Engine trouble

I recently purchased a 1988 Camaro and it originally had a 305 tbi. The guy I purchased it from removed the engine, stripped it down and had the block bored .40 over making it a 355 and installing the nessasary components to make it TPI. The car once it warms up will not stay above 1000 rpm without feathering the gas and brake to keep it above 1000 rpms. I dont want to burn up my brakes and this can create a tremendous safty hazard. He said it was the MAF sensor that was bad but when I questioned him more he said he did not change out the ECM. Im an old school carb guy and i've never played with TPI. My question is do I need to change out the ECM for the car and if so could anyone please provide any info they know on this subject. Id really like to drive my Camaro  

ok... do you have a scan tool...????   that reads live data...

and why do you want to keep it above 1,000 RPMs.. ??????


those engines will idle as low as 450 rpms...  

they also came with alternators that would turn off for 2, 5 or 10 seconds when they drop to that low speed.. the ecm knows what the alternator time out should be.. and just before the alternator starts charging again..  it opens up the IAC a ***

first... i could check the throttle shaft for wear...  with the engine off.. open the throttle slightly..  see how much play there is in the shaft...   you can tell with the scan tool by looking at the throttle position sensor voltage... with you foot off the gas pedal.. note the reading..   open the throttle a few times... each time letting it back to idle...   it should go to the exact same voltage every time..    if you get slight differences... you have a worn throttle body shaft bore... this works on TBI units also...

look at the coolant temp  on the scan tool... TPI and TBI  should be about 215F... with the engine warmed up...

if you don't have a thermostat in there or its broken..  and the coolant temp stays below 190F..  it will usually stick at fast idle... dump extra fuel in to try to warm the motor... this constant extra fuel will melt the converter...

it also has to be a 195F thermostat unless you have  reprogrammed the ecm...

now.. once the car is up to temp... and you know that the throttle body is not worn..

look at the IAC...   Idle Air Counts..   this is the 4 wire idle speed control motor...  it is a stepper motor... and the ecm can move it one step at a time..      normal idle the IAC counts should display between 20 and 50 counts...    if it is lower than 20...  you have too much base idle... the throttle blades are open too far..  or you have a vacuum leak.. the ecm is trying to slow the motor down... so it closes the IAC valve ...


if your IAC counts are above 50...   your base idle setting may be too far closed.. so there is no air passing the throttle blades at all..  your ecm is trying to increase the idle speed....

the IAC motor is also used as a DASHPOT... when you open the throttle... the ecm totally closes the IAC motor for a split second.. then it backs it out to around 135 counts...   when you take your foot off the gas.. the throttle closes..  this prevents the intake manifold vacuum from going too high... lets the ecm bring the engine speed back to idle slowly..   less chance of stalling the engine.. but when it does drop the engine speed below 500rpms.. the alternator will shut off for the few seconds..  as it lets the engine stabilize before it starts charging..


so.. why do you want to keep the idle above 1K... when a 500rpm idle will do.. unless you have a lumpy cam... and it won't run...

when increasing the engine displacement... when you operate the engine  in closed loop where the mixture is set off the oxygen sensors...   it will run fine...  but acceleration above some point.. reverts the system into open loop preprogrammed..   if the injectors are too small...  you will have a flat spot off idle..


one other thing...   you have to verify that there is NO un commanded upstream air into the air injection tubes that thread into the exhaust manifold... the Diverter valves sometimes fail... they are plastic... if the check valves fail the exhaust back pressure can get to the plastic diverter valve and melt it..     if while idling.. you take off the hose from the diverter valve and put your finger over the end...  if you have air coming out.. you really need to check it..   this will dilute the oxygen sensor... and freek out the ecm..      the air injection system pumps air in the exhaust manifolds during WOT>. and for 20 seconds when the engine is cold... to help warm the cat and burn off excess emissions...    it also keeps the cat from getting too much fuel and overheating..   the down stream air.. that is the pipe that dumps into the side of the exhaust pipe infront of the converter or into the side of the converter can get air injection at any time...  the ecm controls this..  i have not studied when this down stream air is on or off..    just that it can be switched..