Jeep 4.0L Two-Rail or Single Rail EFI?

[60Bubba]

Happy Thanksgiving to all!

I've posted before that my CJ-7 has a Cherokee 4.0L in it, but it's not really that simple Here is the history as I know it or can guess from some evidence:

1. Originally a 258/4.2L straight six with the FrankenCarb--this one was sold in California, so I'm sure it was extra-special.

2. MOPAR EFI kit added to the 4.2L.  It seems this addition happened sometime after 2000, as the MOPAR labels for the kit have a 2000 copyright on them.  I learned from the shop that did most of the work on this Jeep that the lady I bought it from purchased it sometime around 2001.  It was owned by 1 family prior to that (1983-2000?).  I suspect the Skyjacker lift and sketchy paint job happened about the same time to make it ready to sell.

3. The 4.2L finally gave up in 2014.  This engine was replaced with a junkyard 1995 Cherokee 4.0L last May; engine had about 155K on it at that time.

I never really knew which version of the MOPAR EFI kit I had, OBDI (with fuel return line) or OBDII from the newer models (returnless).  There was also the option that they pulled a complete 1995 engine including the EFI and junked the old MOPAR kit.  I just didn't really know. 

I've done some more research and taken some pics to see if anyone can confirm what I've come to believe.  It seems like my system matches the pictures I've seen of a single rail EFI system.  That would mean it is:

1. 1997 and newer

2. Returnless

3. OBDII

I mostly ask these questions because I've been looking at performance options for whenever I'm forced to rebuild this engine.  I really think I want to leave a Jeep engine in there, as opposed to a small block Chevy, for example.

It seems like most systems (supercharger, programmers, etc.) are dependent upon the OBDI VS. OBDII distinction.  With a 1995 engine, I wasn't sure which items I should be looking at.  Assuming this is a 1997 and newer-style EFI kit, I assume I should only look for performance items for the newer model engines, correct?  I don't think there was much other than heads (I have the supposedly better casting) that varied in those mid 1990's engines, other than the EFI/PCM.

1. Can I conclusively say this is a returnless/1997 and newer EFI kit?

2. Is this the OBDI/OBDII port I'm showing in the picture?  It is about 8" down the main wire harness from the MOPAR EFI kit PCM.

Regards,

Case

 

 

 

[Moses Ludel]

Here's some fodder, Case...The distinction for OBD-1 is the pressure regulator at the fuel rail.  (Don't confuse a '97-up fuel damper for a pressure regulator, they have a similar appearance.)  The later Mopar EFI Conversion Kit still uses the 60-pin PCM, so that's not a deal breaker.  However, the fuel pressure regulator is at or near the fuel tank.  (The stock '97-up pressure regulator is part of the fuel pump module at the fuel tank and can be seen atop the tank.)  

Note: Whether in-tank or in-line, the OBD-II (later) systems or a later Mopar EFI kit still uses the in-tank style regulator.  Mopar kits use a modified, externally mounted version of this regulator, while a stock Jeep XJ, TJ, ZJ or WJ application has its regulator as part of the fuel pump module at the tank.

The fuel rail looks like a later (OBD-II) application.  Below are photos of a two-rail system used from 1991-95.  Note the two fuel pipes/lines, one is a pressure line, the other a tank return line.  It's highly likely that you have a later EFI system.  The wiring could be either a Mopar EFI Conversion Kit harness or an XJ Cherokee wiring harness that may have come from a donor engine.  Your plug in hand looks like an OBD-I diagnostic plug, similar to a stock pre-OBD-II harness---or it could be a Mopar EFI Conversion Kit harness!  Of course, this could be a mix of parts, so let's break it down:

1)  What type and location is the pressure regulator?  If OBD-II, it's either atop the fuel tank or looks like a '97 up tank regulator that has been modified for a remote mount.

2)  Do you have an actual return fuel line to the tank?  This will be clear with two fuel lines at the fuel rail, a pressure line and a return line.

3)  Since you can run the late system from a 60-way/pin PCM (the later Mopar EFI Conversion Kit does so), this is not a distinct clue.  The Mopar EFI Conversion Kit harness, however, has a simpler 4-wire interface with the Jeep chassis wiring.  If your system has a harness with splices that look like way more than a simple 4-wire hookup, the wiring harness is from a donor 4.0L vehicle.

 

This is a two-rail EFI system fuel rail, used from 1991-1995.  Photos courtesy of a Google search.

You could easily have a mixture of Mopar EFI stuff from the 4.2L conversion to EFI.  The wiring harness and fuel pressure regulator would be telling.  (When we discussed this some time ago, I thought you had a mix of pieces.)  Some of this is academic.  The crucial concerns would be running the right fuel pressure for the injector type, the sensor feedback devices, and the overall performance of the engine.  

If you are lacking feedback signals, like the coolant temp sensor signal for the engine warm-up cycle or MAP, fuel efficiency will suffer profoundly.  If the tailpipe emissions are way too high, that would be telling as well.  Measuring fuel pressure at the rail test port should indicate which regulator is in place and whether the system is working properly.  (Later single rail EFI operates at a higher fuel pressure than the two-rail systems.)  

Without a regulator, the fuel pressure would be as high as the fuel pump pressure!  The two-pipe/rail system requires a return line to the tank and a regulator at the EFI fuel rail.  (The regulator has a conspicuous vacuum hose nipple; a single rail damper does not.)  The tank-type regulator (later single rail EFI) controls fuel pressure at the rear chassis; this same regulator, when modified for external use (like the later Mopar EFI Conversion Kit's modified regulator), still requires a return fuel hose from the remote regulator back into the fuel tank.  Both systems have returns, the two-rail return is from the engine's fuel rail, the single rail return is from the later type regulator (mounted atop the fuel tank in the stock configuration).  

Note: Any type of EFI or TBI system requires fuel pressure regulation.  Without a regulator, the fuel pump's higher pressure (by design) would feed to the injectors and over-fuel the engine.  If you can measure the pump's direct output pressure and the fuel rail pressure, I can suggest which fuel pump and regulator are currently in place.

  

Google search turns up these two single-rail images.  Left photo shows single pipe inlet and a damper mounted mid-rail.  At right is single rail without damper.

As long as all sensors are in place, the PCM is operational and the excess fuel can return to the tank through a preset pressure regulator, you have a functional EFI system.

Moses

[60Bubba]

Moses,

I hope you had a great Thanksgiving! I spent a few minutes digging around through some old pictures of the restoration.  I am going to post up a few pictures that may help identify what I'm working with. One is my fuel tank prior to cleaning, another is the external fuel pump which left me stranded this spring, and the third is a question mark for me.

I have zero doubts I'm working with a single rail system, and I'm 99% sure this setup uses the MOPAR kit wire harness.  My rebuild included a new Centech wire harness, and as you said, it was a simple 4 wire hook up.

I assume the round canister is the fuel pressure regulator or a filter, but I'm really guessing there.  I don't know what a fuel damper looks like, but I didn't see anything on the fuel rail that looked like the little button head item in the middle of the fuel rail picture you posted. It looks like the canister with the strap hinge across the top has three fuel lines on it.  I'm betting this is:

1. Supply in from the pump

2. Supply out to the rail

3. Return to the tank

I'm thinking this is a later fuel rail with an external fuel regulator and an OBDI PCM. Does that seem sensible?

Case

[Moses Ludel]

Case, the photos confirm that this is the later Mopar EFI Conversion Kit that uses a '97-up single rail EFI and the pressure regulator that would normally mount atop the fuel pump module at the fuel tank of a '97-up TJ Wrangler.  The regulator (yes, shown in your photo) is a modified OE '97 and newer tank module regulator designed to fit externally on 4.2L CJs and YJs that have no in-tank fuel pump.  The externally mounted fuel pump is also part of the Mopar Performance EFI Conversion Kit for a 1981-90 4.2L inline six.

 1997 Jeep TJ Wrangler Fuel Pump Module with Regulator.pdf

This PDF is the OE Mopar Fuel Pump Module for 1997-up TJ Wrangler.  The "Filter" (item #5) actually doubles as the pressure regulator.  In the Mopar EFI Conversion Kit (later style), the custom made pressure regulator assembly is modified to work externally and not as part of the in-tank fuel pump module.  The filter/regulator is mated to a custom machined fuel bowl in the EFI kit version of the pressure regulator.  60Bubba's photo shows the custom machined, free-standing regulator with its filter fitted above the bowl. This free-standing regulator is supplied with the later version of the Mopar EFI Conversion Kit for 4.2L 1981-90 engines. It has an inlet, outlet and fuel return port to the gas tank.

The Mopar #04798301 fuel filter is actually the pressure regulator as well.  This dual-purpose part has the regulator built into the unit.  Although the filter can be removed from the top of the TJ Wrangler fuel pump module, this service step requires dropping the fuel tank on a stock Jeep TJ Wrangler.  For that reason, the fuel filter/regulator is consider a non-serviceable item that only requires attention if you need to drop the tank or change the fuel pump module on a '97-up Jeep TJ Wrangler and other Jeep models with similar regulator/filters.  The filter/regulator is readily accessible on 60Bubba's Mopar EFI conversion kit.  Cost for this stock replacement part, which doubles as a Mopar Performance part, ranges from $108 (as an OE replacement part) to the $225 range (Jeg's) if offered as a Mopar Performance part number.  Another reason to leave the filter/regulator in place if it's working properly!

Yes, you're thinking correctly.  The later kit still uses the 60-pin PCM (OBD-I), a modified version of the '97-up regulator and the external fuel pump common to the first generation Mopar EFI conversion kit.  This works with a single rail EFI and manifold; the fuel return is from the modified/standalone regulator back to the tank.  The preset regulator maintains correct pressure by taking the higher fuel pump pressure and diverting excess pressure back to the fuel tank.  This all takes place before the EFI rail at the intake manifold, so there is no need for a regulator at the fuel rail.

As you describe, the pump should be near the tank.  The regulator is inline between the pump and fuel rail, and the fuel rail receives regulated fuel pressure.  The injectors don't care whether this is a two-rail or single rail system, they only require regulated, consistent fuel pressure with enough volume.

Since the later single rail system operates at slightly higher pressures that the two-rail systems, there is no need for the vacuum diaphragm on the regulator (at the rail) that boosts fuel pressure during engine cranking.  Some single rails use a damper to steady the fuel pressure pulses at the injectors, some single rails do not.  Your rail does not.

You mention that the engine is from an XJ Cherokee 4.0L donor, and this inline six should be very happy with the EFI conversion pieces.  The parts are essentially the same as 1991-up Mopar design OBD-1 and OBD-2 4.0L XJ Cherokee and YJ Wrangler/TJ Wrangler factory MPI components.  Mopar did a good job by providing off-the-shelf parts.  This aids service needs.  As a footnote, the EFI conversion kit's intake manifold is actually a 4.0L type that has been port matched for the 4.2L engine.

If you build a 4.6L stroker, you will need to follow the tuning measures I describe at the magazine, including use of 302/5.0L Ford fuel injectors.  Review the tuning tips.  Your current EFI system will prove useful.

Moses

1997 Jeep TJ Wrangler Fuel Pump Module with Regulator.pdf

Edited December 1, 2015 by Moses Ludel

[60Bubba]

A stroker is definitely on the list.  I love the idea of an SBC, but I don't have the time in my current jobs (real work and Dad duty) to source and prep the parts needed for a junkyard V-8 swap.  The Novak kits are great, but you really pay for the convenience.  A 4.8 or 5.3 Chevy V-8 with an AOD transmission and necessary adapters is well over $10,000 for a salvaged engine.  That's a little rich, especially considering what I already have invested in this project.

That said, I think next summer I will start planning for a 4.6L build.  I want to have this engine decked and otherwise prepped while it is still in good working order, rather than wait and risk losing the block if something fails.

I'm sure when that time comes, I'll refer back to your magazine articles and videos (which I've already watched a few times!) As always, you are a wealth of information.

Case

[Moses Ludel]

60Bubba...With your CJ-7's weight and accessory package, you'd be very pleased with a 4.6L build.  A 230-250 horsepower target is attainable with great drivability.  Should be a success story and not taxing to the rest of your geartrain...

Moses

[Moses Ludel]

60Bubba...Here's a photo of the fuel filter/regulator on a stock TJ Wrangler.  I copied this at the magazine's freshly updated platform.  On that note, check out the magazine, the new platform went live during the night!  You'll like the search engine, try "4.6L" or "4.6L stroker" without the quotes.

Here's the pic, should look familiar:

Note that the return fuel simply pours back into the tank at the module beneath the filter/regulator.  On your modified, free-standing regulator, the machined aluminum body serves as a substitute for the enclosed fuel tank.

Moses

Edited December 4, 2015 by Moses Ludel