misfire

scottac · November 28, 2003

I started having trouble with my truck the other day. It seems to be misfiring and I don't really know why.

I get the flashing service engine soon message on the dash. but then it just stops.

I looked in the manual. it isn't the gas cap.

Will a diagnostics test be able to locate the exact problem or is it going to be a waste of money considering I already know that it is misfiring.

thanks

JayMan · November 29, 2003

Well, the flashing indicator means a misfire in progress. If the MIL (Malfunction Indicator Lamp) stays off otherwise, the fault is clearing itself. Could be as simple as a plugged fuel filter or a tank of bad gas.

All OBD2 engines must maintain a misfire history. You will then see if the misfire history is constant across all cylinders, or if one or two have all the misfires.

I'm guessing it's out of warranty?

Wouldn't hurt to pull one of the plugs and examine it. Since the Vortec has aluminum heads, and they put the plugs in dry with no anti-seize, that might be tough. Hey, I warned ya!

sjspor · December 1, 2003

Scott.. Which engine is in your truck... ??

If you have a V-6 your cap and rotor have become Contaminated with corrosion because of excess moisture.

hcvone · December 1, 2003

I started having trouble with my truck the other day. It seems to be misfiring and I don't really know why.

I get the flashing service engine soon message on the dash. but then it just stops.

I looked in the manual. it isn't the gas cap.

Will a diagnostics test be able to locate the exact problem or is it going to be a waste of money considering I already know that it is misfiring.

thanks

As said above, check your fuel filter, cap and rotor (if you have them) and you may want to check your spark plugs, the gap from the factory in my last two Vettes and my Escalade was way off (to big)

Shaners · December 1, 2003

My buddies 8.1L Avalanche was recording a missfire condition. It ended up being a bad injector on #8 cyl.

scottac · December 1, 2003

I can really feel this misfire though. it has only done it the once and it was after a road trip(500 miles nonstop - fuel stop). the symptom was not just the misfire being reported but the truck was actually rocking back and forth. I drove it down the block to see if it would stop, and it did. but for about the first 200 or so yards the motor would only respond in a sporadic fashion to the gas pedal.

I am taking it in today to get looked at.

scottac · December 1, 2003

oh yeah. I have the 4.8 .

Wingnut · December 1, 2003

By the way...Most auto parts stores will read those codes for free.

At least they will around here, but you should call ahead and ask.

scottac · December 1, 2003

I may check that out because I decided to not take my truck in until thursday. So like NAPA, Pep Boys, or Auto Zone may check it for free. If anybody else knows just drop a line.

thanks

JayMan · December 1, 2003

OK you have the Vortec 4.8. That means no distributor cap, as you have Coil Per Cylinder.

Whenever the Malfunction Indicator Lamp flashes, it's detecting a misfire. Depending on whether a Drive Cycle can clear it, the lamp may remain on or stay off. Since you still have the misfire condition, it's important to fix this soon or the O2 sensors and/or cats could fail.

I don't think those cheap code scanners can retrieve cylinder data and Misfire History. Autotap scanners should be able to retreive the Misfire History, and the OTC Tech2/Pathfinder/Genisys is GM-Certified to retrieve all the extended GM data.

Don't forget the basics like fuel pressure. Since it's so easy to check fuel pressure on the truck (Schraeder valve on the pass-side fuel rail) this should also be looked at. FYI 55-64 lbs is normal range.

Good luck and let us know what the results are.

scottac · December 2, 2003

where exactly is the fuel pressure guage.

I kinda looked for a second. but didn't have any luck.

JayMan · December 2, 2003

Look along the passenger side fuel rail. Near the front of the rail is a Schraeder valve (Like on a bicycle tire). You need a fuel pressure gage with a flexible high-pressure hose and a Schraeder fitting to screw into the fuel rail.

I got mine at NAPA 4 years ago, and recall paying $35 for it. There is also a bleed valve on the gage so you can aim the bleed into a small can and bleed down the fuel system residual pressure.

That makes changing the fuel filter one HECK of a lot safer! No more gas at 62 psi spraying all over the place!