Topic: 31's and 4:10's and 65mph's

[mitchh]

My 97' is having troubles keeping hwy speeds these days.  Some days I can run it at 75mph no problem.  Im running 31's 4:10's on a 2.5 rough country lift. Ive just replaced my upstream o2 sensor. Full tune up cap rotor plugs wires air filter. I was wishing that that would help me out. My mpg's are also suffering a bit to 10-12 I still have a slight bucking studder with what sounds like a back fire through the Tb. with the engine cold, second gear @ 1800ish rpm's. Any Ideas what could be the problem?

[sharpxmen]

backfire thru t/b usually means lean mixture

btw, is that speed corrected or indicated? (since you have 31'' tires with stock gearing).

[mitchh]

I should have stated that the speed is taken with my Gps. Ya know When I changed my plugs they were a bit on the white side. Not overly but they did look lean. Thats also why I changed the upstream o2. What else should I look in to. Map or vac leak?

[sharpxmen]

I should have stated that the speed is taken with my Gps. Ya know When I changed my plugs they were a bit on the white side. Not overly but they did look lean. Thats also why I changed the upstream o2. What else should I look in to. Map or vac leak?

a vac leak would result in high rpm at idle, won't lean your mixture (this is b/c it uses a map sensor, a carbed or MAF would result in leaner mixture) unless the leak is on the MAP vacuum line.

bad tps or cps could cause this as well, but without a CEL code would be hard to tell for sure. Fuel filter or pump might cause this also.

Is it doing it when you're at WOT (pedal to the floor)? or doesn't matter?

if it's at WOT then it's not related to the O2 sensor and could be a fuel delivery issue, if it's across the board then it could be something else and not related to fuel pump/pressure/filter/injectors

[chrisfranklin]

MAP sensor is something I never got a code on, but replacing my original, probably 17 year old unit with a new one -- approx. $60-$70 -  seemed to help my throttle response and drivability (unexpectedly).   Currently I am getting ready to modify and install a new CPS also.   

The other sensors to pay attention to are the TPS, CTS, IAT sensor.  IAC valve also.  None of them are that expensive to replace but if you want to save a little money, you can check the voltages on them and make sure your originals are up to spec.   

I just mention this, because, if you've done all the basic tune-up work and your fuel pressure is also up to spec, then you might be looking for other things that could be effecting driveability. If you don't find any vacuum leaks and your sensors look to be the originals, maybe give the sensors a cleaning, voltage check, and/or replacement could help. 

31s and 4:10s is my setup also

[sharpxmen]

iac only matters at idle, has no influence when you're on the hwy - if it holds idle fine there's nothing wrong with it.

[mitchh]

I just took the Tb off today and gave it a good cleaning. The last time I checked the tb it was so clean. This time it was pretty gummed up. I also noticed that my Intake tube was folded inward on the bottom side of the tube. (Between the airbox and the box that sits on the tb) Im sure that it was sucking dirt and crap through there.  I havent took it out on the hwy yet. I was going to clean the IAC. but I didnt have the damn tamper proof torx. Either way my idle is good.
No the bucking happens a t 1/2 throttle. I let off and hit the gas again and it goes away. What are the symptoms of a bad Map and tps?

[bootguy]

Have you changed the speedometer gear?  It sends info that the other sensors use.  If it is not changed out your engine runs good at idle but when driving all the sensors get out of whack.  Cost about $65.

[sharpxmen]

Have you changed the speedometer gear?  It sends info that the other sensors use.  If it is not changed out your engine runs good at idle but when driving all the sensors get out of whack.  Cost about $65.

i'm not saying he shouldn't change the gear but it won't cause what he describes, i drove mine with the stock gear with both 31s and 33s. I had the same thing happening and it was a bad tps in my case (could be something different with his, not saying that's what he should replace) - i did get a CEL and the tps code for it.
sensors won't get out of whack due to bad speedo gear other than the speed sensor (but it's a good idea to change it, will give accurate information to the PCM)

[jcrockarell9937]

I just noticed this too.  I was running stock wheels with 31s. Then i replaced the wheels with ones that have a bit more offset and a more agressive tire(still a/t). Could the wheel offset and more agressive tires slow the jeep down?

[sharpxmen]

I just noticed this too.  I was running stock wheels with 31s. Then i replaced the wheels with ones that have a bit more offset and a more agressive tire(still a/t). Could the wheel offset and more agressive tires slow the jeep down?

yes, you have more frontal section which is what uses most of the power at speed. same goes if the tire is taller, if there is lift and if the tires are wider (all ads up).