91 2500 gmc no start

I have a 91 gmc w/ 350 engine it has spark & fuel , but will not start sometimes it just cranks , but when it does run it will misfire . it has new cap,rotor,wires, spark plugs & ignition module , I was messing with it today I disconnected the the single tan wire for the spark advance on the firewall & the truck starts fine . could this be an ecm problem ?

 

Are you sure it wasn't coincedence that it started after unplugging the tan wire? the ECM has no control over the spark until engine speed reaches a threshold(different for every vehicle,year, etc. but usually around 400 rpm), So I doubt it's an ECM problem. You might have spark, but not strong enough to get it to run when the engine is cold. We use what's called an ST-125 for checking spark, and makes the spark jump a calibrated gap to ensure the coil is strong enuff. In lieu of the tool, the spark should be able to jump at least an inch, or so. You also might have fuel, but not enuff pressure to run the engine. Assuming it's a small block, 9-13 psi is the spec.

 

your "base timing" it sounds like your timing may be off. (Note: check me on this, a lot of newer cars the computer completely controls engine timing) Did you remove the distrubutor from the block? Or phsyically move the distrubutor at all? If you did you may just need to reset the timing by moving the distrubotor. If you pulled it from the block then you may be just a tooth or two off on the gear.

BTW, i've done this to myself, both at the same time!! (Engine jumped time, did both while replacing timing chain).

Hope this helps!

 

tried to start today with everything connected & it started ,it stalled 5 times & barely started , did not remove distributor , has plenty of fuel & recent fuel pump , I think its a spark issue spark seems weak right from coil wire , but I have also replaced coil & wires , can distributor pickup cause weak spark ?

 

If you have alot of miles on the truck I would see how many degrees of play the timing chain has.I would also match mark the dist and pull it to check the gear condition on it.If it runs ok with the timing check connector disconnected I think the ecm could be changing the timing when it shouldn't be.A timing light would confirm cranking timing and timing with the connector disconnected.Does the truck have any codes that were not self induced by you?