Diagnosing a miss, wild AFR readig one bank only

[CDMBill]

I have finally got my car back togeather after a prolonged mechanical repari/upgrade down time and have an odd issue. I have an EMS pro setup with a dominator style throttle body, aeromotive fuel system. None of which was changed at all. I did change to a Slightly bigger cam 268.276 vs 266.272 same lift, .768 112 vs 111 Lobe sep. With the ditributor in the identical location as before and the same tune in the EMS I now thos light throttle ocaasinal pop or miss. The other item of note is that I reinstalled a FAST dual channel AFR meter taking of the the old Innovate unit I had on the passenger bank. The Innovate unit read approariate values, but the FAST shows a wild variation on the drivers bank while the passenger bank is stable at similar values to wht the Innovate unit unit showed. The EMS is set up as batch fire.

So, am i getting a lean miss, a occasional missed pulse on an injector or a an occasional spark plug misfire or skip? I am still very new to injection and am looking for ways of diagnosing the problem. WOT is fine as the engine was happy at the irwindale T nT Thursday night and the new torque converter was blowing the tires off the car. MPH was consistent with previous outings in 100+ high DA temps. I'll admit I was hoping for a bit more but with either spinning or walking it out on three differnt runs (all I got) there was no tuning time. The EMS data logs did not show anything that i thought was unusual other than higher than normal coolant, intake air temps.

Ideas?

[Matt Cramer]

Re: Diagnosing a miss, wild AFR readig one bank only

A missed pulse will show up as an RPM spike in the data logs. I kind of suspect it's ignition related - exactly what sort of ignition setup do you have?

[dieselgeek]

Re: Diagnosing a miss, wild AFR readig one bank only

First thing I'd try to do is find the offending cylinder. Seems like you know pretty reliably it's on one bank, so you're halfway there ;) (might want to swap sensors around just to make sure it's not a faulty sensor or wideband controller though)


Anyways, I like to pull the injector connectors while the engine is running, making sure you observe a loss in vacuum for each cylinder that's disabled. If this is an ignition misfire, the O2 signal should clear up pretty quickly when you disable fuel to the misfiring cylinder.

Are you 100% sure there's no intake leaks in one cylinder? no valvetrain issues? always good to check before we spend many hours thinking there's something wrong with the electronics.

See if you can identify the offending cylinder(s) first, perhaps the plug looks a little different in one jug than the other four... then we can go from there.

-scott

[dieselgeek]

Re: Diagnosing a miss, wild AFR readig one bank only

A missed pulse will show up as an RPM spike in the data logs. I kind of suspect it's ignition related - exactly what sort of ignition setup do you have?

He has an MSD Pro Billet (or equivalent) with a VR sensor, going into the VR input of the EMS. Then the EMS drives a points-input on an MSD CDI ignition box. Typical setup, although I don't think he's going to spot misfire in his tach datalog because it's a low-resolution input.

[Matt Cramer]

Re: Diagnosing a miss, wild AFR readig one bank only

I've sometimes seen misfires show up in a tach data log, though they have to be pretty bad before they show up. I was thinking more along the lines of checking for a dropout from the distributor trigger - this will at least let us rule out a missed pulse, as those usually show up pretty clearly. Pretty rare to have a missed pulse show up only on one cylinder though.

[CDMBill]

Re: Diagnosing a miss, wild AFR readig one bank only

Thanks guys. A couple more data points. My timing light showed a fair amount of timing scatter at idle, almost 10 degrees which is higher than before. I also note in the data logs that the idle speed variance shows up along with movement between two adjacent cells in the fuel and spark tables with 2 points of timing and six points of fuel between them if I'm reading it right. It seems that might account for the idle speed and timing variability. My data logs are drag strip only so basically idle to the lanes, burnout, pass, idle back to the pits and done. I'll need to log the actual light load cruising speed to get better data.

I would agree that the EMS itself is not the likely issue. Wiring, vacuum leaks, and tuning tweask seem more likely.

As to mechanical issues, I think the valve train is stable and set properly, but the new variable is a dry N2O fogger system with individual nozzles at each port. It ceratinly seems possible that there is a vacuum leak there which could be one cylinder.

The plugs were recent but not brand new so I may or may not see obvious differences there. Unfortunatly I had to go out of town for work the next day, I'm writing this from my hotel room in Amman, Jordan so the diagnosis will have to wait a few days.

I will swap the AFR leads to see if the problem follows the cabling. The injectors are batched in roughly firing order, so I will be able to narrow down the problem cylinder with Scott's pull the injector lead idea. I'm not sure the mixed bank batch connections accomplish anything but thats how i did it.

I will try Scott's injector lead idea to see if there is an obvious change at idle although its a light load cruise where it is most notable so that precludes doing while driving. A vacuum leak should be constant and the old starter fluid test should work there. I'll post again when I get back to it next Tuesday.

[BangShiftChad]

Re: Diagnosing a miss, wild AFR readig one bank only

Good luck Bill. Let me know if you need me to come over and help on the car once you get back in town. I'm available anytime but Tuesday night.

[shaun callaway]

Re: Diagnosing a miss, wild AFR readig one bank only

x2 man hit me up too if you want an extra pair of eyes.

[CDMBill]

Re: Diagnosing a miss, wild AFR readig one bank only

Chad, Shaun, that'd be awesome. I'll be doing last minute tuning before loading the car & trailer for Sonoma late Wednesday afternoon. I'll buy the pizza and beer and I bet we can solve all kinds of problems.

[dieselgeek]

Re: Diagnosing a miss, wild AFR readig one bank only

sounds like I am going to miss out on some fun. PCH test & tune in the 'vertible!!