97 B2300 5-speed. Starts rather quickly EVERY time, engine speed flies up to 3000 rpm, then immediately dies. Every time. Can keep it running (barely) by using the gas pedal to "catch" it. Replaced a defective IAC, no change. Have unplugged MAF, blocked off all vacuum lines, no change. TPS sits at 20% with no throttle, but tracks from 20 to 93.3% smoothly with throttle application. I'm going to run through the sensor data in the next few days to see if there's anything obvious or that doesn't change like it should using my scanner. In the meantime, getting some other ideas would be nice. The only vacuum line other than a major intake manifold leak would be the EGR could be stuck open sucking from the exhaust. Previous owner stated he could drive it in its current condition, until one of the brake lines rusted through.

Jon

 

Welcome to the forum

Just a heads up, you need to use a Motorcraft(Ford) IAC Valve, 3rd party will give you problems, even a used Ford IAC Valve from wrecking yard is better than new 3rd party

If you can read sensors, then see what engine temp reads when engine is cold, ECT sensor data
Computer needs to Choke the engine when its cold or it will stall
If ECT reads 150degF when engine is cold, 40degF, then engine would start and stall, until it actually warmed up

When you first turn on the key the computer should open the IAC Valve all the way for startup, so its normal for a fuel injected engine to REV on startup, cold or warm
Then computer starts to close the IAC Valve to set "target" RPMs
Cold engine with Choke should idle at about 1,200rpm
Then as engine warms up, ECT sensor warms up, the computer will slowly close the IAC valve, as the "target" idle RPMs are reduced to match engine temp

Stuck EGR valve should set a DPFE sensor code

And you may have a vacuum leak causing higher RPM
If you can run the engine for 8 minutes or so it should be warmed up
Let it idle and unplug the IAC Valve
Idle should drop to 500, barely running, or engine may even stall, either is GOOD, it means no vacuum leaks

If idle doesn't drop then you have leaks

 

For all those that are looking for answers, and never find them - here's one. Read above description. Only additional "starting" information was a P0102 for low MAF sensor circuit.

The engine code obviously clued me into the issue. The MAF reading on a scanner was 0.0 lbm/hr even when running, which was an obvious tell for a bad MAF/circuit. I pulled the MAF and found a bad jumper at the sensor end (the little conductor that bridges posts), so I bought a new one. Strangely STILL 0.0. I then went to voltage checks at the MAF sensor harness: Red was +12V, Black was O ohms to ground (post on battery). The OTHER two wires, in my case peach with lite blue and lite blue with red were the sensor ground and sensor signal. The sensor ground DID NOT show continuity to ground with the key on, engine off (otherwise known as open, infinitely high ohms). I pulled the ECM/PCM plug from the firewall, and checked continuity to the MAF sensor ground wire (pin 36), NO continuity (open, infinitely high ohms) - looking for a break in the wire somewhere in the engine bay. I went through several of the engine bay grounds to no avail. Then I simply jumpered the ground wire on the MAF harness to the sensor ground wire as a "test" - WALA! MAF has function now, truck starts and idles just fine. Hopefully someone at some point can find this useful. Costs $0 (assuming you have some spare wire and wire cutters).