pimpmaro

I've got to say i'm kind of confused. I tried to search for a similar instance, but kept finding problems with cam cars and the P0300 DTC. My car is internally stock and I'm not throwing any codes. It hasn't been feeling the smoothest lately and I had a chance to log a few EFI Live runs when I was doin the TB bump stop mod with my buddy.

First run, I was gettin some KR in a few instances, none of them at WOT. I checked all the plug wires, and then decided to log a couple of runs in which I monitored the current and history misfires for each cylinder. I had no knock at all during these two runs, no matter how hard I tried to duplicate any conditions that might have caused it, but at and close to idle, I was experiencing misfires on several cylinders that I can't seem to track down.

I'm at 39,500 miles, with about 15k on some TR55s. I've got a new set of plugs on order and I plan on using some Top Engine cleaner at the same time to see if that clears up the problems. The most concerning aspect of this is that I've got a lovely dieseling sound at low rpms that I'm assuming is related to both the misfire and the knock.

If anyone has any advice, I'd be more than happy to accept it. If it helps, I've got all 3 EFI Live logs that I'd be happy to send out for analysis.

Thanks guys.

Redlinez

iTrader: (5)

Those years are known for carbon build up on piston tops. Some of these cars have overly sensitive KR, could be caused by poly trans or engine mounts as well.

pimpmaro

I don't have any poly mounts anywhere in the car, the carbon build up is what I'm pretty much thinking of. I got hassled at the dealership trying to get a can of top engine cleaner... the guy there basically told me it was 100% crap and instead got me a can of air intake cleaner. Do I really need to use top engine cleaner to get a good cleaning out of it? If so, how do you all get the liquid sucked up into the intake without using the spray can or pouring it in the sparkplug holes?

XtinctHawk02

This might be a long shot. On my "daily-driver" 1996 Tahoe. I had the friggin' random misfire code for a long time. I ended changing out everything from plugs,wires,cap,rotor to even running the highest octane gas.The problem still existed. I eventually did more research and ended up changing out my 02 sensors. I changed out the front ones and the engine light went out after driving it for 10 minutes and it's been three days and the light hasn't come on sense, and te car is running a whole LOT better now.
Has your car had any type of hesitation? or smelling of any excessive exhaust? If so, it might be your problem.
Just a guess.

pimpmaro

I'm not sure that that would be it. The car is a 2002. Not only that, but as I said, I logged three efi live runs and the 02s are cycling just fine... at least as far as I can tell. If you'd like to look at the logs, I can email them to you.

Classic

iTrader: (3)

The solution is to buy a cam from a buddy and then turn the PO300 code off

tim99ws6

iTrader: (31)

i had very small vaccum leak on my car and would only show itself on rare occasions driving around........under WOT, it was perfect.....keep your eye on the vaccum....anything is worth a shot

Team ZR-1

check for a slipping belt, something wrong with a pully or damper.
Even a warped brake rotor can cause uneven crank rotation and trip false misfires.
Leaking injector sucking in air, partly plugged Cat and of course running too rich

samz28

i noticed a vague vibration at part throttle low load that flashed knocked retard. I felt some vibration in the tunnel(? where the shifter console was). I was riding shotgun holding the laptop. This is the only real time i saw KR.

Misfires of course are not necessarily KR on efilive. What i need to find is the pid to log the Octane table so we can see where the misfires + KR are putting him between the low and high maps.

Lemme go meander over to the efilive forum and see if i can figure out what pid it was. For the record. My car throws about 10-20X more misfires at idle(specially cold). I attribute it to the poor mounts (being replaced now all poly) and the pacesucker LT/ORY which bangs everything all over the place.

Taking the crowbar and rapping the LT would throw about 1 KR on efilive. Thats how sensitive the knock sensors are. However we must not mix up Misfires and KR. Both are occurring on this car, but not under great amounts. Well i don't have another stock car to base that on...

pimpmaro

A warped brake rotor??? How does that happen? Just out of curiosity.

I gather I'm going to need to go through everything with a fine toothed comb to figure this one out.

samz28

yah i'd like to see how you can trip false misfires at idle with a warped brake rotor.

j/k

Team ZR-1

As per GM :
The PCM detects engine misfire by detecting variations in crankshaft deceleration between firing strokes. For accurate detection of engine misfire the PCM must distinguish between crankshaft deceleration caused by actual misfire and deceleration caused by rough road conditions.
The antilock brake system (ABS) can detect if the vehicle is on a rough road based on wheel acceleration/deceleration data supplied by the wheel speed sensors. If the ABS detects rough road above a predetermined threshold, this information is sent to the PCM via serial data. The PCM can then take the rough road into account when calculating misfire.
Even if the ABS is malfunctioning and cannot detect rough roads, the misfire diagnostic will continue to run. However, if a misfire DTC sets, this additional DTC also sets indicating that rough road data was not available during the misfire calculation due to an ABS malfunction.

Warped rotor causes uneven load on drivetrain which causes slight variation on crank which the CKS sensor picks up and trips false misfire.
ABS tripping due to dragging of brakes of one part of rotor will also cause false misfire.
Slipping belts, pully or unbalanced, anything that causes crank to not have smooth rotation can fool CKS and reason why EPA allows nameplates to lighten up the internal misfire tests in PCM but still cannot catch all false misfire conditons.

samz28

team zr-1 thanks. I will note i was riding shotgun, i did notice some vibration in the center console, during the part throttle situation that was showing some KR (note wasnt watching misfires).

Is it possible that gutting the cats and using sims can cause the front o2's to misread the a/f (untuned car) due to the fact the o2 sensors may not have the heat from the cats/restriction? Could this cause the o2 sensors due to increase airflow and less heat to read skewed?

Team ZR-1

Yes gutting the Cats will change the exhaust flow/temps,
fool the O2s some and without tuning for that would cause problems.
Not usre how the subject was about misfires and now its mainly about knock but clearly with exhaust changes the AFR would change and can cause knock being the O2s may be reporting too rich when its really too lean.

01SLPSS

iTrader: (47)

I took my cats off, put on ORP and a 160T-Stat on at the same time and had not purchased a tuner yet and killed both Front O2 Sensors. The car was getting 12mpg!!! THAT Sucks!!!! We put the car on the laptop and found out the O2's were the problem, new ones fixed the problem. Purchased tuner finally!!!!