MPG going upward

racertracer · **Posted:** Thu Mar 14, 2013 6:18 pm

I have been having bad MPG for a while now and trying to figure out why they were so low..... 17 in town and 22 hwy.

A seized universal joint was the culprit. The MPG jumped to 22 and 25.

naturist · **Posted:** Thu Mar 14, 2013 6:24 pm

Good catch!

One more thing to suggest to folks who come on here asking for tips on improving their mileage.

I just filled up this morning after 535 miles cruising the highways at 55-70 mph and got 28.74 mpg. Not bad on winter diesel, although those are the very conditions that should deliver peak mileage (save for the winter diesel part).

geordi · **Posted:** Thu Mar 14, 2013 6:43 pm

That is dammn good mileage for winter diesel in these things... But I couldn't ever go that slow for that long - I'd lose my mind with boredom.

Summer diesel and that trip could be good for another 3-4 mpg.

MtnClimber · **Posted:** Thu Mar 14, 2013 6:57 pm

racertracer wrote:

I have been having bad MPG for a while now and trying to figure out why they were so low..... 17 in town and 22 hwy.

A seized universal joint was the culprit. The MPG jumped to 22 and 25.

Which universal joint?

racertracer · **Posted:** Thu Mar 14, 2013 7:19 pm

Rear drive shaft at differential. I replaced them both, front and rear since I was in there.

The tranny re-learned shift points and now the gears flow into place smooth as silk.
I never knew that a universal joint could be such a drag on the transmission and the whole drivetrain.

thermorex · **Posted:** Thu Mar 14, 2013 7:44 pm

Good to know, thanks for sharing!

papaindigo · **Posted:** Thu Mar 14, 2013 8:59 pm

Just a reminder that bad drive shaft "U" joints can cause all sorts of odd vibration problems although lost mpg is a new one on me, makes sense though, and keep in mind that it's apparently easy to miss bad "U" joints if you attempt to check them with the drive shaft on the vehicle. There are a number of posts to the effect that "U" joints looked fine on the vehicle but when the drive shaft was dropped the "U" joint bearings were found to be shot.

LS12FAST4U · **Posted:** Thu Mar 14, 2013 10:45 pm

535 out of one tank? Dude i drive all freeway to and from work everyday about 80 miles round trip with my cruise set at 65 and I get 430 per tank and I neverlet my gas light come on. It's usually half way between a 1/4 tank and empty.

CATCRD · **Posted:** Fri Mar 15, 2013 12:26 am

LS12FAST4U wrote:

535 out of one tank? Dude i drive all freeway to and from work everyday about 80 miles round trip with my cruise set at 65 and I get 430 per tank and I neverlet my gas light come on. It's usually half way between a 1/4 tank and empty.

Well there are almost 3 gallons left in the tank after the light comes on, so you're probably leaving 4 or 5 in there. That's enough to get you equal to his 535. My record is 575, and maybe had a gallon left.

Anchorism · **Joined:** Wed May 23, 2012 6:31 am **Posts:** 71

racertracer wrote:

I have been having bad MPG for a while now and trying to figure out why they were so low..... 17 in town and 22 hwy.

A seized universal joint was the culprit. The MPG jumped to 22 and 25.

I should check on mine then thats exactly what i was getting!! BUT seeing as how its been so long since auto tec i forget how to properly check for such a problem. Stupid me...

DOC4444 · **Posted:** Fri Mar 15, 2013 7:32 am

Racer, so what DID lead you to the u-joint having bad bearings? Isn't your current KJ pretty low on miles?

DOC

dirtmover · **Posted:** Fri Mar 15, 2013 10:39 am

papaindigo wrote:

Just a reminder that bad drive shaft "U" joints can cause all sorts of odd vibration problems although lost mpg is a new one on me, makes sense though

Yeah, mine had a vibration on deceleration at low speed. I've just checked my fuel economy data around this time and nothing stands out immediately prior to having the u-joint replaced.

Of course, this mpg drop doesn't make any sense at all. Basic physics tells us that dissipating the amount of energy required to cause this mpg drop in such a small part would cause it to melt :ROTFL:

Look at it this way, brake drag causing a similar mpg drop would cause the rotor to become pretty darned hot and it's massive compared to the u-joint

The mpg change is more likely seasonal variation coupled with the transition from winter blend. My own fuel economy has gone up by 3 mpg over the last 6 weeks, as it does at this time every year.

thermorex · **Posted:** Fri Mar 15, 2013 11:26 am

I never had a u joint or cv joint failing on me, probably bc I lube the heck of them very often... But it does have sense if you think bc brake can drag just enough to make the pads get warm, not hot, and that can knock down couple miles a gallon in a long drive. Same as with a joint, even though a rusty-stuck one would become unstuck in matter of few miles of spinning... So on the other hand it doesn't sound logical... But if racer said that, I don't see why wouldn't be true. Could be possible to be other factors he missed?

racertracer · **Posted:** Fri Mar 15, 2013 1:13 pm

DOC4444 wrote:

Racer, so what DID lead you to the u-joint having bad bearings? Isn't your current KJ pretty low on miles?

DOC

It is pretty low on miles, but the Previous owner never washed the underneath, he just left the road salt to sit.

Rusted and fused toghether.

racertracer · **Posted:** Fri Mar 15, 2013 1:22 pm

thermorex wrote:

So on the other hand it doesn't sound logical... But if racer said that, I don't see why wouldn't be true. Could be possible to be other factors he missed?

At first I thought I needed a new TC ... again. The shake was so bad that it caused my teeth to tickle my gums. the ujoint was frozen.

The jeep came from the Boston area. I believe that the previous owner just let it sit in his driveway after going through mounds of wet road salt and never washing the underneath.

I just can't believe the difference, really. It's night and day, I love it even more now.

kjjet · **Posted:** Fri Mar 15, 2013 1:28 pm

Your the 2nd i've heard had to replace them. The 1st was due to vibration. Going to have a close look at mine. racertracer....did you see anything prior to removing?

My MPG went up when I rebuilt the Emergency brake's. I can get as much as 550 out of a tank in the CRD. But over 700 in the TDI. That thing rocks!

kissfan79 · **Posted:** Fri Mar 15, 2013 1:30 pm

Racer.....how difficult was it to remove the u-joint? Looking at mine....they are pretty rusty and have that "these are going to be a pain to break loose" look. Did you use a large socket on one end and a small socket on the other end with a c clamp to press the end cap out?

Jim

thermorex · **Posted:** Fri Mar 15, 2013 6:08 pm

racertracer wrote:

thermorex wrote:

So on the other hand it doesn't sound logical... But if racer said that, I don't see why wouldn't be true. Could be possible to be other factors he missed?

At first I thought I needed a new TC ... again. The shake was so bad that it caused my teeth to tickle my gums. the ujoint was frozen.

The jeep came from the Boston area. I believe that the previous owner just let it sit in his driveway after going through mounds of wet road salt and never washing the underneath.

I just can't believe the difference, really. It's night and day, I love it even more now.

There are people that don't even think those things are supposed to be lubed/greased every 3000 miles... White grease on the rings, grease all over.

And yes, never heard of such a stuck joint, lol. Glad you figured it out!

racertracer · **Posted:** Fri Mar 15, 2013 6:40 pm

kissfan79 wrote:

Racer.....how difficult was it to remove the u-joint? Looking at mine....they are pretty rusty and have that "these are going to be a pain to break loose" look. Did you use a large socket on one end and a small socket on the other end with a c clamp to press the end cap out?

Jim

Just go easy and take your time. Use a lot of break free rust penetrant. Try not to break the bolt.

Srytrucker · **Posted:** Fri Mar 15, 2013 6:52 pm

kjjet wrote:

Your the 2nd i've heard had to replace them. The 1st was due to vibration. Going to have a close look at mine. racertracer....did you see anything prior to removing?

My MPG went up when I rebuilt the Emergency brake's. I can get as much as 550 out of a tank in the CRD. But over 700 in the TDI. That thing rocks!

I need help with that as my e brake setup is rusted and causing sticking on rear brakes , in turn loss of MPG, actually that also caused my rear rotors to wear uneven too.
So can you please give me the part numbers and source from where you got them.thanks

MPG going upward

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