Rich on decel, modified fuel system tuning help...

Hey all, so I don't have an exactly stock or normal fuel setup. Here it is:

-93 XJ
-01 injectors
-Returnless fuel rail and fuel check valve cleared out to go to external fuel pressure regulator, pressure set at just under 50 psi.
-01 intake manifold
-Wideband

So my XJ has been getting very poor fuel economy. I hooked up my wideband and noticed that on hard decel, its going really rich (~12 AFR, slowly eases back up to 14.7-ish after 5 seconds or so) According to the manual it should be basically shutting down the injectors and going lean. it is definitely not doing that.

Any ideas on a sensor that might be causing this? I can't remember, is there a closed TPS switch that tells the ECU when the throttle is completely closed?

of note: coolant temp is a bit cool due to exterior temperatures hovering around 0*F, but still around 195 or so. Quality 195 degree thermostat.

Other sensors i've tested/replaced in recent history:

MAP
stock narrowband O2
Coolant temp
air temp
TPS, though it may have failed since then?


Any ideas, those with experience tuning and modifying XJ fuel systems (seems to be very few people with that sort of knowledge, but thought i'd try.)

Thanks for the help!

Vaccum should increase dramatically upon deceleration triggering the fuel injectors to basically shut down. Maybe an adjustable MAP sensor might help. Why did you put a returnless fuel rail on? What are your other mods?

The injectors are a little too large for what the PCM wants, by 1.7lbs or so. However, that shouldn't cause an issue.

What is your AFR at WOT at higher RPMs?

You can check the TPS output voltage to see if it is bad or not. Search on here for the correct values or the FSM if you have one. I don't remember them off the top of my head.

Have you watched the fuel pressure as you are driving? I wonder if the pressure spikes as you let off the throttle.

Do you have a return line from the pressure regulator back to the fuel tank?

jeeperjohn said:

Vaccum should increase dramatically upon deceleration triggering the fuel injectors to basically shut down. Maybe an adjustable MAP sensor might help. Why did you put a returnless fuel rail on? What are your other mods?

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-The FSM lead me to believe that the computer also has a part in shutting the injectors down.
-I would try the adjustable MAP (and am planning on using one regardless) However, it seems that it would be a band-aid fix in this situation, as I feel something has gone wrong that needs fixing first.
-Sorry if I was unclear about the returnless thing. I am using the check port as a return line, that goes to adjustable FPR and then to the tank. Reasoning: Didn't want to buy an overpriced Hesco (which seems like the only option for using the stock rail) and the stock FPR location seemed hard to block/plug, or adapt for just a straight pipe. Returnless rail has schrader valve. And I wanted to adjust FPR to be apropriate for the 01 injectors. Does that make sense?

Talyn said:

What is your AFR at WOT at higher RPMs?

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It will spike rich for a bit when I really hammer on that, but generally it pops back to 14.7 pretty quick. Maybe drops to 13 when I'm really on it.

Talyn said:

Have you watched the fuel pressure as you are driving? I wonder if the pressure spikes as you let off the throttle.

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A very legitimate suggestion. There very well might be something going on with that. I have a remote fuel pressure gauge sitting around, I now have an excuse to slap that sucker in there to monitor while driving. I'll post results.

Talyn said:

Do you have a return line from the pressure regulator back to the fuel tank?

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Of course! I hope I don't seem that dumb see my post above.

decathect said:

It will spike rich for a bit when I really hammer on that, but generally it pops back to 14.7 pretty quick. Maybe drops to 13 when I'm really on it.

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Under WOT (floored) and higher RPMS it should not be at 14.7:1. Too lean. Should be around 12.

I guess I haven't completely floored it, but its probably around 12. I only gave it probably 3/4 throttle and eased off. I'm not worried about the WOT conditions, really.

From the 94 FSM, Fuel System, "Modes of Operation":

DECELERATION MODE
When the engine is at operating temperature, this
is an Open Loop mode. During hard deceleration, the
powertrain control module (PCM) receives the following
inputs.
Air conditioning select signal (if equipped)
Air conditioning request signal (if equipped)
Battery voltage
Engine coolant temperature sensor
Crankshaft position sensor
Intake manifold air temperature sensor
Manifold absolute pressure (MAP) sensor
Throttle position sensor (TPS)
Camshaft position sensor signal (in the distributor)
Park/Neutral switch (gear indicator signal—auto. trans. only)

If the vehicle is under hard deceleration with the
proper rpm and closed throttle conditions, the PCM
will ignore the oxygen sensor input signal. The PCM
will enter a fuel cut-off strategy in which it will not
supply battery voltage to the injectors. If a hard deceleration does not exist, the PCM will determine the
proper injector pulse width and continue injection.
Based on the above inputs, the PCM will adjust engine idle speed through the idle air control (IAC) motor.
The PCM adjusts ignition timing by turning the ground path to the coil on and off.
The PCM opens the ground circuit to the A/C
clutch relay to disengage the A/C compressor clutch.
This is done until the vehicle is no longer under deceleration (if the A/C system is operating)

Joe,

I have the FSM for my 93, and I read that section. Its almost exactly the same as mine. Based on that, do you have any other tips or suggestions? Or are you trying to say that nothing is wrong?

and I gave it completely full throttle today a few times (a dangerous proposition in MN when there is snow on the roads, even on the freeway it wants to bust the rear tires loose) and the AFR drops way down to 12 or 11 at times. So I think that is kosher.

Well, your first post says you are going very rich on hard decel, so the PCM is not getting the inputs its needs to determine hard decel with your modded system.

The only suggestion I have is to step through the above list of inputs for the decel mode, try and get some measurements on decel, and then determine a course of action from there--or rig up an injector pulse cutoff tied into the TPS, maybe with a manual override function too.

I wish the manual was more helpful on this. What exactly do they define as being 'hard deceleration"?

anyone tuned an XJ 4.0 before on a dyno? I might have to be the first...

decathect said:

I wish the manual was more helpful on this. What exactly do they define as being 'hard deceleration"?

Click to expand...

That would be throttle blade at 0%.

Makes sense. Sounds like I should re-test my tps

Just in case you need the factory fuel pressure vs flow rates.

Model Year, ...Part #, ......Colour, .....Fuel Pressure, ...Static Flow,
'87-'90, ........53003956, ...Black, ........39psi, .................18.6lb/hr
'91-'93, ........33007127, ...Brown, .......39psi, .................21.0lb/hr
'94-'95, ........53030343, ...Tan, ...........39psi, .................21.0lb/hr
'96-'99, ........53030778, ...Grey, .........49psi, .................23.2lb/hr
'00-'01, ........04854181, ...Blue tip, .....49psi, .................22.5lb/hr


http://www.jeep4.0performance.4mg.com/tech_specs.html

Thanks, does anyone have any other suggestions as to what to look into to get the mileage back where it should be? Cruising on the freeway the AFR is hovering around 14.7... that is where it should be, right? I'm kinda stumped, but most things I can conjur up are trumped by the fact that my AFRs seem correct.

14.7 is where it should be at cruise and light acceleration. I don't recall what my XJ does under heavy decel. Its a 2000 but I will have to check my AFR graphs later.

As for dyno tuning.. your minor mods won't be worth tuning on a dyno.

I would hook a voltmeter up to the TPS and see what it does while driving. As well as the fuel pressure. And test the MAP sensor as well.