rd_ls1_z

Hello all,
Got a misfire problem with 2001 Z-28, all stock except K&N filter and Borla Cat-Backs and 40,000 miles. What's going on is that in the mornings, the car starts and idles normal. However, when I start going down the street, the car starts to run real rough, shaking and shuttering type of thing, but no SES light. When I stop, the idle drops to about 500-600 RPM. After a minute, the idle goes back to the normal 800 RPM and the car runs fine for the rest of the day. I fiugred it could have been a dirty fuel system or bad gas, but ran some cleaner, same thing. Finally, yesterday during some hard rain, the car ran rough, SES flashes, then quit and the car was fine again. Took it to the dealer, they told me about the misfire, but could not determine the cause or the cylinder(s). I think it's a moisture problem somewhere but the plug wires are tight. Any suggestions????

777

iTrader: (21)

Pull the plugs see if they are still good. Do you have a K&N? You might want to try and clean the maf with some electric cleaner spray. Be careful they are easy to break. Do you still have you cats. One could be clogged. Just throwing out ideas. First thing to do is check plugs though.

rd_ls1_z

I figure it's a plug, but I haven't had the time to check them. The MAF looked clean, but what kind of cleaner is good? I did forget one thing though...I noticed an extreme sulfer smell one day, but haven't smelled it since.

mrr23

iTrader: (3)

did you get a specific P0300 code? like P0301, P0302, etc... that'll tell you exactly which cylinder is misfiring. sulfur smell would bet CAT related. might want to look for P0410 or P0420 codes as well.

Y2K SS

iTrader: (1)

I got the same problem but all those duggestions don't work with me. Got brand new plugs and wires and no cats. Also just cleaned the MAF when I did my cam install. Its pissin me off as well. I've had it since I can remember also.

mrr23

iTrader: (3)

i know some cams will set off random misfire codes. there's a sticky about it somewhere.

AdamSS

iTrader: (3)

yea, you need to "de-sensitize" misfire values at low rpms. Do a search on hptuners.com and you'll find your anwser. You'll need editing software to do it...

98z28chevy

I know my 95 Blazer has Vortec 4.3 recently got a P0300. The truck is doing something similar to your car. The RPM's in the truck would go up and down and at times it stalls out. If i am at a stop light i have to keep one foot on the brake and the other foot on the gas to keep the rpms up. I believe the reason for the truck doing it is because i used a scanner and found out the EGR is stock open at 84% causing the truck to run bad. I also hear they have that problem alot with EGR's on the Vortec 4.3's. It had a EGR code also in the scanner so tomorrow i am gonna take it apart and clean it up see if theres carbon build up.

87GMC

mine was doing the same thing turned out to be a clogged injector

rd_ls1_z

As for the code, all it was giving was the P0300. Their scanner couldn't bring up a specific cylinder. I was there when the service manager was scanning the car. I think its a moisture issue with the spark plugs, because it only happens in the morning when the humidity is high, or as in the last time, during a downpour.

Y2K SS

iTrader: (1)

LOL..... thats another thing I changed when I changed my cam. I got some 01 injectors. So thats out too.

CAT3

iTrader: (9)

Well, since you have new heads, I am sure its not carbon buildup in your case. In some, like Brians (99 Firehawk), there is such a massive build up of carbon you'll get misfires.
Typical things to check and recheck:
Plugs, gapped, clean and screw on tip tight.
Wires, double clicked on coils, single click on plugs, and conductivity is good ~ 50ohms
MAF, clean front side and back, K&N oiling will collect, as well as blow-by, skewing the readings and causing misfire detect
Larger Cams, often known to set of misfires, can be tuned out using misfire table.
Injectors, make sure they are fully seated to not draw in air, and clean to not drip instead of spray.
Coils, not sure exactly what like internally but, bad coils...test by swapping them out from side to side, or fore and aft cylinders to check if misfire is following.

Other than that I am not sure what causes or detects misfires. Anyone else?