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Bad Fuel Pump Cause Poor MPG?

FernGreen

FernGreen

NAXJA Forum User
Location
Connecticut
2000 4.0 230K
Is it possible for a failing fuel pump to cause a drop in mpg? I recently had problems with rough starting so i swapped out the injectors to ksuspension's 4 hole injectors but now i have decreasing fuel mileage. The last two fill ups at the gas station i've gotten roughly 13mpg and now 12mpg. I usually average 15.5mpg. It sounds weird but it seems like the mpg is getting worse and worse every time i drive it. I'm under the assumption that the fuel pump might be going out because the fuel pressure at the rail drops below 30 psi within 2 mins of sitting after being shut off. According to the FSM it should hold above 30psi for at least 5 min. The fuel pressure at the rail was a problem before i changed the injectors and the injectors didn't seem to change the leakdown pressure time at the rail since I checked the pressure before and after swapping the injectors. One thing I will add is the jeep has always idled high, rough, and inconsistent w/ surging rpms since I've owned it which is about 6 months. I have heard the failing fuel pumps cause rough idle and poor gas mileage but I'm having a hard time wrapping my head around how a failing fuel pump could cause low mpg.
 
What is the fuel pressure while driving?

I could see low fuel pressure resulting in poor atomization of the fuel, but I would think the O2 sensors should pick up on that, and I would expect the computer to throw a code.

Considering you have only owned the Jeep for 6 months and it has the idle issue, have you gone through and looked for vacuum leaks? How about exhaust leaks?
 
Anak said:
What is the fuel pressure while driving?

I could see low fuel pressure resulting in poor atomization of the fuel, but I would think the O2 sensors should pick up on that, and I would expect the computer to throw a code.

Considering you have only owned the Jeep for 6 months and it has the idle issue, have you gone through and looked for vacuum leaks? How about exhaust leaks?
Click to expand...


I didn’t think to check the pressure while driving I’ll have to try that out. I have no idea when the o2 sensors have been changed if ever. I’ve had the Jeep for a year but have only really driven It 6 months and it’s always idled high from day 1 of owning It. I cleaned the IAC but they didn’t make a difference. It idles around 800-900rpms I think but I can confirm that when I get a live data readout of It. It does have an exhaust leak but it’s after the last o2 sensor. I’ve had long crank problems with It intermittently (It doesn’t do It everytime i go to start it just sometimes). I’m unsure of any vacuum leaks I don’t have a smoking machine I’ve wanted to pick one up though.


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I would check the o2 sensors or just install new ones , then you said that you have a high idle problem check for vacuum leaks or a bad idle motor and maybe it was that way when you got it , some one could have moved the throttle stop because it has a bad idle motor .
 
If the idle was high before, changing the injectors (with new o-rings) may have solved that issue. However, k-suspension injectors have a poor reputation apparently. The only 2 things that can cause fuel pressure to drop off after shutting the eninge off are leaky injectors and a bad fuel pump check valve, but once the fuel pump comes on and the engine is running the check valve is wide open anyway.

Pull the fuel rail, flip it over and turn the key on. Check for any dripping injectors. It's HIGHLY likely that the ONLY thing that you changed caused the drop in MPG... unfortunately.
 
Agreen said:
If the idle was high before, changing the injectors (with new o-rings) may have solved that issue. However, k-suspension injectors have a poor reputation apparently. The only 2 things that can cause fuel pressure to drop off after shutting the eninge off are leaky injectors and a bad fuel pump check valve, but once the fuel pump comes on and the engine is running the check valve is wide open anyway.

Pull the fuel rail, flip it over and turn the key on. Check for any dripping injectors. It's HIGHLY likely that the ONLY thing that you changed caused the drop in MPG... unfortunately.
Click to expand...


Great! This is what I was looking for! In the back of my mind I was starting to think the injectors might be part of my problem. I still think my pump has a bad check valve but I can’t confirm because I don’t have the adapter for my fuel pressure garage that would allow me to see the pressure before the rail. I’m doing all my testing through the Schrader valve on the rail. I’ll pull the rail and report back on my findings. Is there any good place get quality injectors? I don’t care if their 1 hole, 4 or 12 I just want good injectors lol.


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Unfortunately, Bosch injectors are pretty pricey. I generally grab some used ones from a junkyard and have them professionally serviced. Witch hunter performance did me right when I used them last.

You're testing pressure at the right spot. Your system is returnless, so pressure will be equal throughout.

By the way, did replacing the injectors solve anything? Or did the high, erratic idle continue and now you have bad mileage in addition to the previous issues?
 
On the subject of exhaust leaks, pay close attention to the exhaust manifold/header. And while looking at that give a critical look at the motor mounts, especially the driver's side.

It is common for XJs to have exhaust manifold cracks and for those to be related to worn out motor mounts. The motor mounts wear out allowing the engine to move more than it should and the movement creates the stress that creates the cracks in the manifold. The cracks in the manifold can alter the exhaust gas mixture which then leads to false readings from the O2 sensor. Long series of issues, but also pretty typical for an XJ.
 
Agreen said:
Unfortunately, Bosch injectors are pretty pricey. I generally grab some used ones from a junkyard and have them professionally serviced. Witch hunter performance did me right when I used them last.

You're testing pressure at the right spot. Your system is returnless, so pressure will be equal throughout.

By the way, did replacing the injectors solve anything? Or did the high, erratic idle continue and now you have bad mileage in addition to the previous issues?
Click to expand...


Replacing the injectors didn’t really solve anything. I pulled the rail and pressurized the system but didn’t see any noticeable leaky injectors but I swapped them out for a known good pair of oem Chrysler 1 hole injectors and I’ll see if I have any changes. I hooked the rail back up to the tester and I’m still bleeding down below 30psi within 2.5 minutes so I’m guessing the check valve in the pump is bad?


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Anak said:
What is the fuel pressure while driving?

I could see low fuel pressure resulting in poor atomization of the fuel, but I would think the O2 sensors should pick up on that, and I would expect the computer to throw a code.

Considering you have only owned the Jeep for 6 months and it has the idle issue, have you gone through and looked for vacuum leaks? How about exhaust leaks?
Click to expand...


I just checked the pressure while driving and It stays pretty consistent hovering at 49psi. I did a live data readout of both o2 sensors and they seem to be reacting well to the change in fuel.


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FernGreen said:
I hooked the rail back up to the tester and I’m still bleeding down below 30psi within 2.5 minutes so I’m guessing the check valve in the pump is bad?
Click to expand...

That would seem to be the issue.
 
If you have idle issues, suspect injector troubles. injector wiring damage, and/or a worn out or faulty Idle Air Controller.


The check valve on my 2000 has been bad for more than 8 years. Other than having to crank twice to start, it has not been an issue. Gas MPG's are normal.

A 30% or more drop in gas MPG's is almost always an O2 sensor issue. Confirm the exhaust manifold is not cracked, ( most 1999 and older are cracked ), and confirm that the O2 sensors are NOT Bosch. Jeep O2 sensors last about 150,000 miles, and Bosch O2 sensors and cheap Chinese generic O2 sensors do not play well with the Jeep PCM. Use NTK O2 sensors.
 
Tim_MN said:
If you have idle issues, suspect injector troubles. injector wiring damage, and/or a worn out or faulty Idle Air Controller.


The check valve on my 2000 has been bad for more than 8 years. Other than having to crank twice to start, it has not been an issue. Gas MPG's are normal.

A 30% or more drop in gas MPG's is almost always an O2 sensor issue. Confirm the exhaust manifold is not cracked, ( most 1999 and older are cracked ), and confirm that the O2 sensors are NOT Bosch. Jeep O2 sensors last about 150,000 miles, and Bosch O2 sensors and cheap Chinese generic O2 sensors do not play well with the Jeep PCM. Use NTK O2 sensors.
Click to expand...

Whats the chances my MAP sensor is causing the bad gas mileage and rough idle?
 
In line with what Tim said, fuel pressure leaking down back into the tank is not going to cause any more trouble than a slow startup/double crank. That fuel isn't being lost.


If you have 49 psi while driving you don't have a real problem with your fuel pump.
 
Anak said:
In line with what Tim said, fuel pressure leaking down back into the tank is not going to cause any more trouble than a slow startup/double crank. That fuel isn't being lost.


If you have 49 psi while driving you don't have a real problem with your fuel pump.
Click to expand...


Okay that makes sense I could see if the pump couldn’t maintain pressure in the rail but It seems to have no problem doing that. So I just swapped the map sensor with one off a parts Jeep and I don’t think there’s any difference It seems to be sucking gas pretty good now I can almost watch the gas gauge needle drop while driving. So what’s my next option? Change the o2 sensors?


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I would change O2 sensors and do a serious hunt for an exhaust leak before the 1st O2 sensor.

If you still have a high idle and surging RPMs I would also be hunting for reasons for those. My old school habits say look for vacuum leaks, but in this day of computer systems I expect there could be issues with sensors which could also create such a situation.
 
Anak said:
I would change O2 sensors and do a serious hunt for an exhaust leak before the 1st O2 sensor.

If you still have a high idle and surging RPMs I would also be hunting for reasons for those. My old school habits say look for vacuum leaks, but in this day of computer systems I expect there could be issues with sensors which could also create such a situation.
Click to expand...


Thank you [mention]Anak [/mention] I’ll change the o2 sensors tomorrow with the NTK brand and report back.


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lawsoncl said:
Does your live data show the engine getting up to temp? Is the torque converter properly locking up?
Click to expand...


I believe I do have that option in live data, how would I use that to help my diagnosis? The torque converter seems to be functioning properly I haven’t noticed any issues shifting.


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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?
 
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