Any Mechanics out there?

[First-light]

2007 Chevy Tahoe. 196000 miles. Lately its been having a rough idle. Engine light came on and stabilitrack warning came on then shut off. 

Checked the error code which was P0300. This is an emission system exhaust error. Brought it to my mechanic and he checked it again and said # 8 cylinder was mis firing.

Changed plug wire, coil and injector on 8. Still has the misfire. We are currently running Techtron through the fuel system. He wants me to do this 4 times. If this doesn't help he said I probably need a new motor. He said by the time you do valves and lifters a new motor would be a better option. I'm at a loss, other than the misfire and stabiltrack going off the truck runs great. Any thoughts out there?

Thanks,

FL

[turkeyfeathers]

Doc Emmett Brown would clearly check the flux capacitor 

[reeltime]

new cap & rotor?   valves, guides, springs, and lifters would be as much as a motor??  none I have ever done.   these new motors are so finiky even excessive moisture in the fuel will make it throw a missfire code.

[First-light]

Basically he was saying you are better off spend a few more grand and getting a new motor. 

[wdswtr]

Thing is you wont get a new motor for a few grand, your gonna get a reconditioned motor with everything that he says yours needs, but I would expect a warranty of sorts would also come with the reconditioned motor.  I wouldnt give up just yet on your motor.  

[NFA-ADK]

Possible it is a bad computer, test fuel filter and presser?  Otherwise you covered injector, wire (Hope you did them all.) and plugs.  I would do an injection test and possibly a compression test.  Injection test will tell if the computer is firing the injector, compression will tell you if you have mechanical issues. 

[stubborn1VT]

I'd bet money it's injectors.

[NFA-ADK]

11 minutes ago, stubborn1VT said:

I'd bet money it's injectors.

Said he replaced the bad one.

[Jeremy K]

One last check after replacing coil,wire plug,injector for just one cylinder . Swap the new components with a different cylinder and see if the problem stays on number 8 . If it stays on cylinder 8 ,that will pretty much tell you its a mechanical failure on #8 and not electrical.

[stubborn1VT]

With that many miles there isn't just one bad injector.  The injectors stick and throw off the computer, and it does downhill from there.

[First-light]

I hate to give up on it also. Going to try a few more things this weekend. My buddy owns a Chevy dealership I'll bring it there for a second opinion.

[Jeremy K]

Can you spray the intake manifold with carb and choke cleaner on the number 8 intake port to see if the idle raises? That would indicate an intake gasket leak. 

Edited January 17, 2017 by Jeremy K

[doebuck1234]

I was going to say coil aswell.may even be a sensor of some sort throwing it off

[b3h]

Compression test


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[stubby68]

While truck is at idel hold a piece of paper by the tail pipe. If paper always blows away from pipe valves AR ok. If it blows away then pulls back toward pipe you got 1 or more bad valves. We're the codes cleared after changing everything? Could still be reading old codes. Have it cleared not just disconnect battery. Also try a new battery. Sometimes the battery will te SD this fine and work fine yet have a short in it. The short will cause problems when there are none.Some will say no to battery especially a dealer but I have seen this be the case more then once. 

           You said it is a chevy. No need to change engine for a simply miss fire unless it is a dodge. How does it run? Run good just showing code? If it running good why rebuild or replace? Just my 2 cents.

[hunter49]

  With that many miles, Have you checked the oxygen sensor ?  changed  All plugs ,wires, injectors, coils , compression check,  gas filters? If all else fails throw lots of $$$$$ at it!    Warren Buffet has never bought a new car but he does upgrade to a newer one.   Good Luck!

[16. ga hunter]

At that mileage I would put money on a broken valve spring.they snap off on the top by the lock clip. I wouldn't run 4 Techron thought it either one will do if it's a carbon or dirt issue.cylinder leak down would be the most precise diagnostic aid at this point on the number 8 cylinder but not everyone has a leak down tester.


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[PREDATE]

A failing purge canister solenoid can cause a rough idle.
As always when your mechanic is calling for drastic measures, I'd recommend getting a second opinion. Good luck

[Jeremy K]

It could still be a short in the wire to the coil that tells it when to fire.

[stubby68]

Just remembered is it a 5.3 AFM  engine. If so if could have a flat spot on cam. With that many miles it would be better replacing it. Wouldn't be the first one to have this happen.

[First-light]

Stubby,

I'll try the battery and cables. That is an easy one. My mechanic also talked about the cam having a flat side causing the problem. Thanks to all of you I will try everything talked about. It is really a bad bad time for me to come up with this money. I hope something works. 

[chas0218]

15 hours ago, First-light said:

Basically he was saying you are better off spend a few more grand and getting a new motor. 

Umm been there did this. Call up A&P auto in Palmyra http://www.apautoparts.com/ .I put about $500 into the motor in just sensors, I bent a valve trying to run it like the video. I called up A&P got a used motor and haven't looked back also only cost $550 for the used motor from intake to oil pan and that was with the extended warranty ($50 extra). I think what causes this problem is an exhaust leak at the exhuast manifold then plugs the cats and causes too much backpressure and basically just ruins the motor over time. I'm glad you got almost 200k out of yours my 03 has 99k on it and replaced the motor at 81k.  Here is what my 03 silverado was doing. That was also with the exhaust cut off just in front of the cats.

Edited January 17, 2017 by chas0218

[Arcade Hunter]

12 hours ago, b3h said:

Compression test


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Do a compression test.  Try to rule out mechanical failure first.  If it is, you may just need to do the heads.  Your 07 has (or at least my 07 Tahoe) has cylinder deactivation, meaning four cylinders drop out when engine power is not required.  It does this by redirecting oil to certain lifters to cause the valves to stay open hence, dropping out the cylinder.  Compression test may tell you to look at this closer at this system.

Also.... You should be using an AC Delco (original equipment) oil filter as they have a modified oil bypass circuit that can handle the cylinder deactivation system.  Using any other brand could starve part of your engine of pressurized oil.  Ask me how I know!

Also, there is a second oil filter on your vehicle (if equipped with cylinder deactivation).  It is located under the passenger side valve cover.  I have not had to mess with mine yet (157,000 miles on it) but if I recall it is serviceable.

[Arcade Hunter]

19 minutes ago, chas0218 said:

Umm been there did this. Call up A&P auto in Palmyra http://www.apautoparts.com/ .I put about $500 into the motor in just sensors, I bent a valve trying to run it like the video. I called up A&P got a used motor and haven't looked back also only cost $550 for the used motor from intake to oil pan and that was with the extended warranty ($50 extra). I think what causes this problem is an exhaust leak at the exhuast manifold then plugs the cats and causes too much backpressure and basically just ruins the motor over time. I'm glad you got almost 200k out of yours my 03 has 99k on it and replaced the motor at 81k.  Here is what my 03 silverado was doing. That was also with the exhaust cut off just in front of the cats.

Keep in mind that he does not have the same engine in his 07.... great price on your motor by the way.

[chas0218]

7 minutes ago, Arcade Hunter said:

Do a compression test.  Try to rule out mechanical failure first.  If it is, you may just need to do the heads.  Your 07 has (or at least my 07 Tahoe) has cylinder deactivation, meaning four cylinders drop out when engine power is not required.  It does this by redirecting oil to certain lifters to cause the valves to stay open hence, dropping out the cylinder.  Compression test may tell you to look at this closer at this system.

Also.... You should be using an AC Delco (original equipment) oil filter as they have a modified oil bypass circuit that can handle the cylinder deactivation system.  Using any other brand could starve part of your engine of pressurized oil.  Ask me how I know!

Also, there is a second oil filter on your vehicle (if equipped with cylinder deactivation).  It is located under the passenger side valve cover.  I have not had to mess with mine yet (157,000 miles on it) but if I recall it is serviceable.

I forgot the later model 07's had the 4 cylinder deactivation this might just be a lifter issue. I know they had some issues with 07-11? not sure but if you look up lifter issues and afm you will find a plethora of info on it.