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A couple of possibilities. I'm assuming that it all worked before the modifications. The idle air control valve allows extra air into the engine when a load such as the A/C is placed on the engine. If the idle air control system isn't functioning properly, either because it was removed, or the valve itself is faulty, or there is a wiring issue, the engine will stall when the AC is turned on.
I'm also assuming that if you are driving down the road, and you turn on the ac, that the engine carries on, and the AC works! This would rule out a seized AC compressor.
The engine computer also needs a signal to tell it to increase the idle speed. You have made no mention of computer or wiring modifications.
If the system worked before the engine modifications, then something happened during the installation.
Hope that helps. Send me a note if you need more information. Canuck.
How is your Mustang repair going?
OK - so does the engine turn over by the starter, or it won't crank over at all?
Have you checked for spark?
Checked fuel pressure? Verified ignition timing with the new distributor? Cleaned the mass airflow sensor?
14 psi fuel pressure?
i see - timing is at 14...
After looking at the wiring, I don't think this car has a mass airflow sensor... I'll check again...
OK. I'm assuming there was a computer change to go with the mass air conversion...
Is there still a vacuum line going to the fuel pressure regulator?
when you snap the throttle, does the fuel pressure go up?
The idea with the fuel pressure regulator is that as manifold pressure goes up (you are stepping on the throttle) the fuel pressure needs to go up as well. Should be about a 7 psi difference between closed throttle idle, and key on, engine off. Under what conditions did you measure 40 psi?
key on, engine off? Or engine running?
Alright. So we need to verify if the regulator is working. If you can get a number with the key on, engine off, and then a number at idle, that is a good start. The system also needs to be able to hold pressure, which if it doesn't can be an indicator of other fuel system issues. Typically, an engine going "flat" indicated a fuel delivery issue - going lean for some reason -
Do you have a functional vehicle speed sensor?
OK. So we still need to verify the fuel regulator is changing the fuel pressure as the throttle is opened. VSS is an input to the computer.
You have many variables in this car. Could it be the mass air conversion? Possibly. But check out fuel pressure first. I would like to see about 45 psi key on, engine not running, it holding pressure, and then down to about 38 psi at idle. Let me check on the vss wiring...
I don't know that the vss would give you the symptoms you have described. I see on the diagram that this car did come with a mass airflow stock.
OK, so it has a different computer, and that was part of the mass air conversion? Is it a larger mass airflow sensor? I'm also wondering about the computer and its compatibility. I want to have some answers before we chat, because at that point... you are committed with cash! LOL
Ok. I'm looking at a 1988 diagram, and its got a MAF, but all good.
Let's verify that fuel pressure... so long as the processor has a program for those injectors, it should work out. As part of the conversion, did the MAP disappear?
I've gotta take off for awhile, but I can check in later if that is OK...
Yes, to a certain extent. You can do the fuel check and get back to me tomorrow. No problem.
Happy to help. I'll check back in periodically today.