Vapor Lock Issues on 4.0/CJ

adman02

Well-Known Member
Joined
May 15, 2009
Location
Durham, NC
I have a 79 CJ7 with a 4.0 (from 93 wrangler; no idea of mileage) swapped in. I've been dealing with a vapor lock issue and can't figure out how to eliminate it (I'd rather not do hood louvers).

A little background - Was driving to a trail ride in VA in April. While driving on the highway in the heat of the afternoon, the engine would cough, sputter, lose power and stall. It wouldn't immediately restart. After waiting 10-15 minutes with the hood open it would restart. Resumed driving and in about 30 mins, the same thing would happen.

On that trip, I replaced...
• Fuel filter (have an electronic pump mounted on the frame under passenger floorboard)
• Battery
• Coil
No change.

After that trip, I added heat shielding to the injectors and the fuel rail. Was driving around town this weekend (in the 100 degree heat) and it started to do the same thing, but not as bad - loss of power, some sputtering, but no stall.

I don't have a fan shroud, but am looking at getting one (I have an 8274 mounted up front and it damn-near blocks the entire radiator opening). I'm now wondering if I should replace the water pump and the mechanical fan clutch.

Before I throw more money at this problem, has anyone encountered this? Any ideas on what to do next?
 
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Sounds like a typical 4.0L. Your injectors probably number one is getting hot and vaporizing fuel. Even with the factory fixes mine will do it if I pull off and it heat soaks. It has all the insulation and stuff. I just lift my hood anytime I am sitting on the trail and not moving or spotting, etc.
 
I've read that the 2000+ intake manifold is an upgrade worth considering for slight performance gains. Any chance that manifold would help lessen heat soak/vapor lock issue?
 
Did you reroute gas lines from tank to engine or reroute exhaust? I had a problem once with the exhaust being too close to the gas lines causing a vapor lock type of condition.
 
J.C. said:
Did you reroute gas lines from tank to engine or reroute exhaust? I had a problem once with the exhaust being too close to the gas lines causing a vapor lock type of condition.
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The more research I do, the more I think this is the cause. My supply line is on the driver's side, but the vent and return lines are on the passengers side...about 3 inches away from the exhaust. I'm thinking the return line is dumping hot fuel into the tank. When the tank is low on fuel the entire contents of the tank boils and the bump can't suck it out until it cools. Seem plausible?
 
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adman02 said:
The more research I do, the more I think this is the cause. My supply line is on the driver's side, but the vent and return lines are on the passengers side...about 3 inches away from the exhaust. I'm thinking the return line is dumping hot fuel into the tank. When the tank is low on fuel the entire contents of the tank boils and the bump can't suck it out until it cools. Seem plausible?
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Maybe things were different with the '81 and older CJs (and maybe depending on engine options?), but this is the exact opposite of my Jeep. Fuel supply is on passenger side and return and vent are on drivers'.
 
orange150 said:
Maybe things were different with the '81 and older CJs (and maybe depending on engine options?), but this is the exact opposite of my Jeep. Fuel supply is on passenger side and return and vent are on drivers'.
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Doh! You're correct. My description above was wrong. It should have read exhaust, return and vent are on the driver's side; supply on the passenger's side.
 
What pump are you running, and what material are your fuel lines? I have roughly the same setup, but with a walbro 255lph pump and braid AN hoses up driver's side, probably about the same distance to exhaust, and don't have the same symptoms.
 
Fuel pump is an Airtex 8248.

I also have DEI heat shielding on all injectors and the fuel rail.

I'm going to move the vent and return lines to the pssenger side and see if that makes a difference. I'll report back with an outcome.
 
Stock 4.0 exhaust manifold looks like a header. However it is simply just a pipe manifold. Airtex pumps tend to be hit or miss sometimes you get a good one I've had bosch pumps fail as well as any other brand. Simply change it out for another one and I bet the problem will go away. Advance used to carry airTex but changed over to car quest brand, which may still come out of the same factory. I swear they only test 1/100 of a batch.. the vanes clearance is probably too loose when it gets hot and loses delivery pressure and volume.
 
Since we're on the topic...is there a pump that's a little quieter but still gets the job done for a 4.0? The 8248 seems to have a really loud whine.
 
If you do have an aftermarket header I'd check on your underhood heat. I've never had luck with them on various CJ motors - even using professionally wrapped ones from a speed shop. The temps stay so high leading to various issues.

Now headers may not be the main cause of your issue but I'm sure they aren't helping.

Post a pic so we can see if what you have is stock or aftermarket
 
rerouted the return line to the passenger side and replaced the pump with an E2000 and the problem is fixed. So it was one of the two changes that made a difference (I'm betting it was the lines).
 
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