Bad Fuel Pump Cause Poor MPG?

[CJR]

When in-tank gas check-valves fail, the injector spray patterns vary significantly. If enough gas flows back from the injectors and into the tank , while driving, the injectors dribble out excessive gas and gas mileage goes down. Liquid gas (i.e more mass) is flowing through the injectors instead of "atomized gas"(i.e. less mass). The dwell-open times of the injectors ARE BASED on "gas atomized flow patterns" from the injectors, NOT liquid dribbling gas flow patterns. If enough pressurized gas stays in the line to the injectors, while driving, the injector spray pattern stays proper(i.e. controlled atomized gas metering and normal gas consumption).

My suggestions;
1. Run lot's of injector cleaner through your gas tank .
2. Install an inexpensive readily available all aluminium check-valve in the external rubber hose exiting your gas tank to the injectors.

Best regards,

CJR

 

[Anak]

When in-tank gas check-valves fail, the injector spray patterns vary significantly. If enough gas flows back from the injectors and into the tank , while driving, the injectors dribble out excessive gas and gas mileage goes down. Liquid gas (i.e more mass) is flowing through the injectors instead of "atomized gas"(i.e. less mass). The dwell-open times of the injectors ARE BASED on "gas atomized flow patterns" from the injectors, NOT liquid dribbling gas flow patterns. If enough pressurized gas stays in the line to the injectors, while driving, the injector spray pattern stays proper(i.e. controlled atomized gas metering and normal gas consumption).

My suggestions;
1. Run lot's of injector cleaner through your gas tank .
2. Install an inexpensive readily available all aluminium check-valve in the external rubber hose exiting your gas tank to the injectors.

Best regards,

CJR

NO!

Do not do this.

An external check valve without a return line is a recipe for a gasoline fire. Here is a link to one example: https://www.jeepforum.com/forum/f9/fuel-line-check-valve-no-no-1242481/

IIRC you can also find examples of this in the Corvette world.

Without a way for excess pressure to bleed off you are going to eventually burst something and spray fuel all over the place. Very bad idea.

And new injectors should not need cleaner run through them either.

 

[Tim_MN]

DO NOT install an external check valve on a 97-01 XJ Cherokee.

1) It is not necessary
2) It creates a fire hazard.

If the fuel pressure maintains 49 +/- 5 psi while driving, the fuel injector pattern is correct. As I noted before, the check valve on my 2000 failed years ago, and engine performance and gas mpg's are unchanged, on long multi-state road trips, in town, and on the 4x4 trails in all off-camber situations.

So i just replaced both o2 sensors with NTK brand and i did a long trip with it today and i'm still at 12.5mpg. What am i missing? Could a bad manifold gasket cause a drop like this in mpg?

What does your OBD-II Trouble Codes reader with live data say about fuel trims and O2 sensor function ? Have you inspected and tested the O2 sensor wire circuit and fuses for failure ? Did you re-boot the PCM after the new parts ? Is the exhaust manifold cracked ? What diagnostics makes you suspect the manifold gasket, or is that just a wild arse guess ?

 

[FernGreen]

DO NOT install an external check valve on a 97-01 XJ Cherokee.

1) It is not necessary
2) It creates a fire hazard.

If the fuel pressure maintains 49 +/- 5 psi while driving, the fuel injector pattern is correct. As I noted before, the check valve on my 2000 failed years ago, and engine performance and gas mpg's are unchanged, on long multi-state road trips, in town, and on the 4x4 trails in all off-camber situations.




What does your OBD-II Trouble Codes reader with live data say about fuel trims and O2 sensor function ? Have you inspected and tested the O2 sensor wire circuit and fuses for failure ? Did you re-boot the PCM after the new parts ? Is the exhaust manifold cracked ? What diagnostics makes you suspect the manifold gasket, or is that just a wild arse guess ?

I’ll try to post some screenshot of my live data of my fuel trims later. The o2 sensors seem to be fluctuating between their acceptable range but I’ll get you pictures of the data on them as well. I have not checked the sensor wire circuit. Do you have any diagrams of those for a 2000?

I do have a brand new Delphi fuel pump sitting in the box, should I just replace my pump and at least solve my long crank issues and see if It fixes my rough idle? I had my rough idle before I had bad gas mileage so I don’t think they’re related but not sure. Is there a good way to check for exhaust leaks before the first o2 sensor? I had the Jeep running on the lift and carefully moved my hand around the exhaust trying to see if I felt any hot air blowing out but I didn’t really find any. I know I have an exhaust leak because I can hear It under acceleration.


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[Agreen]

Post # 41 and page 5 says it for me. I would put the old injectors back in.

https://www.naxja.org/forum/showthread.php?t=1148084&page=3

 

[CJR]

The 2000XJ does indeed have a fuel return feature to the gas tank. It is a feature built into the Pressure Regulator which is contained in the Fuel Pump Module mounted on the gas tank. See Figures 3 & 23 in the 2000XJ FSM. If the fuel pressure, at the pressure regulator exceeds approx. 49psi, an internal diaphragm closes and excess fuel is routed back into the tank through the pressure regulator. A SEPARATE GAS RETURN LINE IS NOT USED in the 2000Xj. See also Fuel Pressure Leakdown Tests in 2000XJ FSM. In my view, what I have stated previously should work.

However if you disagree, the only other option I see is to simply pull the gas tank and replace the entire Fuel Pump Module ( pressure regulator, check valve, fuel pump etc.) $$$$$$.

Best regards,

CJR

 

[FernGreen]

Post # 41 and page 5 says it for me. I would put the old injectors back in.

https://www.naxja.org/forum/showthread.php?t=1148084&page=3

I just read through some of that thread and i see it's quite the touchy subject. You've got a guy claiming that his stuff is genuine and a lot of discrepancies in the product he's claiming to sell. Either way I've already swapped in a set of used oem Chrysler 1 hole injectors and the gas mileage has not improved or worsened.

Therefore I'm ruling out injectors. I do live in CT so there is winter gas blend here but i would think that i would've noticed a drop at least a month ago since winter has set in for a good bit now. I've tried 3 different fuel stations so far and no change in mpg. So i'm ruling out bad gas as well. What i haven't ruled out is the possibility of a vacuum leak, exhaust leak, plugged cat (which i feel like is rare to happen on a xj), or a bad fuel pump? I know my check valve is bad and im trying to understand how the pump could be the culprit if the pressure in the rail is staying consistent at 49 psi. I guess it would make more sense to me if i saw an inconsistency in the fuel pressure where it dropped and then rose again after the injectors open & close.

Does this sound more like a vacuum or exhaust leak?

 

[Anak]

DO NOT install an external check valve on a 97-01 XJ Cherokee.

1) It is not necessary
2) It creates a fire hazard.

That should be '96-'01. The '96 pump assembly is different, but the lack of a return line is the same.

 

[Tim_MN]

FernGreen needs data and to use that data to diagnose, and not make random guesses. The OBD-II SELF-DIAGNOSTICS should provide the necessary data to focus the trouble shooting.

I am done here ......

 

[FernGreen]

FernGreen needs data and to use that data to diagnose, and not make random guesses. The OBD-II SELF-DIAGNOSTICS should provide the necessary data to focus the trouble shooting.

I am done here ......

@Tim_MN ...

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[FernGreen]

If you pm me your email I can send you a video of the live data while driving but I can’t post a video on the thread. You also mentioned the electrical circuit of the o2 sensors and I did not find any chafing or melting of the wires for either o2 sensor circuit. [mention]Tim_MN [/mention]

 

[RCP Phx]

How are you determining the MPG? What gears do you have and what size tires are you running? Has your speedometer been corrected?

 

[FernGreen]

How are you determining the MPG? What gears do you have and what size tires are you running? Has your speedometer been corrected?

So the speedo is off. I’m running the stock speedo gear in It with 32s and 3.55s. But even with It being off I’ve still lost 3mpgs since I haven’t done anything differently like correct the speedo (which would yield better mpg anyways) or change tire sizes or gears. The mpg drop happened on its own without me doing anything differently. I did talk to a local XJ’er who recently notice a drop in mpgs too and maybe they held off on switching to winter blend until now because of all the extra gas left over from the summer with covid and people not driving as much?


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[RCP Phx]

You added bigger tires! Change the gears and fix your speedometer before you do anything.

 

[FernGreen]

You added bigger tires! Change the gears and fix your speedometer before you do anything.

Yes but not anytime recently. I’ve been running the same 32s since the day I bought the jeep and It always averaged 15.5 mpg until now. Obviously I was getting marginally more than what the math came out to be since the speedo gear isn’t correct for the tire size but my other Cherokee with the same size tires is getting right around 16mpg without regearing

 

[RCP Phx]

I would also replace the t-stat since it seems to be running cold and where you are that can definitely affect mileage.

 

[maxbraketorque]

The fuel trims look good using the new front O2 sensor, but if the exhaust leak you mention is just after the front O2 sensor, I would be slightly concerned about that the leak is affecting front O2 sensor reading. Although it may seem counterintuitive, as the exhaust pulses move through the exhaust, they can create a vacuum condition relative to atmospheric pressure, and if the leak is close enough to the front O2 sensor, it may be affecting the reading. You could try some exhaust tape.

However, if this is this is your first winter with this XJ, you may be just seeing winter time mileage reduction. If its not garage-kept, it would take a while for everything to warm up. You could experiment with letting some warm air from the engine bay into the airbox. That may help atomize the fuel.

 

[Unclewolverine]

Also you will lose mileage when they switch to winter blend fuel.

 

[Tim_MN]

Expect a winter blend gas mileage and extended warm-up times before closed loop variance of about 5-10% at the most. A 30-40% drop in gas mpg's is almost always O2 sensor related.

 

[FernGreen]

Expect a winter blend gas mileage and extended warm-up times before closed loop variance of about 5-10% at the most. A 30-40% drop in gas mpg's is almost always O2 sensor related.

Does It look like my o2 sensors are lagging or does closed loop last this long during initial running period after cold startup?

What voltage readings should I be seeing within the first 20 secs of running after a cold start?



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