TJ is elderly Basset Hound slow - JeepForum.com
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post #1 of 11 Old 08-14-2020, 12:11 PM Thread Starter
Magnetix
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TJ is elderly Basset Hound slow

This started off as a discussion about if I should tow it to the lake because it can’t keep up on the highway, so I figured it would be more appropriate to start a new thread.

97 TJ with 2.5 and the AX5 tranny, stock gearing and 31” tires, about 235k on the clock. Added some stuff and took some off so weight should be the same if not lighter.

Replaced rotor, cap, plugs and fuel pressure regulator. Also replaced vapor canister and previously ran some SeaFoam through the engine via a vacuum line (didn’t smoke). Replaced valve springs when I first bought it.

Noticed a slight improvement on highway speed with the fuel pressure regulator but only +5 or so top end. Still cannot get past 70 without her nearly exploding (or at least it feels that way, floored in 4th or 3rd).

Checked cat with a laser thermometer - inlet temp was about 480 or so, outlet was about 500, so it does not seem to be a clogged cat. Temp at header though was damn near 700.

What else can I check before I start breaking down into doing compression checks and other more involved things?

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post #2 of 11 Old 08-15-2020, 07:43 AM
Mousejockey
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You say stock gearing? 4:11 or?
Lots of miles on the motor, may simply be tired. My 4.0 had 160K when I rebuilt/replaced it, the increase in power and usable rpm was very noticeable
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post #3 of 11 Old 08-15-2020, 10:14 AM
Jonny Jeep
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I replaced pistons in my TJ after loosing a skirt on the #1. There was (maybe) a small difference in power. Several months later the fuel pump failed. There was loads of extra power after replacing that. Check fuel pressure, maybe the pump is getting a bit tired.
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post #4 of 11 Old 08-16-2020, 02:02 PM
joe_jeep
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has the cat possibly emptied into the muffler? partial clog?
random idea.

compression test is super easy.

glfwda #6624
2005 rubicon sahara lj #592
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post #5 of 11 Old 08-16-2020, 09:38 PM Thread Starter
Magnetix
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I guess it still could be an exhaust issue, I will have to try that unplug the O2 sensor and see if it is better test. Regarding the engine, yeah it could just be that it is near the limit of what one would consider worn out.

I cannot however determine if the engine is original or if it was already replaced/rebuilt. I assume it was original because everything else (even the carpet uck) was original. I tried running down the engine serial to tell but was never able to get anywhere with that so I just gave up.

The gearing is the stock 4.11 for the 2.5, and according to everything I have read and heard about, this should still be within acceptable for 31s.

I have however noticed a pronounced fuel smell from the exhaust, leading me to believe that something is making it run rich. I will need to take a look at my plugs to double check this and might as well check the gaps while I am there. Would it be worth it to go ahead and replace my coil too?

So if I wanted to check and see if my engine was getting enough fuel from the pump how would I go about checking that?

Thanks for the ideas!
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post #6 of 11 Old 08-17-2020, 04:07 AM
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A 2.5L with 31s? How much faster than 70 mph do you think it should go? My old 2.5 was screaming at 70 too. That's the only way it will go that fast. Change the upstream O2 sensor (use NTK). What plugs are you running?

p.s. If the cat was clogged you wouldn't do 50 mph.

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Originally Posted by jeepinmichguy_ View Post
TheBoogieman is a jerk.
GOT IT BACK 6/17. 03' Rubicon/Zone 4.25" combo lift with 4" lift coils up front/Zone hydro shocks/5.13 gears/35" Mickey Thompson MTZ P3 tires/Black Magic brakes. Jeep #17 & 19.
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post #7 of 11 Old 08-17-2020, 11:17 AM Thread Starter
Magnetix
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I honestly do not know what brand they are - maybe Champion? I purchased a tune up kit shortly after I bought it a few years ago that had a new cap, rotor, wires, plugs and air filter. Thought that was a good starting point being a new owner.

I do not expect Autobahn performance, as I’m well aware of the limitations on the 2.5, but unfortunately due to financial constraints I cannot drop the thousands or tens of thousands of dollars on an engine swap. But at the very minimum I would expect it to be able to keep up with traffic on the highway and not be a safety hazard or annoyance to other drivers. Many of the highways between where I live and where I camp have 75mph speed limits and are two lane with limited passing.

Others I have talked to with the same engine and even bigger tires state they can go 80 and have plenty left. Me, at 70 even if it is floored I just hit a brick wall. I should not have to re-gear for 31 inch tires. I had 31s on my old Cherokee for years and never had an issue, but it had the 6. My old Chevy truck had 32s and did not suffer from the wall on the highway either, neither vehicle was ever re-geared, but both were 2wd and were 6 cylinder.
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post #8 of 11 Old 08-19-2020, 01:16 PM
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You can't compare the 2.5 with a 6 cylinder. There's around 100 cubic inches of difference and then the fact that your 2.5 has very high mileage doesn't help. My friend had a 94 YJ with the 2.5, AX5 and 4.10 gears. Everything was OK until he put a set of 31's on it. Felt like the brakes were dragging. Lost what little power he had to start with. A gear change made all the difference and 5th gear was a lot more useable.
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post #9 of 11 Old 08-20-2020, 09:22 AM
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Quote:
Originally Posted by 80cj View Post
You can't compare the 2.5 with a 6 cylinder. There's around 100 cubic inches of difference and then the fact that your 2.5 has very high mileage doesn't help. My friend had a 94 YJ with the 2.5, AX5 and 4.10 gears. Everything was OK until he put a set of 31's on it. Felt like the brakes were dragging. Lost what little power he had to start with. A gear change made all the difference and 5th gear was a lot more useable.
So true. My first Jeep was a 2.5L with no AC or power steering. I never bought another 4 banger and I'm on Jeep 19. If I could get one in mint condition with AC and under 100,000 miles, I'd buy it if it was under $3,500. So NO.

Quote:
Originally Posted by jeepinmichguy_ View Post
TheBoogieman is a jerk.
GOT IT BACK 6/17. 03' Rubicon/Zone 4.25" combo lift with 4" lift coils up front/Zone hydro shocks/5.13 gears/35" Mickey Thompson MTZ P3 tires/Black Magic brakes. Jeep #17 & 19.
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post #10 of 11 Old 08-21-2020, 12:15 PM
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Quote:
Originally Posted by TheBoogieman View Post
So true. My first Jeep was a 2.5L with no AC or power steering. I never bought another 4 banger and I'm on Jeep 19. If I could get one in mint condition with AC and under 100,000 miles, I'd buy it if it was under $3,500. So NO.
I tell friends that if they plan to go to bigger tires, don't buy a 4 banger because you'll wish you didn't.
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post #11 of 11 Old 08-21-2020, 04:24 PM
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Quote:
Originally Posted by Magnetix View Post

What else can I check before I start breaking down into doing compression checks and other more involved things?
Borrow a compression tester and a fuel pressure tester from a parts store,
Both are very easy to do, and will give you a starting point. you need to know
these two basic things are good.

What size 31's are you running and on what wheels? Heavy wheel and tire combo
will kill performance in a hurry...

Afterthought, Is your speedo correct? larger tires will make your speedo read slow. so maybe
your going faster than you think?

Whats over that next ridge?
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