GMC Repair: 2007 GMC Sierra, 2007 gmc sierra, gmc sierra

Question
I have a 2007 GMC Sierra 4.3 automatic. My truck is very sick and nobody seems to know what is going on. I have been told several different fixes and all are very costly. About a week age my truck started sputtering when I started but then would clear up. As time passed it has become much worse. Here is the kicker,,sometimes you get in it and she runs perfect then you try again and it spits and bucks and does not want to go anywhere then all of a sudden it will have like a power surge and run perfect. I have changed all plugs and wires and put a new air filter on it. I have added dry gas thinking maybe water in the tank no luck there, injector cleaner added also still no luck. I have the codes checked and they are not showing any. The check engine light blinks when she is acting up but that is the only time. I have been told these fixes..head gasket..fuel pump..fuel filter..fuel injectors. What is your thoughts? I bought the truck brand new and have already put 2 transmissions in it and a computer..Needless to say she is a money pit.lol

Answer
Dear Karyn,

Here's something that's good to know:  If it's a head gasket, that is a condition that does not reverse itself once it happens, not even for short periods.  So if it's only acting up part time, it can't be the head gasket.  Injectors don't temporarily fix themselves either, so if it's running perfect some of the time, it absolutely is not the injectors.  Hint:  Injectors are very seldom the problem, although mechanics love to change them because they are easy and expensive.  Here's how fuel filters behave.  They begin to plug up and the truck will run perfect and normal except when you demand more fuel, like you try to accelerate faster, then it will act up because not enough fuel will come through the filter.  This condition gets steadily and progressively worse over a period of days or weeks until it barely goes down the road and finally won't run at all.  Just like you would expect from a filter that is accumulating more and more sediment.  When injectors go out, it can be a steadily increasing (but constant) problem as they wear or become restricted, or one injector can suddenly stop working, but it's not an intermittent problem. And I can tell you right now that your injectors are extremely likely just fine on your 2007 Sierra.  Spark plugs and wires are usually a predictable, steadily increasing problem, they can be intermittent for a time, but it's usually a steady miss that happens part time or all the time, and the check engine light will not flash, it will just come on and stay on indicating a misfire on one or more cylinders.

Here are the things that can produce recurring but intermittent symptoms like that:  Crank position sensor.  Those are a fairly common problem on the 4.3 Vortec.  They can act up one minute and be fine the next, and they get progressively worse.  Often the check engine light will flash while they are misbehaving.

I've seen O2 sensors do that.  The car will seem fine, then it will cough and sputter and run terrible while the check engine light flashes, then suddenly there is a surge of power and it's fine for a time.  Usually when the O2 sensor does that it's one of the upstream sensors and you will see a spot of black soot on the driveway near the exhaust pipe, or on the fence next to the exhaust exit where you park if the pipe is not facing down.  Most the time with an O2 sensor the check engine light will stay on after the fact.

With your cam position sensor or the crank position sensor the check engine light can turn back off when it's running good, so I would check those.

Don't let anyone rip you off.  The crank position sensor costs about 8 to 10 bucks and you can put it on yourself in 5 minutes with your particular engine.  Look on Youtube and watch someone do it.  Sometimes with that engine the main bearings will be a little worn (normal if you have a bunch of miles on your truck) and the little vane things that the sensor is reading will get a hair too close to the sensor.  You can buy a little shim that goes underneath the sensor and moves it away just a touch, and it will be fine.  Try those things and get back to me if you need to.

I hope that helps.  I appreciate you choosing me, and best of luck to you!