2003 monte carlo high rpm random misfire


#1

heres the deal with this car i just did head gasket, valve job, all new intake gaskets and spark plugs. car idles fine and mid throttles fine but with high throttle it wont shift and flashes the random misfire light p0300 it seems to run out of power. ive heard everything from maf sensors to ignition coils and wires. i was just wonder a good place to start and good test to do. i have most testing equipment including scan tool so i can ride with it. o and all cylinders compression tested above 150


#2

A flashing Check Engine light indicates a major misfire. And a major misfire can damage the catalytic converter.

If there’s a major misfire at high RPM’s, and the engine loses power at the same time, I’d check and see if the cat was damaged.

Tester


#3

" ive heard everything from maf sensors to ignition coils and wires."

You’ve answered your own question. At this age, replace 'em all.


#4

is there a good way to test the coils while accelerating because i know the changing variable make it harder for the coils to ignite under load and may cause misfire but random means more that one is there a chance that multiple ones are bad or is it unlikely. also whats a good easy way to test for a clogged cat


#5

http://www.aa1car.com/library/exhaust_backpressure.htm

Tester


#6

If you have a scan tool drive the car while the misfire is there, record the data under misfire counters and take a look. It will tell you both current and history misfires, so you should be able to identify if one, two, or all are misfiring.

While you’re at it, look at your fuel trim data while on hard accelleration.

Graphing your data, specifically TPS, MAP, MAF, and O2 will give you a clue as to your exhaust being plugged or not.


#7

Considering the work you’ve just completed on it, you may want to check the valve timing and the manifold vacuum too.


#8

i didnt mess with any of the valve timing and just off the top of my head i think the vacuum would affect idle and mid more than full power rite? and all the gasket are new, there the better gaskets and i took my time with them im not ruling it out though. just the way it was probably driven before im thinking the catalyst converter is a pretty good culprit


#9

disconnected the catback system and the car had full power. so im off to get a new cat or test pipe


#10

nice…


#11

"disconnected the catback system and the car had full power. so im off to get a new cat or test pipe "

I think you mean you disconnected the cat. The catback system starts after the cat.


#12

nope it was all disconnected right after the header


#13

"nope it was all disconnected right after the header "

You misunderstand what a “catback” system is.


#14

sorry i was thinking cat and back. catback systems are everthing behind cat and dont include it. i meant everthing behind the headers