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Old 05-01-2014, 05:34 PM   #1
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Rpm problem

1999 4 banger auto trans only gets premium fuel

When it's warm out side and I stop at a light it will rev back and fourth from 900 rpms to 1100 rpms in drive but if it's in neutral or park it will rev steady at 2100 rpms or just below 2000 rpms anyone know how to fix this or what's even wrong?

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Old 05-01-2014, 05:36 PM   #2
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Possible IAC (idle air control) or throttle body is dirty internally and sticking. With engine running, can you manually turn the throttle shaft and make it slow down to idle?

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Old 05-01-2014, 05:39 PM   #3
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Possible IAC (idle air control) or throttle body is dirty internally and sticking. With engine running, can you manually turn the throttle shaft and make it slow down to idle?
1. The throttle shaft is on the throttle body correct? 2.posting on the go so I will check when I get home
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Old 05-01-2014, 05:40 PM   #4
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Yes, it is the shaft that your throttle cable hooks to.
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Old 05-03-2014, 07:53 PM   #5
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Yes, it is the shaft that your throttle cable hooks to.
okay so that's not sticking at all and something new today is it was reving while driving from 2000 rpms to 2500 rpms
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Old 05-03-2014, 08:10 PM   #6
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First, you should not run premium fuel in your engine. Higher octane fuels are harder to ignite, slower to burn, and thus are known to leave deposits behind in engines not designed for high octane gasolines. The ONLY purpose of additional octane is to make gasoline harder and slower to ignite so high compression engines won't prematurely ignite it and cause pinging or detonation. It adds no power and it definitely does not cause the engine to run cleaner.

Not to mention standard octanes like 87 have the same cleaning additives 89 and 91 octanes do, as mandated by the EPA back in 97 or so.

There is no upside to running a "premium fuel" at all, only downsides like unburned fuel deposits left behind. Really. The gasoline manufacturers like having that "mystique" about calling it premium since it's not really a premium grade at all, it just includes more octane that is not usable in a conventional engine like the 2.5L or 4.0L.

Your high rpms could be caused by a vacuum leak or a sticking IAC (idle air control) module. Sometimes by a bad TPS (throttle position sensor). Vacuum leaks are commonly caused by an old split/cracked vacuum hose. Sometimes by an intake manifold that wasn't quite seated properly which would allow extra air into the engine.
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Old 05-03-2014, 08:42 PM   #7
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X2 well said post. I bought premium when I was a teenager. Damn advertisers. Plus I totally Agee with IAC or vacuum leak.
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Old 05-03-2014, 08:49 PM   #8
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1. I'll have to look into the premium thing my parents always used premium in their cars so that's where I get it from.

2. IAC seemed to be working as it should.

3. How do I check for vacuum leaks? How do I check the TPS

4. It only happens when it is warm out side like 60 and up so that really confuses me
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Old 05-03-2014, 08:54 PM   #9
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You can spray carb cleaner or starting fluid on your lines to see if RPMs go up. To check for leaks. If your parents drive Porches or some other high compression vehicle it may be in order. For Jeeps no way. Waste of money. It will say in the owners manual. Like my r1 4 cylinder 186 mph bike needs premium. High compression. My Jeep Id feel like I'm giving it slug gas since I know better.
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Old 05-03-2014, 08:56 PM   #10
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It only happens when it is warm out side like 60 and up so that really confuses me
That points to the TPS. Above idle, the IAC opens fully. If the PCM doesn't get the signal that the throttle has closed, it doesn't attempt to control rpm through the IAC. An open IAC will typically cause rpm's in the 2000 range, with the throttle closed. TPS failures are frequently heat related.
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Old 05-03-2014, 09:47 PM   #11
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How do I check the TPS?
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Old 05-04-2014, 12:46 AM   #12
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How do I check the TPS?
You can check the output resistance for smooth change as the throttle opens/closes. That's not always conclusive, however. On some applications, unplugging the TPS will result in an immediate rpm drop to normal if a high rpm condition exists at the time.

It's also possible that your IAC is sticking. Cleaning the pintle would not be a bad idea in any case.
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Old 05-04-2014, 12:48 AM   #13
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Also where is the TPS located?
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Old 05-04-2014, 09:56 AM   #14
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Also where is the TPS located?
You can access the service manual for your Jeep here:

Jeep Information Center, Jeep Parts Catalog, Jeep Repair Manual & More
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Old 05-04-2014, 09:59 AM   #15
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I found where then TPS is and when it did it this morning I wiggled it and it fixed the problem I will be getting a new one soon
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Old 05-04-2014, 10:10 AM   #16
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I found where then TPS is and when it did it this morning I wiggled it and it fixed the problem I will be getting a new one soon
In that case, the problem could be the connector.
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Old 05-04-2014, 10:42 AM   #17
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In place of the "Premium," gas, I suggest using a gas additive and lower octane like 87 or 89. the gas additive will ignite the fuel evenly and cleanly. It will also help to keep the fuel system cleaner by breaking down any deposits already there.
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Old 05-04-2014, 12:22 PM   #18
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There is NO NEED for any sort of gasoline additive or additional fuel system cleaners in U.S. manufactured gasolines. AutoZone pushes that crap on me every time I buy oil.

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