Jump to content

87 4.0 fuel pressure? need conformation NEW QUESTION!

Recommended Posts

ok, my truck has been running (barely) like crap. it is un-driveable and because of it I was not able to get it inspected in march. I have been trying to get this fixed without much luck.


well testing the fuel pressure I'm getting about 23 psi, I have been told that this is ok. I also have been told I should have about 40 psi and the 88 FSM I have says it should be about 31-39 psi. I have tested the pressure at the rail and I also tested it between the fuel pump and the fuel filter. same reading so it is not a clogged filter to kinked line.


I am just looking to see if anyone with a 4.0 (87-90) can verify what their trucks are running at for fuel pressure.


I am hoping it is just a bad new pump,

here is my other post just for reference



any help would be awesome, I just want to drive my truck. I have only been able to drive it a hand full of times since it has been legal to go down the road.



Link to comment
Share on other sites

What........you didn't want to add to the other 3 pages :dunno:



The fuel pressure at the rail for a 4.0 should be 31 psi with the vacuum line connect, and 39 with the vacuum line disconnect from the pressure regulator.


I don't know where your reading that 23 psi is with in range :hmm:


And the range you wrote......."the 88 FSM I have says it should be about 31-39 psi."


You reading that wrong :dunno: is should say 31 AND 39 psi, like I explained above^


If your only reading 23 psi, you have a problem. Before you run out and buy a new fuel pump, pull the sending unit out, a lot of guys have found the problem with the small short piece of 1/4" hose inside the tank, from the pump to the metal port, this cracks and leaks after time......like 20+ years :roll:


Also, it could just be the pump sock in the tank that's plugged up and not letting the fuel flow, it's worth check out first.


Also, that give you a chance to double check to see if you have the Bosch pump in that rig, or the Walbro pump.


There are a couple write up on the DIY forum on how to remove the sending unit with out dropping the tank ;)

Link to comment
Share on other sites

I swear I have to worst luck with typing long @$$ well thought out post. I get them all typed out and something @#$%s up and I loose everything I just wrote. :mad: :mad: :mad: ::copy and past from the FSM had a character the forum did not like so that is why I lost it all :headpop: ::


now for the abridged version:


new post because I just wanted to know the fuel pressure of an 87-90 4.0.


I also posted on jeepforum and after just now re-reading realized I misunderstood about acceptable PSI readings.


copy and past from FSM:

"Based upon a manifold vacuum signal, pressure

regulator maintains a constant fuel pressure in fuel system of

approximately 31-39 psi (2.1-2.7 kg/cm) by allowing excess fuel to

return to fuel tank."


I was having the same problem before winter we replaced the pump and I drove it home, then it ran like crap again. figured it was another issue because its a new pump. just the other day I got around to pulling the sender and the sock was not attached to the pump. we also fixed the sender before winter because the tubes were no longer attached to the plate and that allowed the assembly inside the tank to move and hit the bottom probably knocking the sock off. we had JB welded them but that did not hold so I brazed them together this time and put a zip tie on the sock to hold it. put the whole sender back in and then I tested again to get the current 23 psi.


so now that I know that I need more pressure I will pull the sender again and either have this pump warrantied or get a new one.

Link to comment
Share on other sites

Can't help you on the 'puter issues :hateputers:


But, at one time, I too would loose a long type, turned out the be the router I had, it would micro-fizz and I would lose everything :headpop:


Yes, it could be your aftermarket fuel pump, I think mvusse had this issue a couple years ago, and went thru several pumps to get the 'working' correct pump.


Try another pump, take it in for warranty, and it should be the Bosch pump, that was the most common for the MJ fuel tanks.


Like I wrote, and what's a fact, you should be reading 31psi at the fuel rail, not 23psi.


The only thing that comes to mind, is the fuel pump is not working correct, there's a pinched line some wheres, or a blocked line, Or.......your injectors are not holding pressure. But, you said you checked the pressure before the frame fuel filter......correct??


So........what answers are you getting on Jeepforum??? Any thing close to what we're saying????


Man.......I don't have the time to keep up with this forum, let alone to keep up with other forums.......and we all know CC forum is the best :yes:

Link to comment
Share on other sites

that time my computer issue was the forum not liking a character from the cut and paste. usually I use IeSpell check and for some reason when its done and I'm editing and of course the second I am hitting the backspace button which is also a hot key for going back in the web browser it de-selects the typing field and goes to just the browser so instead of backspacing a few letters that I had typed it goes back a page a I loose everything. I know I should use firefox or something that doesn't suck but I am just so used to IE.


as for the fuel pump I'm pretty sure it was an Airtex I got. I believe I can get it warranted and hopefully a new one will work correctly.


at this point I'm 99% sure it is a bad fuel pump. I did test the pressure between the pump and the filter and the reading was 23psi. I wanted to eliminate a clogged filter as the culprit.


Jeep forum was a bit of help. but it did not help that I put in a new fuel pump. I was figuring that because the pump was new it should be working correctly. and after a bunch of testing and getting really frustrated I am now at this point remembering to never assume things are working just because they are new.



and CC is definitely the best, I cannot get over the amount of flaming, putting down and just downright disrespect I see on other forums. I am glad we have such a great community.

Link to comment
Share on other sites

Yes, I'll agree, a new part should work perfect.


That's why we all use that word AssUme.


:typing: Firefox all the way :D I've never used IE, I've been on Mozilla/Firefox for the past 14 years and don't plan on changing now. Plus.......I really need the spell correct :D


Well.......your last statement is one of the reasons I don't make time to visit other Forums :yes:


I think you narrow it down to a brand spanking new defective fuel pump. But I would still pull it and check to make sure the sock filter is not plugged up, and that the soft line is not kinked or off the supply line.

Link to comment
Share on other sites

I did pull the whole unit out today, I also had it out a few days ago and everything looked ok. all connections were fine and the sock was ok. gave the "new" pump to by brother in law so he can take it to work and they can get the warranty from wherever they got it from.

Link to comment
Share on other sites

ok new question. what would cause low fuel pressure? I am pretty sure this is an issue between the tank and the filter. I had tested my fuel pressure between the pump and the filter and got 23psi. I have a new NEW pump (2nd new pump) and it did not solve this issue. I have verified that the fuel pressure tester is working correctly. what would cause an issues with the pump not putting out enough pressure. if the lines were clogged it would have a high pressure reading. pinching my return line did nothing to the pressure. it seams like the system is not getting enough fuel to the rail. it will idle around 500 rpms and run rough and then die.

I have tried running a wire from the fuse box to the + pump wire, I also ran another ground to the - pump wire.



pump including sock (x2)

filter (x2)

fuel pressure regulator (swapped from junk yard for testing purposes)

EDIT:: No ballast resistor


what would cause the pump to not be able to get enough fuel up to the rail?

Link to comment
Share on other sites

You back again :eek:


:hmm: So the new fuel pump didn't get the pressure above 23psi???


And you ran a jumper from the fuse box to the pump, with an extra ground.........so that checks that the pump is getting 12VDC.


And, you checked the fuel pressure before the frame filter.........correct??


If a line is pinched or clogged, it would not increase the pressure, it would reduce "flow" and drop the pressure at the fuel rail.


Only thing I can think of is the rubber fuel lines from the sending unit, to the hard lines, and the rubber lines on both sides of the frame filter. If you replace the lines, make sure you get some high pressure fuel line.


There is a chance that the soft (rubber) lines have internally collapse, or swelled up and are causing a blockage, after all.........they are 20+ years old.


When you replaced the 2nd fuel pump.........did you reach in and clean the bottom of the tank??? How was the sock filter on pump #1??? Clean??? Dirty???


Also, if your getting around 40 psi off the supply line before the frame filter, then you narrowed it down some.........now you need to check the supply line from the front of the filter, up to the fuel rail. Take a good look at the hard fuel line where it's mounted on the frame under the drivers floor board, see if there any major corrosion on it, exterior corrosion and cause a internal blockage.


Last :idea: I have is the pull the fuel lines off the rail, and replace the quick connectors/o-rings. A part of the o-ring could have worked loose, and plugged the in-put of the fuel rail.


Part of what you wrote is........when you pinched the return line, and the pressure didn't change.........then there has to be a bad spot in the supply line, because when you pinch the return line, the pressure should shoot right up, your not getting the pressure, 31psi or the flow that you need.


Hate for you going thru all this work.......like changing out the fuel pump.......again, but your there, we're here, and can't just check this and check that.


Also, from what you all wrote before, it sure sounded like the "new defected fuel pump"


Just for S&G's, list the make and model number of the pump you installed, both :brows:

Link to comment
Share on other sites

I have tested at the schrader valve and got 23psi, I also put the T fitting on the tester and placed it between the pump output no the sender and the soft line that is usually connected to the sender unit and still got 23 psi. was with the old fuel pump, replaced the pump and it had the same symptoms pointing to not enough fuel pressure. I have not tested it again but I am 90% sure it will test the same. (I know that 10% is still there and I do plan on checking it again.)


with that test it would tell me that my issue is still in the pump/sender area. 2 new pumps from different manufactures (looked like a Bosch for new #1 and new #2 is a Carter not sure on model number I will try to get that.). both pumps came with new soft elbows for connecting the pump to the sender inside the tank. the lines on the sender are clear. the sock looked ok when I took it out and on the second pump it began acting like crap almost immediately so I doubt it is getting clogged that fast.


I am 90% sure that my issue is in the tank/sender/pump somehow. I am not getting the correct pressure from the pump which tells me that everything after that (lines soft and hard, filter, rail, regulator, return line) is ok or at least not causing the low pressure issue.


is there anything with the physical tank that would cause this issue? like a bad vent thing or something crazy that I cannot think of?


I am at my wits end and I am running out of niceness with this truck.

Link to comment
Share on other sites

so dumb. :doh:


a friend of mine that works at a shop put the first fuel pump on. and I put on the second the same way he did the first. the issue was that we (he at first and the myself copying his install) did not put hose clamps on the rubber line elbow that connects the pump to the sender inside the tank. the result was that fuel squirted out the side of the hose thus reducing the fuel pressure. I put the clamps on and it runs fine now.


I am extremely happy that my truck is running right and I feel extremely stupid that the problem was caused by such a stupid issue.


thank you for all the help with everthing.



Link to comment
Share on other sites

So.........it turned out to be a simple fix???


Ah, yea, the mini worm clamps are needed on the flex line from the pump to the sending unit output.


Well.........glad you got it fixed :D


I bet the truck runs alot better now :yes:


Just one of them 'ole K.I.S.S. things :doh:

Link to comment
Share on other sites

yea, it kept a nice idle, didn't stall out and didn't hesitate. now all I have to do is replace the clamp that I robbed from the fuel filter (it pushed off the line when I took it for a test drive). and then I have to get it inspected before the end of March... yea so I have to figure out a way to get it to my friends shop without getting arrested for no inspection.


if only I had know this last year it would have saved me A LOT of frustration and I could be driving it around now.

Link to comment
Share on other sites

  • 9 years later...

I want to tell you that this post saved me! Thank you! I’ve been fighting with low psi for the last week and could not figure it out!! I pulled the pump assembly and low and behold, that same 1/4” hose was loose. I tightened it up, put everything back together, and now I’ve got perfect psi! 31 and 39 when I pull the vacuum off. Hallelujah! 

Link to comment
Share on other sites



Here's another tip regarding fuel pumps:



The fuel pump and fuel tank sending unit ground at a sheet metal screw up behind the spare tire on an XJ, XJ-Body-Ground.jpg?resize=300%2C162and behind the driver’s taillamp on an MJ.

Not only is a sheet metal screw a lousy way to ground things, this ground path is long and travels through some connectors that are prone to corrosion and moisture.

Locate the black wire on  the HARNESS side of the fuel pump/sender 3 wire connector. Remove a 6″ length of the split loom covering. Strip back about 1/2″ of insulation from the BLACK wire.1-fuel pump ground MJ

Take your new ground wire, preferably at least 14 gauge and 12 to 18 inches long as needed, strip it about 3/4″, and wrap it around the exposed part of the harness plug wire.1-fuel pump ground mj-002Solder the connection. Tape it up and reinstall the split loom covering.1-fuel pump ground mj-003

At the other end of your new ground wire, add a crimp on eyelet. Attach the eyelet under a bolt that goes directly into the chassis. Be sure to clean the attaching point til shiny and apply OxGard to the contact surfaces.1-fuel pump ground mj-004

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...