Vapor Lock (?) 2000 4.0

255,xxx miles, unknown history. Runs great. Just fixed e-fan today (burned contacts on relay, must have been because of the busted looking fan in there before)


Drove to Frederick and back (about 35 miles each way) in ~85F heat with the A/C on. Stopped at pharmacy to drop off a prescription and when I came back out it would not start at all. Had to pop the hood and sit for about 10 min and keep the pedal floored until it stopped coughing and sputtering. I did run a tank with Techron through it.

I've seen people say add an inline check valve, any recommends? Dropping the tank to replace the fuel pump may be a project that will have to wait as I have factory skids and it might take me a while to bust up the old nutstrips and get the rear bumper off enough to pull them out. I do appear to have the heat shield although it is VERY oily as VC gasket has apparently been leaking for approximately 16 of the last 17 years.

Here's my thought though. What if I took another relay in parallel with the factory fan relay and ran it through a temp probe so the e-fan would kick on whenever underhood temps got over say 220 whether or not the PCM was commanding the fan to be on? I haven't really looked into this too much yet but I'm thinking of getting a couple relay sockets and mounting two relays outboard of the factory relay box, one duplicating the original one, one switching on from a temp probe. Wires would plug into the factory relay socket either through male .250" Fastons or else if I can find a NFG ice cube relay I could use that and just solder wires to it. Only way I can think this wouldn't work neatly and easily and be easily transferrable to another XJ if desired would be if the power feed for the fan's 40A fuse is switched by the ignition, but I don't think it is because my memory says that the factory relay on my '99 stuck closed one day and I had to pull it to shut the fan off. This way if I provided the relay coil's power from a non switched 12V source the fan would run through the heat soak period, keeping the underhood cool and my engine from vapor locking. Thoughts?

Sounds like your pulling the bumper!!! :hang:

A simple search of finds that the preferred cure is an e-fan timer. >>>> http://www.naxja.org/forum/showthread.php?t=1028190

An e-fan timer will likely help with the symptoms, but a failed check valve on the fuel pump assembly is probably the root cause of the problem.

It seems that nothing has changed since I investigated dropping the tank on my '99 years ago - the J-bolts are still stupid expensive.

I see shorter ones sold as repros for old MoPar musclecars for a reasonable price that look like they ought to work, anyone try 'em?

I'm also thinking that something like this

https://www.amazon.com/Derale-16759-Adjustable-Fan-Controller/dp/B000CN4XZI

would do exactly what I want as far as controlling the fan by underhood temp, that and a socket to relocate the stock fan relay out of the box should do just fine with only a little minor mod to the relay box to pass the wires through. I was going to put the controller and the stock relay outside the box and run wires into the box to plug into the stock relay socket. Stock relay is untouched but the fan controller's relay contact will be wired in parallel with the stock relay. Fan controller is wired to an always hot source, probably using an inline fuse to the + post on front of relay box. I think this should work...?

You can get J-bolts at any parts store. The Crown nut strips work fine too. Mostly what you want to do is use an impact on them because you might be able to save them and then you don't have to pull the bumper & change them out. Just go forward first, and then backwards and forwards alternately to see if you can get them free. Keep doing it that way until they come out. If you just go full on backwards you will probably snap the bolts.

Fuel boiling can be the pre-cats not running that well. Do you have any codes? I noticed my Jeep ran cooler and less problems on warm start once I changed them. The biggest headache with changing the pre-cats is the cost. It's not that complicated of a job. A 15mm universal socket is about the only special tool you need. Not much else will fit on those flanges. A short swivel with a low profile 15mm like you see in a serpentine belt tool kit also works, but the dedicated swivel is easier. My pre-cats threw codes for over 200,000 miles before I changed them. On a dyno emissions setup the Jeep still passed...

I'll have to call around, I didn't see any j-bolts online that were "affordably priced." The MoPar repro ones are dirt cheap which makes me happy.

I already have JCR nut strips on deck. Same thing happened last Heep, just about the time I was ready to mess with the rear skid, someone was selling a used set of JCRs so I figured why not. I was already planning on dropping the skid because I want either a hitch or better yet the Rigidco rear bumper that's in my storage locker on this, and I might as well trade skids with my old Heep as I cleaned it up and painted it up real nice when I installed a hitch. I am definitely planning on doing this at the shop, with air tools... also any the backsides of any bolts that are accessable will get a die run on them before attempting removal.

I guess I will have to hope that I can either find new J-bolts or else that there's enough room to run a die on with a Gearwrench.

Edit: I see I didn't answer your question completely. When I got it I had a "pending" code for a Cylinder 3 misfire but no MIL. I had my friend's shop replace the plugs with new NGKs and had no problems until it got hot. Now I threw a MIL after the last vapor lock episode, codes are random misfire, and cyl. 1, 2, and 3 misfire. But it runs great once it clears, no other codes. Basically the only time it messes up is if I drive for a long time/get it good and hot and then make a short stop. Based on the reports I've seen of this being common on 4.0s with the dual precats, I'm thinking I need to do something fan related, plus long term plan on replacing the fuel pump and possibly putting heat sleeving on the fuel rail.

Well, today was even hotter and I had to drop a friend off at the airport and at the same time was running on fumes (I didn't want to fill up too early as I had Techron in my tank) - stopped at Costco between the airport and my office, sat in line for a good 5-10 minutes, then filled up. I popped the hood as soon as I stopped because I wanted to check the oil anyway - good news: only down about half a quart, despite an apparently MIA valve cover gasket and a healthy funk of oil burning off - and thankfully started right up with no issues. Also had to stop at Lowe's and just left the hood on the safety latch while I was inside, I think it ran slightly rough when I started it there but not bad. So I'm hopeful that a fan hack will get me running well even though the ultimate solution may involve hood louvers, a new fuel pump, or both. I've already ordered the Derale thermostatic controller, borrowed what's her face's Amazon Prime so it should be here before the weekend if not much sooner.

You have the classic hot soak issue that any of us have in late model XJs.
I have tried every trick in the book, insulating the injectors and fuel rail, insulating the bottom of the intake manifold, hood vents, you name it.
The fuel pump pressure was and is fine with a very slow leak down rate, indicating the injectors were sealing and not leaking, when the engine is off.
An added check valve would do nothing positive, since the one in the fuel pump is working correctly.

The only thing that prevents it 100% of the time is the fan timer, installed to run the aux cooling fan for three minutes, when ever the 'fan' button is pushed.
https://www.flickr.com/photos/lwade/sets/72157617434915718/

When stopping for a few minutes, in warm weather and the engine hot, the button is pushed before getting out.
Three minutes of fan operation is enough cooling to preventive fuel from boiling in the fuel rail. The hot soak problem only occur during short stops, durations of 10-15 minutes.
Longer or shorter stops are not a problem.
This 'cure' has been in successful use for eight years.

I believe the fan method does work, but it cures a problem that never happened when the Jeeps were new? When my 01 XJ was newer I don't remember this at all. For me it first started happened around the summer of 2011. It was about 90-95% fixed when I replaced the pre-cats last spring. The only time I had a partially rough stumble start since doing the pre-cats was after my girlfriend drove it, that was one time, so who knows what happened. Prior to that it was mostly a regular summer occurrence. The Jeep also seems to run a bit cooler since changing the pre-cats. Takes longer for the aux fan to kick on sitting in traffic, gauge reads a bit lower more often, etc.

Well I guess no brilliant plan to deviate from conventional wisdom goes unpunished... got the Derale controller today and it switches ground not power. Wouldn't be a problem except it's all unitized with the relay socket so I'd probably void a warranty if I opened it up and rewired the relay pins.

1) I'm assuming as with most other things the relay in the relay box switches +12V not ground to the fan yes?

2) the PCM signal to that relay isn't +12V is it? Only way I know to force fan on to test is to heat soak and then restart and don't feel like being that brutal (and ruin my cats.) Unlike my '99 turning on the A/C doesn't seem to turn the fan on automatically, it only comes on every now and then.

N8N_99xj said:

Well I guess no brilliant plan to deviate from conventional wisdom goes unpunished... got the Derale controller today and it switches ground not power. Wouldn't be a problem except it's all unitized with the relay socket so I'd probably void a warranty if I opened it up and rewired the relay pins.

1) I'm assuming as with most other things the relay in the relay box switches +12V not ground to the fan yes?

2) the PCM signal to that relay isn't +12V is it? Only way I know to force fan on to test is to heat soak and then restart and don't feel like being that brutal (and ruin my cats.) Unlike my '99 turning on the A/C doesn't seem to turn the fan on automatically, it only comes on every now and then.

Click to expand...

For later years, a new high pressure switch controls the auxiliary fan operation.

that makes sense. I know the fan would come on in my '99 any time the compressor clutch was engaged. That must have started in 2000 then

md21722 said:

I believe the fan method does work, but it cures a problem that never happened when the Jeeps were new? When my 01 XJ was newer I don't remember this at all. For me it first started happened around the summer of 2011.

Click to expand...

I bought my 2001 XJ in May of 2003. I experienced the first vapor lock just a few weeks later, stopping at the store on the way home from work.
It had less than 30,000 miles on it.
The 2000-2001 California versions have two added mini-cats, which pump a tremendous amount of heat into the engine compartment, leading to a vapor lock condition when parked hot..
Mopar responded with a TSB that has insulation added to the injectors. They also came up with a layered aluminum shield that sits on top thin take manifold, under the injectors.
Neither did much to help.
Since buying it 14 years ago, the fuel pump and injectors have been replaced with new parts, without having any influence on the hot soak issue.

So far, the timer is the only 100% cure.
Anxious for someone to come up with a better fix...it's been 14 years and I'm still waiting.

OK so I found this - see post #7

https://www.cherokeeforum.com/f2/cooling-fan-function-133903/

I think I can still make this work, if I hook up the fan controller and provide its switched ground to pin 86 of the stock relay, and then move pin 85 to an always hot source rather than F20, it will work as I want it to, correct? As a bonus, the auxiliary controller will only be switching the relay coil of the stock relay, not the full draw of the fan. Am I thinking right?