Hard/No start then runs fine

[zitkada]

I have been dealing with an issue that turned out to be a seized IAC valve causing it to bog down on decel or at a stoplight. It would bog down only for a few seconds, except at highway speed, then go right back to normal idle. It would almost sound as if it had a lopey cam in it. Since fixing that, it has not started whether the engine is cold or warm, as if it is being starved for fuel. Which it had no problem starting before changing the IAC. The only way I can get it to start is if I press the gas pedal. It doesn't take much, I only need to press the pedal very lightly. But I'll have to keep the pedal engaged for about 30 seconds, then the idle slowly rises enough to stay running. The only other time it will start right away is if I pull the battery and reset everything. After that first start, it's back to the same hard/no start conditions. Driving through town and on the freeway is completely normal. There is no noticeable power loss, drop in idle, or pinging. Since changing the IAC I will get a CEL while cruising, code 43, that was not present beforehand. It will light up when sitting at a stoplight or slight inclines on the freeway then goes away when cruising on level-ish ground.

1994 C1500 350 TBI
-Base idle was low, around 375-400 RPM. Raised it to 525-550 RPM while in gear and readjusting the TPS to .5 volts. (PO already had the idle screw cap removed)
-Tried setting TPS from .4 to .6 volts to see if that would work
-New IAC can be heard engaging when doing base idle procedure
-Initial timing is within spec
-No vacuum leaks that I have noticed
-Replaced coolant temp sensor hoping that might be causing the issue

Truck ran totally fine other than bogging down on decel after "fixing" it. I'm tired of banging my head against the wall. Any help is very welcome TIA

 

[Schurkey]

Connect a scan tool, look at the data stream. Verify IAC counts. Verify ESC system.

GM Code 43 Electronic Spark Control Fault - FreeAutoMechanic

Trouble Code 43 indicates that there may be a malfunction in the Electronic Spark Control (ESC) circuit. ESC is used to sense spark knock (pinging) and

www.freeautomechanic.com

www.freeautomechanic.com/gm-code-43-electronic-spark-control.html

Verify fuel pressure.

How old are the usual "tune-up" items--cap, rotor, plug wires, plugs, air and fuel filters, PCV, etc.

 

[zitkada]

Connect a scan tool, look at the data stream. Verify IAC counts. Verify ESC system.

GM Code 43 Electronic Spark Control Fault - FreeAutoMechanic

Trouble Code 43 indicates that there may be a malfunction in the Electronic Spark Control (ESC) circuit. ESC is used to sense spark knock (pinging) and

www.freeautomechanic.com

www.freeautomechanic.com/gm-code-43-electronic-spark-control.html

Verify fuel pressure.

How old are the usual "tune-up" items--cap, rotor, plug wires, plugs, air and fuel filters, PCV, etc.

I don't have a scan tool available to view live data unfortunately.

-fuel pressure is good
-PCV, cap, rotor, plug wires, spark plugs & fuel filter seem to be in working order but unable to verify their age. Bought the truck up about a month ago.

Kind of at a loss, as it started and ran just fine before I changed out the IAC. Except the fact it would bog down on decel.

 

[HotWheelsBurban]

Not sure about TBI on this (cause my TBI '90 Burb never did this) but I do know on the Vortec that the idle air control valve and the throttle position sensor often fail around the same time.

 

[Schurkey]

I don't have a scan tool available to view live data unfortunately.

Need to take care of that. Diagnosing a computer-controlled vehicle without being able to communicate with the computer is two steps away from hopeless.

-fuel pressure is good

How do you know? What is the pressure under load?

-PCV, cap, rotor, plug wires, spark plugs & fuel filter seem to be in working order but unable to verify their age.

Take them off the engine, test/inspect 'em. "Seem" to be in working order isn't good enough.

Kind of at a loss, as it started and ran just fine before I changed out the IAC. Except the fact it would bog down on decel.

Again...you need a scan tool.

 

[Donald Mitchell]

Isin't there a procedure to replacing the iac. Was that done?