Occassional Random Misfire on All Cylinders?
 

Hey y'all,

So this is my first post here. I joined to maybe get some more opinions/insight on what may be going on with my car. So I recently acquired a 2005 Scion tC with a Turbonetics kit installed, tuned running 10lbs of boost. I got the car for $2400 and it pulls like a mother****** so I couldn't pass it up, certainly a fun "toy" I guess.

A couple of days after I got the car, I had a check engine light light up for 5 codes, all misfire codes (Cylinder 1, 2, 3, 4, and random misfire) when I scanned it on the scanner. The light came on at idle shortly after starting the car. I cleared the code to see if it would come back, and the next day, about 2 or 3 trips in the car later it came back. Each time it comes back I clear it to see if maybe there's a pattern and I can't find one at all. About the only thing in common each time the codes are thrown is it is (almost, not always) all cylinders and it typically comes on at idle, or at really low engine speed just taking off. I've yet to have it come on under boost or highway speed that I can think of. It's also very random. I could drive the car on multiple trips in one or two days and never get a code, and then another day it will throw one on the first trip. One thing that's odd, I never notice any difference in the motor or feel any kind of misfire. I feel like I certainly would if I'm getting a misfire on any cylinder on an engine that only has four! Yet this car continues to run like a freaking beast - certainly faster than my last sports car which was a 323 HP 5.3L V8 Pontiac Grand Prix GXP.

I have already replaced the spark plugs with one step colder NGK iridiums that are pre-gapped at 0.033". The plugs I pulled seemed like the stock plugs with a heat rating of 6. I didn't really notice a difference in the car except maybe a smoother idle, just maybe. Otherwise it runs and pulls the same (hard enough that this morning a Challenger R/T I was taunting had a difficult time pulling away from me lol). I have also ran a bottle of Seafoam through the tank and Techron through another and there wasn't really a difference.

My next guess is that the throttle body could use a cleaning (I hear that's an issue on these cars). The car does occasionally "hunt" for an idle when rolling to a stop in neutral, oscillating between 500-1200ish RPMs before stabilizing at a dead stop around 700ish RPMs. Otherwise everything else does seem fine. I'm new to turbo cars, and I know there is lots of tuning involved as well so it could possibly be the tune, however my A/F gauge seems to read a nice 14.7 +/- 0.4 at idle and certainly richens up nicely under boost.

I just wanted some additional opinions before I start throwing money at this car, which I don't really want to do. It's a $2400 "toy" like I said that I probably won't hang on past this summer, and the CEL seems more of an annoyance than a problem based on how this car runs.

Normally misfire are spark realated.
But the way the misfire code shows up is a bit weird. It has a count so after so many misfire it will then set the code. It doesnt set it right away.

It could be a failing coil pack or the tune being off and you getting small misfire under boost. (Either to lean or too rich will show on your afr gauge).

I would also suggest looking at the injectors and the tuning for them. See what flow rate they are and if the tuning before boost has been adjusted for the increase in flow rate.

Any idea how I should go about looking at the tuning and the cheapest route to do it. The original owner gave me a Dastek tuning cable (has a controller with a "fuel" knob and "timing" knob on it), but said that the laptop his tuning software was on crashed. I'm also not sure where the heck this cable connects to either. Like I said I'm new to this area of the car scene. Cars beforehand for me have always been big torquey naturally aspirated V8s that tuning was a little simpler for right through the OBDII port. So apologies if I sound like an idiot.

I may have found a pattern on when the codes are thrown. It seems to throw codes usually at idle shortly after the first start up of the day, definitely typically at idle shortly after start up, usually only the first start up of the day. Again all misfire codes that could possibly be thrown so it is affecting all cylinders...... Or is there a possibility that it actually isn't happening on all cylinders and it's only happening in one or a couple and confusing the computer to think that it's happening in all of them?

No clue what that thing is tune on.
It could be the tune or a dying cam/crank sensor

Dastek? Then it's tuned on a Unichip. You'll probably have to find a Unichip dealer to be able to access the fuel and timing maps.

I'll have to look into unichip, don't know anything about them. Is the consensus then that this is a tuning issue with this only happening at idle? It's definitely something that is affecting all cylinders since I'm getting codes for all of them, so I don't think my next step which was going to be replacing the coil packs will do much good (unless they all fail at the same time, or even a possibility I guess they failed some time one at a time and now all of them are on their way out). I guess I could also look at fuel delivery and maybe the head gasket (that job will be fun lol). Or just drive the car as it is, cause it still seems to run right and pull well.

It could be the tune, but also it could be your crankshaft or camshaft position sensors. I know these have been touchy with the AEM FIC, but not sure with the Unichip.

If you have an OBD monitor, it might be interesting to monitor the signals at idle. Both timing as well as long and short term fuel trims.

So this morning I happened to plug my scanner in when I started the car. I got a check engine light while I was looking at the live data. The LTFT at the moment was -14% while the STFT was -3%. It looks like the computer is trying to pull a lot of fuel out of the system. Leaky injectors maybe? Would explain why the car cranks quite a bit before starting when it sits for a while and also why I have only been getting misfires at idle or low engine speed (leaky injectors probably not as apparent on the high end of revs). Any thoughts? And any good way to test this theory?

Either leaky or over sized and not compensated for, which of course brings it back around to either the injectors or the tune.

I will pull the injectors and inspect them.
The car takes a lot of cranking to start when it sits for a bit. If I shut down the car and start it up again however it hits almost immediately, which leads me to think they are probably leaking. If I have to replace them then a retune is definitely gonna be needed.

I didn't do the tune, the previous owner did and he said he had it on a dyno when it was done, can't vouch for his knowledge though so I may be forced to purchase the Uni-tune kit for this car and take it to a few of the speed shops here in Cincinnati and see who has the know how and tools to mess with it if the injectors are actually okay.

The bouncy idle you'll just have to get used to, happens to a lot of us, me included, although sometimes just sometimes mine still dies and due to heat soak my starter sometimes won't work. Guess it's nice to take 15 minutes and just admire your car with the hood open cause it's cooling off.

Anyway the random misfire code I had myself. It was driving me insane, because I could hear no misfire. I had bought one of those $7 eBay code readers, that don't check everything. Thought for gits and shiggles I'd had O'Reilly run their ODB 2 scanner that scanned everything. Turned out to be a C1241 code, which was low positive voltage or abnormally high. Ran an alternator test and battery test... It passed. I was at a loss. Then one day my car turned on and ran like absolute trash. Turned it off, ran the code reader and pulled like 10 codes, crank position sensor, cam position sensor, and a bunch of other random codes that had nothing to do with each other. Pulled the battery, reconnected it, car ran fine... Took it back to O'Reilly and had them test battery and alternator again. Alternator failed for the voltage regulator. Replaced the alternator and haven't had the misfire codes again. If you have a good scanner, and it doesn't throw a code then I would still take her to get the alternator and battery checked. I've heard some strange stories of failing alternators from Scion owners. What's even weirder... I had the alternator benched tested and it passed, but after taking it off and spinning the pulley with my hand it was squeaking and caught on something. I think the bearings were shot. Anyway if you've tested for alternator and battery issues and don't have the C1241 code then I would try what everyone else is suggesting. Just figured it would be quick and easy to test one day after work stop by O'Reilly. You want them to use the Bosch ODBII reader. AutoZone and Advanced Auto readers they used wouldn't show this code.

I have an Autel Maxidas scanner and am not getting anything other than the misfire codes. The O2 sensor itself has also popped my mind that it maybe going bad as well. I think I will try testing the battery and alternator first since that will be easier than inspecting injectors. However the car cranks normally, it just takes a lot of it once it has sat for a while for it to hit and start, so I'm not thinking it's battery related, but who knows?

Normally long cranking when starting is due toncam/crank sensor, tune is off, or evap pruge valve.

I still think its in the tune or the wiring

Even if the car usually hits right away on a hot restart? Wouldn't a bad cam/crank sensor usually throw a code? And would a bad cam/crank sensor cause a misfire or just give me a faulty reading as having a misfire?

Its all depends, the aem fic had problems with the cam sensor, but it wouldnt throw a check engine light all the time, just not read correctly whike cranking.
And that could be your problem and why you get a miss at idle after start up.
You should try and datalog start up of the car.

I did this with my aem fic and foubd that with the resistor my car would start up andthe rpms would jump to 5k then to normal idling rpm. This never show on the gauge cluster or even obd2 scan. Just aem fic software. Sometime i got a code for cam, but most of the time i didnt just took a while to start the car. Ran fine tho.
I removed the resistor and tune the startup area a bit and got an oem start up all the time now

When I get a chance I'll plug in the Maxidas and run a datalog of startup.

That is a lot of fuel compensation going on there for sure. While its possible that you have a leaky injector, I'm still suspecting that the tune is not adjusted right for the injectors that are in place. I'm guessing this only because it seems like your car successfully compensates for over fueling at idle with those fuel trims. A stuck injector would typically be one cylinder, and the AFRs would be bouncing all over since the ratio would be so different from the 3 other cylinders. I experienced this with one of my injectors that was stuck closed. The car could never idle properly like that.

I had a series of issue with my tune as well, that are still yet to be fully resolved. From my experience, I observed the following:

-A bad signal from the crank sensor can give you a multiple cylinder misfire code. It my case it was due to the FIC.
-A bad tune can also give a misfire code. For a while I was running so pig rich at wot, that the car would missfire occasionally and trigger a code.
-A bad tune that really screwed with the ignition timing could be a culprit as well. I did not have this problem, but it makes sense logically.

Out of curiosity, what are your AFRs under WOT/boost?

Issue Has Evolved
 

Alright, so last night I was coming back from a 2 hour highway trip. Ended up messing with a 350Z on the way (he got a nice view of my tail lights). When I ended up getting off the highway the car started acting up when taking off from the first stop light. The motor began intermittently losing power (kind of like running out of gas feeling I guess). Any time I would get into the gas it would start "bucking" (probably not the right word) and I would watch my boost gauge while it was doing it and it was dropping sharply during each "buck". I ended up pulling over and shutting the car off. I never got a CEL but the misfire codes were stored as pending. I checked the oil level while I was under the hood and added a half a quart to bring it back to full and then restarted the car after it sat for a few minutes and drove off and everything was fine???? I'm starting to get pretty worried. I have parked the car for now.