I replaced a MAP sensor on a 4.9 Str.6 95 F150 because of rough idling. It still runs bad. But if I unplug the MAP sensor, it runs fine. I replced the connector as well, so my question is what should I check next? Anyone have any ideas?

 

Stick to one thread.....

https://www.ford-trucks.com/forums/1...9-str-6-a.html

 

If I unplug the MAP sensor on my 94 I get a rich condition, actually get some black smoke out of the tailpipe. So by you unplugging your MAP sensor you are likely giving your truck a richer fuel mixture.

I would suspect you are not getting enough gas. In your other thread you said you were going to check the fuel pressure, did you?

 

I have not done that yet. When I was taking my MAP Sensor back for exchange, the people there was trying to tell me that it wasn't their part even with a receipt. I was going to get a fuel pressure tester there, but they eat up a lot of my time and pissed me off a little, so I came back home before it got dark. But I am going to get it tested ASAP.

 

As a preliminary test, try disconnecting the vacuum line that runs to the fuel pressure regulator (FPR). You could do this before you buy/rent/find a fuel pressure tester.

If it idles better, it could mean your fuel pressure is low. At idle (high vacuum), the FPR acts to lower the absolute fuel pressure to maintain the differential between fuel pressure (higher) and manifold pressure (lower). If you disconnect the vacuum signal to the FPR, it will "raise" the fuel pressure in the sense that the differential between fuel & manifold will increase.

 

I just started the truck and let it run until the the rough idle started. When it started, I unplugged the Fuel Pressure Regulator. There was no change in the engine idle. It's a steady idle of up and down, up and down.

 

It took me a bit, but I managed to finally get time to check my fuel pressure and I have 20 lbs of pressure with the key on and running. It was actually a little higher when I first started the truck, like 32lbs, but then it sunk down to 20 lbs and stayed there.

 

Too low, should be >45 I believe.

 

Yeah, I've been reading it's supposed to be anywhere between 45 and 60 psi.