Fuel Tank Alterrnatives--Very Technical

[AZJeff]

Sorry for the following novel, but I have to explain why I am asking the questions:

 

The Previous Owner installed a 2001 XJ engine/trans (and instrument panel) into my MJ, and it of course it employs the late model OBD-II engine control system.  This system requires a fuel pressure of about 49psi into the fuel rail, which an original MJ fuel pump will not deliver.     To achieve this higher pressure, apparently the P.O. chose to use the late model combination fuel pump/pressure regulator/fuel sender from an XJ.  SInce this assembly won't fit into a standard metal MJ tank, so he installed some unknown plastic Mopar tank from some OTHER vehicle, which is capable of taking the late XJ pump assy.

 

This all sounds good, right?  It runs great....just don't look under the vehicle at the fuel tank, because it's RUBBING on the DRIVESHAFT!!   Worse yet, there is no way this tank can be moved over to make clearance to avoid this rubbing.  The tank has GOT to go.

 

So....I want to figure out how to use a more suitable fuel tank that will still allow some sort of fuel pump of higher pressure and the sender from the XJ type assembly.  I am pretty sure that some others have done this, but I am looking for some advice of what is the best route to take here.

[krustyballer16]

Will a dakota tank hold the xj sender?

I've always wondered, why can't a plastic xj tank be placed where the mj tank sits? (can't picture them side by side)

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[MuddFoot]

The xj tank is wider. I used a Dakota tank on my 87mj- with 95 xj swap. My fuel gauge worked with the first fuel pump and sender. Bought a new pump and sender and it doesn't work now. I now have purchased a aftermarket gauge that works just have to install fully.

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[Jeep Driver]

You won't like my answer. 

 

New MJ fuel tanks are available. 

 

Here- https://www.ebay.com/itm/NOS-86-Jeep-Comanche-Fuel-Tank-sending-unit-assy-18-23-5-gallon-tank-52003652/232057535509?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2055119.m1438.l2649

 

 

Now, run an aftermarket fuel pump, not a cheap POS, but a quality pump and regulator. 

[MuddFoot]

Sorry running a 98 Dakota tank and fuel pump.should have posted that also

The xj tank is wider. I used a Dakota tank on my 87mj- with 95 xj swap. My fuel gauge worked with the first fuel pump and sender. Bought a new pump and sender and it doesn't work now. I now have purchased a aftermarket gauge that works just have to install fully.

Sent from my Z982 using Tapatalk

Sent from my Z982 using Tapatalk

[Eagle]

Skinnier drive shaft?

[HOrnbrod]

Get rid of that kludge setup you have now and stick in a new MJ tank as above with the 91-92 fuel sender and the stock Bosch fuel pump. I have been running the late Mopar injectors with an adjustable fuel pressure regulator in my stroker at 49psi for years - the 91-92 pump handles it no problem.

[AZJeff]

1 hour ago, HOrnbrod said:

Get rid of that kludge setup you have now and stick in a new MJ tank as above with the 91-92 fuel sender and the stock Bosch fuel pump. I have been running the late Mopar injectors with an adjustable fuel pressure regulator in my stroker at 49psi for years - the 91-92 pump handles it no problem.

So the earlier pump should work with the later injectors no problem.  That's good to know.

 

Now does anyone know if the guage sender in a stock MJ tank will work with the late-style XJ gauge?  

 

I ask because the late model XJ guage does NOT get it's signal directly from sender.  On the late XJ, the sender provides a variable voltage to the ECM, which in turn provides the guage with it's signal.   The XJ sender is expected to send 0.6V at "FULL"  and up to 8.6V at "EMPTY".

 

What range of voltage comes out of the MJ sender over it's range of sweep?

[HOrnbrod]

The 91-92 Bosch fuel pump has the same pressure vs. delivery as the 00-01 pump, so it will work, not should work. The 00-01 injectors are actually smaller than the 96-99 injectors I'm running; the 96-99 injectors have more fuel delivery at the same pressure than the 00-01 injectors. I don't know what fuel rail you are running now, but you might have to use the earlier 91-95 HO rail with the regulator. You don't have to use an adjustable FPR; there are plenty of fixed GM regulators you can stick in that run at 49psi w/o vac.; I've got the p/n's somewhere. As far as the dash fuel gauge working, I'm not sure because I don't know it's resistance requirements or polarity. But there are plenty of guys over at the stroker forum running a similar setup in XJ's using the early HO fuel rails so they can use adjustable FPRs, and it's not a big problem. Or you can run an aftermarket fuel gauge matched to the in-tank HO sender too.

[Dzimm]

5 hours ago, AZJeff said:

Sorry for the following novel, but I have to explain why I am asking the questions:

 

The Previous Owner installed a 2001 XJ engine/trans (and instrument panel) into my MJ, and it of course it employs the late model OBD-II engine control system.  This system requires a fuel pressure of about 49psi into the fuel rail, which an original MJ fuel pump will not deliver.     To achieve this higher pressure, apparently the P.O. chose to use the late model combination fuel pump/pressure regulator/fuel sender from an XJ.  SInce this assembly won't fit into a standard metal MJ tank, so he installed some unknown plastic Mopar tank from some OTHER vehicle, which is capable of taking the late XJ pump assy.

 

This all sounds good, right?  It runs great....just don't look under the vehicle at the fuel tank, because it's RUBBING on the DRIVESHAFT!!   Worse yet, there is no way this tank can be moved over to make clearance to avoid this rubbing.  The tank has GOT to go.

 

So....I want to figure out how to use a more suitable fuel tank that will still allow some sort of fuel pump of higher pressure and the sender from the XJ type assembly.  I am pretty sure that some others have done this, but I am looking for some advice of what is the best route to take here.

Sounds like you have a Dakota tank under there.  If it's rubbing the driveshaft the OP didn't install it correctly.  You must dent top corner of the tank to clear the frame rail bracket and it will suck it far enough away from the driveshaft that you will have no issues.  Super easy to do and will cost you $0 and about an hour of your time.  Post up a picture to confirm what tank it has. 

 

All the info for the Dakota tank swap is in my build thread linked in my signature (fuel tank info is on the first page in the middle of the first post).  I'm running a 98 Dodge Dakota 22.5 gallon fuel tank and sender in my truck and it works great, and yes it clears the driveshaft by about 3 inches.  Bought a Mopar adapter to plug the Dakota sender into the factory Jeep wiring and works with the 97+ gauge cluster.

[DirtyComanche]

I'm lost on why the original MJ pump wouldn't make 49psi, unless he had a carbed or TBI truck.

[shelbyluvv]

I have a TJ tank under my 97 converted MJ. As far as I know I'm the only one who has used a TJ tank.

[cruiser54]

The stock fuel pump makes plenty of pressure. It's the fuel pressure regulator that keeps the pressure correct.

[HOrnbrod]

1 hour ago, cruiser54 said:

The stock fuel pump makes plenty of pressure. It's the fuel pressure regulator that keeps the pressure correct.

.

[HOrnbrod]

13 hours ago, Jeep Driver said:

You won't like my answer. 

 

New MJ fuel tanks are available. 

 

Here- https://www.ebay.com/itm/NOS-86-Jeep-Comanche-Fuel-Tank-sending-unit-assy-18-23-5-gallon-tank-52003652/232057535509?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2055119.m1438.l2649

 

If the p/n given, 52003652, is correct in the Ebay ad, this is for the Renix 23.5 gal. sending unit, not the tank.

 

However, there are still plenty of decent aftermarket MJ tanks still available in all sizes for not much money. Can't understand why anyone would swap in something different..  

[AZJeff]

9 hours ago, DirtyComanche said:

I'm lost on why the original MJ pump wouldn't make 49psi, unless he had a carbed or TBI truck.

FSM says the spec. for the MJ pump is 31-39 psi.   

[AZJeff]

Just now, AZJeff said:

FSM says the spec. for the MJ MPI pump is 31-39 psi.   

 

[cruiser54]

That's what the fuel pressure regulator  regulates it to. The pump will make in excess of 100 PSI!!!!

[AZJeff]

9 hours ago, cruiser54 said:

That's what the fuel pressure regulates it to. The pump will make in excess of 100 PSI!!!!

That makes sense.  It's odd that the later injectors are fed higher nominal pressure than are the older ones, but an aftermarket regulator should give me the pressure I need without resorting to the old vacuum controlled regulator rail.

[DirtyComanche]

1 hour ago, AZJeff said:

That makes sense.  It's odd that the later injectors are fed higher nominal pressure than are the older ones, but an aftermarket regulator should give me the pressure I need without resorting to the old vacuum controlled regulator rail.

 

Higher pressure allows for them to force the fuel charge in over a shorter duration, which makes more power and keeps the emissions happy at higher RPM.

[HOrnbrod]

13 hours ago, DirtyComanche said:

 

Higher pressure allows for them to force the fuel charge in over a shorter duration, which makes more power and keeps the emissions happy at higher RPM.

 

Everything is geared to air/fuel ratio to the cylinders to best satisfy the emissions regs in fashion at the time of vehicle manufacture. The cheapest way for the manufacturers of fuel injected vehicles to comply is to change the fuel pressure first, then if more change is needed, increase/decrease injector flow and set the A/F ratio that best satisfies the emissions regs.

 

 

[Dzimm]

9 hours ago, saveevryjp1998 said:

Its my understanding that people use dakota tanks since they use the same fuel sending units as 97+ xj so the conversion is an easy plug and play for sender and guage for the swap. I have pulled them from yards for a few people who have done this and I have 1 sitting at my house for a member that I have had for near a year and need to get it to him in southeastern Ky. I hope he has started his project again. Kinda slipped away from him over the cold months. I can post a pic up in morning. There are a few different size tanks. I would run a small tank setup(what I will post in the pics tomorrow for people) from what I have pulled and learned from others. The large tank would be a p.i.a. and impossible to fit in a swb mj. You could run a dual tank setup with the smaller size I imagine. I could be wrong but, I thought there may even be a couple of different size smaller tanks depending on the year of Dakota donor too.

That's exactly why I did it, direct plug and play but I used a Dakota pump/sender with an adapter so the pickup would reach the bottom of the tank, it sends the same fuel pressure and signal as the XJ sender, just a different plug.  The XJ pump assembly is too short and leaves a couple gallons at the bottom of the tank from what I've read.  The larger 22.5gal Dakota tank does fit under the swb, all you need to do is bend a small section of the tank in to clear a frame brace and it will end up with a couple inches of clearance from the driveshaft and rear diff.  I wasn't sure if it would fit but I didn't want to deal with the 15 gallons the small tank gave so I tried the big one and it worked. 

 

Below are pictures of it, the tank, the dent it needs, how it fits, and how much clearance there is from the rear axle.  I don't have any pictures of the driveshaft clearance but it clears by a couple inches because of the dent without the dent it would rub for sure.  I also used 97+ XJ straps to hold it in place.