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Old 11-05-2008, 12:11 AM   #1
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A miss?

My 02 TJ seems to have a miss in it. When I drive down the road and get under a little load I get a little what feels like a miss. Ive changed the plugs and had the coils checked. I replaced the fuel pump b/c a friend suggested it and it still does the same thing. Also it seems to get worse on days that its really warm or hot. The local mech. said if it was a miss that the check engine light would come one. Please help!! Its driving me crazy!!

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Old 11-05-2008, 10:36 AM   #2
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My 02 does the same thing, usually under hard load up hills. Ive got a scanner hooked up and am in the process of trying to find out whats going on. No codes or anything, changed the plugs and its still there. Im thinking it might be a broken spring in the coil pack or something else trivial. It only happens once or twice on a 30 min drive, so its more of an inconvenience than a problem.

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Old 11-05-2008, 10:49 AM   #3
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Mine did that just before the TPS (throttle position sensor) went out. Also I've heard the CPS (crank position sensor) will do that too.
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Old 11-05-2008, 10:57 AM   #4
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Bad fuel (with water in it ) will do that too. I suggest a can of STP injector cleaner and a full tank of gas from a different vendor then re-evaluate.
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Old 11-06-2008, 08:31 AM   #5
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My problem was the TPS. I was monitoring it live and felt the skip as the TPS readout dropped to 5 for a split second, the reading at the time was 27. Stuck a new one in there and so far the skip is gone.
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Old 11-06-2008, 09:37 AM   #6
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Its driving me crazy!!
bring it over behind the house, here is the gun
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Old 11-06-2008, 11:39 AM   #7
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i got the same problem, sorry but where is the tps and how can i check to see if thats the cause
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Old 11-06-2008, 11:56 AM   #8
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Its attached to the side of the throttle body. Easy to get to and replace. Maybe someone has a pic.
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Old 11-06-2008, 12:30 PM   #9
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Your description of "a miss" needs to be defined a little closer, otherwise it's just guessing in the dark.

The "miss" under load - is it a thump thump thump regular beat, like a cylinder is dropping out, or partially dropping out?

Or is it more like a quick "burp" like everything shuts off, then back to running OK again? That would indicate it's something common to all cylinders, not just an individual one.
And if that's true, does it happen when you move the gas pedal position or when you hold it still, or both?
Have you steam or pressure washed the engine compartment recently? That seems to be a killer for TPS's.

How did they check the coil rail?
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Old 11-06-2008, 06:21 PM   #10
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sounds to me like you have engine ping. try a tank of 89 and sea foam the motor to get the excess carbon out of the motor.
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Old 11-06-2008, 07:00 PM   #11
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engine ping would be greatly noticeable when starting the rig on a cold early mourning wouldn it?
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Old 11-07-2008, 12:11 AM   #12
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sounds to me like you have engine ping. try a tank of 89 and sea foam the motor to get the excess carbon out of the motor.
Tried that a# of times and it still didnt help. I did just try the water down the intake to clear the carbon. Ill see how that works.
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Old 11-07-2008, 12:30 AM   #13
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Your description of "a miss" needs to be defined a little closer, otherwise it's just guessing in the dark.

The "miss" under load - is it a thump thump thump regular beat, like a cylinder is dropping out, or partially dropping out?

Or is it more like a quick "burp" like everything shuts off, then back to running OK again? That would indicate it's something common to all cylinders, not just an individual one.
And if that's true, does it happen when you move the gas pedal position or when you hold it still, or both?
Have you steam or pressure washed the engine compartment recently? That seems to be a killer for TPS's.

How did they check the coil rail?
Its more of the burp. And as far as I can tell It doesnt change when the pedal is to the floor or half throttle. As far as the coil rail goes Ill have to call the mech. and ask him how he checked it b/c I dont know. If it was a coil though, wouldnt it do it all the time? Ive heard that once its bad its bad Oh and I havent washed under the hood ever but from what Ive been reading I may want to go ahead and change the TPS and give it a try.
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Old 11-07-2008, 09:39 AM   #14
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my guess would be a coil, spark plug, or injector. can you make it miss just sitting in park or neutral?
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Old 11-07-2008, 11:26 AM   #15
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can you make it miss just sitting in park or neutral?
Ive never felt it miss in neutral at idle or high idle
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Old 11-07-2008, 11:44 AM   #16
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Ive never felt it miss in neutral at idle or high idle
When was the last time you replaced the ignition wires?
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Old 11-07-2008, 12:41 PM   #17
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It's an '02, no ignition wires to replace. It uses a coil rail instead.

It's a long bar running alongside the engine covering the spark plugs. Inside it is 3 coils. Each coil fires 2 plugs at the same time. One fires on the firing stroke, the other at the same time fires the compliment cylinder with a "waste spark." The waste spark burns off a little more unburned fuel during the back side of the cam, reducing emissions.

Most places, including the dealer can only check to make sure that it's firing. Nothing more.

But that's not the symptom you described. In the old days with plug wires sometimes the spark would jump out of the insulation to ground, sometimes it only did it under load, hard acceleration etc. That's when the voltages rise.

The coil rails can short internally the same way, only under conditions when the voltages are high. The short is between the runners potted inside the plastic, not fixable. Depending on which runners are shorting it varies the symptoms from a complete shut down and restart, a "jerk," to a miss - thump thump thump, to backfires both up and down.
We used to be able to easily check for insulation breakdown with a scope when we had wires- but now we can't. (I've been experimenting with my scopes trying to come up with an easy way, so far, no luck.)

The only way I know of is swapping it with another coil rail and try it.
Mine did the same or similar, but the dealer said it was fine. But I swapped it out anyway - it did the trick. That's what got me started on it.

Since then I've had 3 or 4 customers with the same ailment. I keep an extra coil rail now to test swap with.

Dealer's get about $250 for a new rail (but check with them, I heard they may have lowered it) - pricey, Car Quest carries ones made by Standard Ignition, last I bought one it was about $165. (I suggested to MSD that they make one with higher voltages and better insulation - and a way to test them. "There aren't enough Jeeps out there to justify it!")


But to justify the price, the coil rail replaces the wires, cap, rotor, module, pick-up coil, and distributor - replacing all those would be much higher $$$. (But who shotguns like that?)
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Old 11-07-2008, 12:42 PM   #18
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If it is anything like mine, and it should be, its a coil on plug system with all 6 units interconnected on the rail. I have no idea how to check this. I ended up replacing the TPS, but only because I saw the reading fluctuate erratically.
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Old 11-07-2008, 05:28 PM   #19
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Easy to check the TPS:
Back probe the connector to it, leave it connected. Use a needle or T pin, slip it alongside the wire into the back of the connector till it makes contact.

DON'T PIERCE THE WIRES!

KEY ON - 3 wires, +5 volts, ground, and the wiper - .5 to 4.5.

Put your analog volt meter - meter type with a needle, not digital - on the wiper, probably the middle of the 3 wires. A digital won't see the glitches.

Watch the voltage slowly change while you SLOWLY open the throttle. It should start about .5 volts and go up to 4.5 or so - SMOOTHLY, NO BUMPS OR CLITCHES.

I can't believe how fragile those things are - a tiny piece of dirt or dust kills them.

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