Adapting 97+ fuel system for a 92 HO. (NOT 97+ swap).

[Dzimm]

So I'm bored at work and therfore doing some thinking and research.  I am trying to figure out a way to run the 97+ fuel system (tank, pump, fuel rail, injectors) on my 92.  I want to get some thoughts from you guys to see what you think. 

 

So here are the problems needed to overcome to make it work

1 - Fuel tank

2 - Fuel pump (fuel pressure regulation) 

3 - Fuel level on stock 92 gauge

4 - Single fuel line

5 - Fuel rail

6 - Injectors

 

1 - So for the fuel tank, my choice would be a Dakota fuel tank because I know it works.   Easy enough. 

 

2 - Fuel pump I would use a Dakota pump so it matches the tank height and uses the same wiring as a 97+ XJ pump.  This wiring uses the same 4 wires that are on an HO just with a different connector based on what I found so I would need to make an adapter or change out a plug. 

 

3 - Fuel sending unit is a problem.  The 97+ resistance range is 33ohms full and 240ohms empty, the 92 resistance is 0ohms empty and 90ohms full. Couple options I can think of.  Option one, buy the Fuel Link #SN34, which changes the resistance to match the gauge but it's expensive.  Option 2 would be making my own custom resistance regulating circuit, not too hard but would take time and some money.  Option 3 would be taking the fuel level arm off the old pump and attaching it to the Dakota pump and splicing that into the Dakota sending wire so it sends the proper resistance.  I like option 3 the best but wouldn't want to destroy my working HO fuel pump, nor do I have any experience with the older pumps so I have no idea what the mounting of the fuel level arm is like. 

 

4 - The Dakota tank and pump doesn't have a fuel return line so that's easy enough to just remove the stock return line and only use the supply line. 

 

5 - As far as I know it would be plug and play to swap in the 97+ fuel rail other than removing the HO fuel regulator and plugging the vacuum line. 

 

6 - The fuel injectors would have to be swapped.  The HO injectors are 36lb and the 97+ are 49lb from what I'm reading so I would have to run 49lb injectors, and of course run the 4 holes.  Question here tho is will the computer play nice with the higher flow rate or will it cause problems or would the computer be able to compensate?  

 

This truck will eventually have either a piggy back or a stand alone computer but I need to do these upgrades in stages so I can't go all in at once.  I also have a leaking gas tank when I fill over 3/4 tank so I've got to do something soon to the fuel system and I'd rather not buy a new tank just to replace it a year later with something else.  I'm also genuinely curious if this is a possible/feasible fuel system upgrade for guys with bad tanks and fuel pumps that will allow for easier replacement part sourcing. 

 

[gogmorgo]

You could get the fuel level sensing part off an XJ fuel sending unit if you don't want to trash your MJ one. The XJ units are still available new from the aftermarket.

[Dzimm]

14 minutes ago, gogmorgo said:

You could get the fuel level sensing part off an XJ fuel sending unit if you don't want to trash your MJ one. The XJ units are still available new from the aftermarket.

That's a good point, didn't think about that.  

[Pete M]

sure sounds like you've got the bases covered.   

[Dzimm]

5 hours ago, Pete M said:

sure sounds like you've got the bases covered.   

You don't see any potential issues with any of it?  I couldn't really find anything that would be an issue in my research but there isnt much useful info that I could find in regards to how the computer measures and adjusts for the fuel/air ratio. 

[Pete M]

take my opinions with a grain of salt as I don't have much direct experience with the later stuff.  hopefully one of the guys that does will see it and chime in.  

[Ωhm]

The first and last thing I'm going to say is "I don't know". Now with that in mind, I think you must maintain the fuel pressure regulator that senses manifold vacuum. Not sure if returnless regulators do that. Could depend on which ECU controller you use. Also, Mass Air Flow (MAF) sensor vs Manifold Absolute Pressure (MAP) sensor for fuel control may come into play here. Now remember the last thing I was going to say.

[Dzimm]

3 hours ago, Ωhm said:

The first and last thing I'm going to say is "I don't know". Now with that in mind, I think you must maintain the fuel pressure regulator that senses manifold vacuum. Not sure if returnless regulators do that. Could depend on which ECU controller you use. Also, Mass Air Flow (MAF) sensor vs Manifold Absolute Pressure (MAP) sensor for fuel control may come into play here. Now remember the last thing I was going to say.

The newer pump has the regulator on the pump and doesn't use a regulator on vacuum so retaining the regulator on the HO rail would cause two devices trying to regulate fuel.  

 

I suppose now thinking about what you said, if the HO fuel is regulated by manifold vacuum, I'm curious how the 97+ is controlled.  It sits atop the fuel pump and has no vacuum or electrical connection.  

 

Does anybody know if any of the HO factory service manuals say anything about fuel control?  I'll have to look at my 98 FSM to see if I can find anything on that. 

 

[Ωhm]

I wonder if the two (2) styles of fuel rails (regulator w/vacuum and regulator wo/vacuum) are based on the type of fuel control systems used in fuel delivery. In other words, is it speed density vs mass air flow, for fuel control. I do know that speed density requires a constant fuel pressure (always 39psi) at the tip of the injector. This is achieved by varying fuel rail pressure as manifold vacuum varies. Between the two forces, pressure at the injector tip stays the same.

[Dzimm]

2 hours ago, Ωhm said:

I wonder if the two (2) styles of fuel rails (regulator w/vacuum and regulator wo/vacuum) are based on the type of fuel control systems used in fuel delivery. In other words, is it speed density vs mass air flow, for fuel control. I do know that speed density requires a constant fuel pressure (always 39psi) at the tip of the injector. This is achieved by varying fuel rail pressure as manifold vacuum varies. Between the two forces, pressure at the injector tip stays the same.

I came across this article and it actually answers some questions and probably shuts this whole thing down. 

http://injectordynamics.com/articles/fuel-pressure-explained/

 

So both the HO and 97+ are return fuel systems, the HO just has the return and regulation done in the engine bay whereas the 97+ is done right there at the pump.  

 

The 97+ rail is constantly at 49psi, I haven't tested an HO rail to see what that pressure is doing but I would guess it is constant as well given that the article makes it sound like everything is controlled by the injectors themselves as long as they maintain the pressures they need.  

 

As far as the different regulation systems, the vacuum controlled just seems to be a more primative system and Chrysler moved away from it in the late 90s for the newer style fuel regulator on the pump that, I'm guessing is just preset at providing 49psi since it has no other controls. 

 

The problem I forsee now though is that the article states the higher fuel pressure in the rail creates a higher effective pressure across the injectors.  If the tables in the stock ecu are set for a certain fuel pressure, I suppose it wouldn't work to run the higher pressure fuel system and still retain the stock HO ecu without the Jeep running super rich because the computer is assuming the fuel pressure in the rail. 

[Ωhm]

Great article, good read.

[Pete M]

wouldn't the o2 sensor come in to play if it was running rich due to another reason? 

[Ωhm]

O2 sensor does have a 'range of authority' over injector pulsewidth, but this is limited control. Once STFT and LTFT are pegged one way or the other, open loop becomes the default. Under extreme conditions (extra high fuel pressure, injectors too large/too small, air leaks) even open loop is no good.

[Rockfrog]

The 97+ (96 technically) runs a constant pressure system, the fuel pump assembly supplies 49psi to the rail regardless of anytbing else. It just does what it does.

Th HO 91-95 runs a demand regulated pressure system, running between 31-39psi based on vacuum signals.

With the HO system Th ecm controls injector firing, and timing based on parameters based around a variable fuel pressure.

With the later (96+) system, the 'demand' variation is controlled by the ecm, but now is done so at the injector.

The injectors themselves are completely compatible with each other. I have run 97 injectors in my 88 without issue and they actually worked quite well.

49psi is the system rail pressure, not the injector pressure. The later injectors only flow more as the system pressure is higher, and will flow less with the lower HO pressures.

I myself have wondered about running a late model tank/pump with the HO return rail (just have to add the return line port). As the tank will push 49psi constant (with it's own regulator) and the HO regulator should handle the drop. But, it is likely the HO regulator will drop too low as it's based on seeing full pump pressure. Disabling the 96+ regulator should enable normal HO rail function in theory, but I haven't reached that point in my build.


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[Dzimm]

On 1/12/2019 at 12:41 PM, Rockfrog said:

The 97+ (96 technically) runs a constant pressure system, the fuel pump assembly supplies 49psi to the rail regardless of anytbing else. It just does what it does.

Th HO 91-95 runs a demand regulated pressure system, running between 31-39psi based on vacuum signals.

With the HO system Th ecm controls injector firing, and timing based on parameters based around a variable fuel pressure.

With the later (96+) system, the 'demand' variation is controlled by the ecm, but now is done so at the injector.

The injectors themselves are completely compatible with each other. I have run 97 injectors in my 88 without issue and they actually worked quite well.

49psi is the system rail pressure, not the injector pressure. The later injectors only flow more as the system pressure is higher, and will flow less with the lower HO pressures.

I myself have wondered about running a late model tank/pump with the HO return rail (just have to add the return line port). As the tank will push 49psi constant (with it's own regulator) and the HO regulator should handle the drop. But, it is likely the HO regulator will drop too low as it's based on seeing full pump pressure. Disabling the 96+ regulator should enable normal HO rail function in theory, but I haven't reached that point in my build.


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That could actually work... Hmmmm