Okay so back story, I’m new to this site.
I have a 98 Jeep Cherokee xj with the 4.0. I’ve been trying to get this thing “restored” for a couple years. I’ve done A LOT of work to it. Anyways, I’ve completed everything underneath, new suspension components, steering, everything. Brakes fully rebuilt, from brake booster to brake cylinders in rear. I recently pulled motor, did a top end tear down to do head gasket. Got it all back together, put back into Jeep and I did timing wrong. Had a “knock” like a valve bent or a rod. Took head back off, checked it all, cut the valves because why not. Redid timing correctly on cam and crank side. Put it all back together. But as of now, the Jeep idles perfectly fine, to me at least. But once you raise RPMS, once it comes back down to idle it stalls. The previous owner did plugs, which he did completely wrong and they were very fouled. So I did plugs, wires, cap and rotor recently. Still the same outcome, stalls when coming down from an idle. I’ve swapped TPMS, and IAC, no change. The other day, when I cleaned my throttle body and where IAC mounts, I put throttle body back on and accidentally left the PCV hose off, from rear of valve cover to intake, and it ran awesome. Didn’t stall or anything. Hooked the vac line back up and it stalled. Side note, the IAC connector is tough to get on. I repinned a new connector on and it’s still rough to get on the sensor, not sure if it’s making a connection or not but I hear a resounding “click” when I seat it. I have no idea what could be the issue. Not sure if anew O2’s and MAP will solve the problem, or I’m looking at something much bigger. Hopefully some of you may have ideas… because I’m fresh out.

 

Could be running rich with the vacuum line connected. By pulling it, you created a lean condition. Just my guess tho.

Check the fuel pressure. I think it should be 49psi +/-, IIRC.

What IS the issue, i dont know. Curious why the PO had the plugs wrong. You said they were fouled. You could be dealing with whatever issue they were having.

 

I will check fuel pressure. I just put in a new fuel pump not long ago, but never really ran it. Hopefully it’s not my fuel pump causing this whole mess. Thank you

 

1... codes?
2...sure sounds like it's way rich. Does your scan tool data confirm that.
3. I'm assuming "tpms" was a typo and you meant throttle position sensor rather than a tire pressure sensor. Otherwise I would wonder about the throttle position sensor.
4. And like above check fuel pressure .

5 did all the plugs look equally rich or maybe just a few of them.... thinking funky injector (s)

 

yeah sorry, TPS is new. All plugs were fouled pretty equally. I did put in new injectors, however one of the orings wasn’t perfect but it seemed to still seal correctly. Codes in PCM were battery over temp which is unplugged and TPS high voltage, which could be from the old sensor. I’ll be bringing a scan tool home tomorrow to check. And the scan tool did show it was rich, but the Jeep wasn’t running so it wouldn’t have changed. With Jeep off but key on, MAP volts was sitting at 4.49V and tps was at 4.63V.

 

This might sound like a desperate and silly suggestion but hey,it won't cost anything.
when you disconnect the PVC line you are doing 2 things... creating a vacuum leak and eliminating whatever hydrocarbons are going thru that line. Just for the heck of it pull the PVC line off of the manifold and plug the fitting.at the manifold. This will eliminate the vacuum leak but also eliminate whatever was coming thru that line. If it runs better start wondering about blowby, hacked PVC plumbing etc

 

so I did shove my finger over the hole there and it stalled immediately. If there is blow by, am I looking at a whole new motor?

 

Hmmm didn't see that coming but as it sayt on the cover of the hitchhikers guide to the Galaxy "Don't Panic"

Couple things.
Were the plugs just sorta dark and sooty or were they covered with oil? If you worried about the condition of the engine do a compression test.

You might try the thumb over the hole test again but this time slowly gradually cover the hole to maybe give things a chance to settle down and see if it will keep running. I don't know if the thing is running long enough to be going into closed loop but it's worth a try. It almost sounds like it's running at idle on the air coming thru the PVC rather than the IAC.

This trying to diagnose weird problems remotely is tough

 

it was like a carbon texture I guess? Not wet, but dry and black. Possibly stupid question, where that PCV hose goes into the intake, that should have a suction correct? And when I have it running, it’s maybe a minute or two, as my valve cover is leaking bad due to it being bent. I have a new one, just waiting on a chance to install it.

 

Yes there should be vacuum at that port

So this am I tried the following on my 2000 XJ.

With it in closed loop.

Took off filler cap on valve cover to simulate leak...no change.
Pulled PVC line off manifold port...slight change for a few seconds then recovered.

Put thumb over port... slowed for a few seconds then recovered.

Put thumb slowly over port..think being a throttle plate. You can pretty much maintain same RPM if you do it slowly.

I know you said you replaced the IAC but it or it's circuit certainly would get my attention and I'm out of ideas
And I would check the IAC connector for bent pins etc

 

okay I’ll try to slowly cover port. See if it’ll stay running or not. The IAC connector is kinda tough getting on. I checked for vent pins on sensor and it looked okay. I repinned a new connector and the pins on connector side looked okay. But something is clearly wrong.

 

so went back ojt in the garage after work. Redid my valve cover and still leaking, no idea why. Anyways, I took the IAC sensor out, turned Jeep on and the sensor moved. So I believe IAC is fine. However, the TPS connector doesn’t secure, I don’t think it’s making a connection. Will be checking pins later on to see if any are bent. I didn’t look hard but I think the locking pin is broken. I will check later

 

also slowly put my thumb over the pcv on intake, and it still stalled. Went as slow as I could to let it smooth out but it wouldn’t.

 

Another question..once you are off idle does the thing run ok?

Other than that it really sounds like that at idle it's only running on the air from the PVC rather than getting any air thru the IAC. If the IAC is working ok,is the passage/inlet in the throttle body where the air gets into the IAC gunked up?
As much as I don't like rattle can rebuilds try spraying some tb cleaner into the inlet slot in the tb when it's running. Manifold vacuum should suck it thru the passage......if the IAC is open

 

nope IAC passage isn’t gunked up. I’ve been in there and used TB cleaner to clean TB and that passage