'96 sport, 225k miles, DD

I have been running around chasing issues lately before I take my XJ on a cross-country move. I was having a long/weak crank issue, and replaced the fuel pump and ignition coil after some pressure/spark tests. I am also having some weird intermittent rough idle--I figured this was due to the fuel-delivery/spark issue, but fuel pressure is good now and holds, and I have no CEL, so I started trying to check other things.

Looking at my scanner, my "Intake Manifold Absolute Pressure" is reading steady around 12inHg (and it has for 3 years, looking at my old logs), but the forums seem to say spec should be around 20inHg? My AC cuts to defrost when the engine is under load, and the engine kind of bogs on hard acceleration, but the AC runs great otherwise and my cruise control seems fine (I rarely use it though, too much traffic here). Idle is normal and sits around 750 RPM, ignition advance timing is around 15-18* at idle with random dips, 33* at 2500 RPMs.

I'm assuming my low pressure + HVAC issue is a vacuum leak? I see people saying things about "blowing smoke into the lines" but I'm not sure how. I also don't mind replacing vacuum lines, almost everything in my engine bay is basically dry-rotted to hell and crumbles when I touch it, but I'm not sure which connectors/elbows I would need. Does the '96 have a check valve I need to replace? My PCVs (the little tube things on top of the engine?) are loose as well.

And I still have a long crank issue, could a vacuum problem cause this?
I've been looking at this thread and checked the reservoir and connections, but I'm not sure what exactly to do next/how to prioritize.

Parts replaced/cleaned/checked in the past few months:
grounds cleaned/wires checked
TPS (mopar)
CPS (mopar)
Cam sensor/dist pick-up (cheap part, waiting on a mopar)
IAC (cleaned)
Air filter
Fuel pump + Filter
Spark plugs (NGK copper) + wires
Cap + rotor
Ignition coil (NGK)

 

Did you check the check valve in vacuum line? The post your linked says that was his problem. The check valve prevents loss of vacuum to the HVAC and CC systems when the engine is under heavy load (like accelerating onto a highway, towing a heavy load or climbing up a long hill). Engine vacuum drops and the loss of vacuum can make the HVAC system default to Defrost.

If you have a vacuum gauge, try disconnecting the HVAC vacuum at the manifold and attach the vacuum gauge and see what you get at idle. Then blip the throttle a few times and watch the gauge. It should return to the same number. 12" hg does sound low. For a data point, my '88 with 209K runs about 18-ish at idle.

 

So you just take it out and blow into it, and if both sides blow through/don't blow at all it's toast? I know for sure I have at least one cracked line by the battery (because I broke it), but that's fairly recent and the low vacuum has been happening for at least a few years, apparently. I know the defrost under load has been happening since last summer, but I think it's getting worse?

And I think after staring at a few diagrams I understand the smoke test, but do you put smoke through each individual line (i.e. the three at the manifold and the one coming from the reservoir?) or is there just one place you blow into? Or is there something else I'm missing? I'm great at following directions and turning a wrench, but the diagnostic stuff is still a little over my head.

I'll grab a vacuum gauge on the way home today to check things out too.

 

Yep, the check valve should only allow air to pass one way. Flow should only go towards the intake manifold so if it's good, be careful to install it in the proper direction. Not sure about a '96 but you may have more than one so be on the lookout.

Small cracks/splits in any of the lines may show the same symptoms so check each one carefully. Since you have a normal idle, any leak can't be very large.