NAXJA Forums -::- North American XJ Association  

Go Back   NAXJA Forums -::- North American XJ Association > NAXJA Unibody Jeep Technical Forums > Jeep Cherokee XJ (1984 - 2001) > OEM Tech Discussion
HOME Member FAQ Sponsor Info Rules Bylaws E-Mail

OEM Tech Discussion Forum for OEM (Original Equipment) or stock XJs and MJs.

Reply
 
Thread Tools Display Modes
  #1  
Old December 28th, 2005, 23:44
Wiley Coyote's Avatar
Wiley Coyote Wiley Coyote is offline
NAXJA Forum User
 
Join Date: Dec 2003
Location: Anchorage, AK
Posts: 430
fouled spark plug

My XJ went from driving great to running like crap in a matter of 2 days.
For some reason it likes to foul out spark plug # 3 and ONLY #3. In the last 2 weeks Iíve installed new injectors, cap, rotor, wires, coil, TPS, CPS and several sets of different spark plugs. I end up with the same thing every time: it runs great until I foul out spark plug #3. The rest of the plugs look fine, but #3 is nasty, sooty black and smells of gasoline. Iíve tested the plugs/wires and Iím getting good spark (put the plug/wires on the valve cover while friend cranked the engine). I also checked the injector harness with the node lights. The compression is #1-80, #2-80, #3-90, #4-85, #5-80, #6-80. Every thing checks out fine, just keeps fouling the same plug. If it was a horse, Iíd shoot it. Anyone have ideas? Yes I did a search and came up with nothing useful.

Coyote

BTW Itís a í91 Cherokee Limited, 4.0L, Auto, 4x4 with ~110,000 miles
Reply With Quote
  #2  
Old December 29th, 2005, 01:30
5-90's Avatar
5-90 5-90 is offline
NAXJA Forum User
 
Join Date: Mar 2003
Location: Hammerspace
Posts: 18,821
Re: fouled spark plug

Hmm...

A couple things I'd try...

1) Get a "non-fouler" extension for the plug. They're about an inch long, and screw in between the spark plug hole and the plug.

2) "Index" the plug - mark a line down the socket corresponding to the ground electrode, and install the plug so that the line is directly up (and the gap will, therefore, point straight down.)

3) Install a plug one step hotter in #3 (if you're running Autolite 3924, use Autolite 3925.) The "hotter" plug retains slightly more heat, which will help to "burn" the nose clean.


These assume that you're getting carbon or fuel fouling on the plug (which are dry, sooty deposits - if wet, they smell of fuel.) Oil fouling might indicate a failing oil control ring (even with good compression) - oil fouling presents as black, almost greasy deposits - think used oil mixed with finely ground charcoal. You'll have to figure out where that's coming from - it might be easier to change the valve guide seals before servicing the rings, and if you service the rings, you should change the seals anyhow. Oil deposited on plugs typically comes from one of two places - either past the oil control ring, or past the intake valve guide seal.

Since you said the deposits are black, I shan't go into detail on any other plug fouling you can run across - black deposits are either oil or fuel. Since you said they smell of fuel, I'm more inclined to think that the #3 plug isn't burning all of its fuel - I listed possible correctives in no particular order, but I'd try #2, then #3, then #1 (the non-fouler is typically for oil fouling...)

5-90
Reply With Quote
  #3  
Old December 29th, 2005, 23:27
Wiley Coyote's Avatar
Wiley Coyote Wiley Coyote is offline
NAXJA Forum User
 
Join Date: Dec 2003
Location: Anchorage, AK
Posts: 430
Re: fouled spark plug

Thanks for the advice. Iím almost positive that its fuel and not oil causing the fouling. I tried a hotter plug (Autolite 3926, standard is 985), but it didnít seem to help. It still fouled out #3, but the rest of the plugs looked great. I understand the concept of indexing the plug, but if I donít screw it in all the way wonít it be loose? Or is there some other trick to getting it lined up (spacer?)?
Reply With Quote
  #4  
Old December 29th, 2005, 23:40
5-90's Avatar
5-90 5-90 is offline
NAXJA Forum User
 
Join Date: Mar 2003
Location: Hammerspace
Posts: 18,821
Re: fouled spark plug

As I recall, the plugs we use have crush washers - so you tighten it down to "contact," then turn it until the gap is in the right place. That way, you're not loose.

It's possible to get "dead soft" washers in various thicknesses - check Summit or Jeg's with the term "Spark Plug Index Washers" to find them - I think the kit includes 24 washers - 8 each in three thicknesses.

Besides, it's not strictly necessary to point the gap dead downwards - generally downwards will usually help. Unless you're trying to wring every last bit of power out of the engine, this is one of those cases where "good enough" actually is.

5-90
Reply With Quote
  #5  
Old December 30th, 2005, 01:27
Wiley Coyote's Avatar
Wiley Coyote Wiley Coyote is offline
NAXJA Forum User
 
Join Date: Dec 2003
Location: Anchorage, AK
Posts: 430
Re: fouled spark plug

Yep, the spark plugs have a crush washer on them. Unfortunately mine is already crushed, looks like Iíll be getting another spark plug. Iím toying with the idea of going to a 3927 spark plug (the hottest they make for our style engines). That in conjunction with indexing the plugs might just give me the extra little oomph I need to keep from fouling. How much of a difference does indexing the plug make? I know they do it on race engines, but I though that was just to squeak out every last bit of horsepower. Iíd still like to know what is causing that and why only 1 cylinder.
Reply With Quote
  #6  
Old December 30th, 2005, 01:32
5-90's Avatar
5-90 5-90 is offline
NAXJA Forum User
 
Join Date: Mar 2003
Location: Hammerspace
Posts: 18,821
Re: fouled spark plug

The idea of plug indexing, if the leak is from the valve guide seal, would help to keep the gap cleared of oil.

Come to think of it, it you put a new plug in there, pull it in a day or so and see what side it's fouled on - upside, it's the valve guide seal. Downside, it's the oil rings (probably.) Draw a line on the plug so you know how it was installed before you pull it out...

5-90
Reply With Quote
  #7  
Old January 8th, 2020, 21:42
vimoto vimoto is offline
NAXJA Forum User
 
Join Date: Aug 2019
Location: Mexico
Posts: 3
Re: fouled spark plug

i have the same problem on 3rd cyl, my engine was rebuild it has 34 miles on it. spark wets wet of gasoline never oil, so if a bad lifter is not opening the exahust valve, compression could damge the piston rings? i have 120 psi compression in all cylinders except 3rd. on 3rd cyl i have 30 psi
Reply With Quote
  #8  
Old January 9th, 2020, 07:55
lawsoncl lawsoncl is offline
NAXJA Forum User
 
Join Date: Jan 2006
Location: North Idaho
Posts: 5,161
Re: fouled spark plug

Quote:
Originally Posted by vimoto View Post
i have the same problem on 3rd cyl, my engine was rebuild it has 34 miles on it. spark wets wet of gasoline never oil, so if a bad lifter is not opening the exahust valve, compression could damge the piston rings? i have 120 psi compression in all cylinders except 3rd. on 3rd cyl i have 30 psi



Whoever rebuilt it needs to fix it. If you rebuilt it, then I'd start by pulling the valve cover and verify the valves are opening/closing correctly. I'm guessing you'll be pulling the head next.
Reply With Quote
  #9  
Old January 9th, 2020, 08:45
old_man's Avatar
old_man old_man is offline
NAXJA Member
 
Join Date: May 2003
Location: Loveland, Colorado
Posts: 15,734
Re: fouled spark plug

Your compression numbers are crap. No wonder it is low on power. Anything under 100 is cause to look at a rebuild.

Verify that the injectors have the correct plugs on them. Look up the color code and verify the wire colors. Swap the injector with a different cylinder.
Run the cheapest Champion plug. The multi-electrodes do not work well on the early 4.0L's.

Just because you get a spark out of the cylinder does not mean it will spark well under pressure. Swap the plug wire with a different cylinder if swapping the injectors does not yield positive results.
__________________
Tom Houston - Former Colorado Chapter Treasurer , Terminal Adrenaline Junkie, and Rocket Scientist
Loveland, Colorado My teardrop build
You're just jealous because the voices don't talk to you.
Reply With Quote
  #10  
Old January 12th, 2020, 12:07
Driver86 Driver86 is offline
NAXJA Forum User
 
Join Date: Nov 2019
Location: Northern VA
Posts: 5
Re: fouled spark plug

You guys do realize this thread was started 15 years ago
Reply With Quote
  #11  
Old January 13th, 2020, 12:23
Flyfisher's Avatar
Flyfisher Flyfisher is offline
NAXJA Forum User
 
Join Date: Sep 2003
Location: Billings, Montana
Posts: 835
Re: fouled spark plug

Yes, a 15-year old thread that was resurrected 5 days ago by a person with a similar problem.
Reply With Quote
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
cleaning spark plug threads... mikeny59 OEM Tech Discussion 12 June 27th, 2005 18:38
Spark Plug wire layout Cherokee91Red OEM Tech Discussion 4 November 15th, 2004 18:45
Which Spark plug Wire??? Jeepster4wd OEM Tech Discussion 9 September 3rd, 2003 22:58
Quick Spark Plug Question Karlm OEM Tech Discussion 11 July 22nd, 2003 07:30
Spark Plug Gapping & Which One's FJH97XJ Modified Tech Discussion 9 April 12th, 2003 09:19


All times are GMT -7. The time now is 03:31.


Powered by vBulletin® Version 3.8.9
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
NAXJA and NAXJA logo's Copyright NAXJA. All content/images Copyright NAXJA 1999-2014