Code p0340, sensor replaced, check engine light gone, but car backfiring...help!!!

Tinkerbell_too · Mar 10, 2008

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?

nyuk98gt · Mar 11, 2008

Tinkerbell_too:

I am not a whiz when it comes anything electrical but I have to say that the alternator (and, specifically, the voltage regulator) is the likely culprit. You can measure the voltage and if it is below 15 V (14.5 V is the "normal" peak voltage), then the alternator is probably OK. If it is higher, then you should replace the alternator. FWIW, I got my 130 A alternator from PA Performance and it is a high quality unit (it is made in the USA, too!!).

HTH,

Chris

joshheat25 · Mar 11, 2008

It's your alternator! I had the same thing except I didn't waste my time with the sensor.

Jmanbigd · Mar 13, 2008

Im in the same boat my 02 GT same code same problems. The only difference was my light(check engine) was always on and i never felt a loss of power. It was the alternator for me. It was sending of a magnetic wave that was disrupting the cam sensor and setting the light off. Sounds strange by My Uncle/Mechanic was dead one new alternator and it was fixed. Now you try can try a new alternator or it could be the ECU im not sure its strange the light isn't on. With that amount of mileage it could be the computer.

Tinkerbell_too · Mar 13, 2008

That's what has us baffled...the fact that the check engine light isn't on. It's back to backfiring every time I start it....but still, no light.

Tinkerbell_too · Mar 17, 2008

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+).

jstreet0204 · Mar 17, 2008

You can get codes without the light coming on. The light will wait until the code repeats itself a few times to make sure it isn't a fluke. You should have had them check the alternator while you were there. I've had the alternator throw that code as well.

Tinkerbell_too · Mar 17, 2008

The car has been repeating the problem over and over - every time I start the car, it backfires.

jstreet0204 · Mar 17, 2008

Tinkerbell_too said:
The car has been repeating the problem over and over - every time I start the car, it backfires.

And? I'm telling you it isn't uncommon that the code is there but no light. I have my own scanner and most of the time I can pull a code before the light ever comes on. You need to have your alternator tested. They will test it for free, and that is the most common cause of this code.

Tinkerbell_too · Mar 17, 2008

We had Autozone check the alternator while it was on the car....they said it was testing fine. I just had someone suggest that we take the alternator off the car and have it tested.....does that make sense?

jstreet0204 · Mar 17, 2008

Tinkerbell_too said:
We had Autozone check the alternator while it was on the car....they said it was testing fine. I just had someone suggest that we take the alternator off the car and have it tested.....does that make sense?

Yes have them check it off the car. When mine threw that code the voltage on the system was fine but the bearings were starting to make noise. Also spin it and comare it to a new one.

rabidscoobie · Mar 17, 2008

I had this same prob,it was a bad diode in the altenator.Make sure they check everything on it,mine held voltage but would cause it to do exactly what you have said.My light would very seldom come on,but that thing would backfire every time I started it,My code was also the same one,cam sensor.Tell them when they check it to check for faulty diodes in the altenator.The car ran fine or so I thought,but my milage dropped alot.They did the test on my car too,twice and said it was fine.The 3rd time I went to Advance Auto Parts and the first time he ran it he said bad altenator,the diodes are bad,replaced it at $250 and never had that problem again.

rabidscoobie · Mar 17, 2008

Also forgot to add they said the reason it kept showing at autozone as good was because the alt was disharging as it was charging of which in turn killed a 6 month old battery that I had to also replace.

Notchbck93 · Mar 17, 2008

rabidscoobie said:
I had this same prob,it was a bad diode in the altenator.Make sure they check everything on it,mine held voltage but would cause it to do exactly what you have said.My light would very seldom come on,but that thing would backfire every time I started it,My code was also the same one,cam sensor.Tell them when they check it to check for faulty diodes in the altenator.The car ran fine or so I thought,but my milage dropped alot.They did the test on my car too,twice and said it was fine.The 3rd time I went to Advance Auto Parts and the first time he ran it he said bad altenator,the diodes are bad,replaced it at $250 and never had that problem again.

right!

When one of the diodes takes a dump, AC voltage is no longer fully rectified to DC and funky stuff goes down.

An AC waveform will show the bad diode.

http://www.picotech.com/auto/waveforms/alternator_dc.html

http://www.bcae1.com/charging.htm

Read and learn... Autozone is full of parts people (people who couldn't diagnose)...waste of time... Go to an ASE Master tech that has earned the L1 and get your stuff diagnosed correctly.

Or you could cough up the green, buy a good alternator, not a cheap POS from any local junk aftermarket place and save yourself the head ache.

rabidscoobie · Mar 17, 2008

Notchbck93 said:
right!

Read and learn... Autozone is full of parts people (people who couldn't diagnose)...waste of time... Go to an ASE Master tech that has earned the L1 and get your stuff diagnosed correctly.

Or you could cough up the green, buy a good alternator, not a cheap POS from any local junk aftermarket place and save yourself the head ache.

Exactly,Autozone is the ones I took it to the first 2 times,it was late only ones open at the time,then went to advance of which most are certified.As for Autozone I will not buy parts from them,just normal maintance stuff,fluids and what not,had very bad luck with their parts in the past,to be more precise,altenators.But thats just me.There is a reason that NAPA is more expensive..always had good luck with them.

Notchbck93 · Mar 18, 2008

They're cert in customer service. They are usually C1 and P1 certified.

Even at a quick care center, most are brakes and a few are A5, A7 & A8.

Means they understand how to look up parts in the computer and ask questions. It does not mean they can properly deduce a real conclusion from your information. Once in a while you find a decent parts guy.

I run a Napa "new" alt on my notch... it has worked well. I didn't feel like doing a 3G upgrade and was strapped for cash at the time my alt crapped out.

Don't get me wrong, I've met plenty of book smart ASE master techs but you really have to question their shady practices.

Tinkerbell_too · Mar 24, 2008

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 :SNSign:

rabidscoobie · Mar 24, 2008

Good to hear,I think that is a common problem when these alt's go bad.Same thing about Advance here on the bracket thing.I like getting mine tested while its on the car anyway.I got mine from advance with the lifetime warranty,cost about $250 best I remember.

jstreet0204 · Mar 24, 2008

Tinkerbell_too said:
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 :SNSign:

Good to hear, they could have tested it though. They didn't have the bracket for mine either, they just had to clamp it in there good.

Notchbck93 · Mar 25, 2008

Tinkerbell_too said:
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 :SNSign:

Thanks for the follow-up!

Code p0340, sensor replaced, check engine light gone, but car backfiring...help!!!

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