Code p0340, sensor replaced, check engine light still off, but backfiring...help!!
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Old March 10th, 2008, 11:23 PM   6 links from elsewhere to this Post. Click to view. #1
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Code p0340, sensor replaced, check engine light still off, but backfiring...help!!


I have a 2003 4.6L GT with 92,000 miles. Last week, we had the dreaded code p0340 (camshaft position sensor). Along with the check engine light, the car was backfiring at start up, gas mileage dropped significantly and car felt sluggish. We tried for the easy fix and replaced the sensor and cleared the code. For about the next 5 times that we started the car, it still backfired, but the check engine light didn't return. Then, the backfiring stopped. We assumed that it was just the computer relearning. Then again today, the backfiring at startup began again. Stilll no check engine light.

We have read multiple threads and posts that say that the alternator can be a culprit, however, what concerns us is the fact that the check engine light hasn't returned. Is it possible for the alternator to be confusing the sensor but not triggering a check engine light?

Any thoughts or suggestions for other things we should check?
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Old March 10th, 2008, 11:32 PM   #2
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I had the same problem a few months ago. Very strange no shop could tell me what was wrong. Someone told me it was the timing chain so I was going to get a whole new chain and would cost like $800 i think. Well I have an o/r x pipe and hadnt put the mill eliminators on yet and had been driving around for about 2 months without them. when I put the mills on it stopped. No idea what caused it to stop, but I was having the same backfire problem and when it would backfire the car wouldnt start so I had to crank it till it didnt backfire. Still dont know if the problem is fixed, but it doesnt backfire any more. I would like to see if anyone else had this problem.
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Old March 11th, 2008, 09:43 AM   #3
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I also have an x-pipe and Flowmaster, but have had the mill eliminators from day one. I had a problem with the exhaust at one point which caused the car to backfire constantly when I was driving, but not just at startup.

Any other thoughts?
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Old March 11th, 2008, 10:49 AM   #4
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Most likely the alternator, me and about 3 or for friends of mine had this problem.
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Old March 11th, 2008, 11:29 AM   #5
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I had this problem, but mine was a bad PCM. Take your car to a parts store and have them check your alternator before you do anything with the PCM. Here are the things that could cause your problem...Bad crank position sensor, bad cam position sensor, bad pcm, bad diode pattern in alternator, bad wiring. If you have electronics knowledge i can get you a pin out tonight.

Is your computer stock, or is it tuned? If it is tuned, (if possible) put it back to stock and see if problem goes away.
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Old March 11th, 2008, 12:17 PM   #6
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It's a stock computer.
We went to Autozone and had them check the alternator. According to them, it was putting out the proper voltage. What are we looking for - how do we know if there is a bad diode pattern in the alternator?
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Old March 11th, 2008, 12:51 PM   #7
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Originally Posted by tinkerbell_too View Post
It's a stock computer.
We went to Autozone and had them check the alternator. According to them, it was putting out the proper voltage. What are we looking for - how do we know if there is a bad diode pattern in the alternator?
take it off the car. i work at autozone. the machine they use to test it on the car isn't anywhere near as accurate. i've had some that are good on the car but are bad when they take them off.

Just make sure they have the adapter first, it should be a goofy looking wing type deal
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Old March 17th, 2008, 09:41 AM   #8
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Update......we took the car to Autozone and asked them to check it again for a code, even though the check engine light isn't coming on. We were told that they couldn't pull a code unless the check engine light was on. Told them to humor us and check it anyways. So they did - pulled the same code (p0340 - camshaft position sensor) AGAIN. And just to reiterate, we disconnected the battery to clear the code on the computer when we changed the sensor last week. When we reconnected the battery, the code never came back, but the backfiring did.

The bulb isn't burnt out - in the accessory position, it comes on.

Now what -- does this sound like the computer? I don't want to replace the alternator ($200+) and then still have to replace the computer ($500+).
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Old March 17th, 2008, 05:50 PM   #9
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Computer errors are very rare... but if it is i have a computer that i bought thinking that was the error and now i cant return... its with my car in the shop but if thats what you narrow it down to i can sell it to you cheaper than the dealership.
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Old March 17th, 2008, 10:57 PM   #10
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Check engine light decided to come on this evening....and my gas mileage went in the toilet. So, I would guess that the computer is working. I plan on ordering a new alternator tomorrow. If I've got to replace it, I might as well get one that has better amps and looks better, so I'm going to order the 130 amp polished pa performance alternator. Hopefully this will solve the problems. I'll update as soon as I get the new one on and drive it for a few days.
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Old March 17th, 2008, 11:10 PM   #11
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Diode pattern test is a standard test when taking to places that check alternators....basically a diode pattern test ensures you are getting a regulated voltage to your system.

Like i asked earlier, do you have some electrical background? If i give you a tolerance at a connector would you be able to use a volt meter/ohm meter to ensure proper levels?

Before you just go pissing away money, have a parts store load test the alternator out of car a few times to ensure it checks out correctly.

Alternators are very simple devices...basically it is an AC (alternating current) motor (magnets and coils) that when gets spun up to high speeds create a voltage...your alternator is going to create lets say 50,000 volts AC. Well your car runs on a D/C (direct current) voltage, so we need to convert the AC to DC...so you have a basic rectifing circuit that converts it to DC and then you have a "smoothing" circuit that regulates it to a certain tolerance....So with all that said, if the alternator is providing a fairly constant DC output that is within tolerance (no AC ripple) the alternator is good and wont cause your problems. If you have a degraded diode in the regulated section this can cause all sorts of problems...problems like you currently have.

If you take the car to the dealership, they should be able to do a function test/reload factory tune for around $100 or so....might be a more viable starting point.
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Old March 17th, 2008, 11:25 PM   #12
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Actually it might be a DC motor, its been a while since i've dealt with the motors, i'll do some more research on it tomorrow...but wether it be a DC motor or an AC motor the rest of it stays the same....bad diode patter = voltage not being regulated properly.
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Old March 24th, 2008, 10:50 AM   #13
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Replaced the alternator this weekend (got a 130V from PA Performance). Car is running perfect now.....no check engine light, no backfiring.
Took the old alternator to Advance to have tested OFF of the car just to see what the results would be, but were told that they couldn't test it because they didn't have a bracket to fit it.
Thanks to everyone for your input on this issue
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Old March 24th, 2008, 12:15 PM   #14
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Originally Posted by 1979rs/z28 View Post
Most likely the alternator, me and about 3 or for friends of mine had this problem.
Me too, replaced the alternator and the problem went away
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Old March 24th, 2008, 12:16 PM   #15
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Originally Posted by tinkerbell_too View Post
Replaced the alternator this weekend (got a 130V from PA Performance). Car is running perfect now.....no check engine light, no backfiring.
Took the old alternator to Advance to have tested OFF of the car just to see what the results would be, but were told that they couldn't test it because they didn't have a bracket to fit it.
Thanks to everyone for your input on this issue
same thing at my advance lol.. glad to see that you got it fixed
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Old March 24th, 2008, 04:09 PM   #16
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What other problems could the alternator cause? I would have never guessed it would have to do with his or my problem.
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Old April 11th, 2009, 01:51 AM   #17
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Sorry for bringing an old thread back to life, BUT everything tinkerbell said - well it's happening to me too. I have replaced my cam sensor 3 times already and the backfiring comes, gas mileage goes to SHIT! and I can't figure out why, I have been driving my car for 8 months like that, real sluggish, hp decreased a lot, and well when I drive it felt as if the car "backfired" as in it kind of shook and then went back to normal, like a gear got jammed or something. This thread gave me Hope! This is why I love this forum, a lot of answers and help. I just ordered the exact same part as tinkerbell did. In hopes that it will fix my problem as well. *cross fingers!*

Again - Sorry to bring back old thread, I just got lifted with hope and felt good and had to say something ^_^
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Old April 15th, 2009, 11:09 AM   #18
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Originally Posted by jmscobra View Post
Sorry for bringing an old thread back to life, BUT everything tinkerbell said - well it's happening to me too. I have replaced my cam sensor 3 times already and the backfiring comes, gas mileage goes to SHIT! and I can't figure out why, I have been driving my car for 8 months like that, real sluggish, hp decreased a lot, and well when I drive it felt as if the car "backfired" as in it kind of shook and then went back to normal, like a gear got jammed or something. This thread gave me Hope! This is why I love this forum, a lot of answers and help. I just ordered the exact same part as tinkerbell did. In hopes that it will fix my problem as well. *cross fingers!*

Again - Sorry to bring back old thread, I just got lifted with hope and felt good and had to say something ^_^
I'm glad you found hope in this thread. I still have not had any more problems. Hopefully you'll be as lucky! Come back and post your results.....GOOD LUCK!!
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Old April 15th, 2009, 12:17 PM   #19
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one quick way to check the diodes if you have a dvom. put it on a low ac volt scale,put red lead on output stud of alt,black to ground(I use the alt case). turn on acc. and if you read more than .5v it is bad
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Old April 15th, 2009, 03:22 PM   #20
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thanks for the feedback tinkerbell. Hopefully my alt will get here this week or next. Is the PA shipping good? or slow? I used standard.
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