......you folks will know. About a year or so ago, one of the old timers around here - maybe farmer123 or Greg Simms gave some wonderful explanations about the higher octane fuels in our machines. I never knew that higher octane fuels actually burn slower, which allows the piston time to reach the top of the stroke, so you don't get the pinging and pre-ignition blues. If an engines valves chatter on regular unleaded fuel, is there anything you can do other than run that $1.69 a gallon premium fuel in it. Even on an older car I could mess with the timing, but these new fangled engines are so preset that you can't adjust anything anymore.
My Magnum is running great, but my Yukon is not. Can y'all help ?

Houndog

 

Look on your fan shroud and it should have the timing setting marked on it. What year is yours? It is probably 0 degrees. To set the timing, you need to disconnect the ECM wire (on my '90 K1500, anyhow). If you don'e disconnect that, then you will never get it because you will always be fighting the computer. Unless you have changed your cam or something like that, then you should leave your timing alone and leave it at the factory setting.

Different octanes of fuel, as far as what I have seen, make little to no difference on EFI vehicles. Probably the most important thing to do may be to run a good injector cleaner through the tank occasionally. I have heard that Chevron makes a good product, but can't remember the name. If I find it, I will try to let you know.

What is your Yukon doing? Give us some more info about it and what it is doing and maybe we can help.

Snowdrift

 

Some don't know this, but the timing on these computer-controlled viechles CAN get off. Some use a timing sensor on the flyheel to tell the computer where TDC is. I am not too familiar with GM products, as far as working on them, but there are 2 things I would try. If your machine will run with the ECM unhooked as Snowdrift mentioned, try it, and see where your timing mark hits. Most engines I know of use an initial timing reference with it unhooked. If you can't do that, why not just see where the timing really is anyway, to help narrow down what is happening? If the timing jumps to 50 BTDC or something, you KNOW you have problems.
My personal guess...check the Oxygen sensor & wires to it. If you have Fuel injection, it adjusts the 'richness' of the mixture according to what the oxygen sensor tells the computer is needed. If the sensor, connector, or wire is going bad, false readings can go to the computer. The computer can lean out the mixture to where it will ping.
Also, how old are your plugs and wires? We had a Dodge Dakota here on the farm that would ping & rattle from every stop sign. Checked EVERYTHING we could, and were ready to give up until we saw a crossfire. After talking to a mechanic, we THINK we figured out what was happening. The wire was bad, and occasionally, it would jump to the block, rather than spark at the plug. This caused the oxygen sensor to think the mixture was too rich. The computer then leaned out the mix to the point of pinging. Naturally, whenever we sat there with it idling, it ran fine. Whatever the case, we put in new plugs & wires and cured the pinging.
Good luck on whatever you try.
I was assuming you have fuel injection. If not, reply back & I'll try again.

 

The Gm's are real bad for O2 sensors going bad. On my 98 GM company truck, I have replaced the back O2 sensor 3 times. (There are 4) When the sensors get out of range the engine light comes on, the mileage gets real crappy, and it will occasionally ping. Also, a good fuel injection/combustion chamber cleaner may help. With the combustion chambers all carboned up, the compression is actually higher due to the extra material in there. Our company found that performing a proper "motor vac" once a year really helps the mileage and performance. This is something that a shop has to perform, not a can of gunk you add to the fuel.

 

"these new fangled engines are so preset that you can't adjust anything anymore" Your correct, they are designed to adjust themselves, if they are not doing it properly, then there is a problem somewhere. Don't try adjusting anything, it is very rare that any manual ajustment should be needed.

What model year is your Yukon and what engine do you have? Does the chatter stop when you use premium? Does it chatter all the time or only when under load? How many miles on engine?

 

Thanks to all of you for your responses. I have a '97 Yukon ,4x4, 350 engine, with 98,000 miles on it. If I'm driving at 45 mph and give it some gas, it will make small pings. If I put my foot in it enough to make it downshift, it won't ping until the rpms get high and then it makes a little pinging. When I'm pulling my trailer with Polaris and stuff, it pings exactly the same way. Climbing up a hill on the highway, I notice that my performance isn't as good, and of course it pings then, especially with trailer in tow. I changed the plugs, cap and rotor about 2 weeks ago - didn't change wires - still pinging.

It's times like these when I wish I had an even older model 350 that I could work on, like the old days.

Houndog

 

First of all, if the pinging continues, you are going to burn a hole in the top of one of your pistons. Back off the throttle when it does this.

Second, get this thing hooked up to a scanner to read what is happening while it is pinging. I believe the knock sensor controls the predetonation you are getting, but I won't promise it. It is about 1 1/2" diameter thick disc looking thing in a water port on the bottom of your block just above the oil pan. Yours is probably on the passenger side.

The scanner, too, will tell you what temp the ECM (computer) is reading to tell the other sensors what to do and where to set timing. It sounds like yours is reading cold when warmed up. That sensor reads resistance in ohms across the two leads. Not sure where yours is on your truck. Mine is in the front of the manifold. You should have a single wire lead for the guage, and a double wire lead for the ECM. It's only a $5-10 part, so you may try to replace that to start with if you can't get it on a scanner. Stick with OEM parts on the sensors - you will be much more satisfied, even though they will be more expensive.

Are you getting any codes? (check engine light)