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Old 05-19-2011, 12:22 AM   #1
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Slight hesitation when accelerating

I have a 2001 Sahara 6 cyl automatic with 60,000 miles and just recently noticed that after the first start-up in the morning during the first acceleration to start moving there is an ever so slight quick hesitation, can't even change the accelerator position before it disappears, almost as quick as you can blink your eyes. Then every thing is fine from then on with no more hesitation for the rest of the day's driving. I just changed plugs using the Autolite App-985, which it did need but didn't change the hesitation.
If it was carb fueled I'd think either bad accelerator pump or chock needed adjusting since it happens before the engine has warmed up any. With no distributor it can't be plug wires. Seems to do it more in moderate temps like 45-60 degrees, but still haven't tried letting it set and warm up for several minutes before starting to move. Will do that next time I drive it.

Any thoughts, suggestions, ideas?

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Old 05-19-2011, 12:54 AM   #2
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It sounds like your O2 sensors taking a few seconds to warm up and assist balancing your fuel/air. Or something like that... You're not the only one.

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Old 05-19-2011, 01:06 AM   #3
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Same here. And then once the car warms up it's over. That's pretty common in wranglers
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Old 05-19-2011, 05:49 AM   #4
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Could be a lot of things. A good TB and IAC cleaning helped me out.I Also had some hesitation and I replaced the coil rail. She now runs like a champ
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Old 05-19-2011, 08:37 AM   #5
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The O2 sensors don't come into play for about 2 minutes after a cold start. The exhaust and the O2 sensor's heaters haven't warmed up enough yet for them to start working. During the first two minutes or so the O2 sensors don't have any influence at all.

The PCM is going by a pre-programmed curve based on the temperature sensor's input.

Have someone look at it with a good scanner - one that will read what the temperature the PCM "thinks" the temperature is. The temperature sensor may not be giving the right info - it's telling the PCM the engine is warmer than it really is. Making it a little lean.

After the 2 minutes is up, the O2 sensors correct the error.
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Old 05-19-2011, 08:57 AM   #6
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Quote:
Originally Posted by rrich
The O2 sensors don't come into play for about 2 minutes after a cold start. The exhaust and the O2 sensor's heaters haven't warmed up enough yet for them to start working. During the first two minutes or so the O2 sensors don't have any influence at all.

The PCM is going by a pre-programmed curve based on the temperature sensor's input.

Have someone look at it with a good scanner - one that will read what the temperature the PCM "thinks" the temperature is. The temperature sensor may not be giving the right info - it's telling the PCM the engine is warmer than it really is. Making it a little lean.

After the 2 minutes is up, the O2 sensors correct the error.
So what the op is experiencing is probably
Normal especially with higher mileage, untill the jeep warms up and 02's come into play?
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Old 05-19-2011, 11:21 AM   #7
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Maybe something else to check, the TPS.

Could be on the cold start it's hanging up a tad. Once the throttle is moved it loosens up and works as intended. It's signal tells the ecm how deep you have your foot on the throttle. And if it's hanging up that little bit of throttle advancement is not going to be met with the proper amount of fuel feed.

You get a tiny stumble, then the TPS snaps into action and all is well again.

You can check it with an ohm meter. Check it both stone cold and hot. Readings should be a smooth climb up and down, if it skips a spot, or if you have it off and you can feel a bind, most likely it's on it's way south.
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Old 05-19-2011, 08:38 PM   #8
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Quote:
Originally Posted by kjeeper10 View Post
Could be a lot of things. A good TB and IAC cleaning helped me out.I Also had some hesitation and I replaced the coil rail. She now runs like a champ
What is a TB or IAC, and about what does a neew coil rail cost?

Appreciate all the responses. Didn't realize this was something common to Wranglers, thought any vehicle could be made to run smooth. Guess I don't have anything to worry about, that is unless the coil rail can crater completely and leave me sitting beside the road.
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Old 05-19-2011, 08:52 PM   #9
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Originally Posted by Shelby427 View Post
Maybe something else to check, the TPS.

Could be on the cold start it's hanging up a tad. Once the throttle is moved it loosens up and works as intended. It's signal tells the ecm how deep you have your foot on the throttle. And if it's hanging up that little bit of throttle advancement is not going to be met with the proper amount of fuel feed.

You get a tiny stumble, then the TPS snaps into action and all is well again.

You can check it with an ohm meter. Check it both stone cold and hot. Readings should be a smooth climb up and down, if it skips a spot, or if you have it off and you can feel a bind, most likely it's on it's way south.
I assume TPS is the Throttle Position Sensor, but I have no idea what it looks like nor where it is mounted. What is the symptom or problem if it does crap out? I'm not very familiar with all these new contraptions our vehicles have nowadays. I grew up with distributors, rotors, points, condensors, generators, carburators, plugs with wires and a coil, and fuel pump mounted on the side of the engine which the earlier model furnished vaccum for the windshield wipers. You pressed the throttle, fuel went to the engine, the plugs fired it, and you started moving. Hard to teach an old dog new tricks, or maybe any tricks at all.
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Old 05-19-2011, 09:38 PM   #10
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Lol, know where you are coming from, I grew up with jeeps. Started out with those lovely vacuum operated wipers. Accelerate and the wipers would slow down or stop. Jeep was first with intermittent wipers.

Anyhow. Basics.

TB is Throttle body, the somewhat looking carburetor on yours. All it does is control air into the engine.

IAC is Idle Air Control, think of it as the idle jets in a carb, but ones that can adjust themselves.

TPS is Throttle Position Sensor. Little black box attached to the TB butterfly shaft.


Things that go wonky on them.

The TB and the IAC can act up if they get a build up of varnish/deposits on them.
What happens is #1 the TB's butterfly will not close fully so it keeps sending a signal via the TPS to the ECM (engine control module, or computer if you will) telling it that you got your foot on the gas when you don't.

The IAC gets into the act (remember it can adjust, in it's case it adjusts airflow for the idle circuit) if it hangs up, it will cause a stumble, poor idle speed etc.

Easy fix for both. Carb cleaner and a tooth brush,

TB first. Manually open the butterfly on the TB. Look for a ring all the way around the bore, that's varnish/deposits. Bore should be clean as a whistle. Spray carb cleaner and if the varnish/build up is being ornery, hit it with the toothbrush. Another blast of carb cleaner and you should be good.

The IAC mounts to the side of the TB. See photo below. It has a pin that moves back and forth to control the idle air amounts needed. That same old varnish/deposits can cause the pin to bind up.

Same as before with the carb cleaner and toothbrush for it, plus one other step.
Shoot a bunch of carb cleaner in the bore that the IAC came out of. This will clean out the passageway the the air has to travel through. That dang varnish/deposits can plug it up/restrict the flow, so no matter what the IAC does the air doesn't.


Now on to the TPS. All it is, is a variable resistor, as said connects to the butterfly valve shaft on the TB. As the shaft turns the TPS changes it's resistance reading to the ECM. The ECM in turn adjusts the fuel mixture.


If the TPS goes wonky on you the ECM will have no idea what mixture you need. You can move the throttle and the ECM doesn't know it. So your jeep hesitates/stalls/ lacks power.

It has to be checked with an ohm meter to see if it's working correctly.

Photos 2 and 3 are versions of a TPS. I'm not familiar with the 01's set up but will give you a general idea of what you are looking for. Like said it bolts to one side of the TB connecting to the butterfly shaft.



There you go, you now know what a TB/ IAC/ TPS/ and even an ECM are.
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Old 05-19-2011, 10:30 PM   #11
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Thanks for some very informative information. I'll take a look at my engine and see how many parts I can identify.

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