Bad o2 sensor causing knock?

[Talek]

I logged my car awhile back and was getting like 30+ counts of knock at wot. I figured it was my fuel pump choking, stock still stock wiring with 650cc injectors and an evoIII 16g, was running at 15lbs, so I cut it to 12lbs. Just logged it again, and while I'm not knocking quite as bad, I'm still getting 20-25 counts of knock.

So I reviewed my log and my o2 voltage fluctuates. It seems to cycle between accurately reading and not reading at all. If I sit at idle for example the graph for my o2 will look like a wave. Could inaccurate o2 readings be causing my car to run poorly? 20 counts of knock is a bit high for simply running a little lean, and my maft is set correctly for the base fuel maps for my injectors so it shouldn't be _that_ far off.

I'll try richening up the band a bit and see if it drops my knock... If not, something else is going on.

 

[kenamond]

At WOT, the ECU is in open loop mode. It's not using the front O2 sensor for anything. When you're not at WOT (I forget the threshold, but it's like 95% or something), the ECU is in closed loop mode. It adjusts fuel based on the O2 sensor reading. It tries to keep the O2 voltage fluctuating above and below a certain voltage. If it stays too high, it leans it out. If it stays too low, it richens it up.

So if you're doing a full throttle pull and seeing oscillating O2v, check your TPS reading and see if it's actually getting to the high 90% range. If not, you need to adjust your throttle to read near 100% with the pedal floored. Another thing it can be is something like the floor mat under the pedal keeping it from going all the way down.

Or maybe it's something else.

Try running stock boost (run vaccuum line straight from the compressor outlet to the WGA to get about 10psi) or dial down the boost. Log a pull with that and see if you still get knock.

 

[Spln_Hrd]

I dont think the stock fuel pump should be poopin out enough at 12 psi to cause any knock.

Like he said you o2 shouldnt be oscillating at WOT. What do you have to modify your fuel maps?

 

[Talek]

I have a gm maft for tuning, logging with mmcd on my palm. I richened it up a bit and tried another run, still getting knock but it seems to be lower, around 10-15. I'll bump it up another 5% at wot and see if that helps. If I remember correctly, it switches to the wot fuel settings at 8+ psi, on my rx7 it was throttle based. I'm reluctant to floor it simply because of the knock I've been getting... I put it pretty close though, and it only read 70% tps. An example of a 3rd gear pull would be at like 5k rpms, 70% tps, 12lbs boost, 65% idc, 14 knock, 91v o2. I just think it's strange to need to go 10+% over base maps with my mods.

I'll bring my palm in and copy over a log tomorrow.

I also get knock occasionally at pt but only like 1-5 counts. I'll just watch the rpm range and tweak it once I get my base map set.

 

[kenamond]

I have a gm maft for tuning, logging with mmcd on my palm. I richened it up a bit and tried another run, still getting knock but it seems to be lower, around 10-15. I'll bump it up another 5% at wot and see if that helps. If I remember correctly, it switches to the wot fuel settings at 8+ psi, on my rx7 it was throttle based. I'm reluctant to floor it simply because of the knock I've been getting... I put it pretty close though, and it only read 70% tps. An example of a 3rd gear pull would be at like 5k rpms, 70% tps, 12lbs boost, 65% idc, 14 knock, 91v o2. I just think it's strange to need to go 10+% over base maps with my mods.

I'll bring my palm in and copy over a log tomorrow.

I also get knock occasionally at pt but only like 1-5 counts. I'll just watch the rpm range and tweak it once I get my base map set.

I don't know much about GM-MAFT, but if it's like SAFC in that it reports lowered airflow numbers to the ECU in order to compensate for the larger injectors, then you have a side effect that may explain some of the knock. The ECU uses those altered airflow values when computing timing advance. If you're running larger injectors, you're lying to the ECU saying that you have less airflow than you actually have. The ECU will compute a lower g/rev from the lower airflow and rpm to look up the timing advance. At lower g/rev, the timing advance map value is greater. So when you use a piggyback and larger injectors, you end up with more timing advance and potentially more knock.

Another thing I've seen in the past is that folks have 550 injectors when they think they have 650. So the base "recommended" trims for 650s are too lean when you actually have 550s. And you end up wondering why you have to richen things up so much. I'm not saying that's your problem, but if you aren't positive about the injectors, it might be something to consider.

 

[Talek]

I don't know much about GM-MAFT, but if it's like SAFC in that it reports lowered airflow numbers to the ECU in order to compensate for the larger injectors, then you have a side effect that may explain some of the knock. The ECU uses those altered airflow values when computing timing advance. If you're running larger injectors, you're lying to the ECU saying that you have less airflow than you actually have. The ECU will compute a lower g/rev from the lower airflow and rpm to look up the timing advance. At lower g/rev, the timing advance map value is greater. So when you use a piggyback and larger injectors, you end up with more timing advance and potentially more knock.

Another thing I've seen in the past is that folks have 550 injectors when they think they have 650. So the base "recommended" trims for 650s are too lean when you actually have 550s. And you end up wondering why you have to richen things up so much. I'm not saying that's your problem, but if you aren't positive about the injectors, it might be something to consider.

I double checked my injectors, and I purchased 650cc ones from dsmparts, so I doubt they would have sent me the wrong ones. It's possible that the maft is reporting different airflow numbers, but I'm pretty sure it's designed to compensate, but maybe not.

I just keep getting weird knock numbers... *shrugs* oh well.