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Unread 08-14-2012, 04:17 PM   #1
Volvo802
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1994  
 
Join Date: Aug 2012
Location: Wilmington, NC
Posts: 6
1994 Jeep cherokee stalling... Oh boy.

Alright. So after replacing the starter on the jeep yesterday and having it now crank 100% of the time, I'm on to the next problem to be solved. Stalling when idling.
(1994 Jeep Cherokee Sport 4.0 ManTrans)

So on my brothers cherokee, he keeps the battery terminals OFF the battery posts to keep that mysterious electrical gremlin to stop draining the battery. (The terminal clamps soon to be replace, the new battery put on has posts that were smaller, so its been over tightened and "bumped on" with a hammer one too many times).

Firstly, the jeep idles around 13.9 volts (tested with my multimeter), with about .2 volts either way, at about 80F. Is that within spec? I'm not sure how accurate the dash gauge is, because that says it never goes above 14 even when driving, and doesn't change with rpm speed does.

Secondly, would constantly leaving the battery off the posts, resetting the ECU constantly, cause the ECU to have problems? When It turns over (and it turns over well now) it idles around 900rpm, than dips to around 4-600 (bounces as it fights stalling). I've never owned a car with a computer this intelligent lol, so I wasn't sure if constantly reseting would cause it problems as I can't imagine it would.
After dipping down for about 5-10 seconds by itself - it revs up to 3000rpm, than slowly comes back down to around 900rpm. Than goes back to choking, and repeats. Its about a 50/50 chance of it dying when it starts choking, and thats where the problems lies.

Battery and alternator both have been tested at autozone to work fine. But than again, the folks at autozone aren't the best at what they do

So I'm going to head over to try and rule out the alternator with my MM (if possible, will research that aspect myself here).

I don't have the cash to be throwing in parts to see which one fixes it, so it will be more of a diagnosing each thing that can come into play with the problem, where lies you guys, as I haven't had much of a chance to work on jeeps til now.

so far:
Fuel filter recently replaced
Starter recently replaced
fuses checked and replaced as needed
Some sea foam was recently put in as well as fresh gas
I'm assuming we have good spark or else it wouldn't drive very well when its going right?

Thanks guys!

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Unread 08-14-2012, 05:58 PM   #2
tjwalker
It's the crank sensor!
 
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1999 XJ Cherokee 
 
Join Date: Nov 2007
Location: Central Minnesota, MN
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*If it measures 13.9 volts at the battery at idle, that is a good quick and dirty test that verifies that the "charging system" is working properly.

*So my gut tells me that the battery is still in play, regardless of how it tested at Autozone. How old is this battery? The first thing I would do is to get a known good battery installed and see if that resolves it.

*The engine management sensor I would suspect is the "throttle position sensor". More on that below.

*Cleaning both the idle air control and the throttle body is NEVER a bad idea for symptoms like this. More on that below too. Good luck!
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The throttle position sensor is connected to the throttle shaft on the throttle body. It sends throttle valve angle information to the PCM. The PCM uses this information to determine how much fuel the engine needs. The TPS is really just a simple potentiometer with one end connected to 5 volts from the PCM and the other to ground. A third wire is connected to the PCM. As you move the accelerator pedal with your foot, the output of the TPS changes. At a closed throttle position, the output of the TPS is low, about a half a volt. As the throttle valve opens, the output increases so that, at wide open throttle, the output voltage should be above 3.9 volts. Testing can be performed with an electrical meter. Analog meter is best. You are looking for a smooth sweep of voltage throughout the entire throttle band. While slowly opening and closing the throttle, take note to the movement of the voltmeter needle. There should be a direct relationship between the needle motion to the motion of the throttle. If at anytime the needle moves abruptly or inconsistently with the movement of the throttle, the TPS is bad

You should have 5 volts going into the TPS. At idle, TPS output voltage must be greater than 200 millivolts. At wide open throttle (WOT), TPS output voltage must be less than 4.8 volts.. The best is to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5 volts. One of the other wires should show .26V (or so). The other wire will be the ground and should show no voltage. Move the throttle and look for smooth meter response up to the 4.49 at WOT.

Perform the test procedure again and wiggle and/or tap on the TPS while you watch the meter. If you notice any flat spots or abrupt changes in the meter readings, replace the TPS.

The TPS is sensitive to heat, moisture and vibration leading to the failure of some units. The sensor is a sealed unit and cannot be repaired only replaced. A TPS may fail gradually leading to a number of symptoms which can include one or more of the following: -

NOTE: The throttle position sensor is also DIRECTLY involved with transmission shifting characteristics! It should be verified early in the troubleshooting process, when a transmission issue is suspected!

• Poor idle control: The TPS is used by the ECU to determine if the throttle is closed and the car should be using the Idle Air Control Valve exclusively for idle control. A fault TPS sensor can confuse the ECU causing the idle to be erratic or "hunting".
• High Idle Speed: The TPS may report faulty values causing the engine idle speed to be increased above normal. This is normally found in conjunction with a slow engine return to idle speed symptom.
• Slow engine return to idle: A failing TPS can report the minimum throttle position values incorrectly which can stop the engine entering idle mode when the throttle is closed. Normally when the throttle is closed the engine fuel injectors will be deactivated until a defined engine RPM speed is reached and the engine brought smoothly to idle speed. When failing a TPS will not report the throttle closed and fueling will continue causing the engine to return to idle very slowly.
• Engine Hesitation on Throttle Application: The TPS is also used by the ECU to determine if the driver has applied the throttle quicker than the Manifold Air Pressure sensor can read. The fueling is adjusted accordingly to cope with the sudden increase in air volume, however a faulty sensor can cause the ECU to ignore this data and the engine will "hesitate" when applying the throttle. In extreme cases with the engine at idle, a sudden application of full throttle can stall the engine.
• Engine Misfire: A fault TPS can report values outside the denied acceptable range causing the ECU to incorrectly fuel the engine. This is noticeable as a slight misfire and can trigger the misfire detection software and/or Malfunction Indicator Lamp (MIL) light on the dashboard. Extreme cases can cause excessing misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.
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The Idle Air Control (IAC) is mounted on the back of the throttle body. (front for 87-90) The valve controls the idle speed of the engine by controlling the amount of air flowing through the air control passage. It consists of a stepper motor that moves a pintle shaped plunger in and out of the air control passage. When the valve plunger is moved in, the air control passage flows more air which raises the idle speed. When the valve plunger is moved out, the air control passage flows less air which lowers the idle speed. Over time and miles, the IAC can get carboned up which can have an adverse affect on idle quality. Cleaning the IAC may restore proper function and is an easy procedure to perform and good preventive maintenance so it is never a bad idea.

CLEANING THE JEEP 4.0 IDLE AIR CONTROL

Remove the air filter cover, associated hoses and the rubber boot that goes from the air filter cover to the throttle body. Remove the IAC with a torx driver (2 bolts; one can be kind of hard to get to)

“Gently” wiggle out the IAC from the throttle body. Gasket on the IAC can be re-used if it is not damaged

Clean the IAC with a spray can of throttle body cleaner; inexpensive and available at any place that sells auto parts. Throttle body cleaner is recommended rather than carburetor cleaner as it is less harsh, safe for throttle body coatings and is best for this task. Use cleaner, a rag and a toothbrush and or Q-Tips. Be gentle; don’t twist or pull on the pintle that protrudes from the IAC as it is fragile and you could damage it.

Thoroughly spray clean and flush where the IAC seats in the throttle body with the same spray cleaner

It is also a good idea to clean the entire throttle body itself, the butterfly valve inside of the throttle body and all associated linkage as long as you have things disassembled

Reinstall IAC and check idle quality
__________________
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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Unread 08-15-2012, 08:02 PM   #3
Volvo802
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Cleaned IAC, the whole throttle body and still studdered. Proceeded to check the TPS and it went from .56-.59 base to 3.55(about) at full throttle. I couldn't get it any higher, but it runs very well at higher rpm so I believe that to be OK. No set number was distinguished. Thought it may be the PCM (err that little black blox on the driver fender) and pulled it out cleaned the dirt a bit from the surroundings (connections still looked new) and proceeded to put it all back together. Made sure it started fine, and it worked great (seriously). Maybe it has to do with the only time I drive it being during the middle of the day. Its seems to be OK when cool. But we ran it for about 2-3minutes, than proceeded to flat out floor it (in one of those, I hope it breaks so I can fix SOMETHING that must have been causing problems) and hit redline great up to the top off second before shifting to third, than downshifting (once or twice).
It never missed a beat.
However, pulling back in (we only drove less than a mile because we didn't want to get stuck in traffic) around 3,000 she sounded to be "working hard" but all systems seemed to be good.

Ran check engine lights, got a code 12. Not sure if that mean exactly as what it says (Been leaving the battery terminals off the battery for quite some time and it never gave me a code 12) so its not to drain the battery that is (another problem soon the be checked).

Hmm.
Nothing else really checked or done. Are these cars prone to Vapor lock in hot weather? (its been a sunny and humid 98 all day).
Seems to run okay cold. My brother drove it home from work and litterally had to drive with the hazard lights on an not stop moving or else she would have died. Which makes me think it was the alternator, but it tested good, and we can't afford to replace/do it ourself (no tools for the belts and whatnot).

Its so random it sucks. It gives you the confidence to run around than it just keels over during specific start-ups.

EDIT*****
during idle, I pushed on the gas and the shift light would flicker at around 3k rpm also..... (this was in nuetral)
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Unread 08-16-2012, 05:10 AM   #4
tjwalker
It's the crank sensor!
 
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Quote:
Originally Posted by Volvo802 View Post
EDIT*****
during idle, I pushed on the gas and the shift light would flicker at around 3k rpm also..... (this was in nuetral)
Your shift light should not flicker as you tip in the throttle; that could indicate a bad ground somewhere. I would refresh every engine ground; more on that below.

Also, that code 12 is informational only and related to you pulling the battery terminals, so don't worry about that.
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Grounds can be the root cause of many electrical gremlins. Refreshing grounds is NEVER a bad idea, and the investment of your time in this procedure is always well worth it!

You can't tell much of anything by looking at ground connections!! You must remove, scrape, clean until shiny the cable/wire ends and whatever they bolt to. Be sure to remove all paint from any ground connections.

Start with the one on the back corner of the head, and where it attaches to the firewall, as it deteriorates over time and is an area that makes it susceptible to damage. Best to replace that woven cable with a #4 or #2 gauge cable. You can attach the one end to the intake manifold if you would like.

Next go over to the engine dipstick tube stud. Remove the nut and clean the wire ends and scrape the block until shiny at the stud. Reattach tightly.

If you are so inclined, add at least a #6 cable from the negative terminal of your battery to one of the bolts on your radiator support.
__________________
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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