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[Eagle]

Kyle, if your Jeep still has the factory axle ratio (3.54), those 33-inch tires are killing you in the gas mileage department. The Cherokee is basically a brick on wheels at best, and gas mileage goes down quickly as speeds increase. I did a test once with a brand new 1999 Grand Cherokee. I drove from Connecticut to Pittsburch, PA, for a job interview. On the way out, I drove at 65 and 70 MPH. On the way back, as an experiment, I very strictly limited myself to a top speed of 60 MPH just to see what difference it might make. It did make a difference. IIRC, the gas mileage was 3 or 4 MPG better when I kept the top speed to 60 or less.

 

For any internal combustion engine, the most efficient engine speed is the speed that produces the peak torque. In an automobile, this isn't directly reated to speed because drag increases with road speed so at higher speeds you need more power just to overcome resistance, but a good rule of thumb for street driving is to gear so that your highway cruise falls on the engine's torque peak.

 

You have an '87 4.0L. The torque peak for your engine is 2,000 RPM. With your 33-inch tires, at 65 MPH in overdrive your engine is running at around 1800 RPM -- below the torque peak. At speeds below 65 but fast enough to put you in overdrive, you're even farther below the torque peak. That means when with the O2 sensor functioning, you're never giving the engine a chance to work at best efficiency. With those tires, you'd be much better off with 4.10 or even 4.56 gears.

[Kyleinreallife]

Kyle, if your Jeep still has the factory axle ratio (3.54), those 33-inch tires are killing you in the gas mileage department. The Cherokee is basically a brick on wheels at best, and gas mileage goes down quickly as speeds increase. I did a test once with a brand new 1999 Grand Cherokee. I drove from Connecticut to Pittsburch, PA, for a job interview. On the way out, I drove at 65 and 70 MPH. On the way back, as an experiment, I very strictly limited myself to a top speed of 60 MPH just to see what difference it might make. It did make a difference. IIRC, the gas mileage was 3 or 4 MPG better when I kept the top speed to 60 or less.
 
For any internal combustion engine, the most efficient engine speed is the speed that produces the peak torque. In an automobile, this isn't directly reated to speed because drag increases with road speed so at higher speeds you need more power just to overcome resistance, but a good rule of thumb for street driving is to gear so that your highway cruise falls on the engine's torque peak.
 
You have an '87 4.0L. The torque peak for your engine is 2,000 RPM. With your 33-inch tires, at 65 MPH in overdrive your engine is running at around 1800 RPM -- below the torque peak. At speeds below 65 but fast enough to put you in overdrive, you're even farther below the torque peak. That means when with the O2 sensor functioning, you're never giving the engine a chance to work at best efficiency. With those tires, you'd be much better off with 4.10 or even 4.56 gears.

I have to hold it at 2200 just to be able to keep it at 65, which was making me think a fue system issue. I’ll have to confirm my gearing this week and get back to everyone.

On the opposite end, could it have been overgeared? From a stop, tapping the throttle will pop me up just about anything, could this be why it’s having to overwork at the top end to keep speed?

[Eagle]

48 minutes ago, Kyleinreallife said:

 

 

I have to hold it at 2200 just to be able to keep it at 65, which was making me think a fue system issue. I’ll have to confirm my gearing this week and get back to everyone.

On the opposite end, could it have been overgeared? From a stop, tapping the throttle will pop me up just about anything, could this be why it’s having to overwork at the top end to keep speed?

 

 

 

On 33-inch tires, in overdrive 2200 RPM would be approximately 78 MPH, not 65. However, 2200 RPM on stock tires would be about 68 MPH. Have you checked your speedometer to see if it's reading accurately? If the speedometer drive gear wasn't changed when the big tires were installed, the vehicle will be going a LOT faster than what the speedometer indicates.

 

Considering that 2200 RPM is barely above the torque peak for the '87 Renix, and below the 2400 RPM torque peak for the '88-'90 Renix, I wouldn't consider running at 2200 RPM to be "working hard" or "overworked." For these engines, I consider gearing to run about 3,000 RPM at 70 MPH to be optimum for OEM tires, and less than optimum for larger, heavier tires.

 

One other thing to consider -- do you know if your torque converter is locking up as it's supposed to?

[Kyleinreallife]

 
On 33-inch tires, in overdrive 2200 RPM would be approximately 78 MPH, not 65. However, 2200 RPM on stock tires would be about 68 MPH. Have you checked your speedometer to see if it's reading accurately? If the speedometer drive gear wasn't changed when the big tires were installed, the vehicle will be going a LOT faster than what the speedometer indicates.
 
Considering that 2200 RPM is barely above the torque peak for the '87 Renix, and below the 2400 RPM torque peak for the '88-'90 Renix, I wouldn't consider running at 2200 RPM to be "working hard" or "overworked." For these engines, I consider gearing to run about 3,000 RPM at 70 MPH to be optimum for OEM tires, and less than optimum for larger, heavier tires.
 
One other thing to consider -- do you know if your torque converter is locking up as it's supposed to?

Haven’t been able to get an accurate speedo reading. I know it shows at least 10mph slow, but that was done using a speedo ap on my phone, which themselves aren’t very accurate. That being said. If I’m holding 2200 and my speedo reads 55(~65) It still seems slower than it should. For reference I’m I’m not in traffic, I'm holding up the slow lane lol.

How would I tell if the torque converter is the issue? I do notice that it seems to not want to shift into overdrive until about 21-2200 rpm and even then if I bring it up slowly to that it will have a tendency to stay in 3rd, with a slight tap of the throttle it’ll go into 4th

[Eagle]

First you have to know how fast you are actually going. Then correlate that to engine RPM. An automatic transmission is based on a fluid connection between the engine and the driveshaft(s). There is slippage, so the engine speed isn't always constant relative to the road speed. That leads to inefficiency, so in the 1970s the manufacturers started making torque converters that lock up. Once the converter is locked, then there is a constant relationship between engine speed and road speed. The way you can tell is if the engine RPM isn't correct for the road speed but, to figure that out, you have to know how fast you should be going for a particular engine RPM (or what the engine RPM should be for a particular road speed).

 

For 33-inch tires, with 3.54 gears in 4th (overdrive), the engine should be turning 1830 RPM (plus or minus about 10 RPM) at 65 MPH. But that's 65 actual miles per hour -- if you don't have the correct speedometer drive gear installed, you can't go by the speedometer. You could use a GPS -- your cell phone app may be accurate enough. Remember, speedometer error isn't a constant speed difference. If it's off by 10 at 60, it's only off by 5 at 30. If it's off by 10 at 30, then it's off by 20 at 60. Knowing your actual speed-to-RPM relationship will tell you whether or not you have the correct speedo gear, but probably you don't.

 

If you find that the engine RPM varies even if the road speed stays the same, that's a clue that the torque converter isn't locking up. Also, if the engine RPM is higher than it should be for the road speed, the converter my not be locking.

[carnuck]

Renix are .70 O/D and

HO AW4 are .75 O/D




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[Kyleinreallife]

First you have to know how fast you are actually going. Then correlate that to engine RPM. An automatic transmission is based on a fluid connection between the engine and the driveshaft(s). There is slippage, so the engine speed isn't always constant relative to the road speed. That leads to inefficiency, so in the 1970s the manufacturers started making torque converters that lock up. Once the converter is locked, then there is a constant relationship between engine speed and road speed. The way you can tell is if the engine RPM isn't correct for the road speed but, to figure that out, you have to know how fast you should be going for a particular engine RPM (or what the engine RPM should be for a particular road speed).
 
For 33-inch tires, with 3.54 gears in 4th (overdrive), the engine should be turning 1830 RPM (plus or minus about 10 RPM) at 65 MPH. But that's 65 actual miles per hour -- if you don't have the correct speedometer drive gear installed, you can't go by the speedometer. You could use a GPS -- your cell phone app may be accurate enough. Remember, speedometer error isn't a constant speed difference. If it's off by 10 at 60, it's only off by 5 at 30. If it's off by 10 at 30, then it's off by 20 at 60. Knowing your actual speed-to-RPM relationship will tell you whether or not you have the correct speedo gear, but probably you don't.
 
If you find that the engine RPM varies even if the road speed stays the same, that's a clue that the torque converter isn't locking up. Also, if the engine RPM is higher than it should be for the road speed, the converter my not be locking.

Okay, I’ll have to try and get an accurate speedo reading. The rpms don’t vary except when starting to climb a hill, they will drop accordingly with the load, however when I give it more gas to compensate, the rpms will climb very slowly, given the gas pedal a “soggy” feeling, not mechanically mind you, but as if I’m having to give it more gas than I should to compensate for the hill climb.

[Kyleinreallife]

Renix are .70 O/D and

HO AW4 are .75 O/D




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I’ll have to do some reading to better understand this info.

[Kyleinreallife]

Also, I’m thinking my issue may be a combination of curing and something in the fuel system. Because it definitely smells rich on start up

[carnuck]

Also, I’m thinking my issue may be a combination of curing and something in the fuel system. Because it definitely smells rich on start up

If you didn’t change the fuel filter, as it clogs up the flow and pressure can drop. Take the MAP hose off and it’s like running WOT, so more fuel is dumped in. If it’s running too lean, it can help by adding more gas.
Bad gasoline will make it run cruddy too. Especially if diesel is put in instead. EGR jammed part open will be an issue like that too.


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[Eagle]

3 hours ago, Kyleinreallife said:

Okay, I’ll have to try and get an accurate speedo reading. The rpms don’t vary except when starting to climb a hill, they will drop accordingly with the load, however when I give it more gas to compensate, the rpms will climb very slowly, given the gas pedal a “soggy” feeling, not mechanically mind you, but as if I’m having to give it more gas than I should to compensate for the hill climb.

 

 

Do the RPMs drop with the speed staying constant, or does the speed drop too?

 

You have to give it more gas than you think you should because you're operating below the torque peak of the engine. You don't have the proper gears for those tires.

[Eagle]

3 hours ago, carnuck said:

Renix are .70 O/D and

HO AW4 are .75 O/D
 

 Thanks, I forgot about that difference. My numbers are for the .75:1 overdrive ratio.

[Kyleinreallife]

If you didn’t change the fuel filter, as it clogs up the flow and pressure can drop. Take the MAP hose off and it’s like running WOT, so more fuel is dumped in. If it’s running too lean, it can help by adding more gas.
Bad gasoline will make it run cruddy too. Especially if diesel is put in instead. EGR jammed part open will be an issue like that too.


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I’ve changed the fuel filter, map and egr recently. Fuel pump could be going out? Would that make it sluggish? It’s been whining occasionally.

[Kyleinreallife]

 
Do the RPMs drop with the speed staying constant, or does the speed drop too?
 
You have to give it more gas than you think you should because you're operating below the torque peak of the engine. You don't have the proper gears for those tires.

Speed drops too, it’s like it’s having to “try” harder than it should. I think y’all may be spot on on the gearing. I’ll look into it, but that wouldn’t explain it running rich and thus underpowered.

In 4L it smells rich but will idle up anything.

[omega_rugal]

definitly somethign is wrong with your fuel system, check it out first...

[Eagle]

4 hours ago, Kyleinreallife said:

Speed drops too, it’s like it’s having to “try” harder than it should. I think y’all may be spot on on the gearing. I’ll look into it, but that wouldn’t explain it running rich and thus underpowered.
 

 

That's normal when starting up an incline ... but that's useless for determining if your torque converter is locking up or not.

 

I don't know how you can say it's trying harder than it should if this is your first Cherokee. You have nothing to compare it against. I bought an '88 XJ new in 1988. Mine's a 5-speed. Of course, mine has 3.07 gears, but my overall final drive ratio on stock tires s probably close to yours on 33-inch tires. And the '88 will not go up hills in 5th gear, not even on interstates.

[Kyleinreallife]

It’s not my first Cherokee, nor my first car. I’ve been driving since I was 11. Maybe you don’t know cars, but if you do, you can tell when something is off. Mechanical intuition, if you’d like. Also, if you’d like, you can start a thread to talk about your vehicles and experience, but in here, let’s keep to the issue at hand and keep the condescension and assumptions to a minimum.

“Try harder than it should”: i give it more gas while going up a hill and there is a disproportionately lower rpm response than there should be. Understand better?

Have you heard the term “bogging down”? Slightly similar to that, but not quite, it doesn’t lose power, but doesn’t quite accelerate as it should.

[carnuck]

It’s not my first Cherokee, nor my first car. I’ve been driving since I was 11. Maybe you don’t know cars, but if you do, you can tell when something is off. Mechanical intuition, if you’d like. Also, if you’d like, you can start a thread to talk about your vehicles and experience, but in here, let’s keep to the issue at hand and keep the condescension and assumptions to a minimum.

 

“Try harder than it should”: i give it more gas while going up a hill and there is a disproportionately lower rpm response than there should be. Understand better?

 

Have you heard the term “bogging down”? Slightly similar to that, but not quite, it doesn’t lose power, but doesn’t quite accelerate as it should.

 

Does it only happen after warming up? Did you say your cat is an empty can? Did you clean out the fuel tank? I found two gallons of water in mine.

 

 

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[Kyleinreallife]

It smells rich at all times. And yea the cat is empty, which would give a rich smell. Duh, Kyle. Lol

Something about how it’s running makes me think it’s not just the open cat though. I’ll have to check for exhaust leaks, I just retorqued my manifold bolts though.

And good point on the tank, I’ll have to do that soon.

[Ωhm]

Does the O2 heater work?

[Kyleinreallife]

Can you elaborate? What’s an o2 heater?

[Eagle]

3 hours ago, Kyleinreallife said:

It’s not my first Cherokee, nor my first car. I’ve been driving since I was 11. Maybe you don’t know cars, but if you do, you can tell when something is off. Mechanical intuition, if you’d like. Also, if you’d like, you can start a thread to talk about your vehicles and experience, but in here, let’s keep to the issue at hand and keep the condescension and assumptions to a minimum.

“Try harder than it should”: i give it more gas while going up a hill and there is a disproportionately lower rpm response than there should be. Understand better?

Have you heard the term “bogging down”? Slightly similar to that, but not quite, it doesn’t lose power, but doesn’t quite accelerate as it should.

 

Look, Mate -- I'm trying to help you. If you don't want help, I'll move on to someone who appreciates my efforts.

[DirtyComanche]

1 hour ago, Kyleinreallife said:

Can you elaborate? What’s an o2 heater?

 

The O2 sensor on a Renix has a heating element in it to ensure accurate readings regardless of exhaust temperature.  You can test them with a multimeter.

 

Personally I haven't really read the rest of the thread, but you need to verify all sensor functions and that no injectors are stuck/leaking or plugs are bad before doing anything else.  Otherwise it's just the typical guessing game as to of what might be going on, if there even is anything wrong.

[Pete M]

where does it smell like fuel, out the back or in the engine bay? 

[Kyleinreallife]

Okay, previously in thread: i replaced iac, tps, o2, map, egr, plugs, wires, cap, rotor, water pump, fuel filter, oil filter, oil pressure sender, vac lines and alternator. Injectors aren’t leaking, but have been replaced with ford orange tops.

Additional symptom: will idle normal except it will drop down every 20 seconds or so and then pick back up.