1990 XJ running rich

[GGXJ]

Ok, here's a little background... it's a 1990 4.0, 5 spd, 4wd. It's relatively stock with a few minor mods. The engine is all stock. At 210k miles, 90k of me beating on it like a red headed stepchild, I swapped the motor. It was complete block plus the head from a 1996 donor with 80k miles on it. So the intake, exhaust, accessories, transmission, etc was all from the original motor. Since the motor swap, it has been running REALLY rich. I'm getting less than 10 mpg. The only thing that's changed since the motor swap is the exhaust. I had been running with only a cat (ripped off the rest on a rock ledge). Since the swap, I cut out the cat and put on a glass pack. The only other thing that I can think of is that we had to use the temperature sensor from the 96 (located in the thermostat housing) as opposed to where it used to be located on the 90 (in the side of the block). I've double checked the vacuum system to make sure all hoses are where they're supposed to be and all looks well.

Another strange thing about this is that everytime I start it, it will just barely start and immediately die. Then the next time I start it, it starts and stays running. Rarely, I have to start it a third time but never more than 3 times. The idle is sometimes ok but usually surges. Also, when I'm driving, it accelerates fine and will cruise pretty smooth, but if I let off the gas for any amount of time and then push the clutch, it wants to die. If I release the gas and press the clutch at the same time, it's fine. It always starts back up though, if it dies.

I checked the plugs a while back and they were all pitch black, no sign of any single cylinder having more of an issue. Also, I noticed that my oil level actually ROSE. When I drained it, it smelled of gasoline and was very watery. I'm assuming it was running so rich that the gas leaked past the piston rings.

My first instinct is fuel pressure? Any ideals on what else could be causing this? The plugs were brand new after the motor swap, so them being black is just a symptom, not the cause of the problem. I'm not sure if there's maybe something else that I may have missed when doing the swap.

[BlueXJ]

First I moved your post to where it will get more exposure. Secondly in answer to your problem. You have two temp sensors on one engine and only one on the other. This is due to two different computer systems. You can't use the thermostat housing sensor in a Renix setting. The Renix system wants to know what the block temp is not the constantly changing thermostat temps.
Thirdly the rich problem may be an air restriction problem. When I got some leaves in my airbox I had a similar result. On my old engine when it was getting blowby it saturated the air filter and alsio caused an airflow problem with a richness to the engine.
On the temp sensor it needs to be block mounted. Some 4.0L blocks just have a plug where the sensor goes if it is not a late 96 block

[4.3LXJ]

Yup, use the old temp sensor

[Cheromaniac]

So the intake, exhaust, accessories, transmission, etc was all from the original motor. Since the motor swap, it has been running REALLY rich. I'm getting less than 10 mpg. The only thing that's changed since the motor swap is the exhaust. I had been running with only a cat (ripped off the rest on a rock ledge). Since the swap, I cut out the cat and put on a glass pack.

Agree with what the others said about the temp. sensor but I think that may be only part of the problem.
Your '90 intake manifold runners won't be properly aligned with the intake ports of the '96 head unless you raised the manifold to match the higher ports. If you didn't, there'll be a step and your injectors will be firing fuel at the port floors instead of the air stream.
Did you reuse your original O2 sensor? You might want to check the output voltage when the engine's idling. It should be pegged at the maximum if it's working normally on a pig rich engine. A low voltage will indicate a bad O2 sensor and that could be the reason behind the richness.
One more thing you can do is disconnect the vacuum tube from the fuel pressure regulator and see if any fuel comes out. If it does, the diaphragm inside the regulator is cracked and you may be having excessively high fuel pressure. Attach a pressure gauge to the Schrader valve to find out.

[GGXJ]

Ok, I have pulled the fuel pressure regulator vacuum line off an no fuel is coming out. It didn't seem to have any impact on how the engine was running either. I probably need to go back through and double check all of my vacuum lines though, because I did have to redo some of the plumbing. Shortly after the engine swap, the ring gear in the D-35 rear end got a bad case of gingivitis and lost a few teeth... so I replaced the rear with a C8.25 from an automatic (for the better gear ratio and strength) and the front with a non-disconnect D-30 (to match the rear gear ratio and get rid of the POS vacuum disconnect axle). It's possible I may have missed a vacuum line somewhere.

As far as the temp sensor, it would make sense how having the sensor reading from the thermostat housing could affect the mixture. I remember when we were putting this all back together and I tried to get the plug out of the block and failed. If memory serves me correctly, we broke off a 3/8" breaker bar tip in there and it wouldn't budge. I might need to give that another go and see if I can get it out.

I haven't been able to check the oxygen sensor, but it's likely the original sensor that was installed on the Jeep from the factory. However, I've seen O2 sensors go bad before, but never cause an engine to run THAT rich. I'm not gonna rule it out as a contributing factor just yet, though.

And in regards to the intake ports not lining up because of the difference in port height between the 90 and 96 heads, I did notice that. However, it was a very small difference and I wasn't too terribly concerned with it. It actually ran freaking awesome when we first got it running. And the last time I changed the plugs, it was accelerating better than it ever did before the swap, just having the dying issue.

So, it looks like I need to look at relocating the temp sensor back to the original location, check fuel pressure, check the O2 sensor and double check the vacuum system. Now, if I could just get a day or two off work to do all of this...

I appreciate the input, guys, and I'll let ya know what I find out.