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The purpose of this write-up is to outline an inexpensive and simple time delay for the e-fan circuit. This article is specific to a 2000-2001 Jeep Cherokee XJ but the mod described should be pretty easy to adapt to other years of XJ’s with or without a Power Distribution Center (PDC).

The purpose of this mod is to address the heat soak issue that is common in Jeep XJ’s with the 4.0 liter inline 6 engine and this heat soak issue is especially pronounced in the 2000-2001 XJ’s. For those that aren’t familiar with the heat soak issue, it occurs on hot days when you’ve driven long enough for the engine to be at full operating temperature. If the engine is shut-off for a short time (5-15 minutes… such as when running into a convenience store), heat from the exhaust and intake manifolds heats the fuel rail and fuel injectors which are located just above the manifolds. This can vaporize fuel in the fuel rail and/or injectors. When the engine is started, the fuel vapor can result in misfiring and stumbling and can set the CEL with codes related to cylinder misfires and/or injector malfunctions.

People try many different solutions to the heat soak issue such as heat shields, insulation, hood vents, etc. Most of these seem to have limited success. The solution that seems to work most reliably for most people is some sort of timer to keep the e-fan running for 3-4 minutes when the engine is turned off. The e-fan is on the same side of the engine compartment as the intake/exhaust manifolds and fuel rail and injectors.
When the engine is shut-off, the e-fan time delay will cause the fan to blow air over the manifolds and out the back end of the engine compartment and will prevent the fuel rail and injectors from getting quite so hot. An OFF delay of 3-4 minutes will generally cool things sufficiently to prevent the heat soak issue.

When I decided to address this issue in my XJ, I looked at various circuits I found online for implementing an e-fan delay. To my way of thinking, they were generally more complex and expensive than they needed to be and so I came up with my own solution. I used a standard 5-pin automotive time delay relay in parallel with the normal e-fan relay. On a hot day when stopping the engine for a short period of time, I push a momentary pushbutton switch on the dash to trigger a 3.5-minute time delay. The fan runs for 3.5-minutes and then automatically shuts down. I spent less than $20 and completed the mod in about an hour and a half (and since I’m 67 years old, I took a short nap part way through… just kidding).

I used a Buehler BU508TD automotive time delay relay. I purchased the relay along with a standard 5-pin automotive relay socket online for a little over $7 online (including shipping). I then ran to my local Radio Shack and picked up a normally-open (NO) momentary pushbutton (PB) switch and some wire. The relay has a 2 sec to 5 min adjustable time delay (some literature says 2 sec to 3 min, but I adjusted mine for 3.5 minutes).

Except for the two wires running to the dash-mounted PB switch, all wiring is done on the underside of the Power Distribution Center (PDC) mounted on the right-side fender well. I have attached a few wiring diagrams and photos of the PDC with the TD relay next to the PDC and a photo of the PB switch. I should have snapped a couple of photos of the underside of the PDC when I was splicing the wiring, but didn't think to do it. However, it's all pretty straight forward.

Step-by-step instructions are as follows:
1. Remove the battery positive cable.
2. Remove the small cover at the front of the PDC and remove the ½-inch nut and then lift off the cables between the PDC and the battery and alternator.
3. Release the two clips that hold the PDC to its mount and then lift out the PDC.
4. Flip the PDC over and then release the 5 or 6 plastic clips holding the bottom cover in place and then remove the bottom cover to gain access to the wiring inside the PDC.
5. Install the momentary PB switch where desired and route the two connecting wires to the PDC.
6. Connect the wiring from the 5-pin relay socket and the PB switch as shown in the diagrams. I connected the white ground wire coming out of the 5-pin relay socket to an existing sheet metal screw on the fender well adjacent to the PDC. The other wires were spliced to existing wires as shown on the diagrams. In my case, I removed a little insulation from each existing wire and made soldered connections and then insulated with electrical tape.
7. Reinstall the bottom cover on the PDC with all wiring exiting the back side of the PDC. Flip the PDC back over and clip it into its mount on the fender well. I simply let the new TD relay lay next to the PDC on the fender well.
8. Reconnect the battery and alternator cables to the PDC and then reinstall the small plastic cover. Then reconnect the positive cable to the battery.
9. If you want to test the connections, then hook a voltmeter between pins 85 and 30 and verify unswitched +12 Vdc on pin 30. Then hook a voltmeter between pins 85 and 86 and verify you get a +12 Vdc trigger when the PB switch is depressed. Finally, jumper a short piece of wire between pins 30 and 87 and ensure the fan runs.
10. Now install the relay into the 5-pin relay socket. You can adjust the time delay by removing the relay cover and adjusting the small potentiometer inside. Adjust it in small increments… it’s touchy. I recommend unplugging the fan while trying to adjust the time delay. In my case, it took about six adjustments before I got my desired time delay of 3.5 minutes. You can hear and see the contacts in the relay when they pickup and also there’s a small LED inside the relay when the contacts are closed.

You’re ready to go. Momentarily pushing the switch starts the fan for its 3.5-minute time delay. You can trigger it anytime whether the engine is running or not… makes no difference. The normal fan control circuit is unaffected and you won’t get any CEL’s because you haven’t connected anything to the computer (powertrain control module). Besides triggering mine when it’s hot and I’m stopping for 5-15 minutes, I also sometimes trigger it when I want extra cooling and I don’t want to wait for the computer to start the fan at 218-deg… for example, when starting up a long, steep hill on a hot day.

I hope others find this useful.

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Added to my to do list. Very simple.
 

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Great write up. I have looked at a few ways of doing this and this is much simpler.
I must ask, can the fan be triggered to be on fully for as long as you want if using a standard on/off toggle without causing CEL to come on? I want to have the ability to turn fan on fully when I am wheeling and also do a heat soak purge when needed as well.
This would be on my 2000 XJ Limited with a crap ton of mods
 

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The purpose of this mod is to address the heat soak issue that is common in Jeep XJ’s with the 4.0 liter inline 6 engine and this heat soak issue is especially pronounced in the 2000-2001 XJ’s. For those that aren’t familiar with the heat soak issue, it occurs on hot days when you’ve driven long enough for the engine to be at full operating temperature. If the engine is shut-off for a short time (5-15 minutes… such as when running into a convenience store), heat from the exhaust and intake manifolds heats the fuel rail and fuel injectors which are located just above the manifolds. This can vaporize fuel in the fuel rail and/or injectors. When the engine is started, the fuel vapor can result in misfiring and stumbling and can set the CEL with codes related to cylinder misfires and/or injector malfunctions.
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In my case, heat soak didn't cause vapor lock, but disguised itself as such. It occurred as you said, after sitting 5-15 mins, especially on hot days. I tried hood vents, water wetter, bored out water neck, new water pump, lower temp thermostat, 3 core radiator, and even some home made fuel injector heat shield boots made out of chrome plated convoluted wire loom, and still had same issue. Took it to a couple different shops. Dealership was totally useless, telling me that if their scanner doesn't say something is wrong, then they cannot do anything. You could hear, see (tachometer), and feel a slight stumble when running normal, and after heat soak, sounded like an old lawn mower.

If we all remember back to late 1999 when Jeep engineers decided to screw up a good thing, namely the cylinder heads cast # 0331, these were prone to warping and cracking.

I went with my gut and took the head off and saw exactly what I suspected. I was getting water leaking between and into cylinders 3 and 4 dead in the middle of the head. I could see a pathway where the head gasket SHOULD HAVE been sealing between the 2 cylinders and dried rusty drip lines down the cylinder walls from that spot down. I brought it to a well recommended builder. He warned me up front that I had a 99% chance of replacing the head due to cracking.

The head was diagnosed with having a slight warp, but NO CRACKS!! The builder was shocked. He pressure tested it multiple times to be sure and it passed. He was able to machine it flat as new.

Installed the head and never had the problem again.

In short, under heat soak, the internal water pressure reaches its highest psi. Due to the head being warped, the water found a way into cylinders and acted just like vapor lock.

My suggestion if you are having "heat soak" issues and suspect vapor lock but nothing you do fixes it, pop the valve cover off and look for the casting numbers between cylinders 3 and 4. If it says ONLY "0331" there is a chance that you may have warped or cracked head that is causing your issue. If it says "0331 TUPY" Congrats, you have the improved Brazilian made head.

You may be able to peek into the spark plug hole with camera to look at the rearward part of the cylinder wall of cyl 3 or the front side of cylinder 4 to look for dried drops of coolant instead of going through the hassle of ripping off the head.

Thought I would add this to help anyone who may have a late 99 to 2001 4.0L from beating their head against the wall and spending a ton of time and money chasing ghosts
 
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