Better MPG.... Ran out of things to check

[ScottH82]

Hello,

I have a 2000 XJ 4L with about 145k miles on it. I did a search and found a lot of feedback from people stating that what MPG they are getting with their Jeeps. Some of them have close to the same mods as me, some have bigger lifts and larger tires and their all getting much better gas mileage then I am.

Here is a list of my mods:
- New Paper Filter
- Neon 784 Injectors
- New Exhaust w/ Magnaflow Cat & Flowmaster 40 muffler
- NGK V-Power Copper Plugs (Gapped .035)
- Rough Country 3" Lift Kit
- Goodyear Wrangler Authority 31x10.5x15 Tires

I've read people with very similar mods as mine getting between 14-16mpg city and 17-19mpg highway. I am averaging 11mpg city and 13mpg highway. It just doesn't make sense to me. The CEL is not on and I even went as far to scan the OBD this evening to see if any codes were hung up and got nothing. I understand that a lot of the mpg has to do with driving style but I hardly ever go above 2200Rpm (even when on the highway).

I am just looking for some advise on things to check/replace to improve my gas mileage.

 

[ScottH82]

just wanted to add.

The engine fires up almost always on the first try. Idles great and warms up to normal operating temp every time.

Also wanted to mention that someone suggested that the magnaflow cat may cause to much back-pressure? My Jeep has two cats built into the exhaust manifold and he said that the magnaflow cat that is located mid-way back the vehicle isn't necessary and could be causing to much back pressure. I'm no mechanic so I left it go since the exhaust shop put it there. And I don't know the effects of "to much back pressure".

 

[VAhasnoWAVES]

how are you calculating MPG? if your on stock gears, have you redone your speedo gear? if your getting between 200-250 miles off of a full tank, your doing pretty decent on mileage actually. go to stock tires, or regear.

one thing i did notice when reading... 2200 rpms seems a little low. im right at 2500 doing 75 on the highway. the 4.0 likes higher rpms, and operates more efficiently in the 2500 range.

 

[ScottH82]

how are you calculating MPG? if your on stock gears, have you redone your speedo gear? if your getting between 200-250 miles off of a full tank, your doing pretty decent on mileage actually. go to stock tires, or regear.

one thing i did notice when reading... 2200 rpms seems a little low. im right at 2500 doing 75 on the highway. the 4.0 likes higher rpms, and operates more efficiently in the 2500 range.

I am using the iphone app "Road Trip". I have not redone the speedo gear. I was reading into that and I think Quadratec said it only makes like a 5% overall difference. Although it's a start, at 5% I don't think its the cure.

My last tank of fuel I got 215 miles out of (17.42 gallons).

 

[4x4wagon]

if you can scan it watch engine temp while cruising at speed shouldnt realy drop below 190* it helps if you let it get to operating temp before you go for the drive

 

[VAhasnoWAVES]

My last tank of fuel I got 215 miles out of (17.42 gallons).

that sounds pretty normal to me, thats about what i got with 31s, 3.07s, and the AX15. i can get almost 300 now with a 245/75/16 (slightly smaller than a 31), and 3.55s on the same tranny.

if you have the AW4, try rolling around in 3rd gear. i believe thats what most do for the best power and fuel economy.

i say pay more attention to your throttle position than your rpms. 31s and 3.07s was a turd in 5th gear. even though i was doing 2000 rpms, i was full throttle, sucking up lots of gas. i stopped using 5th on the highway, and found that even though i was doing 2750 rpms in 4th gear, i was getting better mileage because i was only at half throttle.

 

[Super mud]

if you can scan it watch engine temp while cruising at speed shouldnt realy drop below 190* it helps if you let it get to operating temp before you go for the drive

Any idea how to monitor this on a renix? I have a 180* stat that according to the gauge runs ~200* but would like to make sure it's correct and not lower. Currently getting 14-15 around town driving like I stole it so not too concerned.

 

[Darky]

just wanted to add.

The engine fires up almost always on the first try. Idles great and warms up to normal operating temp every time.

Also wanted to mention that someone suggested that the magnaflow cat may cause to much back-pressure? My Jeep has two cats built into the exhaust manifold and he said that the magnaflow cat that is located mid-way back the vehicle isn't necessary and could be causing to much back pressure. I'm no mechanic so I left it go since the exhaust shop put it there. And I don't know the effects of "to much back pressure".

Your mechanic was incorrect. The downstream cat is there from the factory. 2000 and 2001 XJs run the precats up front plus the down stream cat as opposed to just the downstream on 99 and down models. Gear could be a part of the problem, but probably not too major. I only lost 1-2 mpgs going up to 30s on stock gears.

 

[techno1154]

At what RPM's are you turning those tires at? Try to keep it above 2,500 on the highway. That is where the 4.0L is more efficient.
I did not read where you replaced the O2 sensors. If they have not been changed lately then you should look into that. At one time my XJ lost about 100 mile per tank that had me bafflled. There was also no CEL. The only tell tale sigh was an over rich exhaust smell when standing behind the XJ in the driveway. A new O2 sensor brought back my mileage.
Maximum engine temperature (200+ degrees) do not play a major role in fuel economy on newer XJ's. My XJ changes from Open Loop to Closed Loop before the temperature reaches 100 degrees. That change is dictated by the heated O2 sensor which is functioning properly. It is important to note, the O2 sensor plays a major role in determining how much fuel is pulsed into the engine at all engine speeds and loads.

Along with what the others mentioned above, I could only add proper tire pressure removing excess/unwanted load from the XJ. HTH.

 

[Talyn]

That 784 is an 25 lb/hr injector. Why you need that on a 4.0L I have no idea.

 

[XJCasper]

Unless you know the last time they were done, replace your O2's

At what RPM's are you turning those tires at? Try to keep it above 2,500 on the highway. That is where the 4.0L is more efficient.
I did not read where you replaced the O2 sensors. If they have not been changed lately then you should look into that. At one time my XJ lost about 100 mile per tank that had me bafflled. There was also no CEL. The only tell tale sigh was an over rich exhaust smell when standing behind the XJ in the driveway. A new O2 sensor brought back my mileage.
Maximum engine temperature (200+ degrees) do not play a major role in fuel economy on newer XJ's. My XJ changes from Open Loop to Closed Loop before the temperature reaches 100 degrees. That change is dictated by the heated O2 sensor which is functioning properly. It is important to note, the O2 sensor plays a major role in determining how much fuel is pulsed into the engine at all engine speeds and loads.

Along with what the others mentioned above, I could only add proper tire pressure removing excess/unwanted load from the XJ. HTH.

 

[O-Gauge Steamer]

Just to verify the phone app... Pick a chunk of highway you like and zero out both the trip odometer on hte Heep and your phone. Run down the road for at least 10 miles. The more the better and then stop at another mile marker. Compare the phone and the trip odometer to the actual, physical miles traveled.

What I can tell you for sure is that with P285/75R16s running on 4.56:1 gears, my odometer was exactly 10.2% fast. So, I slowed it down via electronics. I am now less than 1% off.

I get 15+ in town with this combination. But, and it is a big but, the 10.2% speedo correction means that compared to a stock Heep, my engine is turning 10.2% faster at any given speed in any given gear.

Basics first. Make sure you are recording actual miles traveled. The tune-up you have already done. The Oxygen sensor is a possibility. When I ran 31s I also ran 4.11:1 gears. Avoid using 5th gear as you are lugging the engine. It is better to run under partial throttle at a higher rpm thatn to run at or near full throttle at a lower rpm.

Why? Because.

Because when the throttle is in or near the WOT position, the PCM drops out of closed loop and goes into open loop as it thinks you are accelerating and so dumps extra fuel so as to prevent detonation (pinging). At this point the O2 sensor is ignored.

 

[ScottH82]

At what RPM's are you turning those tires at? Try to keep it above 2,500 on the highway. That is where the 4.0L is more efficient.

I always kept the RPM's under 2200Rpm. But after reading this I can see how that could be hurting me in some cases.

That 784 is an 25 lb/hr injector. Why you need that on a 4.0L I have no idea.

I read that the 784 injectors have four pinholes on the tip where-as the stock injectors only had one hole. The four holes allowed for better spray pattern verse the stock ones sprayed more like a squirt gun...?


Thanks for the info guys. Any particular brand o2 sensors you suggest I pick up?

 

[yossarian19]

While the 784 injector shoots a better spray pattern, it is still putting out too much fuel for a stock engine. The computer can only do so much to adjust fuel quantity. I think you are probably running rich - perhaps "very" rich.
If you still have your stock injectors, give them a try for a couple tanks and see what happens. I suspect you'll restore some of your lost MPG.
A 2000 4.0 is spec'ed with ~22.5 lbs/ hour injectors. That 25 lb injector is flowing 10% too much, I think.

 

[Johnnie Walker]

how are you calculating MPG? if your on stock gears, have you redone your speedo gear? if your getting between 200-250 miles off of a full tank, your doing pretty decent on mileage actually. go to stock tires, or regear.

one thing i did notice when reading... 2200 rpms seems a little low. im right at 2500 doing 75 on the highway. the 4.0 likes higher rpms, and operates more efficiently in the 2500 range.

I think that 2.2k-2.7k RPM is fine, at least from what I have read. With my 31's and 4.10 gears I sit about 2300rpm on the hwy with TC lockup. And my mileage is approx the same if not slightly better than when I was on 235/75s with stock 3.55s. I had no intention of running 31's, but got a great deal. When driving with the stock gears it sucked.

I am using the iphone app "Road Trip". I have not redone the speedo gear. I was reading into that and I think Quadratec said it only makes like a 5% overall difference. Although it's a start, at 5% I don't think its the cure.

My last tank of fuel I got 215 miles out of (17.42 gallons).

In your mileage/fuel calcs did you add the 5%? If not then your actual mileage driven would be closer to 226 which would be approx 13mpg. If that was city/mostly city. Depending on the terrain/actual speeds is not that bad.

Your mechanic was incorrect. The downstream cat is there from the factory. 2000 and 2001 XJs run the precats up front plus the down stream cat as opposed to just the downstream on 99 and down models. Gear could be a part of the problem, but probably not too major. I only lost 1-2 mpgs going up to 30s on stock gears.

If you are running stock gears (regardless of manual or auto), the fact you are running 31's is taking a toll on your mileage.

That 784 is an 25 lb/hr injector. Why you need that on a 4.0L I have no idea.

I always kept the RPM's under 2200Rpm. But after reading this I can see how that could be hurting me in some cases.



I read that the 784 injectors have four pinholes on the tip where-as the stock injectors only had one hole. The four holes allowed for better spray pattern verse the stock ones sprayed more like a squirt gun...?


Thanks for the info guys. Any particular brand o2 sensors you suggest I pick up?

I know that the 784 are plug n pray for the later models, whereas the 704/most of the other injectors use the earlier harness connectors. But yes, too much fuel. A FPR could help with this though?

Unless you know the last time they were done, replace your O2's

This can help. If nothing changes, the only thing affected is your wallet.


AS with anything, not all vehicles (even same make/model) are not created equal.

 

[yossarian19]

Other thing I wanted to mention is that O2 sensors can create piss-poor fuel economy without throwing codes. When a code is thrown, it is "bad-bad". It can be "bad" without throwing a code, though. Ask me how I know.

 

[Talyn]

I know that the 784 are plug n pray for the later models, whereas the 704/most of the other injectors use the earlier harness connectors. But yes, too much fuel. A FPR could help with this though?

A fuel pressure regulator for his 2000 would run around $300 from Hesco, which is the only company I know of that supplies one. Its the returnless fuel system. A better fit would be the Bosch XF2E-C4B

 

[techno1154]

A fuel pressure regulator for his 2000 would run around $300 from Hesco, which is the only company I know of that supplies one. Its the returnless fuel system. A better fit would be the Bosch XF2E-C4B

Why would he need a fuel pressure regulator? Would the regulator not reduce the pressure to the injectors causing the spray pattern to change?
I must admit that I do not know too much about gasoline engine injectors but on the diesel engines, the fuel(diesel) injectors need to receive the proper pressure or above (to achieve a proper squirt pattern) for the engine to burn the fuel properly. It was mentioned earlier that the injectors in use now needed higher pressure and flowed more fuel than the XJ OEM injectors. Personally, I would go the other way ie; replace the injectors with the correct ones.

 

[Talyn]

He would need a FPR to reduce the pressure so his injectors would appears smaller. It wasn't my idea. It is true that injectors are designed to run at a certain pressure, but I don't know the range nor how much he would have to reduce the pressure. Best idea would to go to a better sized injector.

 

[bigalpha]

Just to verify the phone app... Pick a chunk of highway you like and zero out both the trip odometer on hte Heep and your phone. Run down the road for at least 10 miles. The more the better and then stop at another mile marker. Compare the phone and the trip odometer to the actual, physical miles traveled.

Are those mileage signs pretty accurate? I've always wondered about that.

I just tested my truck and found the following: After 60 miles of driving (according to the mileage signs), I read 63 miles on the odometer; so my odo is reading 5% too high. I think it's because I'm running 215 tires.