side of the firewall. This cable is undersized for it’s intended use and subject to corrosion and poor connections at each end.

First off, remove the cable end from the firewall using a 15mm wrench or socket. Scrape the paint off down to bare metal and clean the wire terminal. Reattach securely.
Remove the other end of the cable from the rear of the head using a 3’4” socket. Clean all the oil, paint and crud from the stud. Clean the wire terminal of the cable and reattach securely.

A suggestion regarding the braided cable:
I prefer to add a #4 Gauge cable from the firewall to a bolt on the rear of the intake manifold, either to a heat shield bolt or fuel rail bolt. A cable about 18” long with a 3/8” lug on each end works great and you can get one at any parts store already made up. Napa has them as part number 781116.

A further improvement to the grounding system can be made using a #4 cable, about 10” long with 3/8” terminals at each end. Attach one end of this cable to the negative battery bolt and the other end under the closest 10mm headed bolt on the radiator support just forward of the battery. Napa part number 781115.

For those of us with Comanches, it’s very important to remove the driver’s side taillamp assembly to access the ground for the fuel pump. Remove the screw holding the black ground wire. Scrape the paint from the body and corrosion from the wire terminal. Reattach securely.

If you want to upgrade your grounds and battery cables in general, contact Jon at www.kelleyswip.com. He makes an incredible cable upgrade for a very reasonable price.


Revised 03-04-2013
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Quote:
Originally Posted by chris87xj View Post
If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

Cruiser's Mostly Renix Tips
My Jeep Technical Photos

http://www.cherokeeforum.com/g/album/1725214
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Unread 03-24-2013, 08:52 AM   #11
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Quote:
Originally Posted by Dennis88 View Post
Ok. I am about too clean all grounds and fix the ground on firewall. Will update in just a bit. If this fails my next step is replace injectors. (I have extras). Thanks Anymore tips is appreciated
I still think your dizzy is off.




Remove the distributor cap and cut a "window" into the side of the distributor cap at the #1 spark plug wire post . The "window" should be large enough to allow easy visual inspection of the position of the distributor rotor at the #1 spark plug wire post. Reinstall the distributor cap.

Install a wrench or socket onto the vibration damper retaining bolt. Rotate the engine in a clockwise direction until the #1 cylinder is at top dead center. Align the timing mark on the vibration damper with the "0" degree mark on the front cover timing scale. The tip of the distributor rotor should be near the #1 spark plug wire post.

Disconnect the distributor electrical connection. Remove the distributor holddown clamp, holddown bolt and distributor. Remove the distributor cap and rotor.

Place the distributor housing upside down in a soft jaw vise. Scribe a line 1/2 inch from the end of the distributor locating tab. Cut the distributor locating tab at the scribed line with a saw.

Remove any burrs and metal filings from the distributor. Reinstall rotor.

If necessary, using a flat blade screwdriver, turn the oil pump gear drive shaft until the slot is slightly past the 11 o'clock position. The oil pump gear drive shaft is accessible through the distributor mounting bore in the engine block.

Visually align the modified locating tab area of the distributor housing with the holddown clamp bolt hole.

Turn the rotor to the 4 o'clock position.
Lower the distributor into the engine block until it seats. The rotor should now be very close to the 5 o'clock position.

Reinstall the distributor cap with the cutout "window". Rotate the distributor housing until the trailing edge of the distributor rotor tip is just departing from the #1 spark plug wire post terminal .

Reinstall the distributor holddown clamp and bolt.. Reinspect the position of the rotor to the #1 spark plug wire post to insure that it has not moved.

Install the new distributor cap, reconnect the distributor electrical connections.

Revised 07/03/2012
__________________
Quote:
Originally Posted by chris87xj View Post
If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

Cruiser's Mostly Renix Tips
My Jeep Technical Photos

http://www.cherokeeforum.com/g/album/1725214
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Unread 03-24-2013, 09:23 AM   #12
Dennis88
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Ok well after fixing one ground and cleaning the other .. Too my surprise it idles better. Not perfect. And I still have no throttle. When I push throttle or stumbles and pops And still has a jumpy idle but it's not "camming" lol
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Unread 03-24-2013, 10:20 AM   #13
Dennis88
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Ok I am about too mess with my dizzy. Ill update when done
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Unread 03-24-2013, 12:20 PM   #14
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Quote:
Originally Posted by Dennis88 View Post
Ok well after fixing one ground and cleaning the other .. Too my surprise it idles better. Not perfect. And I still have no throttle. When I push throttle or stumbles and pops And still has a jumpy idle but it's not "camming" lol
I'm not surprised, nor will I be when we find out your dizzy is off a tooth or more.
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Quote:
Originally Posted by chris87xj View Post
If you own a Renix Jeep, you'll want to check this out. >

An index to over 25 write-ups can be found in Post #1.

Cruiser's Mostly Renix Tips
My Jeep Technical Photos

http://www.cherokeeforum.com/g/album/1725214
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Unread 03-26-2013, 04:57 PM   #15
Dennis88
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OK well.. I fooled with the dizzy for a while. Re stabbed it. I cut off the alignment tab on it so i could turn it. I couldn't get it better at all.. Hell when i turned it when the motor was running it didn't really change the way it idled unless i got way too far then it would die and not crank.

I found TDC... Put thumb over first cyl. Waited until air pushed my thumb. Took a soft tube and put it into the cylinder until it came up all the way. Air will only push through on cyl #1 when its on compression stroke right? Ah this is frustrating, im ready to get back in the woods lol

ALSO... Where does the vacuum line from the MAF sensor go? I have it going into my TB. Theres a rubber bushing on the passenger side of TB that had a place for a vacuum line.

I may have vacuum hooked up wrong? Detailed Pics of the engine bay would be nice if anyone has some.

Last edited by Dennis88; 03-26-2013 at 05:24 PM..
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