'99 Vortec L31 won't run right

[wrager]

...despite many parts having been replaced. It will start up but then the idle will surge. After a few seconds it will idle up to about 1500, then return to a normal idle (650'ish). At that point it sounds perfectly normal, but any acceleration brings a stumble and backfire thru exhaust.

So, what I've done so far:
Replaced: Fuel pump/filter, new MPFI, plugs, new cats and O2 sensors, IAC, CTS, MAF, Evap valve, MAP, PCV

Cleaned TB, checked everywhere for vac leaks using carb spray and also propane. Timing retard is -.03. EGR delete.

Of course no codes....but the ECM has been "tuned." I thought maybe I didn't insert the new injectors correctly, but once it stops surging, it will idle fine (but no acceleration). I just don't know where to turn.

[61_FL_Apache]

When my CPFI was going bad, it ran real rough with no power because it was dumping too much gas in the ports. I also had a bad IAC which was causing surges at idle like you were explaining. Once I replaced the CPFI with the MPFI and IAC, my truck ran like new..

I cant think if any thing you have not already changed. Maybe cap, rotor and wires?

The injectors just kinda push in the holes. There is a plastic retainer on them so they should be fine. Theres not much room for them to back out.

[speedygonzales]

What the OP was referring to when he said inserting the injectors was that maybe he put them in the wrong locations.

You did not mention the TPS. Perhaps the PCM is getting the wrong signal at first. Have you checked the TPS?

What do you mean by - .03 do you mean "3"? The spec for retard is 0 +- 2

Have you actually tested the pressure? When I finally checked the MPFI unit a year after I installed it, I found the pressure to be way below spec. I had to replace it and then dial in my pressure with a valve. The MPFI unit pressure spec is 55 to 60 PSI. Check it out.

[wrager]

I did actually check the pressure with two gauges (didn't trust the first). It is 56 steady even when I can give it throttle. Does not bleed down at all (key off). I thought that seemed a tad low for a new FP (Delphi) but on another site they said the newer MFPI has a lower pressure setting than the poppets. True?

I might aw well throw a new TPS and rotor at it. Cheap comparing....

My scanner shows -0.3 retard??? It's an EZ Scan 6000.
Thanks for your suggestions. My '71 purs like a kitten, at least

[speedygonzales]

Originally Posted by wrager

Of course no codes....but the ECM has been "tuned."

WTH does that mean? I had mine tuned to eliminate things like EGR and the rear O2s but I still get monitoring. So what gives?

You may want to try something like pulling a plug wire to get it to missfire and see if a SES light pops up or DTC 300.

Originally Posted by wrager

I might aw well throw a new TPS and rotor at it. Cheap comparing....

There are methods to check sensors like a TPS so you don't need to replace them. Unless you are independently wealthy

Originally Posted by wrager

My scanner shows -0.3 retard??? It's an EZ Scan 6000.

When scanners show retard, they are not capable of showing fractions of a degree. I would not trust your scanner. Time for a good one.