BLM's are the long term fuel trims and the INT's are the short term fuel trims. You need to back away and start fresh. What are you O2 voltages when you put the car in closed loop? I personally have no clue why you are checking the MAP sensor, but if you find it necessary, do as Injuneer stated above. Don't just ground things here and there and poke at wires. There are specific checks to make sure things are within spec. The MAF sensor is in operation during open and closed loop. You need to get a log of the car; I think that would tell us a lot.

 

I'm not poking at wires. believe it or not, i do have a method, but between my idiot instructor, my own ideas, and the right ones, i'm not getting very far. the whole map sensor thing.... well, my lab partner (actually a pretty good kid) decided to test the resistance of my map sensor while the car was running. this is where we got 28ohms. well, you're not supposed to do that, but i guess i never thought twice. so we grounded that and got it to 8 ohms and then the short term fuel trim "int's" were holding steady in closed loop for a change. about 108 in bank 1 and 112 in bank 2. If i could get a datalogue, i would. I tried the free scan for datamaster many times with no success on my connection. it wont communicate on any com port, including the same com port i use for tunercat. let me see what i can do with the tech 2 again when i get time. I'll write down everything important. fortunately, this is a very detailed scanner since its manufacturer specific. Electrical is definately a learning process.

 

ok, here's what i got on the tech 2 today....

OPEN LOOP SPECS / CLOSED LOOP SPECS
(RPM) 980ish
(ECT) 170* / 170*
(IAT) 84* / 84*
(MAP) 58-61 Kpa(2.70 volts)/ same
(Baro) 99Kpa (4.82 volts) / same
(TPS) .70 volts / same
(Left HO2 bank1) 452mv/ same
(Right HO2 bank2) 452mv/ same
(Short term FT bank1) 128 counts/ started 128 and dropped until car died.
(Long term FT bank1) 108 counts/ stays at 108
(Short term FT bank2) 128 counts/ starts at 128 and drops until car died.
(Long term FT bank2) 128 counts/ same
(fuel trim cell) 16/ same
(fuel trim enabled) No/ Yes
(spark advance) 24-27*/ same
(MAF) 10.9-11.50 g/s/ same
(Spark knock) 0* / same
(KS activity) 9 counts / same
(Low Res. signal) 15.8-16.4 ms/ 16.5-17.2 ms
(High res. signal) yes / yes
(injector PWM bank1) 3.2ms / 2.8-2.9ms
(Injector PWM bank2) 3.2ms/ 2.8-2.9 ms
(IAC) 65 counts/ 87 counts when dying
(learned IAC) 32 counts/


well, thats everyting in excact order the scan tool listed it. any help??

 

The 452mV reading for "closed loop" doesn't make sense. That's the reading you would get if the O2 sensor was cold and not capable of generating any voltage on its own. The PCM is "seeing" its own bias voltage.

The only thing that will cause the INT's to drop like a rock would be an O2 sensor reading that was VERY high..... 700-900+mV.

 

o2's usually vary from 200-800 continuously in a wve pattern...it didn't show any sign of this on the scan tool. hung right at 452mv. i tried all kinds of special functions on the tech 2 with the fuel related and nothing seemed to help at all or lead me in the right direction. I'll try and hook another scan tool to it and try and read the o2 volts again, but I'm not getting anything so far. readings were the same on both sets of o2's i have. i have the new GM o2's in there now. Is there anything making sense here? all other specs seem to be on the right track. it just kills me because i never had this problem and all the sudden i was driving it one day and it started doing this. didn't change anything.

 

It appears that the PCM is in open loop. Here's a dumb question. Is there any codes for O2's present?

Another way to look at what Fred mentioned is: If the O2 signal is steady at 452, the fuel trims should be steady.

If you unhook one of the O2's you will notice it hangs around 450 (as Fred also mentioned). The PCM will immediately go to open loop.

BTW mine is doing the exact same thing. The first time I investigated it, the left O2 channel of the PCM was defective. changed out the PCM and all was fine for a couple of days. Then the right side went haywire and I haven't had time to finish checking it out yet. In my case the O2 reading is always high, the PCM cuts fuel until it bottoms out, the PCM momentarily goes into open loop, resets the fuel trims and goes back to closed loop and starts the whole process all over again. Already tried another O2 AFTER I confirmed the wiring. Not much left to change at this point. Oh I did confrim the entire bank of cylinders was firing.

 

no codes...oepn loop is the only time it runs right

 

and typical readings form 02 sensors jump up and down in a wave form, so i don't see how holding at 452mv is holding a steady fuel curve.

 

The PCM looks at the O2 sensor voltage. If the voltage is 450mV, it calculates an injector pulse width that will provide exactly 1 part of fuel to go with the 14.7 parts of air that the MAF is reporting. If the voltage is very low, it says "lean" and adds more fuel, if the O2 voltage is very high, it say "rich" and adds less fuel to lean it out. The PCM uses the short term corrections to vary the fuel calculation. If the PCM is always seeing 450mV, its always supplying exactly the same amount (with respect to A/F ratio) of fuel.

The PCM closed loop control control strategy intentionaly varies the fuel mix from lean to rich and back again, about 10 times per second. It does that to allow the catalytic converter to "store" O2, that it can combine with the unburned HC and CO.

 

right. i understand how that works now that you explained it. So i guess what i'm wondering is, where do i go from here??

 

Have you verified that the PCM isn't junk? I don't know of any other reason why the fuel trims would change when the O2 voltage is staying constant at 452mV. It doesn't make sense.

 

yes, new pcm. did the same on the old

 

any ideas?

 

Are you sure it is not a harness issue past the point of the PCM?

 

i'm not sure...care to share what i should look for? i'm stumped