Need to know about Mustang MAP sensor logic

1992 Mustan 302 with a FI harness and parts from a 1990. The car is a real "Bitsa" if you know what I mean.

Just converted back from carb. Aftermarket larger throttle body.

The problem we ran into is that the engine wont start with the MAP connected. Tried 3 different MAP sensors and checked for 0V and 5V at connector. No oscilloscope available to check output (who uses frequency as a sensor output anyways!)

As mentioned, car starts without the sensor but it hunts up and down by a few hundred rpm.

If the MAP sensor is connected with the engine running, it will start to smooth out after 15 seconds, but then die as if it ran out of fuel.

If cranking with the sensor attached, there is fuel pressure and there is spark. Not sure how to test for injectorS firing, but I assume they are not?

If you take the car for a drive without the MAP sensor, it runs great. Just does not like the low rpm stuff.

If anyone is aware of a logic condition that results in these symptoms, I would welcome some guidance. I am conversant with EFI but have no previous EFI troubleshooting experience.

I may be wrong, but my records show that the mustang was changed to a MAF system back in 89.

So what is the complete combination of parts that is in the car? Engine, engine sensors/actuators, harness, and module.

are you sure it is a MAP sensor and not a BARO (atmosphere pressure) sensor?

I can see hooking a BARO sensor to manifold would cause some goofy stuff to happen. Like Alfa said, I thought only the 88 mustangs ran a MAP - speed density, and after that they used a MAF (but also a BARO - located on the firewall, slightly to the drivers side, near the hood seal).

Also, I have found this site to truly live up to it's domain name (it is very useful), and it actually a fun read.

http://www.veryuseful.com/mustang/tech/engine/EECIVInnerWorkings/

It is barometric sensor your are all correct.

I'm telling you something you already know, but - in a MAF system, the MAP is left open to the atmosphere. If there was formerly a vac line from the intake to that sensor, it should be blocked at the manifold.

I'm guessing your problem is actually your Idle Air Control valve.

Jamey_from_Legal wrote: I'm telling you something you already know, but - in a MAF system, the MAP is left open to the atmosphere. If there was formerly a vac line from the intake to that sensor, it should be blocked at the manifold. I'm guessing your problem is actually your Idle Air Control valve.

Yeah, I was kinda headed that way next. Looking at how the two were logically interconnected.

Might never know what the deal was, the car got sent to a shop with the proper scan tools. No more Grassroots troubleshooting.

Like I said, I am just coming on board with laying hands on FI. This was a good execise for when the Molvo engine comes alive. Someday.

I hear you about "good exercise." FI is a cross-platform device. The reason I'm pretty sure your IAC is the problem? Not because of the 5.0 HO I have in the lab right now. Because I just went through that on a Motronic 1.3 system on a BMW motor.

I think this pdf is quite informative. It's for the 3.8 but for learning about EEC-IV, the theory should be the same.

http://foureyedpride.com/pdfs/1986_3.8_CFI_EEC.pdf