Same old problem - 4 banger and highway speed -
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Unread 04-10-2013, 06:02 AM   #1
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1987 YJ Wrangler 
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Same old problem - 4 banger and highway speed

Ok after a nightmare intruing to get the girl running right again I now have the age old highway speed issue to deal with.

4.10 gears factory, 33x12.50 TSL boggers, and a PO botched 3" body lift. Springs are probably shot the have no arch in them the are well not frowning but are flat.

When finally getting her on the highway last night I could get about 55 mph out of her then started getting a vibration. No matter how hard I pushed the old girl 57-58 was it no more. It was like I was out of power. This is my first 4 banger pushing 33s, is this normal? I can deal with the low speed as long as I know I don't have another problem.

By the way the speeds I posted are GPS speeds, speedo shows 7-10 mph less.

I've read lots of threads about highway speed in relation to the gears and tires but don't recall any also naming the 4 banger in the mix.

Thanks for your informative replies in advance.

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Unread 04-10-2013, 06:26 AM   #2
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What gear were you in and what RPMs?
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Unread 04-10-2013, 06:51 AM   #3
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Sounds about normal especially if you were going uphill. You'll need to put some deeper gears in to get that acceleration back (4.56 or 4.88) with the larger tires. Of course it's never a bad idea to give the 2.5L a tune-up. Though it won't be that much better, it couldn't hurt. As for the speedo you'll need a different speedometer gear with more teeth to accommodate the tire size. The required tooth count will change if you put different gears in though.

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Unread 04-10-2013, 07:20 AM   #4
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No thats not normal. I am able to cruise 65 to 70 except for steep hills when it would drop down to around 55. I run 35s and 4.56s now. Still get the same speeds. Cruising right at 65 gets me the best fuel mileage.
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Unread 04-10-2013, 08:49 AM   #5
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If you were trying to use fifth gear then yes that's normal.

Try just staying in forth gear and you can get a bit more power. just a teeny bit anyway haha
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Unread 04-10-2013, 09:16 AM   #6
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The 2.5L isn't a high powered engine, it's an economical engine. You've basically gone up 5 tire sizes over factory tire size, so what did you expect? The 6 cylinder even has issues pushing big tires on stock gears. Re-gear to 4.88 or drop down to 29" tires is about your only option if you want to maintain highway speeds.
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Unread 04-10-2013, 11:05 AM   #7
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I agree with MLJeeper and BogleParsons...
I have a '92 4 banger with a ton of miles factory gears (4.10) and 33" MTs. On the flats I can run 70 all day no problem in 4th gear. If you throttle body injection ('91 or older) I don't know how well they do, but if yours is'92 or newer, you have a problem somewhere.
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Unread 04-10-2013, 07:21 PM   #8
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Ok guys its an 87 and t-body injection. The engine is fresh and it has a new fuel pump filter plugs distributor and pretty much all new.

Don't have a tach hooked up yet so no rpms to report but the 55 is in 4th gear. I know the timing could use some tweaking.
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Unread 04-10-2013, 07:41 PM   #9
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There is some setup and adjustment that has to be done to the t-body injection systems, that Que knows about, I'm not up on the details of those. By comparison, I'm able to do 65 on the flats all day with 31's and stock gearing on MPFI. Not much more than that, tho - a little rise or a headwind means downshifting.
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