4.0 swap fuel problems

MIKE M

New member
City
milledgeville
State
ga
#1
1983 CJ8- I changed the engine to a 4.0 to have fuel injection and my 258 was knocking. I had a problem getting the engine to start , had spark, compression and 49 psi on the rail. When I choked off the return line to the tank and forced the pressure to 65 psi it would start and run fine. I have a manual ball valve inline after the fuel rail and regulator to make the Jeep run. Any ideas why this has to run that high a pressure to operate?
 

MIKE M

New member
City
milledgeville
State
ga
#3
I have read that article and it has a lot of info, but doesn't help with my 4.0 needing 65 psi to run. But thanks for the help. My jeep runs fine and has for 8 months. I love the engine, I would like it to be right.
 

walkerhoundvm

Typical Know-it-all
LIFETIME
CJ-8.com Member
City
Cave Creek
State
AZ
#4
I admittedly have no first hand experience on the subject, but I wonder if your return line is too large?
 

tower210

Scrambler Junkie
LIFETIME
CJ-8.com Member
City
Olathe
State
KS
#5
I unfortunately know WAY too much about this subject.....

You need to know what year 4.0 you have and where the regulator is. Mine was a 94, the factory regulator is on the return side of the fuel rail. ZJ's had a regulator that happened right after the fuel tank (with the return line there); this will be a massive differnce.

I did this swap over the winter. Tried to re-use the stock return line; it would run at 80 PSI; Idle for a bit, backfire, overheat and die on the test drive.... We learned when we put the hose off the return into an unrestricted fuel jug, that it would run 32 ish at idle; increasing to 35 at WOT and purr like a kitten. Ran a new full size line ss to the tank (same as supply) and the pressure dropped to 34-36 ish. Fixed the problem.

Are you sure your pressure gauge is correct?

I have a FSM for 1994 XJ if you need pages.....
 

Kane

CJ-8 Member
CJ-8.com Member
City
Berlin
State
ct
#6
I heard (but have not verified) that you can swap the 5/16" charcoal canister vent line and use it for the return line, then use the 1/4" old return line for the charcoal canister. Since I am doing my multiport injection conversion right now I was going to see if I could do that since all my lines are only 2 years old.
 
Last edited:

spankrjs

Scrambler Junkie
LIFETIME
CJ-8.com Member
City
Biloxi
State
MS
#7
I am running a two line system, supply and return, regulator on fuel rail, on my red Scrambler. It is a 4.0 with the Hesco MPI.
I am using new, stock size, supply and return lines, stock sending unit, no problems with restriction, fuel pressure is correct.
IIRC, the fuel pressure is around 39psi at idle, with vacuum line hooked to regulator.

I wonder if your fuel pressure gauge is faulty, and you really have low pressure that doesn't get high enough until you clamp off the return line?

My green Scrambler has the single line fuel rail, Cherokee filter/regulator, it runs around 50-60psi.
 

MIKE M

New member
City
milledgeville
State
ga
#8
I haven't checked my pressure gauge. Both of my fuel lines are new and full size to and from tank. I ordered new fuel injectors because I am thinking they where not replaced during rebuild. I didn't rebuild it and don't know who did. I think the injectors my be nasty and are requiring more pressure to spray. The jeep runs great, just not starting quick and I know its not right!
 

Kane

CJ-8 Member
CJ-8.com Member
City
Berlin
State
ct
#9
I am running a two line system, supply and return, regulator on fuel rail, on my red Scrambler. It is a 4.0 with the Hesco MPI.
I am using new, stock size, supply and return lines, stock sending unit, no problems with restriction, fuel pressure is correct.
IIRC, the fuel pressure is around 39psi at idle, with vacuum line hooked to regulator.

I wonder if your fuel pressure gauge is faulty, and you really have low pressure that doesn't get high enough until you clamp off the return line?

My green Scrambler has the single line fuel rail, Cherokee filter/regulator, it runs around 50-60psi.

Stock size as in a factory CJ carb return line?
 

spankrjs

Scrambler Junkie
LIFETIME
CJ-8.com Member
City
Biloxi
State
MS
#10
Stock size as in a factory CJ carb return line?
Yes, 5/16" Supply, 1/4" Return

They are new SS lines from either InLine Tube or Classic Tube

Stock size vent line

I actually have the Supply and Return on driver side, near exhaust. Never had a problem with the 4.2, but the 4.0 manifold is closer to frame.

I will probably "upgrade" this one to a single linef rail, run the fuel line down the passenger side, like I did on my green Scrambler.
 

Kane

CJ-8 Member
CJ-8.com Member
City
Berlin
State
ct
#11
Yes, 5/16" Supply, 1/4" Return

They are new SS lines from either InLine Tube or Classic Tube

Stock size vent line

I actually have the Supply and Return on driver side, near exhaust. Never had a problem with the 4.2, but the 4.0 manifold is closer to frame.

I will probably "upgrade" this one to a single linef rail, run the fuel line down the passenger side, like I did on my green Scrambler.

Thanks. I have the regulator on the fuel rail and will just try the stock return line first to save some work. I will be running an external pump on the passenger side which I have not for the Howel kit so I just need to swap in a high pressure pump.
 
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