05 Aveo - P0300 Random Misfire

[Daox]

Alright, I retested the vacuum again tonight as I think I had a problem with a bit of leakage around the gauge fittings last night. Here is the result. It looks pretty solid to me now.

http://www.youtube.com/watch?v=J3d45biDFC8

[mwebb]

on the vacuum gauge , wander is ok , normal even
if it had flutter the needle would be a blur of motion .
now
measure engine vacuum at 2k rpm hot
should be 2" above idle value
then WOT snap decel
vacuum should be around 24" as the engine slows down from peak RPM of the WOT snap
WOT = Wide Open Throttle
============================================
Long Term Fuel Trim is one of the PIDs, Parameter ID s that is present in
OBD2 Generic Data

i have used scan gauge 2 only 1x and there is a way to scroll thru various PIDs

somewhere in there
is LTFT , Long Term fuel Trim .
a rule of thumb is up to +10% is ignition misfire
above that is fuel related
so 11% and up is fuel related ....

at 2500 rpm hot or better still at cruise at 40 to 50 mph
and
at hot idle
i would like LTFT , 2 values , which may be the same
if the value is zero , the system is really pissed off and is in open loop
in closed loop LTFT will never be zero . may have a + 0r a - value

these are not carved in stone values and are useless with multiple cylinders misfiring ....
but
i am trying to use what you have to solve the problem ....
================================================== ==
if LTFT is adding high percentage at idle
but close to zero at 2k rpm or cruise , you have a vacuum leak after the MAF sensor .
========================================
we can also use calculated load to see if your cat is clogged
but , one thing at a time

[Daox]

Just to clarify, this engine uses a MAP, not MAF.

Also, I don't think the CAT is clogged. Its directly under the exhaust manifold and I had a good view of it while swapping the engine. It looked fine, and as I stated before, it pulls smooth all the way to redline. I know thats not real scientific, but I'd imagine a clogged cat would be noticeable in that regard.

[Daox]

A quick google search and I was able to find the x-gauges for short and long term fuel trim. I'll try to get that info tonight. I don't have too long before this thing is due to get an emissions test (23rd), so I gotta get it fixed soon.

[Phantom]

You could call the local tech school and see if they want a car to troubleshoot and tell them that you will not even charge them for the learning experience lol.

[Daox]

Originally Posted by mwebb

[COLOR="Blue"]on the vacuum gauge , wander is ok , normal even
if it had flutter the needle would be a blur of motion .
now
measure engine vacuum at 2k rpm hot
should be 2" above idle value
then WOT snap decel
vacuum should be around 24" as the engine slows down from peak RPM of the WOT snap
WOT = Wide Open Throttle

It is at 22" @ 2k rpm hot.

The WOT snap produced variable results, a quick snap only got it to 22". A rev got it to 24" and a good rev got it even higher. This engine isn't real responsive though, it revs up real slow.

I still have to check the fuel trims.

[mwebb]

This engine isn't real responsive though, it revs up real slow.

that is one of the symptoms of clogged cat
as is
the inability of the car to reach 100% of calculated load with a MAF sensor
but with a MAP sensor
and a clogged cat

you should reach 100% calculated load pretty easily , so take it for a ride to see

or

loosen the cat converter bolts at the front and drive the car
see if that changes anything , besides creating a load noise

if the engine revs freely or freely er with the cat loosened (relative to what it does now) you are heading in the right direction

again
fuel trim values would help

[Daox]

I tried the fuel trims today. I could not get them working, I tried all the xgauge codes and nothing worked unfortunately.

I did loosen up the bolts before the CAT and ran it. It seemed to rev up even slower with the bolts loosened.

[Daox]

I also took it for a drive to test the load. I would to up to 94 for me at WOT in 2nd gear.

[mwebb]

Originally Posted by Daox

I also took it for a drive to test the load. I would to up to 94 for me at WOT in 2nd gear.

94% calculated load AT WOT in 2nd with a speed density system points us away from the cat converter being clogged as
clogged cat raises MAP which would max out calculated load at 100%

this is kinda hard to do without test equipment test results from
1000 miles away

how about
driving the car with Vacuum gauge connected and under the wiper as you drive
at cruise at about 45 mph in 5th .
along with calculated load from the scan tool at the same condition .