hatched91

When it's around 60°F, my car runs great. However, any colder and it feels significantly less powerful. Also, it seems to dip in idle between shifts when the weather is cold.

I tried checking for vacuum leaks with carb cleaner, no trouble codes, and cleaned the MAF and throttle body. Still, symptoms persist. Any ideas guys (and gals)?

SilveradoSS500

At what oil temp do you start to get on it?

hatched91

I wait for the coolant temp to reach operating temp, or at least 200°F

odthetruth

iTrader: (4)

Sometimes the coolant temp can be at operating temp, but the oil temperature can be under 100*. Whats the oil temp looking like when you decide to go WOT?

hatched91

i'm somewhat confused by these "oil temp" posts. I'll take a look at the oil temp next time, but when the weather is cold outside, my car consistently runs much worse than when it's warmer; no matter what operating temp the engine is at. It's less powerful, and the driveability is worse.

DMM

Open the hood and disconnect the MAF sensor at the air filter. The MAF has the IAT (Intake Air Temp Sensor) built into it, and the MAF sensors have been know to fail within range, which does not cause a DTC (Diagnostic Trouble Code) to set.

When you disconnect the MAF sensor, the computer knows it isn't good and defaults to the VE tables based off the MAP sensor (Manifold Absolute Pressure).

If it clears up by doing this, you know where your problem is.

Not having driven the car, this would be the first thing I would suggest.

hatched91

Excellent suggestion! I did the same thing with the sti I had. Wouldn't I need to reset the ecu to "relearn" more quickly?

odthetruth

iTrader: (4)

I know in the V6 CTS, pulling the ECM fuse and putting it back in after about 10 seconds did this.

hatched91

wow, i'm an idiot haha. By pulling the mafs connection, the computer won't use its previously saved values for the fine tuning afr corrections...it will just revert back to a "safer" map that doesn't incorporate the maf, o2 sensors, or iat sensor (as DMM stated).

If I understand it correctly, the computer relies on a base map to run, using throttle position, commanded tq, etc. Then, it "tunes" based on the map sensor, and finally "fine tunes" based on the mafs, coolant temp, iat, knock sensors, cam & crank sensors, o2 sensors, etc. Turbo Subarus(former troubleshooting experience) obviously use obd2 and fly-by-wire, but i wasn't sure if the same theory applied to NA LS cars.

SilveradoSS500

So is it fixed? What oil temp or do you not know? Go to at least 130 before you give it WOT

DMM

GM uses a blended MAF OS, meaning the VCM uses the MAF for steady state cruising and refers to the MAP VE tables for quick throttle transitions since the MAF does not accurately measure quick throttle blips that induce larger quantities of air quickly. You still have feedback correction from the O2 sensors using the Speed Density VE map, however since the IAT is located inside the MAF it will still be a little bit off. If it is your MAF or IAT causing the problem as I suspect, it will run noticeably better.

The "learned" fuel trims will stay in memory when disconnecting the MAF, to what extent they will use them, I don't exactly know.

hatched91

well, I drove about 20 miles today with the mafs disconnected. once warmed up, it ran much, much worse, even to the point of hesitation. There was no CEL either, which struck me as strange. Any advice on what to do next? (aside from purchasing a programmer for datalogging)

DMM

I believe the MAF must fail twice before setting the CEL, even though the DTC has already been set. Honestly, its good that there was a change...the problem now is that you need to start looking at data to get a better idea of what's going on.

I would focus on looking at the fuel trims, IAT, MAF, MAP, and KS activity. And see what actually happens when you disconnect the MAF. Really pay close attention to the LT Fuel trims...if adding +10 or more, look at fuel pressure, fuel filter, vacuum leak (i know you already tried this, carb cleaner rarely finds vacuum leaks now), etc. If it is -10 or more, the computer is taking fuel away for some reason, I would start with replacing the front O2 sensors (as they may be fuel poisoned) and go from there.

Funny thing is that when tuning my V, I have set the MAF fail to 1 hz and still never get a CEL, but the DTC sets and the sensor is not used. I have not done any others, maybe the OS is designed this way?

Sorry man, lots of variables here and I am trying to give you the best info I can without seeing/driving the car. Hope this helps.

Edit: re-reading your post, I keep noticing that you mention that it runs worse after it warms up, how warm are we talking here? The O2 sensors are not used for fuel trims until the coolant temp is 131*...does it start running bad at this point, or hotter than this?

wcryan

iTrader: (2)

if it's not the MAF, id look into plugs and wires. I say it alot, but there are a lot of people that have gone that route and it's fixed their problem

hatched91

DMM, thanks for the super informative reply. I will pay close attention to how it runs at what temps tomorrow morning. fuel poisened o2 sensors? what the hell? i'll have to research that!

I was afraid that it would come down to this. DMM, do you think a predator tuner will read/log the values necessary for troubleshooting? Or do you have any suggestions as for computer programs and obd2-usb cables(and drivers) which would be most effective in troubleshooting? My friend has access to a snap-on obd2 reader, but i'm not sure if it will give enough info for troubleshooting (and it's a hassle to use).

wcryan, I wouldn't mind doing plugs/wires..except i'm at 30k mi, and the plugs/wires are rated for 100k mi. The only problem I have, is that changing the plugs/wires is a $130 (unlikely) shot in the dark. That money could go a long ways towards a program/datalogging cable, or predator tuner. Given that this car has spent a lot of time sitting, parts that deteriorate due to no use is a place to start looking. (possibly plugs/wires??)

DMM

The Snap-On scanner (if this is what you are referring to) will do everything that you need it to. Remember, right now you need a diagnostic tool, not a programmer. The Snap-On diagnostic tools are hard to beat in terms of functionality and bidirectional testing ability (ie, activating solenoids, cylinder balance tests, etc through the PCM). The only problem is that on some of the earlier "brick" units the screen was pretty small, but still an excellent tool. Never heard of Snap On having just a code reader, is this a Blue Point piece?

An O2 sensor being "fuel poisoned" basically means the sensor is out of calibration and will take a richer mixture to cross the .450 mv split point which the PCM uses to determine the difference between rich and lean. The PCM does not command an actual AFR, it averages an AFR based upon the O2 sensor cross counts (an actual term that states the average of rich/lean as determined by the O2 sensor voltage going over/under the .450 mv value within a defined period of time).

Drivability diagnosis takes a long time to get comfortable with and to fully understand how mechanical problems can "fool" the engine management system to cause a secondary problem. Perfect example of this is a misfire situation...when you have a cylinder misfire the O2 sensors measure the 14 parts of unburned oxygen, not the one part of fuel. Every other cylinder is commanded to account for this, making things even worse.

hatched91

that makes a lot of sense. i'll see if my friend will loan me the snap-on diagnostic tool. I'll see what I can find. hopefully the LTFT, STFT, IAT, MAF, MAP, and KS activity will reveal something.

Can you be specific about what exactly to look for with the iat, maf, map, and ks? I'm sure my friend (who is a mechanic) will know, but I would like to have some sort of idea before going through the hassle of borrowing the snap on diagnostic tool.

hatched91

Well, i payed attention to the coolant temperature vs driveability of the car today. I really couldn't put my finger on a temperature where it changed significantly. I did notice (as with other vehicles i've owned) the car running very well right around 1/4 of the way up the temp gauge...which low and behold is approximately 130*F. Maybe there is something to the o2 sensors. Hopefully I can get my hands on a diagnostic tool this weekend!

serik21

Maybe you have a bad crank sensor, I had a car once that to long it would fire up fine than once it got fully warmed up it would sputter or die, as soon as motor got cold it would run just fine till it got warm again