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Old 02-27-2010, 05:06 AM   #1
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94 YJ loses rpms

I have a 94 YJ 2.5L automaticwith 124k miles, that keeps making go hmmmm. The Jeep was parked for about four months at a body shop when I got it, I was told it ran fine. I have noticed that when driving at 35 mph or so that I am losing a couple hundred rmps. If I step on the gas it stops doing this for a few seconds to a few minutes and then starts doing it again. I have run four tanks of gas through it with fuel injector cleaner and it has not helped at all. The fuel pump and fuel filter are brand new and there are no fuel leaks that I can detect. I think that I am down to replacing the injectors to try to correct the problem. Does anyone have some good advice on anything else I might try before putting in new/remanufactored injects? The jeep was not bought to be an off-road king just a once or twice a month ride with the top down. I sure hope someone can help me out here.

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Old 03-07-2010, 01:32 AM   #2
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Well this post got a few looks but no input from anyone. I guess this is not a very common issue. I guess it might have to go back to the shop for them to try to fix.

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Old 03-07-2010, 01:58 AM   #3
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What size tires, gears? What RPM's is it running at 35 mph? Are you traveling on flat ground? Is it losing speed as the RPM's drop? Sounds like your Jeep is shifting to another gear and when you hit the gas the kickdown is shifting it and bringing the RPM's up.
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Old 03-07-2010, 04:45 AM   #4
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The tires are 33 12.50 15 on 15 X 10 rims, gears are stock. It does it while driving on flat ground. It does not lose speed, it runs about 2400 rmps and then drops to 2100 and then comes right back up to 2400. I step on the gas jsut to stop the annoying drops in rpms. The jeep sat for about 4 months without being start prior to me gettng my hands on it. I am thinking its injector but that would just be a guess.
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Old 03-07-2010, 05:27 AM   #5
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As always, yall correct me if I'm wrong, but I think stock gears w/ 33s and a 2.5 engine could cause RPM loss. I think Fweaky may be onto something about the shifting. Next time it does it, listen to it shift and then listen to it shift when you hit the gas again. I would bet it is up shifting at 35mph and because it's got 33s that's too high so your rpms drop drasticly. When you hit the gas again it down shifts to the correct gear. if that's the case, it's a mater of either going down on your tire size or up (or is that down?, damn ratios are so confusing) of your gear ratio. Before you do that I'd let Jerry or one of the other guys weigh in though, they know a lot more than me.
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Old 03-07-2010, 05:50 AM   #6
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I would have thought it had to do with tires size also but I had the same rims and tires on another 94YJ that was a stock driveline set up. The only thing changed in the driveline is the tranny has been dropped to reduce the driveshaft angles caused by the lift kit. I will drive it again today and listen and see if it is attempting to shift when I hit the gas. I do not think it is but I will try anything right now.
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Old 03-07-2010, 08:03 AM   #7
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a 2.5 with 33's and a auto trans will drive you crazy with the back and forth shifting and power drops.
you need to regear to 4.88's or drop tire size
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Old 03-07-2010, 10:50 AM   #8
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Quote:
Originally Posted by tim2 View Post
The tires are 33 12.50 15 on 15 X 10 rims, gears are stock. It does it while driving on flat ground. It does not lose speed, it runs about 2400 rmps and then drops to 2100 and then comes right back up to 2400. I step on the gas jsut to stop the annoying drops in rpms. The jeep sat for about 4 months without being start prior to me gettng my hands on it. I am thinking its injector but that would just be a guess.
If it does not loose speed, have you considered that the tach is no good? A 300-400 drop in RPM on flat ground should be noticeable in speed too. Or as Scott said, its shifting up and down
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Old 03-08-2010, 01:32 AM   #9
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Well I drove it yesterday and it is not shifting when it drops RPMs. It is loding the rmps for a second and then going right back up. If you do not press on the gas or remove your foot from it the rpms just keep flucutating around 200-300 rpms. It does not drop long enough to cause a loss of speed.
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Old 03-08-2010, 03:58 AM   #10
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Can you actually hear the change in RPM's or are you just going by the tach?
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Old 03-08-2010, 01:05 PM   #11
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Sounds like a faulty tach..
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Old 03-08-2010, 03:18 PM   #12
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You can HEAR the rpm change. The tach was checked and I even put another one in to make sure it wasnt as Garyk said, "Sounds like a fualy tach.."
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Old 03-08-2010, 05:03 PM   #13
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Have you checked for any codes? Checked the fuel pressure regulator?
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Old 03-11-2010, 09:02 AM   #14
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Well I kept checking and checking. The jeep never flashed a code nor courld the local parts stores get one to flash. This morning I replaced the ECU and went for a test ride. POOF the loss or RPMs is gone. I want to thank all of you that tossed info to me and let you know that I really appreciate all the help. Now I can get a lil mud on the tires again.
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Old 03-11-2010, 05:47 PM   #15
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Glad to hear all is well now..with all that rain y'all have been getting, you should have plenty of mud.

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