Vapor Lock Issues

adman02

Well-Known Member
Joined
May 15, 2009
Location
Durham, NC
I have a 4.0 from a 92 wrangler swapped into my CJ. Went wheeling at Wheelin' 4 Hope this weekend in Arrington, VA. On the way up the jeep sputtered and stalled many times on the highway. Each time, I opened the hood and waited about 10 minutes and the jeep fired up and ran normal. Replaced fuel filter, battery and coil but the problem persisted. Only seems to happen in hot weather. I also have an 8274 so there's not a lot of air flow.

Could this be the heat soak/vapor lock issue that affects some 4.0s? If so, what's a good fix (I only see fixes for later model - OBD2 - 4.0s). Also, I don't have a fan shroud but plan to put one on now.

Any other thoughts, ideas or advice? I hate being 'that guy' with the jeep that keeps breaking down.
 
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Ckp, cmp, rotor button, fuel pump... See what you got when it won't start. Do you have spark? Injector pulse? Fuel pressure? You could also hit various components with cold when it won't start and if it does start, you know the problem.
 
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IMO vapor lock is more likely to occur after shutting down when hot not driving. Try a fuel pressure gauge with long enough hose to hang gauge on windshield and drive it. If pressure checks out look at electrics
 
Make sure you don't have an exhaust leak blowing on the fuel line. When I had my YJ with a Chevy Vortec in it, I experienced vapor lock when the exhaust separated at the donut, blowing hot exhaust on the fuel line. Once I got the exhaust fixed, and put a heat shield and header tape on the fuel line, I never had the problem again.
 
I think it would be the other way around. Lower pressure and higher temps boil the fuel ( vaporization). When fuel is hot and pump is off pressure can drop enough to allow vaporization. Motor does not run good or not at all with vapor mixed in liquid fuel. I think your problem is with ignition or fuel supply
 
Crank position sensor.

4.0 vapor lock had only happened to me if I shut it off when everything is heat soaked. This boils the fuel in the rail. Takes a few cycles to flush rail with fresh fuel and then it's fine
 
a_kelley said:
Crankshaft position (sensor)
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Thanks for clarifying.

VortecJeep said:
Make sure you don't have an exhaust leak blowing on the fuel line. When I had my YJ with a Chevy Vortec in it, I experienced vapor lock when the exhaust separated at the donut, blowing hot exhaust on the fuel line. Once I got the exhaust fixed, and put a heat shield and header tape on the fuel line, I never had the problem again.
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How'd you guess!? I do have an exhaust leak, but it's not blowing directly on the fuel lines though exhaust and fuel intake are all on the same side, so I'm sure it's not helping things.

moldman05 said:
I think it would be the other way around. Lower pressure and higher temps boil the fuel ( vaporization). When fuel is hot and pump is off pressure can drop enough to allow vaporization. Motor does not run good or not at all with vapor mixed in liquid fuel. I think your problem is with ignition or fuel supply
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I'm thinking fuel supply

Mac5005 said:
Crank position sensor.

4.0 vapor lock had only happened to me if I shut it off when everything is heat soaked. This boils the fuel in the rail. Takes a few cycles to flush rail with fresh fuel and then it's fine
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That's how I've traditionally understood vapor lock (fuel boils over after shut down making it hard to start). But, there was another CJ with the exact same set up as me at the event and he had the same issue. And a third guy running a 258 with howell FI and he had the same issue – stalls out while running and hard starting/no starting until you let the engine cool a few minutes (all three of us ended up running the trails with hoods removed). What are the odds of all three of us having bad CPSs?
 
Well, I made some changes and the issue disappeared. I rerouted the return line to the passenger side and swapped out my fuel pump for an E2000. Just spent the weekend in VA (driving there, driving trails and driving back) and no issues whatsoever. So, it was either the pump or the lines (my money is on the lines). thanks for your help!
 
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