PCM Diagnostics & Tuning - AFR Spikes

What do you make of my AFR spikes? Obviously, there are two main possibilities: The spikes could be real or they could be measurement errors. If real, I assume they must be due to to fuel starvation, possibly due to loss of fuel pressure. The fact that the engine continues to pull smoothly through the questionable region makes me suspect measurement error. Measurement errors could be caused by air leaking into the exhaust system or sensor failure. I have QTP longtubes, a custom ORY, and my LM-1’s wideband sensor is in the intermediate pipe in front of the rear axle. I have inspected my exhaust system and don’t see evidence of leaks anywhere – everything is tight and there is no external soot at any of the joints. My sensor is relatively new and my LM-1 has always worked flawlessly. I have seen this behavior on more than one quarter-mile run. The spikes appear only at the top of fourth gear. I intend to install a fuel pressure sensor and I might try to pressure-test my exhaust system. I would like to hear from anyone who has seen a similar problem.

What do you make of my AFR spikes? Obviously, there are two main possibilities: The spikes could be real or they could be measurement errors. If real, I assume they must be due to to fuel starvation, possibly due to loss of fuel pressure. The fact that the engine continues to pull smoothly through the questionable region makes me suspect measurement error. Measurement errors could be caused by air leaking into the exhaust system or sensor failure. I have QTP longtubes, a custom ORY, and my LM-1’s wideband sensor is in the intermediate pipe in front of the rear axle. I have inspected my exhaust system and don’t see evidence of leaks anywhere – everything is tight and there is no external soot at any of the joints. My sensor is relatively new and my LM-1 has always worked flawlessly. I have seen this behavior on more than one quarter-mile run. The spikes appear only at the top of fourth gear. I intend to install a fuel pressure sensor and I might try to pressure-test my exhaust system. I would like to hear from anyone who has seen a similar problem.


Those are missifre spikes by the looks of things. either to rich or snuffing out the plug.

Detonation will produce the same spikes

Can misfires produce spikes like that...?

I've seen DFCO produce similar results.

Yes, because a misfire is not just dumping fuel, but oxygen too. And the sensor can only see the oxygen, so it thinks you're lean, even if you're just unburned.

what is happening with commanded there at the same time? are the injectors foing static?

Thanks for the information, guys – I had not considered the possibilities you’ve raised. But my plugs all look like the two in the attached thumb and there are no PCM-detected misfires. My tank was half full of Sunoco 96-octane unleaded and the data shows no knock retard. WOT ignition advance is only 23 degrees. Does anyone have ideas about how I might verify the misfire/detonation theories?

i try to tune by ear to get it relatively close then look at plugs then do the wideband then back to the plugs again.... AFR gauge is just a average... they cant possible change fast enough to give immediate changes...

what is happening with commanded there at the same time? are the injectors foing static?I've been wondering about the simultaneous dip in commanded afr (it drops from 12.6 to 12.5). My PE table is 12.6 throughout. COT is disabled. What means "going static"

.. they cant possible change fast enough to give immediate changes...Depends what you mean by immediate. I don't have specs at hand but I believe wideband sensor response time is on the order of a few milliseconds. Rev-limit fuel cutoff shows up clearly.

I just finished re-reading your post. Could very well be fuel delivery. The lower gear pulls don't last as long. The afr starts going crazy after 4 seconds of wot demand and you're not in the lower gears that long. I'd monitor the fuel pressure to rule it out.

With the amount of air flowing through the exhaust in that rpm range, I don't see air infiltration being the cause. That's more likely in a low flow situation.

I had high-end break up when I was spraying and didn't bring the plug gap down small enough. It was very noticeable when driving though. At the top of the higher gears. Those are some big a/f swings. Seems like you'd feel them.

Depends what you mean by immediate. I don't have specs at hand but I believe wideband sensor response time is on the order of a few milliseconds. Rev-limit fuel cutoff shows up clearly.

it might make changes but unless you hit that cell atleast 50x then its usually a big change