CallmeMaybe Posted December 9, 2012 Share Posted December 9, 2012 So finally took my MJ out after getting everything fixed up and drove it down the highway. With my foot to the floor (i really mean pedal all the way to the floor) in 5th i could barely keep speed at 65mph any ideas? Then took Ol girl out to some back roads to see how she handled on rough patches, went to turn around and sunk in the sand. No biggie 4x4 right?.....Nope, back tires spinning front tires stationary. Dash light comes on but nothing. Link to comment Share on other sites More sharing options...
Pete M Posted December 9, 2012 Share Posted December 9, 2012 one of the stickied threads in the DIY forum will show you how to fix the CAD in the front axle once and for all. :thumbsup: how big are your tires? Link to comment Share on other sites More sharing options...
ftpiercecracker1 Posted December 10, 2012 Share Posted December 10, 2012 2.5 or 4.0 gearing and tire size also play a huge part in power. Link to comment Share on other sites More sharing options...
CallmeMaybe Posted December 10, 2012 Author Share Posted December 10, 2012 4.0, with 31's. Link to comment Share on other sites More sharing options...
Pete M Posted December 10, 2012 Share Posted December 10, 2012 there's your problem. you've got 3.07 gears in those axles. :( 3.07s suck. upgrading to axles with 4.10s (or even 3.55s if you can't find a pair with 4.10s) will bring the power back. :thumbsup: Link to comment Share on other sites More sharing options...
mvusse Posted December 10, 2012 Share Posted December 10, 2012 There are three vacuum lines going to the axle, one to engage it, one to disengage it, and one to turn on the 4wd light when engaged. If the light comes on, the axle is working, so the problem must be in the transfer case (or missing a front driveshaft). As far as no power with 31" tires, it's not usually that bad with an automatic transmission, but that gives you 3.55 gears. I had 29" tires on the only vehicle I have had with a 4.0/5 speed and 3.07 gears, though and had no problem getting up to 65, 75 or even 90, or, on flat ground, maintaining any speed above 35mph. I really doubt 7% less torque because of the larger tires would make that much of a difference, so you may have engine problems as well. Link to comment Share on other sites More sharing options...
Eagle Posted December 10, 2012 Share Posted December 10, 2012 There are three vacuum lines going to the axle, one to engage it, one to disengage it, and one to turn on the 4wd light when engaged. If the light comes on, the axle is working, so the problem must be in the transfer case (or missing a front driveshaft). As far as no power with 31" tires, it's not usually that bad with an automatic transmission, but that gives you 3.55 gears. I had 29" tires on the only vehicle I have had with a 4.0/5 speed and 3.07 gears, though and had no problem getting up to 65, 75 or even 90, or, on flat ground, maintaining any speed above 35mph. I really doubt 7% less torque because of the larger tires would make that much of a difference, so you may have engine problems as well. This is incorrect. I have had the light come on when the CAD was not functioning. Loss of power could mean a failing catalytic converter. It could also mean, especially if the truck has been sitting, that a mouse family has taken up residence somewhere in the exhaust system and the nest is restricting exhaust flow. Link to comment Share on other sites More sharing options...
glundblad Posted December 10, 2012 Share Posted December 10, 2012 I have an '87 with 4.0 and 3.07 gearing/5 speed. I feel the engine and truck in general run like they did when new (I am the original owner). I never understood the comanche commercials when they came out. They claimed the high amount of torque & HP yet mine struggled to move down the highway on it's own. Then I installed a tach a few years ago and saw how low the RPM's were running as I went down the highway. Now I treat it as a 4 speed and use 5th only on highway driving at 65+mph. Jeep should have never sold the 4.0 engine with that high of gearing. If it had 4.10/3.73 or even 3.55, I bet it would have made a world of difference. Link to comment Share on other sites More sharing options...
Eagle Posted December 10, 2012 Share Posted December 10, 2012 I have an '87 with 4.0 and 3.07 gearing/5 speed. I feel the engine and truck in general run like they did when new (I am the original owner). I never understood the comanche commercials when they came out. They claimed the high amount of torque & HP yet mine struggled to move down the highway on it's own. Then I installed a tach a few years ago and saw how low the RPM's were running as I went down the highway. Now I treat it as a 4 speed and use 5th only on highway driving at 65+mph. Jeep should have never sold the 4.0 engine with that high of gearing. If it had 4.10/3.73 or even 3.55, I bet it would have made a world of difference. I agree completely. Same with my '88 Cherokee, which I bought new. The difference is that I bought mine with a tachometer, so I always knew how slow the engine turns, and I have always treated it as a 4-speed except on the highway. Even on highways, I have to downshift for passing if there's any traffic. And that's with stock tires. With 31s, 5th gear becomes almost useless. With 31" tires and 3.07 gears, in 5th gear at 65 the engine is only turning at 1696 RPM. That's several hundred RPM below the torque peak. Even 70 MPH is only 1827 RPM. 3.55s are better, but still not "good." IMHO the minimum ratio for 31" tires is 3.73, and even that only gets you to the equivalent of stock tires with an automatic. 4.10s is what I think is the optimum ratio with 31s. Link to comment Share on other sites More sharing options...
HOrnbrod Posted December 10, 2012 Share Posted December 10, 2012 3.55s are better, but still not "good." IMHO the minimum ratio for 31" tires is 3.73, and even that only gets you to the equivalent of stock tires with an automatic. 4.10s is what I think is the optimum ratio with 31s. :agree: With 27" tires, 3.55 gears, and cruising in AW4 O/D (.75) @ 70MPH, the engine turns @ 2319 RPM. With 31" tires, 4.10 gears, and cruising in AW4 O/D (.75) @ 70MPH, the engine turns @ 2333 RPM. Nearly the same. Link to comment Share on other sites More sharing options...
Dadinator Posted December 10, 2012 Share Posted December 10, 2012 I think mine has the 3.73's. It was originally a auto with the big ton package. It is now a 5 sp and goes down the highway just fine. The reason for the high gearing was the OEM trying to get the fuel mileage up. Link to comment Share on other sites More sharing options...
87MJTIM Posted December 10, 2012 Share Posted December 10, 2012 I have an '87 with 4.0 and 3.07 gearing/5 speed. I feel the engine and truck in general run like they did when new (I am the original owner). I never understood the comanche commercials when they came out. They claimed the high amount of torque & HP yet mine struggled to move down the highway on it's own. Then I installed a tach a few years ago and saw how low the RPM's were running as I went down the highway. Now I treat it as a 4 speed and use 5th only on highway driving at 65+mph. Jeep should have never sold the 4.0 engine with that high of gearing. If it had 4.10/3.73 or even 3.55, I bet it would have made a world of difference. :agree: 5th gear was/is overdrive on the highway. Have to downshift to 4th to get out of my own way. Original owner of an '87 BA10. We had a '91 XJ (AX15) and 5th gear could excelerate up hill, but not the BA10. Link to comment Share on other sites More sharing options...
Eagle Posted December 10, 2012 Share Posted December 10, 2012 :agree: With 27" tires, 3.55 gears, and cruising in AW4 O/D (.75) @ 70MPH, the engine turns @ 2319 RPM. With 31" tires, 4.10 gears, and cruising in AW4 O/D (.75) @ 70MPH, the engine turns @ 2333 RPM. Nearly the same. Who has 27" tires? That's a 205/75-15. Yes, that was the base tire in the early years, but most Mjs and most XJs probably came with 225/75-15s, which are slightly larger than 28". Link to comment Share on other sites More sharing options...
Eagle Posted December 10, 2012 Share Posted December 10, 2012 I think mine has the 3.73's. It was originally a auto with the big ton package. It is now a 5 sp and goes down the highway just fine. The reason for the high gearing was the OEM trying to get the fuel mileage up. Too many people don't understand how that works. All other factors being equal, the most economical engine speed is the speed (RPM) at which it produces maximum torque. In a real-world vehicle, various drag factors increase as road speed increases but, in general, the 4.0L engine will not produce maximum fuel efficiency when run below 2000 RPM. Somewhere around 2500 RPM at highway cruise seems to be the sweet spot. Link to comment Share on other sites More sharing options...
Dadinator Posted December 10, 2012 Share Posted December 10, 2012 Too many people don't understand how that works. All other factors being equal, the most economical engine speed is the speed (RPM) at which it produces maximum torque. In a real-world vehicle, various drag factors increase as road speed increases but, in general, the 4.0L engine will not produce maximum fuel efficiency when run below 2000 RPM. Somewhere around 2500 RPM at highway cruise seems to be the sweet spot. Yup. Link to comment Share on other sites More sharing options...
HOrnbrod Posted December 10, 2012 Share Posted December 10, 2012 Who has 27" tires? Me, that's who. That's what was on my rig when I bought it. Link to comment Share on other sites More sharing options...
mvusse Posted December 11, 2012 Share Posted December 11, 2012 Purple People Eater (originally 2wd 4 cylinder/AX4) is supposed to have 27" tires, but I run 29" (235/75R15). Speedo is off about 6 or 7% which is how I know. Door sticker is missing. Wilbur and Sparkles came from the factory with 28" (225/75R15), both are 4.0/automatic, Sparkles originally 2wd. Booger (4.0/AX15) was supposed to have 215/75R15. These three according to the door sticker. What I find amusing is in two of the four, the glove box lists tire inflation numbers for 195/75R15 (smaller than the smallest from the factory AFAIK), 205, 215 and 225. 235/75R15 is not listed (and can sometimes contact the lower control arms) but seems to be the most common size tire run on our Jeeps after the originals wear out. Actually, I also think it is the most common tire size manufactured. Link to comment Share on other sites More sharing options...
Eagle Posted December 11, 2012 Share Posted December 11, 2012 Me, that's who. That's what was on my rig when I bought it. You're even more of a purist than I am, then (at least in some ways). My excuse is that I have a fleet of multiple MJs and XJs, different years and different models. It's just easier all around to standardize as much as possible on wheels and tires so I can swap across vehicles without upsetting the balance of the universe. Link to comment Share on other sites More sharing options...
Eagle Posted December 11, 2012 Share Posted December 11, 2012 What I find amusing is in two of the four, the glove box lists tire inflation numbers for 195/75R15 (smaller than the smallest from the factory AFAIK), 205, 215 and 225. 235/75R15 is not listed (and can sometimes contact the lower control arms) but seems to be the most common size tire run on our Jeeps after the originals wear out. Actually, I also think it is the most common tire size manufactured. I don't know about the most common, but I do know it's getting more and more difficult to find ANY 15" tires suitable for a Jeep. I thought Goodyear had completely dropped the original Wrangler Radial, then I discovered that Wal-Mart still has it ... only in size 235/75-15. And it is still listed on the Goodyear web site ... in only that size. I think that the original is far better than several of the newer Wrangler XYZ series tires that Jeep has been using as OEM, so I just grabbed a set from Wal-Mart while I could get them, and put them in the garage rafters until Spring. As soon as I can afford it, I'll gather up another set of decent OEM Jeep alloys and buy another set before Goodyear discontinues the 235s. Link to comment Share on other sites More sharing options...
HOrnbrod Posted December 11, 2012 Share Posted December 11, 2012 You're even more of a purist than I am, then (at least in some ways). My excuse is that I have a fleet of multiple MJs and XJs, different years and different models. It's just easier all around to standardize as much as possible on wheels and tires so I can swap across vehicles without upsetting the balance of the universe. :yes: The 205's are long gone. Went to 235's, then 30", now 31". Happy now. Big enough for me. Link to comment Share on other sites More sharing options...
Dadinator Posted December 11, 2012 Share Posted December 11, 2012 For the use my trucks get, 31 is the size for me too. Link to comment Share on other sites More sharing options...
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