slow speed surging
 

At least that's the best description I can give for the problem.

1991 454 TBI. When it's cold out and I'm driving with the throttle just off idle, it feels like the engine has a fuel surge. Throttle response is weak unless I give it a bit more fuel than just a slow pick up the pace. When the engine and the outside temperature is warm, it's not as noticable.

Injectors are still original. Last time this happened I found the fuel pressure was over 15 psi so I dropped it down to 12 psi and the problem went away. Haven't checked it lately.

I've checked and reset the IAC. I can't remember if I checked TPS voltage or not last time.

It idles fine and cruises down the highway fine.

Any suggestions?

Does this happen at speeds below 45 MPH?:D

My investigative list would be:

Fuel Pressure
TPS voltages
IAC cleaning
Coil condition... yes odd as it may be, I'd check it.

Yes below 45 mph. The throttle is open enough above that and the problem either goes away or isn't noticable.

Coil is a MSD Blaster II
Platinum plugs and new wires.

Since it only does it cold, we can rule out quite a few sensor options, I assume it is only doing this in open-loop. Do you have access to a scan tool? a funky CTS might cuase some drivability issues like this, it would also be interesting to see what MAP values it is happening at.

Another option, with the light throttle, the computer is going to be commanding a fair amoutn of sparkl, it might be worth checking the timing, and making sure the dizzy cap didn't slip just enough to cause problem.

I would really suggest hooking up some sort of scan tool though, that is going to tell you/us more than anything else.

It was a little warmer today on the drive home and the engine ran normal with no hesitation or surging.

That's why it has me baffled. It doesn't happen all the time. It usually does it when it's raining too so I'd guess there's a sensor problem but it also runs fine after I pressure wash the engine.

I've also installed a heated O2 sensor last year.