'02 Silverado 2500HD might have big problems

Driving while in 4x4 HI up a hill the truck started to slip. I stopped, shifted to neutral, and engaged 4x4 Low. I shifted back to drive and started moving up the hill. The engine started to lose power and the “Service Engine Soon” light began to blink. I put the truck in park. As I let it run a little bit, the engine started to shake as if something was very imbalanced. I went to check under the hood, and confirmed that it was shaking quite noticeably. There was no grinding or clattering of any kind though.

I turned the truck off for a bit, and then restarted. There was no light or shaking upon starting up; but as soon as I put it in gear and started driving I could tell it was losing power, the light came back on, and the shaking ensued. This occurred in 2wd, and both 4wd lo and hi. I parked it at a neighbors place and there it sits for now.

I plan to try and get the codes read. My other vehicles can’t make it through the snow we have right now, but it is suppose to get warm later today and might melt enough for me to get one out. I also live out in the country, so the nearest auto parts place is about 20 miles. Same with a dealership service shop. I’m hoping I can buy and return a code reader from an auto parts store to assess if this is something I can diagnose myself or not.

It is the 6.0l gas engine with the 4sp automatic transmission. The truck also has about 107,000 miles. Just thought I would throw this on here and see if anyone had some solid advice?

As far as maintenance goes, plugs were due seven thousand miles ago.

Anytime the Check Engine light starts to blink the computer has detected a major misfire.

Do a complete tune-up and hope that turns off the blinking Check Engine light.


You can only defer maintenance for so long…Don’t forget the fuel filter…This stuff is usually easier to do in September than it is in January…

I couldn’t get out today, but the temp is going up tomorrow.

The plan is still to get my codes read first and foremost. I did buy the truck at 99,000 miles, and the previous owner put in new plugs just before I bought it.

If I can get a code on a specific cylinder I will definitely check the plug out. I’m hoping its plugs or wires. I’m worried though. The shaking seemed to be getting progressively worse…

I had debated about transmission or a mount; but that doesn’t make sense if blinking is indicative of a misfire. The engine did not sound any different to me though; and I would have expected to notice the engine sounding a bit different if it was a misfire.

Only time will tell though. I appreciate the knowledge though.

Get a set of spark plugs and wires. When you go to the wires make sure you know which coil type you have. The vin # will help with that. This the way these trucks act when the plugs and wires need changed.

This truck has coil on plug

If it runs downhill, put gas in it. Tank might be low, like below one eighth.

Next update…

The computer is saying two codes. Both are P0300 “Random Misfire Detected”. I was hoping it would be a little more specific as to which cylinder, but that’s where I’m at.

The next step in my plan is to remove and inspect the spark plugs per the guidance above. I took the seller at his word on having changed them prior to my purchase. He didn’t really have any reason to lie about it; but whatever…

If anyone has some more input based on the code; I appreciate it all.

@db4690 This truck can have 1 coil per cylinder with wires. Square or round type. It can have coil on plug if it has Mitsubishi Ignition.

It’s possible that the reason the previous owner put new plugs in it just before you bought it is because he was trying to solve the rough running.

If your setup has the coils each mounted to the side of the engine with a short ignition wire to each plug, like my neighbor’s did, you could try pulling the wires one by one while it’s shaking and see which one has no effect. If it’s only one cylinder, it’s a quick & dirty way to find out which cylinder is bad. From there you can try switching with another wire and see if the “miss” moves with the wire. That code is more than likely suggesting something affecting more than one cylinder like a crank position sensor, but you never know until you look.

Other than that and reading the plugs, the only thing I can suggest is an ignition analyzer.

Why don’t you save some trouble and address the specific cylinder that is misfiring? Have a shop (not the AutoZone monkey) hook up a scan tool and have them tell you which cylinder is misfiring and whether the problem is mechanical, fuel, or ignition (spark) related. This can all be done with a scan tool from the driver’s seat without even opening the hood.

The reason you only got a code for random cylinder misfire is that GM products do not log a fault code for a specific misfire. But if you look at your scan tool live data you will be able to see current and history misfires. So if cylinder #4 is not firing, you will be able to see that in real time. And even if all the cylinders are firing currently, you will be able to see previous cylinder misfires in history.

Sure, this will cost you $50-$100, but what will it cost you for the time and effort of chasing this problem by throwing parts?

This code is tricky, because the cause could be ignition, fuel, or vacuum leak.

Because the symptoms can come or go, I would dismiss vacuum leak for now, and concentrate on fuel delivery and ignition. Test the fuel pressure with a gauge at the test port both when it is running Ok, and when it is misbehaving. The problem could be a pump going bad or a faulty regulator.

I would check the fuel pressure first. Just to be clear a “misfire” does not mean that you lack spark, though lacking spark can cause a misfire. It should be called “miscombustion” since the “fire” part makes people thing about spark.

I’m sure @asemaster knows more than I and has plenty more experience working on cars. But I just have to say that I am assuming P0300 IS the real code. I have owned a GM vehicle and recorded specific cylinder misfires from it. I am also skeptical of pinpointing the exact problem with a scantool. Maybe sometimes, yes. Either way, for me, resorting to paying a shop with full scantool capability is a very last resort and I’ve usually checked scrap prices before making an appointment. Maybe its just something about where I live but I pretty much never get careful and accurate diagnosis out of it.

Just need a Tech 2. Expensive.


I’m well aware that this truck has 1 coil per cylinder, with a short wire between the plug and the coil.

However, this is still referred to as coil on plug

We have tons of these vehicles in our fleet, with both coil types, so I already know about the differences with the connectors

In our fleet, though , it’s usually the 8.1 liter big blocks that have the problem with the plugs and wires.


Here’s something else to consider

GM issued a technical service bulletin about leaking intake manifold gaskets on your engine.

They can, among other things, cause misfires

I’ve seen this several times.

An evap/smoke machine is the best way to find this problem. Carb cleaner will work in a pinch