05 Aveo - P0300 Random Misfire

[mwebb]

GM has misfire counters in Enhanced data, there may be misfire counts present that are not high enough to code the system
so , no Code .

you will need a scan tool with Enhanced GM software , a generic scan tool can not view misfire counters .

and - there' s more
intake vacuum at hot idle should be around 20"
and
the needle must be steady smooth with zero flutter , if there is any flutter , your new used engine has a valve sealing problem

and
if there is a local shop that has Emisfire OR AceMisfireDetective
either of those systems can pick a misfire out of the exhaust pressure pulses synced to spark or IGT on any cylinder.

they work by looking for a vacuum pulse in the pressure pulses , if a cylinder did not fire or did not fire as strong as the rest of the cylinders , when the exhaust valve opens at about 150 degrees after TDC , there will be a Lower pressure or vacuum pulse added to the exhaust pulse train .

not too expensive to connect
i charge 1 hour of labor



the upper yellow trace is exhaust pressure pulses
you can see a regular dip down , a vacuum pulse , and i have overlayed a running compression waveform (720 degrees) below in yellow also with Exhaust valve open EVO time marked by the green cursors.
sync to the pink / red trace .


the software normally picks this out for the user , i just made this screen cap so people can see what the software uses to arrive at the misfiring cylinder
Both programs Emisfire and AMD work very well

[Daox]

I was thinking this over this morning and I'm really thinking its the O2 sensor. The code doesn't pop up for ~20 seconds after the car has been running. Thats just about as much time as you need for it to go into closed loop mode.

I had planned on changing it out last night but had problems getting the old sensor out.

[mwebb]

Originally Posted by Daox

I was thinking this over this morning and I'm really thinking its the O2 sensor. The code doesn't pop up for ~20 seconds after the car has been running. Thats just about as much time as you need for it to go into closed loop mode.

I had planned on changing it out last night but had problems getting the old sensor out.

probably not -
if the 02 sensor was that bad - it would code the system
you do not have a code for the 02 sensor ....
and fuel trims would be out to lunch.
ie; sum of LTFT and STFT would be greater than +/- 25% from zero trim adjustment

test , do not guess .
it is always cheaper and better er

i recommend that you ;
DO NOT add a new problem by installing a FRED's 02 sensor
Frakin - Ridiculous - Engineering - Design .... FRED's
OEM only

a scan tool with GM Enhanced Data will have Mode 6 testing results
for the 02 sensor (s) , those test results will condemn or exonerate the 02 sensor as the culprit .
the system may give the rear 02 sensor some trim authority , an exhaust leak near the rear 02 sensor can skew fuel trim values ,
do you have an exhaust leak near the rear 02 sensor ?
would NOT cause p0300 though ....

[Daox]

Well, this guess was free. I had the O2 sensor from the other engine to swap in. But, it turns out you're right and it wasn't the O2 sensor. It stayed in closed loop while idling, but as soon as I put a significant load on it I got the blinking engine light again.

[mwebb]

the blinking CEL means
the ECM thinks you have a cat killing misfire , stop driving right now .

so
there should be a P030X DTC where X is the number of the misfiring cylinder .
try to get your car to someone with
a scan tool with
GM Enhanced software ,
i believe the scan tool you are using is not up to the challenge.

the other (unlikely) possibility may be
CKP CMP alignment or more likely in compatibility because the replacement engine has a different crankshaft or camshaft wheel / teeth set than the original engine .

IF hot intake vacuum is 20 to 21" then there is no Cam Crank misalignment error , what is your intake vacuum ?

[Daox]

I tried the swapping in the 05 crankshaft position sensor over the weekend. It didn't help. I also removed the intake manifold and installed the 05 manifold in its place. That didn't help either.

I'm going to try swapping a few more parts before I bring it in somewhere. I'm thinking I'll try the EGR valve next. There isn't too much more I can replace though which really makes me wonder whats going on.

[mwebb]

test, do not guess

i was swagging that if there is a problem with CKP CMP sync , it might be caused by a difference in the pickup wheel / reluctor for the CKP or CMP sensor
not the sensor itself.

and
you have not yet measured intake vacuum
which is critical

you seem like a fairly wise individual ...
stop doing this all willy nilly , you need to start at the beginning and finish at the end -
the beginning is to confirm all the mechanical parts of the engine are in good shape
starting with intake vacuum measurement .

which should be about 20" at hot idle with ZERO flutter in the gauge .
what is your s ?

[Daox]

19" pretty steady. There was a tiny bit of flutter, went between 19" - 19.25".

[mwebb]

Originally Posted by Daox

19" pretty steady. There was a tiny bit of flutter, went between 19" - 19.25".

a wee bit on the low side
but good enough to confirm that the CAM and Crankshaft are in proper alignment and

.25" of flutter is a show stopper
is that with the engine at hot idle ? if yes , you have a leaking engine valve OR
the PURGE valve is operating while you are taking the measurement
so
put your finger on the PURGE valve , if it is ticking in time with the flutter
ok , disconnect it (the Purge Valve electrical connector only ) and repeat to see if the flutter is gone , if not

you will need to do a LEAK DOWN TEST on each cylinder in turn
a COMPRESSION test might not find this
================================
and
if possible , it would help if you could provide Long term fuel Trim at hot idle and hot 3k rpm , the generic tool like a scangauge2 can do this

[Daox]

It wasn't completely warmed up, but the gauge was 1/4" of the way up. I'll check out the purge valve tonight and get back to you.

I'm not sure how you check fuel trims with the scangauge though?