My mustang buddy and I just completed a 351 swap into his fox vert. The engine came from another fox that was carbureted. The engine has a pretty nasty cam in it. The problem is that the cam being designed for a carbed application has drastically reduced the amount of vacuum supplied to the engine due to cam duration. This vert being a fuel injected car, is having issues with the computer reading the signal from the map sensor as being "vacuum starved" forcing the engine to run rich and idle erratically. Does anyone know how I can "fool" the map sensor into thinking it has sufficient vacuum. A local hotrod mechanic said to install a vacuum check valve inline with the map to hold vacuum in the sensor, however the map sensors in v8 foxes don't have a vacuum fitting, only an electrical pigtail and a vent.
You are prob not going to like this answer I pulled this from another website
Many people get into problems when a stock cam is replaced with an aftermarket or Ford lettered cam.
Why; is due to lobe centerlines that cause "positive" intake/exhaust valve overlap periods.
The stock camshaft actually has a small amount of "negitive" valve overlap.
Since the SD engine is 'load detected' by vacuum supplied to the MAP sensor; a cam with positive overlap causes a loss of intake vacuum affecting the MAP signal and making the engine run richer.
Since the oxygen sensors detect this, the result is to attempt to lean fuel delivery. End result is an engine that does not idle correct and tends to hunt for it's correct idle but cannot ever arrive at a final loop setting to match the idle set in computer program.
There are at least 4 aftermarket cams that have performance profiles but still maintain zero or negitive overlap lobe centerlines for use in SD engine control.
When looking at camshafts, the first parameter is to look at centerlines of 114 degrees or larger, have higher lifts, more rapid lift rates and somewhat longer durations without getting into light car cams that generally tend to lose to much low end torque .
There are from time to time, people who change cams, get into issues then ask for help and have a hard time accepting the answers to either change the cam again or convert from Speed Density to Mass Air engine control.
The reason Mass Air clears the cam problem is because these system controls do not use a MAP sensor to detect engine load but uses the air meter to measure airflow thus not being 'for the most part' affected by cam overlap. Of course cam overlap taken too far makes for rough idle for a different reason but still created initally by the cam's positive overlap period.
Hope this is of help to those who have an interest cam parmeters vs Speed Density engine control system.
