Problems with O2 sensors?
03-04-2005, 06:04 AM
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Problems with O2 sensors? Yesterday, I tried to nail down the idle on cobra with an Ls1 engine. It's a 6.3l stroker with ported heads, 244/248 cam, FAST intake and FAST 90mm throttle body.
I have tuned C5 corvettes before, but the cobra has the camaro throttle linkage, so it took me a little while to get used to the IAC thing. Fuel trims were a little high, so I rescaled MAF a little bit, and the fuel trims were okay.
Well, at the point where I thought I had almost nailed it down, I noticed the LTFT on bank 2 increasing to 25%, and STFT also going up and up. The left side was okay, just the passenger side was out of control. Voltage on the O2 sensor read low, about 0.15 to 0.2.
My first guess was a broken O2 sensor. We took both sensors out (the car has no cats and no rear sensors), and they were both black. We cleaned them and put them back. After that, the car ran fine for a couple of minutes, then the fuel trims on the passenger side started rising again. There had been a smell of too much fuel before, but now it started to really burn my eyes.
Here is my guess what is happening, and I would like to know your opinion. The car has sidepipes, with really long and big primaries. We measured the temperature of the collectors, and they were at only about 220 F. I guess the O2 sensors never got hot enough, and measured lean while the mixture was rich. While testing, the residue of the unburnt fuel clogged up the O2 sensor, until it almost didn't work at all, which is when the LTFT started going up to max. What do you think?
On reving the engine and releasing the throttle, there also is a loud popping sound from the passenger side. I wonder if this is due to a fat mixture.
Only thing I can think of now is using rear O2 sensors, since they have a stronger heater.
I will also hook up my wide-band O2 sensor to see what is really going.
Any input and advice would greatly appreciated. Thanks, Till
I have tuned C5 corvettes before, but the cobra has the camaro throttle linkage, so it took me a little while to get used to the IAC thing. Fuel trims were a little high, so I rescaled MAF a little bit, and the fuel trims were okay.
Well, at the point where I thought I had almost nailed it down, I noticed the LTFT on bank 2 increasing to 25%, and STFT also going up and up. The left side was okay, just the passenger side was out of control. Voltage on the O2 sensor read low, about 0.15 to 0.2.
My first guess was a broken O2 sensor. We took both sensors out (the car has no cats and no rear sensors), and they were both black. We cleaned them and put them back. After that, the car ran fine for a couple of minutes, then the fuel trims on the passenger side started rising again. There had been a smell of too much fuel before, but now it started to really burn my eyes.
Here is my guess what is happening, and I would like to know your opinion. The car has sidepipes, with really long and big primaries. We measured the temperature of the collectors, and they were at only about 220 F. I guess the O2 sensors never got hot enough, and measured lean while the mixture was rich. While testing, the residue of the unburnt fuel clogged up the O2 sensor, until it almost didn't work at all, which is when the LTFT started going up to max. What do you think?
On reving the engine and releasing the throttle, there also is a loud popping sound from the passenger side. I wonder if this is due to a fat mixture.
Only thing I can think of now is using rear O2 sensors, since they have a stronger heater.
I will also hook up my wide-band O2 sensor to see what is really going.
Any input and advice would greatly appreciated. Thanks, Till
03-07-2005, 10:08 AM
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I figured the problem was too much overlap. So I forced the car into open loop in idle. Idle is at 1200rpm, and I set open loop up to 2000rpm.
I scaled down VE for low rpms, which wasn't quite enough, so I also rescaled MAF for low values.
Now indicated AFR is between 15 and 18, CO is between 1% and 4%. Guessing it might still be a little rich, I made it a little leaner, which resulted in bad banging from the right exhaust, including flames. I reduced timing to a maximum of 20-22 degrees.
Now it runs pretty nice. The sound is impressive. Driving the car at slow speeds is no problem.
There is a noticeable difference between the left and right side. Holding your hand behind the exhaust, you can feel strong pulses on the right side, while it is much smoother on the left side. The exhaust headers are different left and right, with the right one being straight and short, the left one more bent and maybe three or four inches longer to the collector. Maybe that is the reason.
MAF readings at about 1200rpm are still pretty screwed up. Idle bounces between too cells in the rpm vs. map charts, and MAF jumps quite a bit between the two, with the higher values in the lower MAP cell. I guess this is due to the huge cam and the MAF bolted right to the throttle body.
IAC wouldn't go below 100 today, most of the time it was around 130. Using the tps sensor reset trick didn't seem to work, at least not consistently. There is a very small hole in the 90mm throttle blade, maybe I will put in a bigger hole, or a second hole, into the blade sometime. For now the car runs fine.
On wednesday I will take the car to the dyno. Can't wait to get the results. It's a dedicated (road) race car, so we can't test drive it on the street.
I scaled down VE for low rpms, which wasn't quite enough, so I also rescaled MAF for low values.
Now indicated AFR is between 15 and 18, CO is between 1% and 4%. Guessing it might still be a little rich, I made it a little leaner, which resulted in bad banging from the right exhaust, including flames. I reduced timing to a maximum of 20-22 degrees.
Now it runs pretty nice. The sound is impressive. Driving the car at slow speeds is no problem.
There is a noticeable difference between the left and right side. Holding your hand behind the exhaust, you can feel strong pulses on the right side, while it is much smoother on the left side. The exhaust headers are different left and right, with the right one being straight and short, the left one more bent and maybe three or four inches longer to the collector. Maybe that is the reason.
MAF readings at about 1200rpm are still pretty screwed up. Idle bounces between too cells in the rpm vs. map charts, and MAF jumps quite a bit between the two, with the higher values in the lower MAP cell. I guess this is due to the huge cam and the MAF bolted right to the throttle body.
IAC wouldn't go below 100 today, most of the time it was around 130. Using the tps sensor reset trick didn't seem to work, at least not consistently. There is a very small hole in the 90mm throttle blade, maybe I will put in a bigger hole, or a second hole, into the blade sometime. For now the car runs fine.
On wednesday I will take the car to the dyno. Can't wait to get the results. It's a dedicated (road) race car, so we can't test drive it on the street.
03-08-2005, 06:09 PM
#4
WOW!!!!!!!!!!!! Definitely a sweet ride...
I had issues with my O2s as well after switching over to Edelbrock stepped long tubes. I orderred up a set of Bosch 13111 O2's which are the 'vette rear ones. I only was able to go around the block, but the car definitely felt better and atap confirmed that they are switching quicker. I'm going to need some good weather to put a couple miles on the car to test out the tune.
I may need to adjust the tune after swapping from mac mids to the longtubes.
I had issues with my O2s as well after switching over to Edelbrock stepped long tubes. I orderred up a set of Bosch 13111 O2's which are the 'vette rear ones. I only was able to go around the block, but the car definitely felt better and atap confirmed that they are switching quicker. I'm going to need some good weather to put a couple miles on the car to test out the tune.
I may need to adjust the tune after swapping from mac mids to the longtubes.
03-10-2005, 10:31 AM
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The car was on the dyno, and tuned to the point where the owner was happy and wanted to take the car home. It pulled 545hp at the flywheel, 480hp at the wheels. Max power was at 6650, where the final pull was stopped. The owner didn't want to go higher, and the power was still going up, so there is still some power in there.
Also, I didn't feel I was quite done tuning. Everyone else was happy with the result and wanted to go home. I feel I might have squeezed out more power, who knows. Nothing I can do about it.
At the end of the racing season I will try to get the owner to go back to the dyno, spin it up to 7200 and see how much power there really is.
Till
Also, I didn't feel I was quite done tuning. Everyone else was happy with the result and wanted to go home. I feel I might have squeezed out more power, who knows. Nothing I can do about it.
At the end of the racing season I will try to get the owner to go back to the dyno, spin it up to 7200 and see how much power there really is.
Till
03-10-2005, 07:54 PM
#7
Nice!! With headers that long it might need some sort of header wrap in addition to rear sensors to keep the heat up. Any chance there's a problem with the injectors on the passanger side, damaged o2 wiring? Usually both sides stay inline. I've seen mine vary +-3 from left to right at times but nothing major.
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03-11-2005, 12:49 AM
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Thanks. The car is running pretty well now.
The headers are wrapped up to the point where you can see them. Unfortunately, coming from the engine, the headers are straight on the right side, and bent on the left side. The difference in length is something like 3-4". Depending on rpm, LTFTs are different on both sides by 3-4%, with one side being leaner at low rpm, the other side being leaner at high rpm.
The overlap on this cam is huge. With the O2 sensors turned off at low rpms, it idles fairly well at 1200rpm. I am sure it would also run at 1000 or even 900rpm, but since it is a pure race car and extremely noisy under any conditions, there would be no point in lowering idle. And even if the engine idles at a lower rpm, even with a fairly lean idle it gets more smelly as rpms go down.
Till
The headers are wrapped up to the point where you can see them. Unfortunately, coming from the engine, the headers are straight on the right side, and bent on the left side. The difference in length is something like 3-4". Depending on rpm, LTFTs are different on both sides by 3-4%, with one side being leaner at low rpm, the other side being leaner at high rpm.
The overlap on this cam is huge. With the O2 sensors turned off at low rpms, it idles fairly well at 1200rpm. I am sure it would also run at 1000 or even 900rpm, but since it is a pure race car and extremely noisy under any conditions, there would be no point in lowering idle. And even if the engine idles at a lower rpm, even with a fairly lean idle it gets more smelly as rpms go down.
Till
