Any GM Mechanics?

I recently bought a 2014 Silverado 1500, 5.3 engine, 43,000 miles.

It ran good when I bought it, but now it has developed a mid range stumble and intermittent rough in gear idle. The stumble feels like lean mixture, driving in the 50 MPH range, try to slightly increase speed, it tends to stumble. Trying to drive a steady speed it wants to fall off, then push the throttle a little, it picks up too much, like it wants to run any speed but the speed I want to go! LOL

This has the V8-V4 system, but watching the dash indicator, when it switches back and forth does not seem to be associated with the stumble. Watching the average MPG, it's very unpredictable. One time I look at it, it's showing 13 MPG. Drive the same route, same conditions, next day it's showing 17!

Seems every theory I try to pursue hits a dead end...

Fuel filter: Doesn't have one that I can find.

Fuel pressure test: Can't find a port. No fuel rails visible, evidently everything is under the intake plenum.

EGR valve: Don't see one... Is there one?

MAP sensor: Don't see one.

Haven't looked at the plugs, but they are supposed to go 100,000 mi. I'm getting no codes. I've looked on Youtube, nothing for that late model, just the older stuff. Looked on the forums, no success.

Any ideas?

Thanks!

Plugs can fail early. They can crack or have other internal problems. The symptoms seem to be a plug. I have had that problem before.

What are you using to scan it with? If it's one of the cheap ones it may not read the entire code. You may have to bite the bullet and take it to a dealer and have it scanned.

Rick

If you think it is a lean mixture, by what you describe, I think you're right. I would check the throttle position sensor (key on engine off) and see what voltage value you get. Often you can loosen the screws that hold it on and move it just a bit to get a higher voltage then lock it down on that spot. Cheap thing to try but can sometimes give great results (BTDT). When the system goes closed loop it will compensate to what it wants.

scannerdanner has a good youtube channel dedicated to checking electric and electronic problems that affect driveability. He is an instructor at Rosedale Tech in PA. He might even answer online question for you.

Steve,
Maybe an 02 sensor? My 05 Tahoe 5.3 didn't show any code and acted like yours did, after a while it did show a code for bank 1 sensor 2 ...don't know what caused it to go bad,but never had a problem after that....sometimes a good code reader will catch a problem,before the computer system does. Please let us know what it was.

Keith

I would think there is a MAF sensor if no MAP, I do not think you need both, put a scan tool on it and check the values to see if they seem realistic.

Check to make sure it hasn't had one of the "super duper hp adder" K&N oilable air filters installed by previous owner trying to get that elusive mpg and hp . These filters always get oiled too much and the sticky oil carries through and deposits on the MAF sensor and then minute dust particles stick to that and pile up to insulate the sensor wires in the MAF sensor and basically send the signal to the ECM that there is very low air volume/velocity and therefore it shuts down the fuel to injectors and then that in turn makes the O2 sensors read out of spec and screws up everything. Take off intake boot and get a can of MAF sensor spray cleaner and spray clean. Then if it was ,in fact, a "K&N", get rid of the thing. They are for racing in the dust not everyday driving.

Looks like that's a GDI engine

The #1 issue I have seen with GDI engines is using cheap no name fuel AND the engine oil "LOW" are worn out from extended oil changes... Yes I have fix a few by just changing the oil :shock:

GDI engine will break those cheap arses that believe in extended oil changes... Not saying you are one Its a hole new ball game you are going to need a good scanner to diagnosis it... If it is GDI don't go fudgen with the fuel lines under pressure unless you want to die. You use a scanner to check fuel pressure...


http://www.searchautoparts.com/auto...nt-training/industry-leaders-share-their-tips

It should have a 5/100 powertrain warranty. You might want to check on that before you get too involved.

5 year?100,000 mile war. on power train, ask the dealer.

I would try plugs. I have a 2008 impala that drops 3 cylinders when less power is required. Had the same issue with it just like you are saying. No codes and the dealer couldn't help me. That was at 40,000 miles. Fought with it as it kept getting worse finally changed plugs and wires and has been great since.

MAP sensor is a mixture control device. It "measures" the amount of air going into the engine based on the vacuum reading. Sensitive, but less accurate than a MAF sensor. MAF sensor measures the rate of air entering the engine based on the current it takes to keep the "hot wire" up to a specific temperature. Much m0ore accurate, and replaces the MAP sensor. You may still have a BARO sensor.

Your symptom seems to point more to a TP sensor or a gas pedal sensor. A scanner may give you some indication of a problem, but more likely you are getting a "dirty" signal. That is, the signal from the sensor is not a good clean DC voltage signal. The only way to verify that would be with a scope. Also, since you have a "fly by wire" throttle control, you have a sensor in the gas pedal as well as one on the throttle body.

Also, keep in mind that a Mass Air Flow sensor will NOT set a code unless it is a major failure of that sensor. It can go "out of calibration" and cause the mixture to get too rich or too lean.

Timing is another potential problem. Timing is continually varied by the PCM, and a CKP or CMP giving a "dirty" signal can also be a source of trouble. Maybe check that the connectors are clean and tight on them.

As a BTW, by law, emission control systems are covered by a 5 year/50,000 mile warranty. The entire engine control system from the PCM to the lowliest sensor are part of that emission system. A trip to the dealer with your complaint may be in order. They have diagnostic equipment that dwarfs anything that either a mechanic or a third party shop may have.