I have a 1992 4.3W CPMFI that is very hard to start cold and stumbles badly when throttle is opened. Fuel pressure check gives 60 PSI before engine starts, 54 PSI at idle or running, jump to 60+ when throttle opened. I replaced the "Spider" injector/regulator assemble to no avail. When finally running is smooth at steady state. Cold start reaction: fires, runs rough for a few seconds, dies. Eventually runs longer and longer till it keeps running and smooths out.
This engine is a transplant in a non GM vehicle so all non essential wiring is not hooked up, no dash.
The engine sat for about 6 years with ethanol fuel in it so first had to replace fuel filter and clean tank, pump seems to be OK, a bit noisy.
Any suggestions, I'm pulling what little hair I have left out.

 

What comes to mind:
Fuel pressures seems o.k. to me.

Temperature sensor signal, looks like the cold start enrichment is not correct.
Also check all vacuum lines to and from sensors, should not be related but going over that system helps further down the road in diagnostics.
Also check the whole engine wiring harness for scuffing, bad connectors etc.

As it is a transplant it may come down to the quality of the wiring.

 

You installed this in a Mercedes, so it may be a bit difficult to diagnose.

 

BTW, how did they run the engine on Ethanol? That would require at least a new mapping of the software.

Have you done the wiring by yourself?

 

Was the 'ethanol gas' 15% or 85% ethanol gas ? I would again look for contamination at the gas tank, gas filter and gas lines. It is hard to get all the contamination out. The second thing to look at with cold start problems is the ECT sensor and its wiring.
Are you getting any OBD II codes. What are they ?

 

Ed, am I right in assuming you installed the GM 4.3L ECM= computer when you transplanted the V6 in the car ?

 

I just re read the original post and the V6 is a 1992 and therefore prior to the OBD II or OBD I for that matter and so no codes. Even so, it would seem your engine would have to have some sort of ECM ? Right ?

 

The ethanol gas was E10. Before doing anything I replaced to fuel filter, removed and cleaned the tank and put non ethanol fuel in, have replaced the injector spider.

 

Not ethanol E10 ethanol gas. Preserved the entire wire harness un interrupted when removed from original vehicle.

 

Has original stock ECM from Jimmy, preserved entire wire harness uninterrupted when removed and reinstalled.