Hey all. Im new to the forum but after reading through the threads, I have found you guys to be very helpful. Now I have a small problem that I think the cumulative mass of Ford thinkers can help resolve. I have a '90 F-250 supercab 4x4 with a 351W in it. The Truck has 151000 miles on it and it is just a little tired but not too bad. It was only used by a farmer to haul grain trailers and fertilizer tanks around the fields. Now to the problem. I have a suspected out of range MAP sensor. The book says put a tach on it and use a 6-cyl. scale. Should read 310 rpm KOEO. Then apply 20" of vac. on it and it should read 200 rpm. Now when I do this, I get 1100 rpm just sittin there and 650 rpm with the vaccum applied. Thing is I bought a new one and it was only 200 or so rpm lower on both numbers Still not in range. Do I have a defective tester or is this a misprint in the book? It keeps throwing a code 41 at me but there is a section of exhaust pipe just upstream of the O2s I had replaced and the guy who welded it only spot welded it and jammed that putty junk in there. Now its falling out and I think Im getting some O2 from outside in there and thats the prob. I think. Well thanks in advance for the advice. And if you know how to convert 6-cyl. rpm to Hz, let me know. That would be nice to know.

 

Well, if you put a brand new MAP in and you're still getting the code, I see 1 of 3 scenarios being the most likely:

1: You didn't clear the computer after replacing the MAP sensor. Do so by unhooking the positive battery cable for at least 15 minutes, then check codes again after the engine is up to temp.

2: The new MAP you bought is bad.

3: You have some vacuum leaks around the upper manifold, whether its cracked hoses or a torn gasket or something else.

As for the code 41, I'd guess the replaced section of exhaust is probably to blame for that as well, but it also depends on how old the existing sensor is. If it's original or has at least 50,000 miles on it, it's time to check its accuracy. But if its brand new then it's possible the computer hasn't adjusted to the new voltage output of the sensor.

 

If there are no codes indicating a problem with the MAP sensor then why are you going down this path? Personally I like the direct Hertz readout values listed here: Ford Fuel Injection Manifold Absolute Pressure Sensor (MAP)

If you have two sensors reading out of range then I would suspect your testing method/equipment is the problem.

 

+1, stop wasting your time and fix the real problem... the leak in the exhaust system.

 

....I must've misread something because I swear I saw something in his post about a MAP code....