BMW M3 w/ LS1 1500rpm stumble?
12-23-2009, 11:01 AM
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BMW M3 w/ LS1 1500rpm stumble? The set up is a 99' LS1 with a T56 in a 98' BMW M3.
The car has stock MAF, stock LS6 head milled 0.005, CC 232/232 .595 112 LSA cam. The rear gear is a 3.23 which the tuner says could be part of the problem since they don't have any other LS1 cars with this tall of a gear to compare it too. They did turn off the speed sensor on the trans to try and help things to no avail.
I had it tuned by a local shop, it runs great except for a there's a stumble at 1500rpm whenever lightly rolling into the throttle. It happens almost everytime passing this rpm at exactly at 1500 everytime, no where else.
They tuner is stumped and can't find anything wrong in the data logs. Unfortunately I don't have the data logs but I can get some.
Does anyone have any suggestions on where to start looking? or what items to look at when logging?
The car has stock MAF, stock LS6 head milled 0.005, CC 232/232 .595 112 LSA cam. The rear gear is a 3.23 which the tuner says could be part of the problem since they don't have any other LS1 cars with this tall of a gear to compare it too. They did turn off the speed sensor on the trans to try and help things to no avail.
I had it tuned by a local shop, it runs great except for a there's a stumble at 1500rpm whenever lightly rolling into the throttle. It happens almost everytime passing this rpm at exactly at 1500 everytime, no where else.
They tuner is stumped and can't find anything wrong in the data logs. Unfortunately I don't have the data logs but I can get some.
Does anyone have any suggestions on where to start looking? or what items to look at when logging?
12-23-2009, 11:15 AM
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Do you have your own data logging software? Are you giving up on the shop? Logging the basics Fuel, Air, Spark should reveal a bad actor. Short of a leak (up top-uncommanded airflow or down below-exhaust leak) that the shop overlooked, that should be a fairly easy combo to tune. What year pcm are you using in this application?
12-23-2009, 11:37 AM
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Do you have your own data logging software? Are you giving up on the shop? Logging the basics Fuel, Air, Spark should reveal a bad actor. Short of a leak (up top-uncommanded airflow or down below-exhaust leak) that the shop overlooked, that should be a fairly easy combo to tune. What year pcm are you using in this application?
Well they looked at it data logged it, changing a few things in the tune and they gave up. I built they car they just tuned it.
It's a 99' PCM
Last edited by Speed Demon; 12-23-2009 at 11:53 AM.
12-23-2009, 12:10 PM
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Do you have the means to check fuel pressure when it stumbles? Just trying to throw out some things you could do on your own. Could be a tuning issue but, ruling out setup problems and giving them (the shop) the benefit of doubt is where I would start. Odd that they just gave up. Finding and fixing a problem is generally what a shop is supposed to do.
12-23-2009, 06:08 PM
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Do you have the means to check fuel pressure when it stumbles? Just trying to throw out some things you could do on your own. Could be a tuning issue but, ruling out setup problems and giving them (the shop) the benefit of doubt is where I would start. Odd that they just gave up. Finding and fixing a problem is generally what a shop is supposed to do.
I don't have a fuel pressure gauge, but I can't imagine how it would be fuel pressure related. Not saying it can't be though. I'm running a Walbro GSS340 and a C5 corvette stock filter/regulator. It might be a good idea to check it though since my regulator is not adjustable. Yea that's a big oversight on my part.
12-23-2009, 06:16 PM
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Have him log where the stumble is. You will need the RPM and the grams/cyl and timing. You need to add some timng there or take some out. Do it in about 2 deg per try. Or just dont go there that is what most people do.
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12-23-2009, 07:08 PM
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I'm horrible at troubleshooting but when my car did that it was a bad coil. It happened at the exact same rpm every time, before that it was fine and after that it was fine. Just thought I'd share; sometimes we overlook the simple stuff.
12-24-2009, 12:28 AM
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I've read that deleting VSS still leads to bad issues with decel/accel/etc. If your tuner knows what he is doing, he should be able to calculate and configure the ECM with proper VSS either from the 4 ABS wheels sensors, the speed sensor in the diff, of if there is a speed sensor in the T56. I recommend using the speed sensor from the diff, since the gearing size will not make any difference and it will be constant to the proper speed. The ABS sensors should allow the same raw speed as well. Transmission speed sensors are the worst and require extra garbage (like having to change tunes for different gears).
12-25-2009, 10:27 AM
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I've read that deleting VSS still leads to bad issues with decel/accel/etc. If your tuner knows what he is doing, he should be able to calculate and configure the ECM with proper VSS either from the 4 ABS wheels sensors, the speed sensor in the diff, of if there is a speed sensor in the T56. I recommend using the speed sensor from the diff, since the gearing size will not make any difference and it will be constant to the proper speed. The ABS sensors should allow the same raw speed as well. Transmission speed sensors are the worst and require extra garbage (like having to change tunes for different gears).
Very good point. I was wondering about that. Can't the PCM be recalibrated to make the trans speed sensor match the actual ground speed? Currently that the only vehicle speed input connected to the PCM. The speed sensor in the diff is only connected to the speedometer.
I really need to educate myself on tuning.
12-25-2009, 02:27 PM
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How rich is it in the lower RPMs? I've seen cars load up from an excess of fuel, when they catch their breath after that they're able to handle it. Check the spark plugs... Fouled plugs hate rich cars at low RPM.
If the rpm is consistent and it's not easily noticeable in the data logs I'd be working more toward an ignition issue. Like one of the above posters said, you might have a faulty coil.
If the rpm is consistent and it's not easily noticeable in the data logs I'd be working more toward an ignition issue. Like one of the above posters said, you might have a faulty coil.
