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I am working on this vehicle for a friend,it started out having no fuel pressure, I replaced fuel pump and filter and it started. I test drove vehicle and it has really poor accel until foot feed reaches maybe 1/4 throttle. I thought maybe throttle post sensor but it scans fine and no problem. It is like is either is losing fuel or fire but really hard to tell which it is. It goes into closed loop but as soon as U try to accel it goes open until it reaches 1/4 throttle and then accellerates well. Have become a parts hanger with this one and tryed ecm,egr,maf,o2, and ignition module due to a no start problem that arouse in this process, no fire and injector pulse. It U have and ideas I am game to try anything at this point. thank you  

somebody just sent me a data set...

i can see problems with this.. on a 90 TPI 350 chevy..


                                   cold                                       warm
coolant temp-------------65 degrees                              186
throttle sensor-----------.58                                           .58
   "          "       set at .56 not running
injector pulse ----------- 2.5                                         2.2
desired idle--------------712rpm                                   600rpm
actual idle  --------------780/800 open loop                  680/750 closed
map sensor--------------1.68                                         1.98
throttle angle--------------0 %                                        0 %
block learn --------------110                                          110
spark advance ----------14.7                                         14.7
block learn cell------------5                                              5
integrator-----------------126                                         124
idle air mtr steps----------0    ???????????????????????       0
fuel pressure-------------48 psi                                       48psi


can you post your data stream recording from idle and under accel...

its surging...   the answer is the base idle is open too far.. the idle air motor cannot close far enough to bring the idle down to where the computer wants..  so it has a vacuum leak or needs the base idle reset to allow the idle air control motor to settle between 20 and 50 counts.. with the TPS reset to .58 or so.. after the adjustment..
look at the injector pulse width.. 2.2...  at idle should be 1.4.. the ECM is pouring on fuel as it thinks you are ready to run...  perhaps even lowering the TPS voltage lower...  

here is a printed chart to help...



print this chart..  write your scan tool values on it..

see what you can post...

i will be around all thur evening.. probably away all day friday.  

thanks for your reply the only thing that is actually out of range is the as soon as u accellerate it goes from closed to open loop until u reach 1/4 throttle then it goes back into closed loop and really accellerates then and as soon as u let off throttle below 1/4 throttle it goes back into open loop and starts missing or what ever it is doing.  

switching back and forth from closed loop to open loop and back to closed loop does not give me much info to help you...


open loop is where the ECM controls the fuel and timing by preset charts......  it is used  before the oxygen sensors come up to temp and start switching..  its also used during acceleration.. when the system is applying additional fuel..

closed loop is where the adaptive fuel control is set off the oxygen sensor readings...  

lets go over some things...
first..  this is a lumina van..  which motor do you have.. the 3.8 SFI or the 3.1 TBI

i am taking that you have a scan tool that displays live data stream...  which scan tool model do you have.. ?

engine off..

throttle position sensor... look at the voltage with the throttle closed.. under 1.0 volts.. probably around half a volt..    slowly depress the gas pedal.. watch the TPS voltage..    you should slowly see an increase to 4.5 volts  without any drop outs where the voltage drops and then recovers..  

each time you step on the throttle and release.. it falls back to the exact same spot..



if you have the 3.1 TBI  can you hook a timing light to one of the spark plug wires.. or to the coil wire.. tape the trigger on.. lay the timing light under a wiper arm and take it for a test drive..  when the engine acts up.. is it still flashing properly.. or did it begin to studder???

one thing .. if you loosen the distributer.. you will need to remove it and replace the Oring.. or you will create a MASSIVE oil leak..  there is oil galley pressure right below the oring on the housing..   please.. clean the area first..

you will want to examine the pick up coil for proper function..



disconnect it...   use your ohm meter set to 2K Ohms to test the resistance..   should be around 980 ohms..  expected is between 1500 and 500.. ohms..     before disconnecting the test leads from the pick up coil wiring..   flip the meter to 2 volt AC scale..   have somebody crank the engine or spin the shaft while you see if the pick up coil generates just about 1.0 volts AC.    sometimes over..   it needs close to 1.0 volts AC to trigger the ignition module properly..

do also check for a burned cap and rotor..

do a voltage output test on the ignition coil...  i have had to replace so many of those ignition coils.. i cannot count them..  there may be signs of external leakage..

almost all scan tools have a record function..   set it up and record some data stream when the problem is happening..

on the 3.8 motors..   i would start also with the TPS check..   then look at the scan tool for the maf flow changes as you increase the engine speed..  

you might also want to set your digital volt meter to 20 volts scale..

start the engine.. turn on the headlights..

1.   test from the positive battery post to the negative post..  14.1 to 14.8 volts

2.. test from the negative battery post to the engine block..  0.04 volts

3.. test from the negative battery post to the body..            0.02 volts

4.. test from the engine block to the body...                   0.02 volts..

you may have to retest 2, 3, 4, after resetting the meter to 2 volts DC scale.. this takes about 2 minutes to run this test.. it will point out gremlins that are impossible to find..    i run this test on EVERY car as it comes in.. and every car before i close the hood to verify that it is fixed correctly.. remember.. electrons flow from negative to positive.. if you have a loose ground.. you may not have enough electrons to properly power the circuit..   it will also throw off many sensor readings..

be sure to check the ECM ground wires on the engine..   could be on the intake..  or on the back of one of the heads..  the ECM is only grounded to the engine.. not to the body..    

run the voltage drop tests. 1 through 4 above and post the results..  try it on a different car. also..  

It has 3.8l have three scan tools red brick snap on, and a small mac tool scanner and the larger snap on not at the shop and cant remember the names of them. Have been working on these since they came out and usually it has either been a fuel problem. I replaced the fuel pump, the original problem, no start with no fuel pressure. It started up and sounds great. I decided to test drive it and it had this problem. I have changed the egr valve, o2, maf,and changed ign module because during all the testing and starting lost fire and injector pulse, another no start condition. I replace the ecm thinking maybe it was an ecm problem. When u start this thing it goes into closed loop very fast at idle, but as soon as u acel and  up to about 1/4 throttle it goes into open loop. Also u can disconnect the o32 sensor connector and it still has around 433mv reading and flutuates even though it is disconnected.It appears that it could be a short in the o2 circuit and will check it tomorrow. This thing has been kind of long winded and may need to go back to school lol. Thanks for your help  

the MT2500 red brick will record data.. those are a great scan tool...  

set it up to record when you push the button.. so you can page through the saved frames...

log in.. check your private messages...  i posted my #..

do you have one of the harbor freight mini amp meter that plug into the fuse. holder???





this way you can monitor the various amp draws on the circuits..

i had another i was working with on another forum..

that fellow was not a mechanic like us..   took a lot of posts and talking to finally get him to find the problem..

his would die after 15 minutes.. when he finally monitored the fuel pump fuse..  as the pink wire circuit.. was dropping just before the engine died.. the fuel pump went from 6.8 amps to 10 amps to 12 amps to 15 amps at that point the engine died..

i can post the wiring diagrams if you need..

i really worry about the engine grounds down on the bell housing bolts..  

does that motor have just one oxygen sensor.. i think it has 2.. one in each manifold.. but i am probably wrong.. .  

only has one o2 sensor and have checked all pin voltages and ground is under ignition contol module on front of engine and have all my grounds. I have all the wiring diagrams just sometimes when u look at something so long u overlook the ovious. I think I may have done that, cause I think the o2 is shorted to ground. I appreciate your reply. I have worked on these cars since the computers were added and have never had a problem like this. This project is one that if u charged by the hr the vehicle would not be worth the money. I can usually figure out almost anything on all these cars but I think I just got frustated with additional things being wrong too. Sometimes just talking to someone else helps to bring out the problem. I will check it out Monday and see what happens will let U know one way or the other. Do appreciate your replying  

that has a heated oxygen sensor..  could the heater circuit have a bad ground.. and be throwing off the sensor ground..??


i had an 87 300Zx that would just barely run...  looked like it had bad grounds.. i added a few that i found were bad.. but it still acted stupid..

turned out the ground wires on the back of the drivers side head were never connected when somebody installed the reman engine..  i had grounded on of the wires i could reach with a scotch tap as i was NOT aware of where it was supposed to be grounded at..  but the pair of ring terminals were not bolted on behind the cylinder head.. they were tucked up into the harness..   i had to find a factory 87 300ZX manual and go through every page to find the diagram that showed where the grounds were attached on the motor..   once i got the oxygen sensor ground hooked up..  took care of some other screwed up wiring..  the car returned to service with the power it should have.....  

the oxygen sensor voltage was over 1.2 volts..    not quite like your voltage signal..


say... did you know that some of  these cars have a ground buss hidden in the rocker panel..   open the door.. remove the kick plate..  hidden down in through the holes is a buss block with a bunch of quarter inch quick connects pushed on.. the buss bar is grounded under the middle of the floor under the drivers seat and under the rear seats somehow..   individual circuits may have come loose or burned..    you might want to spend another 20 minutes and remove just the front ones ...  to check for loose or burned ground connections..