ECM Problems on an '81

I have a problem with the primitive ECM on my 81. Not too long ago we installed a GMPP 350 HO engine in my 4 speed 81. We kept the Q-jet, distributor and exhaust from original underpowered '81 engine. Wanting to extract a bit more power I installed a set of Patriot short-tube headers, true duals and Magnaflow mufflers. It runs and sounds great --- when it runs! Though I feel rather ignorant to admit it, as a result of the most recent exhaust upgrade the engine runs horribly at anything less than full throttle. I am almost positive that the problem is with the ECM which is not hooked up to any exhaust sensors and is throwing my spark curve/ignition timing way off. As I see it I have only two options. The first is to install new sensors (and I know not what else) to make the distributor conform to the modifications. The other option is to strip the car of the entire ECM system -- installing a new carb and distributor. Have any of you who have '81s had any similar problems? If so, how did you remedy the problem. Thanks much.

Jay  

wow... someone with a C4 computer system in a C3 vette...   i actually went to gm training on these...

and the first thing..  you have to have the oxygen sensors installed.. or it will run in default..

you also need to verify that there are NO upstream air leaks...   upstream air is air from the smog pump through the diverter valve and commanded into the tubes in the exhaust manifolds ..

once in a while.. the diverter valve fails.. leaks air injection air into the air injection tubes ... this dilutes the exhaust stream... can drive the early computers into doing the most nasty things..   like making the car almost totally undrivable.  


be sure the grounds to the ecm are hooked up..

do you have a dwell meter????  analog or digital..  you can hook it up to the green lead with the green connector near the carb... set the meter to 6 cylinder scale...    after you put in the oxygen sensors..  you should have 28 to 35 degrees of dwell at idle..


there is another green connector near the ecm..  grounding that one..  and you get check engine light codes...  there is one thing.. you have to connect the ecm power wire to constant power at the fuse box...   as this computer did not have a keep alive memory..   so you have to keep the ecm powered full time.. then take it for a short drive to see if the check engine light comes on..   if it does.. you can ground the green terminal with a jumper to get codes..    be sure after you fix it.. and before you walk away for the night.. that you reconnect the ecm power to the switched power position in the fuse box...  or you will have a discharged battery the next morning..

there is one thing .. if you intend on keeping the feedback carb also..   81 full size cads.. here in california.. came with a 368 motor and a quadra jet feedback carb..  the larger displacement of the cad motor should have slightly larger jets and metering rods..


oh.. and you will find that autozone.com has the engine wiring diagrams for the really early 80 model..  the 81 and also the 82.. under wiring diagrams..   you should probably print all three..

the 82 actually is the early crossfire system...

so you will have to open the gm mid size or camaro wiring to get models as late as 87 with feed back carbs...

oh.. one last thing.. quadrajets with electronics.. only the primarys are controlled by the ecm...  when you get the secondarys open.. its all mechanical...    so you could actually have the best of both worlds... if you can find the right carb guy..

fuel economy on the primarys..   brutal acceleration on the secondaries..  and you can actually do the secondary tuning with a single small screw driver.. the metering rods and hanger come off with a single tiny screw..  

Thanks much for the detailed tech info.  I take it that you would recommend that we put the emissions wiring and sensors back in place.  Will the absence of a smog pump have any bearing upon how the ecm works?  We removed it when we replaced the engine.  

Jay+  

it will run without the smog pump.. as long as there are no exhaust leaks infront of the o2 sensors..      if you are running a cat...  you might actually want to think about putting it back on.. as it will prevent it from overheating and melting..

during WOT and some percentage of acceleration it will fall into open loop.. this is preprogrammed fuel control..  it will at that point inject air into the upstream manifolds and into the front of the cat..  this is only during open loop acceleration..

you would have to monitor the ground  signal to the diverter valve  to see when it gets activated.. other than that.. you can probably do without it..  as long as you don't need it for smog.. be sure to box it up along with all the other parts... as they are hard to find...


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