Discussion in 'Stock 1991 - 1994 Explorers' started by John Dozer, March 8, 2011.
1991 Navajo with a vapor lock problem and we don't know why?
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put a wooden clothespin on the fuel line towards the firewall on the driverside
With a pressurized fuel system, it is very difficult to have a vapor lock problem in the traditional sense. 1st thing I might do would be to check the fuel pressure and see if the fuel system is pressurizing like it should.
Not sure if this is at all similar to my 94 Ford Ranger, but I was having an issue I thought was vapor lock and turned out being a bad Fuel Relay in the fuse box.
Issue: Drive 20min engine get warm stop for gas and cut engine off. Fill vehicle then it wouldn't start. Let it sit for 3 or 4 more minutes and would start right up like nothings wrong. Swapped fuel relay with A/C relay when it did its wouldn't start issue and it cranked right up, replaced Relay and haven't had the issue since.
Vapor lock is defined as the fuel vaporizing (usually due to heat) while still in the fuel supply system. Your fuel pump is submerged in the fuel tank. It is extremely unlikely that the fuel is vaporizing before the pump or anywhere in the fuel line prior to the injectors. Vapor lock was basically eliminated when fuel supply systems switched from low pressure to high pressure for fuel injection.
If your fuel pressure is not adequate then the problem is probably due to a defective pump, a clogged strainer in the tank, a clogged fuel filter, a defective fuel pressure regulator or an electrical problem.
I thank you all for your suggestions, but its my fault that I made you all in thinking we are using the stock replacement. We had a replacement fuel pump when I first bought the 1991 Mazda Navajo LX 4.0L which is one of the "X" model. Now here is what we did after doing the research on fuel pumps. This is what we bought.
Specs: Free Flow Rate 43 gph
Maximum Pressure (psi) 85 psi
Inlet Size 5/16 in.
Inlet Quantity One
Inlet Attachment Nipple
Outlet Size 5/16 in.
Outlet Quantity One
Outlet Attachment Male threads
Quantity Sold individually.
Notes Fuel pump overall length is 7.250 and diameter is 1.812.
Multi-port EFI systems need a stable fuel supply at any rpm, and Summit® high-pressure, high-flow electric fuel pumps supply it. They feature a free-flow rate of 43 gph at up to 85 psi. Summit® high-flow pumps also have 5/16 in. inlet and outlets, brass stud terminals for secure connections, and two included cushioned clamps for inline mounting. They're ideal as stand-alone pumps for multi-port EFI systems on engines making up to 500 hp, or as boosters for nitrous applications. Mounting bolts are included.
We mounted it horizontal ahead of the fuel tank using the stock steel lines to supply the fuel. We have a fuel pressure reading at 40 psi at the beginning of the start and and drive for about 10-20 minutes then looses the pressure completely. Even with the pump still running. Does anyone have theory my mechanic and I are both confused on this little problem?
To make sure I understand, you are using an external fuel pump instead of of the stock in tank pump? FWIW, Ford used to put a dual pump system in the mid-'80's Ranger/BII's with the 2.9 L engine -- a low pressure pump inside the tank to feed the high pressure pump mounted on the frame rail. What sometimes would happen is the in tank pump would fail and the external high pressure pump wasn't getting enough fuel to feed the engine. If this were a similar dual pump system, then I would suggest looking at the in-tank pump to see what it is doing. What mechanism (if any) are you using to get gas out of the tank to the high pressure pump? If you are using some other mechanism to feed gas to the external pump, I'd probably suggest looking at that mechanism.
We have know about the dual pump system and are currently only using the external high pressure pump's natural draw.
The pickup line is the orginal 3/8, but somewhere between the top of the fuel tank and the filter it changes to 5/16. On the new pump the inlet is 3/8, and the outlet is 5/16. My question is, when I use the orginal line to connect the pump, I had to down size to the 5/16 to connect it to the pump. Would that cause the pump to starve for fuel?
I guess that is the real question with that kind of set up -- does an external pump have sufficient draw to feed itself. I assume the reason Ford used a dual pump set up is because they did not trust the external pump to feed itself.
Have you asked Summit if they expect the pump to be able to draw gas out of the tank without any assistance?
yes the pump can draw on a empty line. I have completly replaced the line from the tank to the pump with 3/8 steel line. It has improved a bit but it still has a vapor lock condition after it gets hot. As long as the engine is cold the pressure @ the rail is 40 psi. Once it get's to op-temp the pressure drops to a steady 20 psi at idle then will drop off sharply as the throttle is applied. I shut the engine off and relieve the pressure at the guage and only vapor comes out very little liquid fuel. I leave it sit for about 20 min. and check the guage and have about 8 psi of pressure, relieve guage and no fuel just vapor.
I am the mechanic tring to figure this navajo out. I have had 87 Ranger with a 3.8L super-coupe, a supercharged 91 xlt 4.0L with the same fuel line, pump setup as the Navajo. And a 4.0L 87 bronco II, and a bone stock 94 xlt.
How much fuel is in the tank? Any suction pump is notorious for creating a negative pressure and boiling the liquid, (vapor lock). What about the temperature of the pump? Is the pump not flowing enough liquid to keep it cool? Pumps use the liquid they are flowing to keep the motor cool. It sounds like the pump you are using is to big.
The pump is cool to the touch, the inlet is cool and the outlet is about the same. Without a temp meter I truly can't tell much of a difference. The specs "Multi-port EFI systems need stable fuel supply at any rpm, and Summit's High-Flow electric fuel pump supplies it. With a clog-resistant roller vane design, our 7 5/8 L x 1 3/4 diameter aluminum inline external pump has a free-flow rate of 43 gph (258 lbs./hr.) at 40 psi. that makes it an ideal standalone pump for multi-port EFI systems on engines making up to 500 hp."
I used this same pump on a 91 xlt about 10 years ago with stock injectors, fuel rail, and comuter. With no problems at all. With out the supercharger and then later with the supercharger the only difference I remember is that then I had wired the pump to a direct 12v power supply with a switch to turn it on and off. The only thing the key did in that xlt was unlock the wheel.
10-years is a long time for injectors and for pumps to stay the same.
Pump flow rates can be estimated by voltage draw. You could check the voltage on the pump before and during to cross the pump off.
How about how is the gas tank vented properly.
Dont flame me,but you have rulled out heat, the fuel line is close to to the exhaust manifold.
More Info Please.
Hi Will, I sent you a PM but thought I would try this as well. I could use your help/advice on the exact same issue I am experiencing with the wife's 94 Ranger. I need to pick your brain a little though .... if you could message me back I would greatly appreciate it.
Talk about a delayed reaction......sorry for not updating this post.
Anyway.... a huge Thank You goes out to wicsmith1997 for his suggestions and the absolute correct solution to my 94 Rangers issues. The fuel system relay in the fuse block under the hood was exactly the problem. After changing that out with a freebe from the wreckers, problem is gone!
Thanks again to everyone for all the help....this site ROCKS!