TJ Downpipe Conversion on YJ -
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post #1 of 1 Old 04-19-2014, 06:12 PM Thread Starter
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1991 YJ Wrangler 
Join Date: Jun 2012
Location: Walcott
Posts: 4,510
TJ Downpipe Conversion on YJ

I have seen a lot of people talk about installing the TJ style 4.0 downpipe/collector pipe onto a 4.0 YJ. I was not able to find much of a writeup(on JF anyway) so here goes.
The stock pipe sits only an inch under my AT pan and was a target for those pesky rocks when wheeling. This pic was after the first time I wheeled it, since then it was nearly pinched in half.

I cut out the bad and threw in a flexy pipe to get by but it leaks and dumps alot of heat+ fumes under the tub(and into the cab).

After having the guys at 2 exhaust shops tell me it cant be ran this way(in front of the oil pan sump) I have decided to do it myself.
Things you need:
  • part #53458 pipe (from 97-99 TJ 4.0) $60 at orielly's
  • fel-pro #60836 -round gasket for manifold connection, about $10
  • pb blaster or penetrating oil of your choice
  • sawzall and sharp blade
  • Welder or exhaust clamps(both in my case)
  • Typical garage tools, sockets wrenches etc.
  • Sixer of your choice(I went with Blue Moon!)

New pipe:

Old Pipe:

I started with pb blaster ahead of time, then unscrewing the O2 sensor and moving it out of the way.

Next unbolt the 2 bolts holding the pipe flange to the exhaust manifold. Move it down out of the way.

Using a flathead or similar, pry out the old donut gasket.

Comparison old vs new

New gasket put on, it's tight!

Next I removed the section of flex pipe(which you probably don't have)

I decided to go ahead and throw the new pipe up now and loosely install the nuts to get an idea on how it will line up to the existing system at front of cat.

The new pipe does not have the 4 bolt flange to connect to the cat converter, so I decided to cut off the old pipe a few inches out from the flange, and attach the new pipe here. This is with it unbolted. All 4 broke off so I hammered them out. Heat was needed for 2 of them.


My little helper

Cleaning the baked mud off the old flange from front of cat.

This is where my plan started to change. The TJ pipe is 2.5", and the yj system from cat back is 2.25". The old YJ downpipe, and thus flange is a mere 1 7/8". I didn't like the idea of reducing from 2.5>1 7/8" and back to 2.25" so I threw the flange in the scrap heap and decided to use a 2.25" pipe into the front of the cat with a reducer.

I will not be winning any welding awards here, but I jammed the 2.25" into the opening on the front of Cat and buzzed it in with the ol' 90 amp. It also takes a little bit of manuvering the new downpipe to get everything to line up, so make sure the nuts are still loose.

Bolted the existing hanger back to the cat flange

This is the result, not finished yet!

Another weld, honestly one of the better ones. Yeah I'm bad

Once everything is welded/clamped in place, dont forget to tighten the flange nuts back up to the manifold!

Now back to the O2 sensor. Plainly put, It will not reach its new home. Follow the wires to a bracket on the side of the oilpan, pop the plastic guy out of the bracket which will free up the slack and allow installation.

Bracket shown behind and left of the plug.

Now you can install the O2. Grab a ziptie and find a place to tie the wire up out of the way so it's not touching the pipe or even too close. If you have the AT be careful, the shifter linkage is very close also, make sure the wire wont get pinched when you shift. (I forgot this picture sorry)

That should just about wrap it up! By now you should be godawful filthy and on the last beer of that 6'er.

Some notes: I had a hard time getting welds to the top sides of the pipes at the reducer I used. So it still leaks a bit there. I will be putting some exhaust clamps on there for now. The better approach would be to drop the TC skid and lower the whole system down far enough to get in there. All in all not a bad job to do though. Hopefully this helps someone who wanted a step by step for this swap.

Here is a test video from before the clamps.

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