I have a 2000 chevy blazer ZR2
about two weeks ago while on the hyway at about 65 mph the started to misfire bad then I heard a noise from the engine. I stopped and found that my pulley tensioner had went and the serpentine belt was shot, which also ended up cutting through the "T" connector for the hoses from the secondary , so I replaced that and resecured the hoses, along with replacing the belt and tensioner.

But the engine was still misfiring, but this time it start after driving for a few minutes and around 40 mph. the engine light was on and I had the P0300 code and a code for the catalytic converter, which was also making a bad rattling noise. Come to find out the honeycombs had come loose and were blocking the hole for the exhaust, restricting the flow through the Cat., the cat has been replaced, but i it misfires, although now only at hyway speeds.

also, I was getting a code for the secondary air injection system, found the fuse blown, replaced that, but it keeps on blowing, although it is not tripping the code for the engine light.

I have replaced all the plugs, wires, distributor cap and rotor. I also sprayed MAF cleaner on the MAF sensor, that seemed to help for a day, but then it started to misfire again, although not as bad. I also every now and then get a code P0440. but after spraying the MAF it hasnt come back. The only code that keeps coming up is P0300

My engine code reader gave the following in the freeze frame
ABSLT - 34.1 %
engine rpm 2251
calc load 27.4
MAF - 47.1
MAP - 80 kpa
coolant 194deg
st ftrm1 -7.8%
lt ftrm1 _ 21.0%
st ftrm2 - 14.0
lt ftrm2 - 23.4
fuel sys 1 & 2 - clsd
speed - 68 mph

a check of the I/M monitors showed all applicable montiors were ready except the EVAP SYS was not ready

Any ideas on what is causing the misfiring?
it runs smooth at idle and driving around town, its only when i get on the hyway and have been travelling at about 2500 rpm (60-65 mph) for a few minutes.

 

I tried to do some research on this because it intrigued me and I'm trying to get smarter on code reader results. Basically I wonder if the VCM is being tricked to lean out the mixtures (which I believe are not supposed to be more than +/- 10% either way) when the enters Highway Mode (or semi-closed loop) because there might be damage to the HO2S after the catalytic converter. Can you get live data to the HO2S while driving at those same conditions? They're supposed to switch between lean and rich conditions constantly every few seconds from what I understand. I'm definitely no expert...just throwing this out there for you to chew on.

 

if im looking at the numbers right, and the dashes in front of the fuel trim numerals are just that, and not negative values.....
then the pcm is commanding LOTS of fuel to compensate for a lean condition.
since you are not at closed throttle an intake leak is ruled out.
one number stands out. the MAF g/s is high for the RPM and engine load.
and were missing what the o2 sensors are doing at the time.
see if you can jot down some data when the miss is happening
add whatever you can and we'll work at it

 

by the way, when that evap monitor shows "complete" that p0440 code will probably come back. has nothing to do with the maf

 

Yes that dashes are there as spaces not to represent a negative value.
My code reader is a basic reader from pep boys, so i will only be able to get limited information from it. I dont recall seeing anything in the menu regarding the 02 sensors. But, I will jot down whatever data I can pull and when it starts missing and add to my post.

So we may be looking at a bad o2 sensor or MAF sensor?

 

Bank 2 is lean. Bank 1 is a little high. Check the fuel trims above 2,500 rpm to see if the short term fuel trim comes down. That would indicate a vacuum leak.

 

You may also want to do a fuel pressure and leak down test.

 

Ok I will give that I look. although, I dont think my code reader will do live monitoring. But wouldnt a vaccum leak have the engine running a little rough at idle? because the engine runs smooth at idle, not even a shutter. even when i rev the engine, its a nice smooth acceleration, no misfiring

 

Though I mistook your dashes as negatives...I still wonder about the HO2S after the cat. Also, I didn't notice before, but what brand did you use when you replaced your cap, rotor and wires?

 

The cap and rotor were ACDELCO, not sure about the wires. But, I was having the problem prior to replacing plugs, wires, etc.


Here are the new engine codes. I cleared out the codes and ran the truck for a couple days after spraying the MAF sensor. the truck ran a little better, but still misfires when on the hyway.

The code reader now picked up the follwing codes:
P0300
P0440
P0420
P0440
P0420

Not sure why it registered the codes twice. Not sure why it is stating the Cat is below efficiency, its a brand new cat. So possibly the O2 sensor after the cat is shot. So i have replaced that sensor.

Also, the fuse on the secondary intake system blew again, so I am thinking that the motor is probably shot also. although, that shouldnt create a misfire situation. from my understanding, the secondary air intake system only comes on when the engine is cold at stat up.

So for the EVAP code, I checked the gas cap and O ring, everthing looks fine, even ran a finger of oil around the O ring and made sure the cap was properly secured.

I replaced the fuel filer, which looked as if it was the original filter from 2000, I also replaced the vapor canister vent selonoid that sits above the gas tank and checked the hose, which was in good shape, still plyable and no cracks that I could see.

I cleared the codes and will run it for a couple days to see what codes if any come back and if the misfiring stops.

The freeze frame from the last set of codes were as follows:
P0300
ADSLT TPS 63.1
Eng Spd 2397
Case Load 38.4
MAF 66.38
MAP 98
coolant 197
ST ftrm1 5.4
Lt Ftrm1 19.5
St Ftrm2 29.6
Lt Ftrm2 13.2
Speed 48 mph
fule sys 1 & 2 clsd

I/M monitors were all ready, except the secondary air system was "not ready"