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Unread 10-01-2015, 10:26 AM   #1
Skerr
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Why does my Jeep lurch at low speed?

'79 CJ5, 4.0L MPFI, T18/D20, 3.54, rear Lock-rite, 33 inch tires

When I am running very slow, 10-15 mph, low throttle, first gear, through the trails in the deer woods, the Jeep will start to lurch, or pulse. I thought I was "bumping" the throttle with my foot due to uneven terrain, but I have ruled that out. I can shift into second and it goes away, but I am lugging the engine. Is it because I have 3.54 gears? Would it go away with 4.10s?

Thanks

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Unread 10-01-2015, 10:31 AM   #2
LumpyGrits
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Cj

Your lug'n it with that R&P.
Put the t'case in 'lo' range and try that.
LG
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Unread 10-01-2015, 11:15 AM   #3
80cj
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Quote:
Originally Posted by LumpyGrits View Post
Your lug'n it with that R&P.
Put the t'case in 'lo' range and try that.
LG
He has a T18 with 6.32 first gear. shouldn't be lugging even with 33's. How does it idle? Does it surge while idling?
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Unread 10-01-2015, 11:18 AM   #4
dirtdudeaz
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You may be past the speed/rpm of the sweet spot for that gear. When you get to that mid/high RPM for first gear, small RPM changes up or down will really be pronounced.

As Lumpy said, 2 or 4 low and choose gears through that range, or pop it into second and let it idle along
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Unread 10-01-2015, 11:35 AM   #5
swatson454
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Can you tune it out at the throttle body? We dealt with a Vette that did that once and the fix was in the tune.


Shawn
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Unread 10-01-2015, 02:01 PM   #6
LumpyGrits
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OK-After more coffee, I do see he's run'n FI-
It could well be a program issue with the FI's 'puter that may need to be addressed.
LG
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Unread 10-01-2015, 02:49 PM   #7
lucdog
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Does this just happen when turning, or when going straight as well? In 4wd? Remember, with the T18 and 6.32 to 1, it doesn't take much gas pedal to make it feel like you describe. Does it do it just idleing along as well? I've noticed this with my Rubicon, and others with lockers, 1st. Gear and with FI this as well, like the pedal is a bit touchy. It's hard to get used to. I haven't noticed this on carb'ed Jeeps.
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Unread 10-01-2015, 03:40 PM   #8
Skerr
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Wow! Wasn't expecting so many replies so quickly!! Thanks, All. This is driving straight, down my access road, in 2WD, hubs locked. You know how you kind of want to creep into the woods and have a look-see while you are making-way to your stand. There are a couple muddy places, so I lock hubs when I open the gate. When in 4 wheel, hi-range, there is no lug. In low-range I can get out and walk beside it, no pulsing.

Throttle response is immediate. Engine does not pulse on its own while idling... runs very well. My thought is too high (numerically) of R&P for 33s. My gut is telling me to go to 4.10 or 4.56 (since I plan on 35s next). But my gut has no basis for accuracy! I just feed it occasionally, and it is happy!
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Unread 10-01-2015, 05:12 PM   #9
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I'd run it in 4low and 2nd gear. There's no way with a 6.32 to 1 first, 33" tires, and 3.54 gears, that you should need to go lower. At least for what your doing with the Jeep at the present time.

I think it's a throttle thing .

It's got a tach right, what RPM's are you turning at 10/15 MPH in 2 hi, and 1st gear.
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Unread 10-01-2015, 05:19 PM   #10
ADDZ71
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I have the same question as Lucdog. What rpm are you running at 10-15 mph. I have similar gearing (T-18, 3.54, 32's) and I know that I would be at 2,500-3,000 rpm at that speed. In other words way above idle and the spot I would expect to see surging. Just to be clear are you doing 10-15 in 2nd gear?
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Unread 10-01-2015, 07:21 PM   #11
Skerr
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Quote:
Originally Posted by lucdog View Post
I'd run it in 4low and 2nd gear. There's no way with a 6.32 to 1 first, 33" tires, and 3.54 gears, that you should need to go lower. At least for what your doing with the Jeep at the present time.

I think it's a throttle thing .

It's got a tach right, what RPM's are you turning at 10/15 MPH in 2 hi, and 1st gear.
Quote:
Originally Posted by ADDZ71 View Post
I have the same question as Lucdog. What rpm are you running at 10-15 mph. I have similar gearing (T-18, 3.54, 32's) and I know that I would be at 2,500-3,000 rpm at that speed. In other words way above idle and the spot I would expect to see surging. Just to be clear are you doing 10-15 in 2nd gear?
According to my tach, I am running between 500 and 600 RPMs, maybe 700. I have parallax issues!! Also, I am calling it 10 - 15 MPH, but my speedo bounces a bit at this speed, so maybe it is actually slower. I barely use any throttle. This is just a notch above idle speed. My engine will stall if I remove my foot from the throttle in 1st gear. Not immediately, however. The engine will lurch at any speed, if I try to maintain that speed, while in 1st gear. It doesn't happen right away, but maybe after a couple hundred yards, after the fuel input tapers off.
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Unread 10-02-2015, 04:41 AM   #12
gmakra
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All this from a guy who claims someone moved his bolt holes overnight! And then stuck to his story.

Skipper as tall as you are you do have parallax issues why not let a normal sized person look at the tach/speedo, then again it could be your noggin hitting tree branches and giving the feeling of lurching.

And thats my story and I am sticking to it!
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Unread 10-02-2015, 05:25 AM   #13
Matt1981CJ7
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Maybe the lurching is from running over liberal tree-huggers. Mine does that when I hit one square

Sure sounds fuel-related to me, but I'm no help with Scott's FI system. Sorry.

Matt
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Unread 10-02-2015, 07:18 AM   #14
John Strenk
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Quote:
Originally Posted by Skerr View Post
According to my tach, I am running between 500 and 600 RPMs, maybe 700. I have parallax issues!! Also, I am calling it 10 - 15 MPH, but my speedo bounces a bit at this speed, so maybe it is actually slower. I barely use any throttle. This is just a notch above idle speed. My engine will stall if I remove my foot from the throttle in 1st gear. Not immediately, however. The engine will lurch at any speed, if I try to maintain that speed, while in 1st gear. It doesn't happen right away, but maybe after a couple hundred yards, after the fuel input tapers off.
Sure you have a T-18 with a granny gear?

That RPM seems too low for 10-15 MPH in 1st.
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Unread 10-02-2015, 09:07 AM   #15
lucdog
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Quote:
Originally Posted by Skerr
Wow! Wasn't expecting so many replies so quickly!! Thanks, All. This is driving straight, down my access road, in 2WD, hubs locked. You know how you kind of want to creep into the woods and have a look-see while you are making-way to your stand. There are a couple muddy places, so I lock hubs when I open the gate. When in 4 wheel, hi-range, there is no lug.
So this is only happening in 2 wheel hi, correct? And at 500 to 700 RPM's. What about 4 low, does it do it then?
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1957 WILLYS pickup,
1973 J 4000,
1978 CJ7 DD.
1979 CJ7 360, TH400/Quadratrac.
1979 J20
1980 CJ5 trail Jeep.
1983 CJ7
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