0331 head swap, mpg change?

[indy_xj]

2000 XJ
54,xxx miles
AW4/NP231

I swapped my cracked 0331 head for a '98 630 and an 'aftermarket' header. The one with the acordian expansion chambers.

That was about 1100 miles ago. Since then, my milage has gone down the drain. I used to average about 17.5 mpg and 230-240 miles / tank.

Now I am getting about 13.5 mpg, and my last tank was a tiny 180 miles.

Its mostly city driving, but it was before the change also. No check engine lights or codes since the swap.

Anyone else have similar experiences, results?


thanks,

ixj

[BIGSLVRXJ]

Originally Posted by indy_xj

2000 XJ
54,xxx miles
AW4/NP231

I swapped my cracked 0331 head for a '98 630 and an 'aftermarket' header. The one with the acordian expansion chambers.

That was about 1100 miles ago. Since then, my milage has gone down the drain. I used to average about 17.5 mpg and 230-240 miles / tank.

Now I am getting about 13.5 mpg, and my last tank was a tiny 180 miles.

Its mostly city driving, but it was before the change also. No check engine lights or codes since the swap.

Anyone else have similar experiences, results?


thanks,

ixj

I can't give you much insight on the head swap, but I know those "headers" with the expansion joints are pretty much a copy of the stock manifolds, and they crack almost as fast. As long as yours isn't cracked though, I would look past that. I honestly can't think why swapping heads would be the problem other than the fact that the 0630 has bigger exhaust ports and thus possibly produces more power since you have the 99+ intake and 99- exhaust. Just a thought, I can tell you that's about what my 99 XJ gets mpg wise. I forgot to ask...what did you do about the dual cats coming off your '00 exhaust manifold, did you hook them back up somehow or delete them?

[indy_xj]

Thanks for the reply.

Originally Posted by 90PioSport99

I can tell you that's about what my 99 XJ gets mpg wise.

But according to your sig, you have a 3" lift and 31s. Mine is completely stock. I would hope for slightly better milage than something that is lifted.

Originally Posted by 90PioSport99

I forgot to ask...what did you do about the dual cats coming off your '00 exhaust manifold, did you hook them back up somehow or delete them?

Mine had federal emissions, not cali (only the '01 were ALL cali emissions) I have the standard 1pre-cat, 1post-cat O2 sensor and only the main exhaust cat.

ixj

[BIGSLVRXJ]

Originally Posted by indy_xj

Thanks for the reply.


But according to your sig, you have a 3" lift and 31s. Mine is completely stock. I would hope for slightly better milage than something that is lifted.


Mine had federal emissions, not cali (only the '01 were ALL cali emissions) I have the standard 1pre-cat, 1post-cat O2 sensor and only the main exhaust cat.

ixj

Okay, you're right, I do have about a 3" lift and 245s which equal out to be roughly 31"s. To tell you the truth, I'm not sure if that much lift affects a whole lot, but it may. I thought all '00s and '01s had the triple cat system so thanks for clearing up that confusion. That's about all I can tell you, but I would really suggest that it might just be because your Jeep is more or less using a 99 setup now, except the coil pack rail. Hopefully someone else will have insight as well that has actually done this, that's just my guess.

[keep0njeepin0n]

Not to nit pick, but according to your numbers you posted, you're actually using less fuel when calculating MPG than you were before...what I mean is plug your numbers into a calculator and it shows that NOW you're filling up just a tad sooner than before....maybe that little bit of difference is where your miles went.

[BIGSLVRXJ]

Originally Posted by keep0njeepin0n

Not to nit pick, but according to your numbers you posted, you're actually using less fuel when calculating MPG than you were before...what I mean is plug your numbers into a calculator and it shows that NOW you're filling up just a tad sooner than before....maybe that little bit of difference is where your miles went.

I think what he's saying is that he has to go get gas more often because it's gulping it down faster now.

[keep0njeepin0n]

No, I know what he's saying....but what I'm saying is that he's getting gas sooner now than he was before...example...he used to get gas after using 15 gallons, but now gets gas after 13.5 gallons....not actual numbers, just an example of what I meant

[indy_xj]

90Pio,

Thanks for your thoughts. You might be right, this may just be what i get now with this 'set up.' I was hoping it would return closer to normal, but after 1000+ miles it looks like i may be stuck with it.

I put some seafoam in this tank of gas to give the injectors a good cleaning, etc. We'll see if that makes any difference.

thanks

ixj

[BIGSLVRXJ]

Originally Posted by indy_xj

90Pio,

Thanks for your thoughts. You might be right, this may just be what i get now with this 'set up.' I was hoping it would return closer to normal, but after 1000+ miles it looks like i may be stuck with it.

I put some seafoam in this tank of gas to give the injectors a good cleaning, etc. We'll see if that makes any difference.

thanks

ixj

That Seafoam can work wonders sometimes, let us know if it helps you any.

[Kittrell]

Originally Posted by keep0njeepin0n

No, I know what he's saying....but what I'm saying is that he's getting gas sooner now than he was before...example...he used to get gas after using 15 gallons, but now gets gas after 13.5 gallons....not actual numbers, just an example of what I meant

Pretty sure that was miles per gallon, not actual gallons of gas...........

[keep0njeepin0n]

Yes what he was saying was MPG, but if you do the math, you can see when how many gallons he's used before filling up...it's just simple math!

[indy_xj]

Originally Posted by keep0njeepin0n

...example...he used to get gas after using 15 gallons, but now gets gas after 13.5 gallons....not actual numbers, just an example of what I meant

I havent acctually got out the pencil and slide rule for this one. Its based on a lot of perception. But here's the deal:

I usually stop for gas when the needle is around 1/8th, but before the idiot light comes on. This usually leads to around 15-16 gallons of fillup. That hasnt changed since the swap. I reset the trip odo ever tank. I used to always get 230-240 miles/tank. If a tank had some highway miles i could get 260/tank. Now I am coming in at 200/tank. You might be right. If i drove that extra 2 gallons out of the tank i would be getting a similar 230/tank. But, my mpg numbers are so far off. The mpg calc i am using is from the overhead console. I know its not exact, but i think its margin of error is better than 20% (17.5 vs. 13.5).

thanks,

ixj

[birchlakeXJ]

I would calculate your mileage pump-to-pump with a calculator and a notebook for a few weeks. See if there is any difference. Concentrate on just the MPG calculation, not the amount of miles. MPG tells the whole story.

Also remember that winter brings lower gas mileage with colder temps, longer closed loop, reformulated oxygenated fuels, etc. I highly doubt that the head is responsible for a significant mileage decrease.

You could try replacing your "upstream" oxygen sensor if it's old as they become sluggish and can't optimize the fuel/air ratio as well over time. Simple tuneup stuff too can help optimize gas mileage. Oh and easy on the right foot can't hurt right??

[indy_xj]

birch,

All good suggestions. I stopped driving the jeep around oct. and it sat for about 2 months until i got the head swapped in dec. Winter gas could be contributing.

I had thought about the pre-cat O2 sensor, but I didnt want to sink $60-70 into a fix if that was not it.

I'll calculate mpg based on fillup for the next couple of tanks to see what is going on.

thanks

ixj

[Hawg130]

Hey Indy, what did you do about securing the coil pack? I'm considering doing this and need some pointers. Thanks.