Topic: Runs poorly after initial start and at WOT

[shovelhead]

I have a 97 TJ, 2.5L automatic, 118K miles.  The engine is stock except for a cold air intake.  Since I got the Jeep, it's had this problem that I can't solve.  After first starting the Jeep, it idles fine, but if I put in in gear, and attempt to drive, it chokes, coughs, protests, and backfires through the throttle body.   When it warms up enough for the fuel system to go into closed loop, it runs almost normal with one exception.  If I press the pedal to WOT, it bogs down.  If I let off the gas a little, the power comes back. 
Here's what I've done to chase the problem down, in to particular order.
Replaced upstream O2 sensor.
Replaced TPS.
Replaced MAP sensor.
Cleaned throttle body.
Replaced vacuum lines on intake manifold
Seafoamed through brake vacuum line.
Add MMO to gas at regular intervals.
Replaced fuel injectors
Replaced plugs, wires, cap and rotor.
Checked for intake manifold vacuum leaks with propane gas.
There is one obvious issue that I have not yet addressed, which is a visible crack in the exhaust manifold, about 3-4 inches, vertically, in the center of the manifold.   I can feel exhaust gasses escaping through the crack.

[shovelhead]

I forgot, I also replaced the coolant temp sensor.

[aka-justin]

Checked the fuel filter or fuel pressure?

[sharpxmen]

Catalytic Converter issue or what he ^ said (fuel pressure related)

EDIT: if it is the cat (sounds like it) then it's probably the most expensive exhaust fix ever considering all the parts you swapped 

btw, how do you check for vacuum leaks with propane?

[shovelhead]

The cat does rattle sometimes, it sounds like a bunch of loose pieces inside.  I'm in California, so it's even more expensive.  I ran a Carfax report on the vehicle, and the last time it was replaced was 2005.  Perhaps the cat was damaged in the previous owner's offroading. 
As far as the propane leak detection method, I read some tips that you can use a propane torch, valve opened and unlit to find intake vacuum leaks.  You let some of the gasses out around the intake manifold gasket.  If the engine speed changes from the propane being sucked in with the air, then you've located a vacuum leak.  I tried that, sparingly and cautiously with no change in engine speed.

[shovelhead]

I haven't checked the fuel pressure.  My fuel rail is the style without the fitting for the gauge.  I have an aftermarket tank, the Crawler, made by GenRight.  There is another gremlin associated with that.  When my fuel gauge reads below 1/8th of a tank, and I make a left turn, my engine cuts out a little bit.  Either my fuel sending unit is off, or I have some other issue with the pump assembly.

[shovelhead]

One more little tidbit related to the cat.  I have a little OBDII scan tool.  When I run the test for the State OBD check, the cat never comes to the ready state, it stays on INC no matter how many miles I put on it.

[sharpxmen]

I haven't checked the fuel pressure.  My fuel rail is the style without the fitting for the gauge.  I have an aftermarket tank, the Crawler, made by GenRight.  There is another gremlin associated with that.  When my fuel gauge reads below 1/8th of a tank, and I make a left turn, my engine cuts out a little bit.  Either my fuel sending unit is off, or I have some other issue with the pump assembly.

no, all the Jeeps are like that , if you're low on fuel and have a long left or right turn you will starve the pump, it's really the bad baffling inside the tank and seems to be the same for your aftermarket one as well. And it is also related on how low really is, 1/8 of a tank doesnt necessarily mean 2.5 gallons (could be less).

I read some tips that you can use a propane torch, valve opened and unlit to find intake vacuum leaks.  You let some of the gasses out around the intake manifold gasket.  If the engine speed changes from the propane being sucked in with the air, then you've located a vacuum leak.  I tried that, sparingly and cautiously with no change in engine speed.

with that exhaust leak right under the intake i'd be careful.

On the other hand you're chasing ghosts, a vacuum leak on your Jeep won't act like one on a carburetted engine or with a MAF sensor, it will just rev up more but WOT power will be the same unless the reading at the MAP is affected (which means the MAP is damaged or the leak is on the vac line going to the MAP).

take a fuel pressure while this bogging down happens, if it stays constant and not dropping then it is not fuel related (so don't replace the fuel pump or tank, test it first and save some cash, no reason to keep throwing money into parts you don't need to replace, it gets expensive) - pressure should read 49psi for 97 TJ if memory serves me well (might want to double check but i'm fairly sure that's the one).

One more little tidbit related to the cat.  I have a little OBDII scan tool.  When I run the test for the State OBD check, the cat never comes to the ready state, it stays on INC no matter how many miles I put on it.

so you have a CEL code as related to cat efficiency? would you pass smog in that case? - the cat would need to be replaced anyway if this is applicable to you, so would be a good place to start and a necessary expense anyway.

[shovelhead]

Thanks for the tips!  I'll see if my autozone has a fuel pressure gauge loaner tool, and shop around for a new cat.

[sharpxmen]

keep us posted, would be interesting to hear what you find and the fix

[shovelhead]

Will do.  It's always disappointing when you scour forums to find your problem, and the thread dies without a solution.

[3Xloser]

I had almost the same issues going with my '97 TJ.  When I first bought it off the PO, it had a glasspack u-bolted on right behind the 'cat and it only bogged down at WOT.  First order of business was a new tailpipe and muffler.

Over time the intermittent rough idle, stumbles, and backfires started up.  After a while, it would only reliably start when it was cold.  Warm starts were a crapshoot.  It got to where it wouldn't idle too long, either.  I could "massage" the throttle to a point where it was idling smooth enough that I could try shifting and driving, but it left me hanging getting into traffic a couple of times.

I did the first four items on your list, plus plugs, wire, cap, and rotor.  I had the same crack in the manifold, too.  I replaced the manifold and it was no help.  It ran quieter, but just as bad.
I also had the CEL lit and it was showing the O2 sensor/lean condition code. 

My Jeep's problem ended up being the fuel pump.  Once I dropped the tank, the strainer at the bottom of the pickup was missing; it was just floating around in the tank.  There was also at least a 1/4 cup of sand down in there. (It's just one of the perks of living in the desert, I guess)  Since the new fuel pump, the CEL cleared immediately.  It starts easy and idles smooth.  As a bonus, I'm back around 17 MPG with everything running right.

[shovelhead]

Thanks for the tip!  I'm gonna start with the cheap stuff first.  The manifold, and gutting the cat.  Fuel pump will be the next thing.

[sharpxmen]

rent a fuel pressure check gauge, that will tell you if it's the pump.

[AWK]

Was there any final fix for this?   My 97 is doing similar.   It starts fine, then after a few seconds in starts to miss and stumble.  I can keep it running by feather the throttle.   Issue goes away when it gets to operating temp.  WOT seems fine.   Awk