Can't get better than 15mpg

[kf_chris]

So I have a slight concern
Running a 4.6 with probably lower than 9 points compression, good cam, good build. 24lb Ford injectors

I cannot for anything get more than 15 mpg. No matter what driving, highway, city, heavy throttle, light...it's all the same
I used to have 3.55 gears and a 8.25, now it has 4.10s and a 8.8. No change

Built an HHO and those are 'supposed' to give 30% mileage increase but I didn't see jack

I'm starting to give up on having a jeep that does good with gas, even though I didn't build it for that. I just thought with all the extra power it would be better. I'm BARELY on the gas most of the time because the low end tq is amazing and the throttle is snappy and crisp. The engine runs great, never misses or pings. I only run 87 octane.

Any thoughts? Advice?
I"m going to pull the plugs probably tomorrow and see if the engine is running fat or thin.

[Bodo]

Take the 'HHO' off. It's crap and will only damage your motor.

My 96 XJ got 18MPG in town and on the highway. You're not that far off. It's the nature of the beast I'm afraid.

[kf_chris]

HHO's been off. It didn't do it's job so it was removed.
I'm ok with it, but I have a hard time with not getting better mpg when I'm properly geared and only have 31's for tires. If they were bigger I'd understand.

Oh well I suppose. The power's worth it and even my fiance really likes driving the jeep so we're both happy. Thankfully her Honda goes far on a tank

[sudo chop]

I would think you are running excessively rich. But I don't think that's your whole problem. I don't think your low compression is helping either. I'm pretty sure higher compression and higher power per unit of fuel also translates to higher efficiency. Therefore the opposite would be true also.

[4.whoa]

The 4.10's with 31's aren't helping you much. I run the stock 3.55's with 31's with a 4.0 and get 19-20 hwy. with all that tq you don't really need the 4.10's without bigger shoes

[yuppiexj]

I may be blind but what model Jeep is this?

A Cherokee is a much more aerodynamic and lighter weight brick shape than a Wrangler.
Automatic or a stick?

I would expect 4.10s and 31s with an automatic trans in a Wrangler to get crappy mileage mo matter what you do.
I would expect 4.10s and 31s in a automatic Cherokee to get around 20 mpg (my stock 4.0 XJ with 4.56s and 32s got 22 MPG highway)

[kf_chris]

The 4.10's with 31's aren't helping you much. I run the stock 3.55's with 31's with a 4.0 and get 19-20 hwy. with all that tq you don't really need the 4.10's without bigger shoes

I'd agree with you BUT I got 15 mpg with 3.55's then a day later when I put the new rear axle in. I thought I'd see some change...but none happened. Gonna pull the plugs this morning and see how it's running.

[4.whoa]

this might be a dumb question ,but you did change the speedo gear to the correct one for the new ratio, right? that would make your figures WAY off.

[kf_chris]

all my calculations were with both a gps and a correct speedo gear


I took the plugs out and they looked really really good. So it's running perfect from what I can see, a touch lean if anything.

Problem now is that when I pulled the TB butterfly off to sand a burr off of it (throttle was sticking at running temps) I pulled the little rod that holds the TB butterfly out of the TPS. Now there is no throttle return at all. I bought a new TPS and it does the same thing. It idles really high and when you hit the gas the throttle stays where it stops and doesn't return. Any tips?

[unlimitedrubicon]

Did you check the throttle return spring?

[4.whoa]

you need to put the spring "tabs" in place.

003.jpg

Sorry, don't know how to turn the pic. the little tabs are in the lower center of pic. they have to be in that slot.

[kf_chris]

Is that on the throttle linkage on the front of the TB? I'll figure it out when I return from Peru, hopefully I have enough time between jobs to get the jeep running (and guzzling!).

Thanks for the help.

I need to figure out what compression I have.

Originally the engine was to have the stock 4.0 rods but when the builder mounted the pistons they came out higher than the top of the block. So we changed to 4.2 pistons. I have kb944 pistons....

[unlimitedrubicon]

You are using the shorter 4.2 rods with KB944 pistons? How far down are they? The stock 4.0 rods w/KB944s should not be proud of the deck unless your block was previously machined, or the machinist took too much off.

[kf_chris]

Actually I'm mistaken
I originally asked and wanted to have 944's in the engine but for some reason, can't recall now, we went with these

http://www.summitracing.com/parts/UEM-2229-030/

[kf_chris]

I used the compression calculator

static 9.92
dynamic 8.65

This looking right?

We didn't have the block decked, just cleaned I think.
I'm not entirely sure which head gasket we used.