Jeep 4.0 Vapor Lock?

Van-go

Van-go

Not an old man
Joined
Dec 11, 2014
Location
Greensboro
my 4.0 is under a yj but came from a '96 Cherokee.
It runs flawlessly on the road but as soon as I take it off road it will run for a while then shut off. then randomly start again, sputtering, then running good, after cranking it for 5 min...

I thought it was the crank position sensor so I replaced it. This last weekend at the flats I ran into the same problem again...
My buddy told me to bleed the fuel rail, and behold, air came out. It ran great until I bounced around going up easy street. Then had to bleed more.

I am running a stock fuel pump and sending unit mounted to a 15gal aluminum RCI fuel cell.
Tank was full, but could the pump still be sucking air in when the gas sloshes around?
Anyone ever had any similar problems?
Trying to keep air out of my fuel lines... Advice is needed.
 
How close are the fuel lines to your exhaust?
My Ranger did exactly what yours is and it was due to my return line getting way too hot. I added some heat shield wrap and cured it.
What fuel pump are you running? An overheating pump can also cause it.
 
What Jody said. When I did the Vortec swap in the YJ I use to have, the exhaust header was too close the fuel lines and it would vapor lock. Heat wrap and shield solved it.
 
X3 on the fuel/exhaust. My buddy's XJ would get just like you describe on the trail. The intake can also get very hot and cause the fuel in the rail to evaporate.
In addition to the wrap, consider hood louvers if you don't already have them. They really let a lot of heat out of the engine bay. I used to ride around with my hood popped but now don't really need to with the louvers.


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Thanks for the advice, I'll give it all a go.

Jody Treadway said:
How close are the fuel lines to your exhaust?
My Ranger did exactly what yours is and it was due to my return line getting way too hot. I added some heat shield wrap and cured it.
What fuel pump are you running? An overheating pump can also cause it.
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I'm running the stock fuel pump fit into the cell.
 
77GreenMachine said:
X3 on the fuel/exhaust. My buddy's XJ would get just like you describe on the trail. The intake can also get very hot and cause the fuel in the rail to evaporate.
In addition to the wrap, consider hood louvers if you don't already have them. They really let a lot of heat out of the engine bay. I used to ride around with my hood popped but now don't really need to with the louvers.


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not trying to start debate here but XJs and WJs 4.0 do not have "fuel vapor lock issues" as people think they do. 00-01 XJ and 00-05 WJ 4.0s all have the dual small cats right under the intake manifold and third one downfield and the fuel lines run right next to them as they do in all the jeeps and there is NO fuel vapor lock issue with them. Every customer that has come to me with this "statement" has turned out being Fuel Pump/Regulator issue as they are all in one unit or the Cat furthest back against the firewall clogged and glowing cherry red causing too much heat or engine running poorly dumping too much fuel causing those cats to burn super hot but under normal driving conditions and off road abuse, they will not cause an issue. Just sharing this in hopes to help someone in the future. Argue all you want but I've encountered too many of these with one of the above mentioned fixes being the culprit. Not saying there isn't a possible "first timer" out there but working on jeeps for long time specifically and driving them for 20years now, I've never encountered such vapor lock issue under normal operating engine and computer/fuel management system.
@Van-go I hope you get it resolved man, don't know anything about the fuel cell setup so sorry I wasn't able to be much of a help last weekend.
 
You're wrong John. Sorry.
You are confusing a long crank time due to torn and/or orings in the filter/regulator assembly with fuel boiling in the lines due to heat.
Once the fuel rail is shielded from the heat of the manifold and converter with reflective heat wrap, the vapor lock is greatly reduced.
I'm sure you, being a 20 year Jeep vet (whereas I only had 14 of mine being foreman at a Jeep dealer), have a roll of aluminum foil in your tool box like I do to properly diagnose this issue. Well along with a fuel pressure gauge too.
 
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Jody Treadway said:
You're wrong John. Sorry.
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well I haven't personally encountered it. So far one of those other issues always turned out to be the culprit so maybe I've gotten lucky with not having to pull my hair out diagnosing a "fuel vapor lock" with a perfectly good running 4.0
I'm all ears though as to how a normal running jeep can cause vapor lock with all parts functioning properly!
 
marty79 said:
I'm all ears though as to how a normal running jeep can cause vapor lock with all parts functioning properly!
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Easy. Fuel gets so hot it boils. Air is trapped in the lines as a result. Fluid pumps (in this case a fuel pump) are great at moving liquids, but trap some air around the impellor and guess what happens? Nothing until that air is moved along.
I'm sure you are familiar with 4.0 engines getting air pockets in the water pumps and that causing an overheat. How do you remedy that? Bleed via the heater hose to rid air from the pump.
 
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benXJ said:
He said parts functioning properly. A stock XJ doesn't suffer from vapor lock. The factory engineerd system performs properly.
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I don't wanna argue anyone (especially whose been working on cars in general way longer than me) however in my 20yrs of mostly Jeep work and "proper" working xj as you/I mentioned, I am not aware of any heat soak issues. Sure many cars get heat soak from what I hear but not if it's working/running properly with cooling system, exaust, and engine/fuel management systems up to par. That was my main statement that something "improper" would have to cause this heat soak issue otherwise yes I totally agree, "factory system" was designed to operate just fine considering especially on those newer 4.0s where the cats sits pretty close to the fuel lines.
 
It's same issue with everyone's "MYTH" that the Jeep cherokee runs hot no matter what and overheats on trails without cooling system upgrades/hood louvers/dual electric fans..blah blah blah. I tell them wrong!!! FIX your cooling system properly and it can run with a 195thermostat all day long beating the hell out of the rpms and never run past 218/220 (factory Aux fan cut on). I know this to be true from way way too many personal jeeps, including the "renix sealed" system that is supposed to be more prone to overheating...Most people I see overlook the basics of "proper" working systems: radiator, water pump, thermostat, heater core, hoses, head gasket, coolant passages inside block/head, and blockage.
I believe someone who thinks their cooling system is "not worthy" but it's good mechanical shape goes and puts in dual/triple core radiator now thinking they fixed a factory design flaw when that bigger stronger radiator just usually masks the ongoing weakness in their system and over time, they eventually see this...been there and seen this too many times, their money though not mine I just install their fancy parts and let em ride when they wanna be the mechanic!
 
benXJ said:
He said parts functioning properly. A stock XJ doesn't suffer from vapor lock. The factory engineerd system performs properly.
Click to expand...

2000 and 2001 XJs do in fact suffer from vapor lock in stock form. The issue was so common that Chrysler introduced a Technical Service Bulletin to install a reflective heat shield under the fuel rail to deflect the heat that was causing the fuel to overheat and vapor lock to develop.
This issue was common on all of the 4.0 equipped Jeeps with front mounted cats from 1999 to 2006. XJs and WJs were the ones with the most occurrences since their unibody design didn't give the hit air anywhere to escape from unlike the TJs which has conventional fender openings.
 
Jody Treadway said:
2000 and 2001 XJs do in fact suffer from vapor lock in stock form. The issue was so common that Chrysler introduced a Technical Service Bulletin to install a reflective heat shield under the fuel rail to deflect the heat that was causing the fuel to overheat and vapor lock to develop.
This issue was common on all of the 4.0 equipped Jeeps with front mounted cats from 1999 to 2006. XJs and WJs were the ones with the most occurrences since their unibody design didn't give the hit air anywhere to escape from unlike the TJs which has conventional fender openings.
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well I'll be...must not be as problematic as that TSB as I've not had issues, the dealer i do work for on mostly WJs never had one come up with it so maybe only in super hot climates and people who like to let cars idle for 20min chattering on their phones :lol:...interesting nonetheless
 
Jody Treadway said:
You are confusing a long crank time due to torn and/or orings in the filter/regulator assembly with fuel boiling in the lines due to heat.
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i never saw this the first time you posted...only saw the "your wrong" lol. Had I seen this my answer would've been fuel pump is worn as I told him already last weekend since 1. it's original, 2. they are very prone to wearing out causing long cranks or priming needed on startup to compensate for worn out pump.
I knew you had to worked at the Jeep stealership for some time to know so much about them LOL. I don't claim to know everything and learn new stuff from the "dealership techs" all the time but have lots and lots of experience with them so I usually give experiences that I've dealt with.
If his jeep runs fine on the street though, i assume it idles fine too when sitting at lights in hot days of traffic, that alone makes me rule out heat soak issue as he says sloshing around the trails and it acts up.....that to me sounds like fuel pump, fuel pump mounting, venting issue, regulator issue, either way something to do with the pump itself since it's the original, long cranks already a symptom of worn pump so back to the OP original issue, pump is what I'm leaning towards.
 
Jody Treadway said:
(whereas I only had 14 of mine being foreman at a Jeep dealer),
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ha ha ha not fair, you have more legit "tech" "book" smarts:beer: with under hood time from working there as I have mostly all everyday common experiences and whatever Haynes, Chiltons and Google has taught me along with long years of just being under the hood dealing with issues!!:D
 
marty79 said:
i never saw this the first time you posted...only saw the "your wrong" lol. Had I seen this my answer would've been fuel pump is worn as I told him already last weekend since 1. it's original, 2. they are very prone to wearing out causing long cranks or priming needed on startup to compensate for worn out pump.
I knew you had to worked at the Jeep stealership for some time to know so much about them LOL. I don't claim to know everything and learn new stuff from the "dealership techs" all the time but have lots and lots of experience with them so I usually give experiences that I've dealt with.
If his jeep runs fine on the street though, i assume it idles fine too when sitting at lights in hot days of traffic, that alone makes me rule out heat soak issue as he says sloshing around the trails and it acts up.....that to me sounds like fuel pump, fuel pump mounting, venting issue, regulator issue, either way something to do with the pump itself since it's the original, long cranks already a symptom of worn pump so back to the OP original issue, pump is what I'm leaning towards.
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Explain how he can bleed the air out and it runs fine? How did the air get there in the first place?
You have to diagnose based on what this one particular vehicle is doing, not based on what has been seen in the past.
 
Jody Treadway said:
Explain how he can bleed the air out and it runs fine? How did the air get there in the first place?
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Good Question....I don't know. That's why I'm on this thread to learn about this new "heat soak" issue he's having as this is how I've learned all my new knowledge and stuff. I was suspecting something with tank/pump/venting but I don't know anything else beside that without looking at it and running tests in my physical presence!
 
and you dont know if your past "fixes" solved the problem or just removed the symptoms...
 
Ron said:
and you dont know if your past "fixes" solved the problem or just removed the symptoms...
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ok I'm lost. What's the difference? solving the problem and removing the symptom seems to be the same result...No more running/driving issues so unless I'm really missing something here apparently lol I would like to know how removing the symptoms isn't the same thing as solving the problem. If you remove the symptom of "fuel boiling" lets say by replacing a fuel pump/regulator assembly and never returns, how is that any different than solving the problem.
 
A couple of incorrect statements here:

1. Both of our TJs will suffer from heat soak after restart from being run hard. Rod them out a couple of times and they're fine.
2. The TJ fuel lines run on the opposite side of the frame from 90% of the exhaust (including both cats), so they're not as susceptible to the vapor lock issues as XJs are. They also have more room around the motors in general and more places for air to escape.

marty79 said:
i assume it idles fine too when sitting at lights in hot days of traffic
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That word will get you in trouble. Stick to what is known.

A bad pump can cause vapor lock. BTDT. But it's a long way from there to "XJs don't actually have overheating issues".
 
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