So I took my car to get dyno tuned and was told it had a misfire on passenger side.

Car is full bolt on w/ a cam and is an A6. It was originally tuned by unknown tuner prior to me buying it, ran fine. I had some exhaust work done and the guy said it has a hesitation but I could honestly not feel one. I wanted to try e85 on my car so I installed 50lb OEM GM injectors and a DSX flex fuel system. After I installed it and did a test loop driving it, it had a misfire/hesitation at low rpm... I assumed it was because of the bigger injectors and not being tuned for them as it was running really rich, I could smell it plainly. So I took it to get it dyno tuned for e85. Dyno tuner immediately said there is a low rpm misfire on passenger side. Switched plugs and wires and it was still there, said it could be the coil pack or injectors. I do have Granatelli coil packs on the car that it came with. He did not like them at all and I tend to agree, a friend in a Ford had same issue and it was the Granatelli coil pack, changed it back to OEM and ran fine.

I dont have any CEL's but I done have a code reader either if there are any that are thrown. I'll get one soon.

Should I buy new coil packs, switch back to my stock injectors? Anything else to help diagnose it? Higher RPMs it doesn't have the misfire as I can tell, the lower the RPM the worse. Highway cruising in 6th its bad, downshift and it feels much better.

 

First thing is to figure out what cylinder or cylinders are misfiring…does your code reader have the ability to read misfire data ??….what do your long term fuel trims look like at idle ??…are the right bank upstream O2 sensors switching normally between 200-800mv’s ??…if it is “stuck” (bad O2 sensor or electrical issue) lean well below 450mv’s the ECM will increase injector pulse width and you will have a rich smelling car…happened to me…if you can’t read misfires you can try a “cylinder drop test”…remove one plug wire at a time from the coil pack and have somewhere for the spark to ground to or use a test light connected to a good ground…if that cylinder is “contributing” the RPM will drop…this is more difficult on a V8 to hear as compared to a 4 cylinder…you can try using the RPM data PID on your code reader…if no RPM drop that is your misfiring cylinder…once you identify you can do some “swaptronics” by swapping plug, wire, coil pack and even injector with a good known cylinder…it can still be coil pack and injector wiring…without a “better” scan tool and a scope the diagnosis is a little more difficult.

 

Thanks! I don't have a scan tool but I could buy one is what I was saying. I'll try the RPM drop test and see what I get, might be hard to do since its cammed lol.

EDIT: OK, so I pulled every plug wire and notice no RPM drop each time, they also had very strong spark so I don't think its the coil packs. Possibly wires, they are OEM from the looks. Possibly o2 sensor since I still smell strong fuel.

 

One of the members here was looking to buy an Autel MK808…looks like a good scan tool…you have to be careful where you buy them …you don’t want to purchase a “gray market” scan tool…best to order right from the manufacturer !!

 

Pull the valve cover and make sure you don't have a broken valve spring.

 

I bought a obdII reader, spark plugs, wires, and o2 sensors.

Gonna read codes, gonna change plugs and wires (I need to do this anyways), replace o2 sensors since I do smell it running rich. It dynoed 434rwhp for bolt ons, 3rd gear pull, and being AOD, so it seems at WOT it’s ok, meaning it’s probably o2 sensors. If not then I’ll swap my new injectors for my old ones and see if that does it.

 

I'd be interested to hear an update on this when you figure it out. Mine has a 0300 misfire but the car runs fine. The only indication is the check engine light. Regular shop couldn't figure it out and said it may be a tune issue (car was already tuned when I bought it). So I'm taking it to a tuner next week. Based on your experience, now I'm not too optimistic. I've already replaced the O2 sensors, plugs and wires with no change. I hope you have better luck.

 

99% of “regular” shops AND dealerships don’t do well diagnosing cars…was it a flashing check engine light ??…if this is a P0300 with no misfires felt you most likely need a “crankshaft variation relearn” procedure done with a scan tool….takes 2 minutes that may solve your problem…I’d take it back to your regular shop and let them do it (especially if they charged you) because they should have this kind of bidirectional scan tool…this procedure is for ANY car with a P0300 misfire that is not felt…the crankshaft position sensor is the PCM’s misfire monitor.

 

As far as O2 sensors go now that you have a code reader what are the O2 sensors looking like with the engine running…are the upstreams switching normally between 200-800mv’s ??…B1S1 and B2S1....also what do your long and short term fuel trims look like...LTFT and STFT ??

 

The light is steady, not flashing. I don't want to derail OP's thread. But Thank you. I'll look into that.

 

Was this what you were talking about?

Not sure how to interoperate that if that was what you were needing.

Also, I took my temp gun and shot each exhaust manifold, they are ceramic coated LT's just fyi. All run around 350-420deg temp except one, it read consistently 240deg. I'm sure thats where my misfire is coming from. It is cylinder #6, passenger side 2nd from the rear.

I'm going to let the car cool down and start swapping plugs, wires, and coil pack so see if temp change moves. I don't have my new parts yet. The more I think about it, the more I suspect its one of my "new" injectors. The dyno shop said they moved the plug and wire and no changes already, they thought either coil or injector. I'll do it again to double check and move the coil this time.

EDIT*

It was the fuel injector, it was the only thing that followed when I swapped it with a known good one.

 

You normally graph the O2’s but your code reader probably can’t do that but it looks like they are working normally…well at least it looks like you found your misfire.

 

It was the injector. I swapped it to another cylinder and the low temp followed it. Going to replace that, thanks!