FPR Vacuum Line on 97+?

I'm hoping some of you guys could give me a quick helping hand here. I've got a 97 with the 4.0/AW4 combo. I bought it recently, knowing it had a few small issues, and now I'm having trouble buttoning up the last few bits.

It acts like it has a vacuum leak. It seems to idle low, though I'm one of the lucky ones that got the idiot lights and no tach, so I couldn't tell you exactly how low. I also can't get the cruise control to set, and get an intermittent P0505 code, even after replacing the IAC with a new unit, cleaning the throttle body, and checking the harness. I have gone through all of the vacuum lines under the hood fairly thoroughly, and haven't found any leaks. I have also sprayed throttle body cleaner around the intake manifold gasket, and did not find anything there either. I have been trying to go through all of the components on the vacuum diagram under the hood, but I noticed this afternoon that it doesn't show a line to the fuel pressure regulator. The only line it shows going back to the tank is EVAP, which is now capped. Does the FPR use that vacuum line, or is there another line that I'm not seeing? Any help or advice would be greatly appreciated.

1. A leaner mixture, via a vacuum leak/intake leak results in a higher idle RPM.

2. A richer mixture has the effect of lowering the idle RPMs.

3. Your EVAP system is supposed to be dumping fuel tank vapors into the intake. Please repair your EVAP so others don't have to choke on the SMOG you are contributing to.

4. Your fuel pressure regulator/check valve/filter is on top of the fuel tank, no FPR/return line on the fuel rail after 95.

1. I understand this, and I guess I should have elaborated on that a bit. My thinking was that OBDII may have been overcompensating for a high idle condition resulting in a low actual idle and causing the IAC counts to move out of range.

2. It could be running rich. I have a wideband, but it's on the other end of the country at the moment, so I guess my best bet for now will be to get a fuel pressure reading and see if it's in range and holding pressure...

3. To be honest, I'm pretty anti tree-hugger. The purge solenoid actually operates very rarely anyway. I've put 100 miles on the vehicle since I capped that line, and the PCM has yet to send the signal to purge the system. At that rate, the gas can I use to fill my lawnmower is a greater contributor to smog than a failed evap system. With that said, I'm also not fond of Check Engine Lights, so it was a temporary fix until I can get my hands on a replacement canister, I just wanted to rule out that line as the source of my problem. It will be fixed. (On a Jeep, is it a CEL or an SES light?)
*I don't mean to be unappreciative or undermining at all, I just truly believe that EVAP and EGR are unnecessary on a vehicle that has been well maintained.*

4. I know the regulator is at the tank, what I don't know is whether it is vacuum operated. If it is, there is a vacuum line running from the engine compartment that isn't listed on the routing diagram...

Thanks for the insight, if I'm missing something, please point it out. For now I guess I'm going to put a fuel pressure gauge on it and try to get a hold of a charcoal canister.

Glad to hear you are going to fix the EVAP.

No, no vacuum line to the FPR at the tank.

P0443 will give a hard CEL, PO441 will not: http://www.kelleyswip.com/obd2.html

The EVAP system came about in the 70s. It's role is to capture, store, and then introduce those vapors into the intake so they can be burned. The vapors, if released into the atmosphere, turn directly into smog without having to undergo photosynthesis. The government claims that something like 147 MILLION gallons of gas are saved in the U.S. each year by the EVAP systems.

Good luck.

Thanks for the info, hopefully I'll get it figured out in the morning...

Just out of curiosity, how does the FPR operate if it's not vacuum controlled? I've never had the tank out of an XJ before, so I have no idea what kind of system I'm even dealing with there...

I believe it has an internal bypass to release excess pressure back into the tank, but I haven't had my hands on one to hack apart-yet.

Here is a picture:

Well if mine winds up being bad, I'll take a hacksaw to it and upload pictures...

The EGR is in fact unnecessary - there is none on your jeep since it's a 91 or later, the last year for EGR on XJs was '90!

I hated those stupid high-temp diaphragm valves... they are now getting difficult to find for RENIX vehicles and are troublesome to service IMO. Good riddance, glad we found something that worked better.

The EVAP system on the other hand is kinda neat, it sucks up the vapors that evaporate out of your fuel tank (can't just seal it unfortunately, or it would inflate like a balloon or a plastic jerry can left closed securely in a hot vehicle) and allow you to burn them, saving a tiny amount of money and gas.

Careful chopping up that FPR, don't want to do anything that could cause a spark, that would suck. I want to see pics though, always wondered what was in there.

Yeah, I had an OBDI controlled F-Body at one point that had issues with an EGR setup similar to the RENIX XJ's. I wound up getting rid of the system and tuning it out of the PCM, and it really had no effect when they hooked the sniffer up for emmisions testing...

As far as the EVAP system goes, I can see some merit in sending unburnt fuel back to the tank, but the way this one works it just dumps vapors into the intake manifold, displacing oxygen that could have been used for combustion. At least that's the way I understand the system to work. I guess that may help to lower temps in the combustion chamber, but it only works when the system purges, and I doubt it makes any difference in mileage or driveability, other than getting rid of the gas smell... It would be an interesting experiment to do a thorough mileage test with and without the system operating.

It doesn't really displace oxygen that could have been used for combustion... any more than the fuel injectors injecting fuel do. That's like saying that adding more fuel to a fire makes it smaller because you displaced some oxygen.

EDIT: also - ever notice a strange rattling sound from what seems to be deep inside the dash on the passenger side while at idle or coasting? That's probably the purge solenoid shuttling on and off - I never really realized what it was till recently. The evap system is most active when the engine isn't loaded heavily, from what I have noticed. I always thought the squirrel cage blower was about to fall off my blower motor and that's what I was hearing, was quite glad to find out it was just the evap purge solenoid.

I've never noticed the rattling sound, but I haven't had this XJ very long and I haven't driven the '89 in quite a while....

If the fuel that got dumped into the intake was metered and accounted for, it could possibly help combustion, but on this system, when the valve purges it releases an undetermined amount of vapor into the intake charge, after it has been metered. The stock PCM enrichment tables are already set up to be on the rich side of stoichiometric, so any added fuel vapor from the EVAP system just can't be burned, since there is even less oxygen in the chamber than the PCM accounted for when determining the injector pulse width. It will lower the combustion chamber temperatures while the system is purging, but it's not going to contribute to power or efficiency. If anything, I would think it would rob you of some...

That would be true if it was an "open loop" system, i.e. wasn't watching how well it actually burns things. The upstream O2 sensor(s) are used by the PCM to determine whether it's running rich or lean, and by how much, and it modifies the amount of fuel injected using that info and the tables you mentioned. So basically the evap system doesn't change the fuel ratio, it just changes how the fuel gets to the cylinder and keeps the vapors from escaping from the tank.