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Oil pressure switch for swaped-in 3.4

85Chief

NAXJA Forum User
Im purchacing a 3.4l engine to put in my 85 Cherokee. I am told that I need to install an oil pressure switch so when you lose it, the fuel pump shuts off and it will protect your engine. With that in mind, I should install a momentary by-pass switch in case you run out of gas. That way I get fuel to the carb.

Why is this? How difficult is it to install this switch. Can anyone give me detailed instructions to preform this.

Thanks a lot!
85Chief
 
Who told you that you "need" this? It's a nice idea, but a great many manufacturers have sold many millions of cars and trucks that didn't have it, and the number of engines lost due to low oil pressure is rather small.

I also don't understand your question about a by-pass. If you're out of gas, how are you going to get fuel to the carb by by-passing an oil pressure cut-off? Doesn't make sense to me. I'm sure you have something in mind, but I don't think you're explaining the idea very well. Either that or I overdosed on stupid pills this morning.
 
It was pabloconrad who said this. This is a list of things he told me to make sure I did, and I knew about all of them accept the one I mentioned (which I didn't really make clear as to what I was asking)

pabloconrad said--
""Several things to consider when upgrading to the 3.4L:
1. If you go with the Weber Carb, you need to get an electric fuel pump that puts out no more than 3.5 PSI. If you can't find one, then buy a Holley fuel pressure regulator with by-pass, or one with two outlets.
2. You need to install an oil pressure switch so when you lose it, the fuel pump shuts off and you protect your engine. With that in mind, you should install a momentary by-pass switch in case you run out of gas. That way you get fuel to the carb.
3. Your flex plate off the 2.8L needs to neutrally balanced as the 3.4L is balanced on the crankshaft and the 3.4L is a three hole rather than four. I just bought a new one for the 2.8 as the teeth were wearing out and had it neutrally balanced.
4. Although many people say the oil pump and the oil pan should be swapped from the 2.8 to the 3.4, they are identical and it is a waste of time. Keep as spares, especially the oil pan. You never know when you're going to hit that too tall rock.
5. Replace your motor mounts, belts, hoses, and clean the inside of your bell housing while you have a chance. Double check everything else as you tear it down.""


#4 is what I was wondering about. I don't understand why I would need it.

Eagle-"If you're out of gas, how are you going to get fuel to the carb by by-passing an oil pressure cut-off? Doesn't make sense to me. "

I was wondering the same thing...
 
Yodels!

Here's the reason why you need an oil pressure switch:
If you somehow lose oil pressure, the switch opens and shuts the fuel pump off. As Eagle said, the number of engines lost due to low oil pressure is rather small. But why take that chance. You just invested mucho dinero in a new engine. Protect it.

I already had this problem. The shaft that drives the oil pump was a tad short and about a week later, the shaft fell enough to disconnect from the distributor and stop turning the oil pump. I lost my oil pressure, opening the switch and shuting off the fuel pump, hence, saving the engine.

The need for a momentary by-pass switch in parallel with the oil pressure switch is that if you run out of gas, you can hit the switch without running the engine and fill the carb. Saves your battery and the cost is minimal. About 5 bucks for the wire and in-dash switch.

The whole system is easy to do. I've drawn a schematic and can e-mail to you in j-peg format. PM me and let me know what it is.

ciao for now

pablo
 
pabloconrad said:
The need for a momentary by-pass switch in parallel with the oil pressure switch is that if you run out of gas, you can hit the switch without running the engine and fill the carb. Saves your battery and the cost is minimal. About 5 bucks for the wire and in-dash switch.
Aha! I see.

You're not talking about magically forcing gas out of an empty tank with a secret switch. What you're talking about is a way to re-prime the float bowl in the carb after you have refilled the empty gas tank.

Makes sense. Like the other switch, not necessary but nice to have.
 
What he also mean by putting a oil pressure switch will save your engine is this:

lets say you have a manual tranny and you stall your engine, the oil pressure will go down and shut off you fuel pump and prevent getting to much fuel in your carb and flood your engine! I know there's a overflow on the carb but don't forget that a mechanical pump won't pump if the engine is not running and if your electric pump continue to pump at full speed there might be to much fuel to go true that overflow fuel line. that mean flooding your engine which will come to getting fuel in the oil pan which will dilute your oil. you'll still have oil pressure but will still wear your engine!

I'm starting pretty soon on that swap and I tought to that point. also the idea of a by-pass in case you run out of gas is damn good. don't have to turn the engine to get gas!! thanks!

hope this will help somebody!
 
Gomer xj said:
lets say you have a manual tranny and you stall your engine, the oil pressure will go down and shut off you fuel pump and prevent getting to much fuel in your carb and flood your engine! I know there's a overflow on the carb but don't forget that a mechanical pump won't pump if the engine is not running and if your electric pump continue to pump at full speed there might be to much fuel to go true that overflow fuel line. that mean flooding your engine which will come to getting fuel in the oil pan which will dilute your oil. you'll still have oil pressure but will still wear your engine!
This cannot happen if your carb and fuel pump function correctly. The carb has a needle valve actuated by the float that shuts the inlet when the level is at the correct height. The electric fuel pump for a carbureted system only puts out 7 psi, and has an internal regulator. 7 psi is not enough to force fuel past the needle valve at the float bowl inlet.

I used to restore Hudsons, and mechanical fuel pumps for old Hudsons were in short supply so I often set them up with Stewart-Warner electric fuel pumps. By your logic I should have been flooding the engine and washing down the cylinder walls every time I turned the key to start the engine. Didn't happen.
 
Eagle:

so basically I can actually wire the fuel pump direct so as soon as i turn the switch the pump can start pumping and don't have to worry with it?? If so it's a lot less assle to worry about. I'll start that swap as soon as I can put my hands on some extra $$$ since I've put a rod through the side of the block of my 2.8!!!
 
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