Mechanics

MISFIRE - ROUGH IDLE

2009 Chevrolet Silverado • 50,000 miles

Hi, I have 2009 Chevy Silveraor with about 88,000 kms. Purchased the truck in December and have been having rough idle conditions and what feels to be misfiring(on morning start up, sitting at lights I can feel it, and during the daytime if it has sat for a couple hours). The problems seem to have started as soon as the spring and warmer weather started.

I have had it in to the dealer several times, and here is progression.
1.) At this time the truck if it sat overnight or for a coupel hours when I went to start it up, it would misfire and feel like it was going to stall out. The dealer thought it was bad injectors so they changed two fuel injectors and also told me I should have some sort of fuel injection cleaning process completed as the fuel rail was dirty and they blamed me of either putting bad gas in or that the previous owner (a farmer and dirt roads was to blame). I never did have the fuel injector cleaned or fuel rail clean as I just bought the truck and did not feel I should pay.
2.) The fuel injector replacement did not help anything. So I then took it to a different GM dealer, who confirmed the rough idle after it had sat overnight and also they plotted some sort of graph confirming it was not running correctly. They root caused it to a bad O2 sensor which they replaced.
3.) This seem to help a bit, but not 100%. I went on vacation with the truck and got a code (did not write it down fortunately) saying that the engine was running to lean. When I got back from vacation I took it to the dealer and they said they forgot to hook something up related to the sensor or something else.
4.) It seems when the re-hooked something up the problem of misfiring got worse on startup. So I took it back to the dealer. They told me the only thing they can think of is to replace all 8 of the spark plugs, which I did and paid for. I also asked are you sure this will fix it, of course they said it should.
5.) The spark plugs I changed about a week ago and the truck is still misfiring and running rough, in fact the engine seems to be running worse with the new spark plugs. When I startup the engine it misfires and shakes and I can see the RPM meter fluttering.
6.) I called the dealer to set-up an appointment for them to look at it as something is not right. They told me they are not sure what is going on. They said had no codes when I took it in last time, the only thing they said it might have something to do with carbon build up and I should have some sort of vacuum cleaning system done on it.

So I am not sure what to do right now.I really want to take this truck back and throw the keys at them. Have had to take several days off from work and the headache alone.

Hoping someone can help me out. Is there something else they should be checking and proving to me they did actually check it.

Thanks for the help.

p.S. It is still under powertrain warranty and I bought the extended GM warranty as well.
Avatar
Gutter
August 20, 2013.




The extended warranty which is really an insurance contract, means you have paid for repairs that may never be needed. The other problems are "what you need isn't covered" is a common reply, and there's usually a pretty high deductible, often for every repair, even during the same visit. When you DO need an expensive repair and it's not covered by this contract, you can ask for a refund of the remaining balance to put toward the repair bill. About 80 percent of the premium you paid goes to pay commissions to the people who sold you the contract. That doesn't leave much to pay for the insurance.

Two different shops recommended a fuel system cleaning which points to a real common GM problem that may have been addressed by 2009. Chrysler buys their fuel injectors from Bosch in flow-matched sets and problems with them are unheard of. GM grabs a handful of injectors from a large bin and throws them in an engine with no regard to flow rates. As the miles accumulate and a little varnish builds up in them one or two cylinders will start to run a little lean. The extra unburned oxygen in the exhaust is detected by the oxygen sensor as a lean condition, and the Engine Computer reacts by commanding more fuel from all the injectors on that side of the engine, even those that weren't running lean. Your mechanic can find evidence of that with a scanner by viewing the long and short-term fuel trim numbers. They will be very high positive on the side that's running lean.

The second GM problem relates to fuel pressure. On my '88 Grand Caravan that recently had a plugged pickup screen in the tank, I monitored the pressure with a gauge tied to the radio antenna, and when it acted up it would still run fine at 20 pounds of pressure, and stall at 15 pounds. Normal is around 50 psi. GMs also run near 50 pounds of fuel pressure but running problems will occur when the pressure is low by as little as 2 psi. The pressure is real critical. You can have a no-start or a hard-start condition if the pressure is 5 pounds low. If pressure is suspect, on the engines that still use a pressure regulator on the fuel rail on the engine, remove the vacuum hose and check inside for any sign of wetness. If there's any fuel in the hose the regulator is leaking and must be replaced. That's another real common GM problem but it usually is so bad you'll smell a lot of fuel at the tail pipe. Most commonly a new fuel pump is the answer to low fuel pressure.

As for that fuel system cleaning, that DOES seem to be pretty effective in solving a lot of rough running and lean fault codes, particularly on GM vehicles. This is not the simple "mechanic-in-a-can" that you dump in the gas tank. Those chemicals are just concentrated versions of the additives you get in every gallon of gas. The professionals connect their equipment to the fuel rail and disable the vehicle's fuel pump. The engine runs on that highly-concentrated detergent for about 20 minutes.

Caradiodoc
Aug 20, 2013.


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