2.5L Bogs down REALLY bad

[Comangi]

So I have finally got the 'Manche running. Replaced a whole lot of parts, including the motor mounts.

 

So i took it on it's first trip up the highway and it was..... bad. The thing bogs down REALLY bad on hills, I downshift to 4th and it doesn't get any power at all. I feel like the second half of the gas pedal doesn't do anything. Where should I start looking? Also, the thing is getting single digit gas mileage, it's worse than my 9MPG SRT8 Jeep.

 

Info: 6" lift and 32's (I know, with stock gears, this is a problem)

It's got vacuum leaks, I'm working on those.

It keeps leaking oil out of the filler cap, I even replaced the seal, still leaks

We removed the catalytic converter, where it was mounted was turning the pass side floor into LAVA. ha!

 

So I'm humming along at what the speedo (not reliable) tells me is 65 in 5th. I hit a hill, downshift to 4th, and it gives me zero extra umph, the thing just bogs and bogs until I'm going 45 and begging the hill to end. I'm afraid of dropping it to 3rd, as I don't know if the RPM will jump too high. The dang jeep is so loud with the tires, I can't hear the motor, and with no tach, I have NO clue where I'm at RPM wise.

 

Help!! What do I do?

[88whitemanche]

Mine does the same...just need to regear or just that your 2.5 is tired...

[Comangi]

Ha, I know my 2.5 is tired for sure. Any ideas on how to freshen it up?

 

I want to regear, but I plan to put 35's under it, so I don't want to have to gear it twice. Do you think i'll regret 35's even after a regear?

[88whitemanche]

Well I'm in the process of the regear process(saving money) I got 4.88s already and do plan on 35s....so we'll see how it plays out...

[Comangi]

I'd be very curious. I'll keep an eye out!

[Eagle]

Info: 6" lift and 32's (I know, with stock gears, this is a problem)

Correction: This is THE problem. The 2.5L was never a rocket engine to begin with. Let's face it, it's a conventionally-aspirated 4-cylinder engine trying to push a 3,000+ pound vehicle shaped like a brick. And the '86 version had less power than the Chryco MPFI version. Then you lift it 6 inches and throw 32" tires under it without changing the gear ratio -- what did you expect?

 

So I'm humming along at what the speedo (not reliable) tells me is 65 in 5th. I hit a hill, downshift to 4th, and it gives me zero extra umph, the thing just bogs and bogs until I'm going 45 and begging the hill to end. I'm afraid of dropping it to 3rd, as I don't know if the RPM will jump too high. The dang jeep is so loud with the tires, I can't hear the motor, and with no tach, I have NO clue where I'm at RPM wise.

 

Help!! What do I do?

Since you have a 5-speed, it's probably safe to assume you have 4.10 gears. With stock tires (for an '86 we'll assume that means 215/75-15), 65 MPH would have had the engine turning at 2512 RPM in 5th gear, or 3349 in 4th.

 

Now we switch to 32s. 2512 RPM is now actually about 74 MPH, so the frictional and aerodynamic drag factors are increased exponentially. And running at 2500 RPM (or even 3350 RPM when you drop to 4th) you are WAAAAY below the torque and horsepower peak of the engine. Your engine was rated (when new) for just 117 HP at 5,000 RPM, and 135 ft-lbs of torque at 3,500 RPM. And for the 2.5L, the torque peak really is a peak -- the 2.5L doesn't have a nice, long, flat torque curve like the 4.0L, it really peaks at 3,500 RPM and falls off sharply above and below that.

 

You unfortunately have unreasonable expectations of what your engine can do.

[Eagle]

I want to regear, but I plan to put 35's under it, so I don't want to have to gear it twice. Do you think i'll regret 35's even after a regear?

Yes, you will.

[Comangi]

Thanks Eagle, I appreciate the feedback. I think I knew, I just wanted to be sure maybe there wasn't another problem.

 

3rd gear gets me all the way to 50 MPH (on the speedo, with 265/70 R16's) so I assume i may be climbing some of these hills at the top of 3rd and bottom of 4th.

 

I'm honestly afraid to rev the motor too high, but it seems as though that's where the power is. so if I'm avoiding high revs, and the power is up at 3,500.... I'm avoiding the power peak, making things worse!

 

Thanks again.

[dasbulliwagen]

I have 32 inch JK take offs on mine with almost no lift and 4:10 gears and 5 speed. My earlier calculations figured 32 inches with 4:10's and only to 4th gear to be very close to the original 3:55 gears and stock tires to 4 the gear. If I were you I wouldn't ever worry about using 5th unless you were on a long downhill run. I don't and I don't have too many issues getting around and my fuel milage is nowhere near as bad as yours. But then again I'm not lifted. I love the 2.5's but I think if you want to go with bigger tires you'll need a 4.0 swap and bigger axles. Keep your expectations low and you'll be OK.

[Eagle]

Link to the tire/gear/speed spreadsheet. This is based on actual tire manufacturers' revolutions-per-mile data, so it's more accurate than most of the other calculators you can find on-line.

 

https://spreadsheets.google.com/pub?hl=en&hl=en&key=0AvuGamS7bzt-dHJxYy1QajhuVUFUbUVDUWdiZnVFbmc&output=html

[Comangi]

Thanks for the info guys. I'm thinking my newbie a$$ needs to stop hitting hills in 5th gear!

 

So, with the 2.5L, will 35's just be a terrible idea? even with gears?

[86FUBAR]

Sounds like my 2.8 when I lifted it 4" and put 31" on it ! What a freaking dog ! I drove it like that for probably 4 years . I regeared to 4:88 and and never looked back ! First I ran 33" then switched to 35" and it's still just as fast . Just stuff 4:88 in it and be done !

[Comangi]

I was thinking 4.88's. The chart says 4.56's give you a decent RPM in 5th, but that's with a decent motor! 4.88's should do the trick. I hope!

[Eagle]

So, with the 2.5L, will 35's just be a terrible idea? even with gears?

On 35s, a true 65 MPH will only be turning 1995 RPM in 5th gear. That's far, FAR below both the torque and horsepower peaks of the engine. Even in 4th gear it's only 2661 RPM -- still way below where the engine makes usable power.

 

4.88s will bring that up to 2375 RPM in 5th and 3167 in 4th. Better, but not great. In fact, it'll be just about the same as what you have now with the 32s.

[Comangi]

wow, so what gearing would you recommend for 35's? something even bigger?

[Jeep Driver]

wow, so what gearing would you recommend for 35's? something even bigger?

Have you gone through cruiser's renix tips yet?

 

EVERY ONE OF THEM?

 

 

Do nothing until you do that........that includes getting rid of your c101.

 

 

There is a reason you are getting single digit MPG and it has nothing to do with gearing.

[87Warrior]

I agree with Jeep Driver. Get that motor running right. Follow Cruisers tips. Don't worry about the C101 connector on the '86....it doesn't exist ;)

 

wow, so what gearing would you recommend for 35's? something even bigger?

Yes. But that will require new axles that can support the deeper gears. 4.88 is as deep as you can go in stock axles. I think my TJ Rubi is a dog on hills with 35's, 5.13's and a 4.0. Mnkyboys 2.5 is something you should look at.

 

I think the root of your problem is a tired motor. You can do a tune up and replace/clean sensors, but a rebuild is the only way to 'restore' what little power the 2.5 made when new. I know my 181K mile 2.5 with 30's and about 2" of lift is tired, and it requires a downshift to 4th at any hill on the interstate usually topping out at 70mph.

[Jeep Driver]

I agree with Jeep Driver. Get that motor running right. Follow Cruisers tips. Don't worry about the C101 connector on the '86....it doesn't exist ;)

 

wow, so what gearing would you recommend for 35's? something even bigger?

Yes. But that will require new axles that can support the deeper gears. 4.88 is as deep as you can go in stock axles. I think my TJ Rubi is a dog on hills with 35's, 5.13's and a 4.0. Mnkyboys 2.5 is something you should look at.

 

I think the root of your problem is a tired motor. You can do a tune up and replace/clean sensors, but a rebuild is the only way to 'restore' what little power the 2.5 made when new. I know my 181K mile 2.5 with 30's and about 2" of lift is tired, and it requires a downshift to 4th at any hill on the interstate usually topping out at 70mph.

 

 

I didn't ketch the '86.

 

 

My statement stands, however.

 

 

 

I've got a compression issue and a new engine waiting when I get the time........that said.......

 

I tool around town in 5th gear at 35-40 all the time on 31s. The idea that you have to be at the top of RPMs to get any grunt out of a 2.5 is just not true.

 

Interstate at 70 and I retain 5th.......overpasses and all.

 

 

Get it running right first.

[gogmorgo]

JP mag just announced that Revolution Gear & Axle has released 5.13's for the Dana 30 and 35. But I'll agree with everyone else that you should sort out your engine first... A good used motor will cost you less than a regear.

[mnkyboy]

I had 3.55s and 33s,i told people my AX4 was a 3spd with overdrive as 4th gear was useless,i daily drove it like that for a long time.

 

I later went to 4.88s and 33s,i regained use of 4th gear but still didnt feel like i needed a 5th gear.

 

Haven't driven the new build yet but i think its gonna turn the 37s just fine.

[Comangi]

Thanks for all the tips guys. I'm going to go do some reading.

 

I took it out on the highway again today and stayed in 4th, my issue was WAY WAY better, also the MPG on the highway was better. I guess I'm a dummy and don't know how to drive an old jeep! using 4th was a huge help. Now to go see about spiffing up my motor!

 

I think my biggest problem is that with the mud tires and zero interior (besides seats) the jeep is SO LOUD above 50MPH, that I can't hear the motor. Without a tach, I've got no idea where my RPM are. I guess I was hitting hills in 5th (didn't know any better) and was probably at very low RPM. Once I hit hills in 4th, at what I'm guessing is 2K RPM, it was buzzing along at 60, no problem.

[Eagle]

If your speedometer read 60, you were going closer to 70.

[Comangi]

Update: The comanche made a 320 mile round trip into the mountains this weekend. The hills were rough, but I only needed 3rd a few times. The jeep did awesome! I got out and back, logging trails, 2 trips up the mountain with 1K+ LBS of wood in the bed, bouncing around the muddy ruts of the trail, then back home. only used 3/4 of a tank! The alignment did a number on the tires, the fronts are almost completely shot now. They were close before.

 

Eagle, above you say if my speedo was at 60, i was doing closer to 70. Are you sure? at some points, I was cruising at close to 70 on the speedo. I can't imagine I was up close to 80 MPH. People were still passing me!

 

All-in-all, the jeep did amazing. Now time to figure out my alignment problems and replace the two front tires!

[Eagle]

Eagle, above you say if my speedo was at 60, i was doing closer to 70. Are you sure? at some points, I was cruising at close to 70 on the speedo. I can't imagine I was up close to 80 MPH. People were still passing me!

Yes, I'm sure. I posted the link to the spreadsheet that shows road speeds for various tire sizes and gear ratios. Look it up. Your factory speedometer is only accurate for the tire size that came on the truck. If you increase the tire size significantly without changing the speedometer drive gear, your road speed will be significantly faster than what the speedoemeter says, because the speedometer doesn't know you changed the tires.

 

Assuming your factory tire size was 215/75-15, the jump to a 31x10.50-15 is an increase of approximately 10 percent. That alone would suggest your true speed was 66 MPH.

 

The spreadsheet I compiled is based on actual tire manufacturers' revolutions-per-mile data, which is the most accurate way to correlate road speed to engine RPM. With 215/75-15 tires, 60 MPH in 5th gear with 4.10 gears is 2319 RPM. If I then go to the data block for 31x10.50s, that same RPM falls between 65 and 70 MPH. Interpolating, it looks to me like it would be about 67 MPH.

 

If the speedo is showing 70, your road speed is about 77 to 78 MPH.

[HOrnbrod]

I recently went from 16" to 18" tires on the JK. Since it was still under warranty I brought it to the local dealer to recal the speedo. The guy whipped out a hand-held computer (looked like a simple code scanner), plugged it in, selected the exact tires I put on from a menu, pushed a button, and the speedo was calibrated. Did it in the parking lot for no charge. Sometimes progress is good.  :yes: