Auto Parts: s-10 fuel injection, crank sensor, exhaust stroke

Question
QUESTION: hi i am working on my 95 s-10 2wd 2.2 litre..it has been having a problem for several years with the engine missing on 2-3 cyl after start. it would do this and then would work on all four after a restart or two making it difficult to diagnose ..but in the past several months it has been getting worse..the other day it would not hit on cyl 2-3 at all..i have checked the ignition and it is working correct so i removed the intake top and checked all the inj.wiring and all is good there..i also changed the ecm and it still does the same thing..i sprayed fuel into the open intake and it fires on all four cyl...is there somewhere else that the two sets of injectors 1-4 2-3 are supplied by power or ground that i could be losing a signal..or could it be that the two injectors went bad ..which dosent seem likely to me since it would miss on both then fire on both after restart..

ANSWER:      The best way to check this thing out would be with the parts that you already have.  First try switching the coils.  If the problem moves to 1 and 4 when they are switched, then a new coil is in order.  Then try switching the injectors the same way.  This is a lot more work, so do the coil test first.  I'm thinking it probably is the coil because that is really the only thing common to 2 and 3 cylinder.  It could be the module, since there are actually two of them in the one pack.  One module fires the 1-4 coil and the other fires the 2-3 coil.  Both are triggered by the crank sensor through the computer.  Each coil fires on both the exhaust stroke and the compression stroke in what is called wasted spark.  It's a pretty simple system and I'll bet that either the coil or module is bad on 2-3.  Either one could be intermittent, although the module is more likely to act like this.  If you feel like investing a little in tools, you could get a "noid" light at a parts store.  It will allow you to check the injectors without taking the whole deal apart.

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QUESTION: HI..thanks for your reply..i have already switched coils and had the module tested..all is good..i sprayed some fuel into the intake with the top plenum off and it made all four cyl. fire..my next step was to switch injectors around ..i was hoping to avoid this and some could tell me somewhere else to look ..the two wires coming from the ecm one blue for 1-4  and a green for the 2-3 i even ran wires into the cab to see if maybe i had a break in the wires from the connector to the ecm and it didnt change anything..i was wondering where the ecm gets its ground for the injectors if maybe there were seperate grounds for the two sets of injectors that i am not finding..i am thinking maybe switching the blue and green wires around which would not be in time but might tell me if maybe i have 2 injectors that went bad at once or if the problem is still elsewhere..i have a good wiring dia. but am not really sure of what actually gives the injectors its negative pulse..all four injectors share the same power wire. i wish i had a good factory manual that might give me more info than these worthless chiltons ....tanks again ..R.C.

Answer
    All of the injectors share the same power wire, and the ECM grounds them in order to fire them.  I'm pretty sure that on these GM trucks that this is done via a relay.  I'm pretty sure that they don't fire the injectors sequentially on these four cylinder engines.  I think that they fire all four injectors at once.  It seems counter intuitive but, except at very low engine speeds, there is plenty of air moving through he manifold to pick up the fuel.  They really didn't even start using sequential injection until the very late '80's, except for some exotic foreign cars.  My old Jag fires the twelve injectors in batches of three.  It allows them to use a four cylinder ECM to trigger all twelve injectors, a big savings in manufacturing cost.  And you can balance a quarter on the valve cover while it's idling.  I wonder if it could be the relay.  There is a diagram showing which relay does what in the owner's manual, you might want to check that.  We had some of these S-10s as delivery trucks back in the '90's and they didn't work out so well.