My Ford Powerstroke is easily the best truck I have ever owned.
But not everything is wonderful in Powerstroke land. About a year ago my truck started running poorly. It had very distinctive symptoms. It
ran fine at lower rpm, but at high rpm (1800+), where you expect the turbo to kick in, there wasn't much power. It would go ahead and
accelerate, but slowly and without the usual snap. Since it is still under warranty I took it to the dealership (the only Ford dealer in town). They
couldn't get to it for a couple of days, but they made an appointment to work on it....Three weeks later, after many phone calls, picking up the
truck once and taking it back (wasn't fixed although they told me it was), yelling at everyone in the dealership including the owner, I finally got my
truck back running properly. They had finally replaced the MAP (Manifold Absolute Pressure) sensor and that fixed the problem. Funny that
trained diesel mechanics couldn't figure it out any quicker. Give me a week and access to the parts department and I guarantee that I could fix
it by changing parts at random.
About a month ago the truck started having the same symptoms. Did a little checking (disconnected the MAP sensor
and the truck ran the same) and decided that it was the MAP sensor again. A new one costs $125. Or I could take it to the service department
at the Ford dealer and only pay the $100 deductible on the warranty. But, if I take it to the dealer it might take another three weeks and lots of
frustration. So I did some research. The Ford-Diesel forums had a number of messages from
other people with MAP sensor trouble. Some of them got really inventive and built a circuit that bypasses the MAP sensor altogether (thanks Mick
and A. Smith). Mick has some pictures and descriptions on his web page and was kind
enough to answer some additional questions by email. There's also a link from Mick's page to a
schematic for the circuit.
Before I explain the simulator and what I did with it, maybe I ought to fill in a little background on the MAP sensor...
The MAP sensor reads the pressure in the intake manifold (through a hose) and sends a signal to the PCM (Powertrain Control Module) that
tells the PCM to adjust the injector pulse width to fit the amount of turbo boost. Essentially it controls the fuel/air mixture as the turbo boost increases.
That explains the symptoms when it quits working. At low rpm there's very little turbo boost, so the mixture is about right and the engine runs
OK. As boost increases as the rpm's rise, if the injector pulse width doesn't increase the engine gets a lean mixture and doesn't run properly.
The signal from the MAP sensor is a pulsed current (~5V) at about 100hz
(at idle). As the boost increases the frequency of the pulsed signal increases. The simulator circuit (left) takes the signal from the EBP (Exhaust Back Pressure)
sensor which is a constant voltage that increases as boost increases and converts it to a pulsed current that mimics the signal from the original
MAP sensor. It performs the same function as the MAP sensor (adjusts the injector pulse width so that fuel mixture is correct at all boost values).
It's a pretty simple little circuit and fits neatly into a small project box. I bought all the components from Radio Shack (for ~$25). Much cheaper than
replacing the MAP sensor.
The back of the circuit board even looks pretty good for me. The two potentiometers control the frequency of the pulse at idle and the slope of the
increase in frequency with increasing boost. It's necessary to adjust these to get the best performance. I pretty much followed the schematic from the
link above, but adjusted the values of some of the components. Partly because I couldn't get the exact values specified, partly to get exactly the
signal I wanted.
I used a length of telephone cable (four conductors) to
wire the simulator to the EBP sensor. The conductors are +5V, 0V, and the signal (~.87V at idle). The fourth conductor is used to run the signal back
to the MAP sensor wiring.
With the circuit all wired up (but not in the box yet) I borrowed a
portable oscilloscope and went to test it. The frequency of the signal has to be high enough (>95hz) to avoid getting a CES (Check Engine Soon)
light on the dash. Higher frequencies will give slightly higher torque at low rpm, but also affect throttle response. I got best results at about 110hz
I adjusted the gain (slope) so that the maximum frequency is about 180hz. Remember that this is an otherwise stock engine. These settings give
just a hint of color to the exhaust at full throttle. Pretty exciting trying to keep the truck on the road while accelerating at full throttle and still read the
output on the oscilloscope.
It didn't really take very long to dial in the simulator. A couple
of runs around the neighborhood and a few minutes out on an empty highway (I probably spent more time blasting up and down the highway than
was absolutely necessary). The truck runs GOOD (more below). When I was happy that it was going to work I went ahead and boxed up the
simulator circuit. Some day I may seal the box permanently, maybe set the circuit in epoxy for durability, and put the circuit under the hood. Right
now it's sitting on the floor in the cab.
The wires under the hood were all taped into place and when the vanity cover was replaced
over the engine the change is hardly noticeable....
...unless you see that the MAP sensor isn't plugged in.
So, final analysis?...I like the way the truck runs with the MAP simulator. The idle is smoother. With the stock
sensor the idle was rough enough to make the truck rock a little. With the simulator the idle is absolutely smooth. The kick at ~1800 rpms when the
turbo kicks in is gone. Yes, it's gone. Took me a little while to figure out. Then I realized that the kick is gone because the truck is already pulling
so strong that there isn't a big surge when the turbo takes over. I didn't expect the increase in low-end torque. The engine pulls strong all the way
to 3000 rpm. Performance is great all around.
What next? I really don't have any idea yet about longevity or durability of the circuit. I used Radio Shak components and I'm not sure of the temperature
stability of the circuit. Will it work the same this summer when it's 200 degrees in the cab? I hope so. I also don't know about fuel mileage. I suspect
it will still be pretty good if I can keep my foot out of it. The engine is running more efficiently so I can shift sooner and keep the rpms down. But it's
so darned much fun to stomp it and hear that turbo whine....
Remember, I'm a biologist not a trained diesel technician or engineer. I'm not providing advice or suggesting you make similar modifications to
your truck. You will at least void the factory warranty. I've just described what I've done for your information (and maybe amusement). If you blow
up your truck, fry your PCM, or injure yourself - don't try to blame it on me.
Addendum....I've actually removed the MAP simulator and replaced it with a stock sensor. The simulator
definitely gave much better performance (MUCH BETTER), but mine wasn't temperature stable. The frequency of the output changed as the temperature
of the circuit board changed. If you ran the AC or heater in the cab it would change the frequency and the truck wouldn't run right. It's partly my fault,
I used Radio Shack components instead of going to an electronics supply house and buying components with better tolerances. The simulator is
still a good idea, just not for me right now. Anyway, I figure I still beat Ford (I bought the sensor from the International dealer for $50 less than the price
at Ford).
The signal from the MAP sensor is a pulsed current (~5V) at about 100hz
(at idle). As the boost increases the frequency of the pulsed signal increases. The simulator circuit (left) takes the signal from the EBP (Exhaust Back Pressure)
sensor which is a constant voltage that increases as boost increases and converts it to a pulsed current that mimics the signal from the original
MAP sensor. It performs the same function as the MAP sensor (adjusts the injector pulse width so that fuel mixture is correct at all boost values).
The back of the circuit board even looks pretty good for me. The two potentiometers control the frequency of the pulse at idle and the slope of the
increase in frequency with increasing boost. It's necessary to adjust these to get the best performance. I pretty much followed the schematic from the
link above, but adjusted the values of some of the components. Partly because I couldn't get the exact values specified, partly to get exactly the
signal I wanted. 
I used a length of telephone cable (four conductors) to
wire the simulator to the EBP sensor. The conductors are +5V, 0V, and the signal (~.87V at idle). The fourth conductor is used to run the signal back
to the MAP sensor wiring.
With the circuit all wired up (but not in the box yet) I borrowed a
portable oscilloscope and went to test it. The frequency of the signal has to be high enough (>95hz) to avoid getting a CES (Check Engine Soon)
light on the dash. Higher frequencies will give slightly higher torque at low rpm, but also affect throttle response. I got best results at about 110hz
I adjusted the gain (slope) so that the maximum frequency is about 180hz. Remember that this is an otherwise stock engine. These settings give
just a hint of color to the exhaust at full throttle. Pretty exciting trying to keep the truck on the road while accelerating at full throttle and still read the
output on the oscilloscope. 
It didn't really take very long to dial in the simulator. A couple
of runs around the neighborhood and a few minutes out on an empty highway (I probably spent more time blasting up and down the highway than
was absolutely necessary). The truck runs GOOD (more below). When I was happy that it was going to work I went ahead and boxed up the
simulator circuit. Some day I may seal the box permanently, maybe set the circuit in epoxy for durability, and put the circuit under the hood. Right
now it's sitting on the floor in the cab. 
The wires under the hood were all taped into place and when the vanity cover was replaced
over the engine the change is hardly noticeable....
...unless you see that the MAP sensor isn't plugged in.