TJ won't hold 70 MPH even on the slightest hill

Kennyroberts70

Kennyroberts70

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Jan 20, 2020
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Peoria, Il
This is my first Jeep. It’s a 97 TJ. Super babies rust free Jeep. It was a summer fun car for the previous owners. At highway speed it won’t hold 70 even on the slightest hill. The tires are 305 70 18. It also has what acts like a it has a cough under load. My thought is like an EGR clog. Has anyone experienced this.
 
Those tires are 34.8" diameter! Sounds like the previous owner didn't regear the differentials for tires nearly 35" diameter!

Check your gear ratios first, then progress from there!
 
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Depends on your gearing. If you look to the left on the diff you should see a tag, you may have to clean it a bit first but it should be stamped 3.07, 3.55, or 3.73.
 
check your Cat too. I believe you only have one. If it is plugged, you will notice a big difference in power loss.

Check your air filter. Like the member above suggested, you may want to confirm what gearing is in the jeep too
 
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You have a fair bit of discovery to do on your TJ!

Your profile says modifications are only bigger tires!

Those are big tires, and needs to be geared accordingly.

You've got quite a bit of lift on that rig as well. You'll want to figure out what route, and what components the previous owner used on the lift.

Too, you'll need to determine what remediation has been done in regard to your driveline angle with that amount of lift.

Just think of it as quality time with your jeep! 🤓
 
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That's a really low mile TJ, I don't know about being so quick to unload it!

Actually just determining what you really have to work with isn't that hard, and lots of folks here on this forum are more than happy to help out! Good photos of various components will go a long way in that regard.

Help is here! Make full use of it! 🙃
 
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Kennyroberts70 said:
wow!!! I think I’m gonna sell it. I haven’t even done the title yet and this is proving to be way over my head.
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Dont overthink it. It is a used vehicle that MAY require some maintenance and/or upgrades/downgrades.

Take baby steps. Check for the gearing stamp on the rear diff to determine what gears are installed OR pop the cover and lots of youtube videos on how to determine gearing. If you find out you have 3.07 gears with big tires like that, you could trade those to someone with smaller tires OR regear.

Any muffler shop should be able to determine IF your cat is plugged. Did it throw a check engine light ?
 
Halfstock said:
That's a really low mile TJ, I don't know about being so quick to unload it!

Actually just determining what you really have to work with isn't that hard, and lots of folks here on this forum are more than happy to help out! Good photos of various components will go a long way in that regard.

Help is here! Make full use of it! 🙃
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But money and resources and or friends to help me do it I don’t have. It’s hard. This think has low miles, absolutely no rust and was used mostly in the summertime. I thought I had a great deal. I got taken it sounds like.
 
It needs to be geared properly. And even then, don't expect that 4.0 to push those 35s very well. The 4.0 has something like 135 HP at the wheels. Top that off with the aerodynamics of a barn (as well as huge, heavy tires), and you'll very easily understand why high speeds in your Wrangler just aren't going to happen for sustained periods unless you plan on swapping in a Hemi or LS.

This thread will help:

What gear ratio should I install in my Jeep Wrangler TJ / LJ? (and other re-gearing FAQs)
 
Was everything fine when you first got it? What is the RPM at 70 mph? You need around 3000 RPM to have much chance with 35s at 70MPH. Fifth gear is not going to work. My Jeep never saw overdrive with 35s and 4.10 gears. It did better with a retune and better yet with regear. I don't think with 35s and a stock 4.0 you will have much power to spare under the best of circumstances at 70 mph.
 
TurboTJ said:
If the Jeep came with 28 inch tires and it has not been re-geared and the speedometer has not been adjusted, 70 mph indicated is 87.5 mph actual.
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Good point, use a a phone or gps to check it.

Although, if you are passing or getting passed is probably pretty obvious.

I had 3.73’s with 35’s and although not quick 70 mph wasn’t difficult.
 
Don't be so quick to give up on it. Looks like a really nice Wrangler. If I had to pick something to fix, wheel size is much easier than rust. 35's are $$, I agree with derek that selling/swapping with someone would be an easy way to go.
 
Kennyroberts70 said:
After this research I have decided I am going to sell the Jeep and not keep it. This is more than I can take on. I hope to recoup my invest.
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I wouldn't be so quick to give up. A '97 in good shape is a good find and regearing the axles to restore the power lost to the big tires is a one-day job at a shop. I've regeared 3X over the past 23 years and it's no big deal. That's really the only significant (but still no big deal) thing that appears to need to be done.

The 'cough' under load is likely a minor thing like a bad O2 sensor which can happen with any gas powered vehicle. At any rate that wouldn't be hard to fix.

I'd reconsider giving up so easily with what promises to be a fun vehicle. I had my '97 TJ stolen ten years ago and I still miss it.
 
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Midwest said:
Don't be so quick to give up on it. Looks like a really nice Wrangler. If I had to pick something to fix, wheel size is much easier than rust. 35's are $$, I agree with derek that selling/swapping with someone would be an easy way to go.
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Going down to. 275/65/18 is a 2” smaller tire. What would that do do ya think?
 
Kennyroberts70 said:
Going down to. 275/65/18 is a 2” smaller tire. What would that do do ya think?
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It’ll make a difference but still won’t be perfect. It’ll probably get you by until you save some money for a re-gear. Maybe a friend has a set you can toss on to get an idea what it’s like before you spend money on them?
 
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