I know that "throwing parts at it" frequently doesn't work that well, but I figure in the long run, I still come out ahead - especially when considering what I would pay a shop or a dealer to fix something. I am also aware of the problems in diagnosing these older OBD1 vehicles...sometimes you take your chances.
I am 'probably' going to change out the MAP sensor - IF I can finally rule out the fuel pressure issues.
KOEO - key on engine off, as opposed to KOER - running...
I don't remember the pressures dropping with KOEO before changing out the pressure regulator???
Dead giveaway is having a vacuum line to the pressure regulator that eased pressure at idle. Opening the TB would decrease vacuum so the pressure would go up.
I have a 82 27' Pace Arrow that gets 12 mph when keeping up with traffic, like 72 mph, but if I hold it to 55 gets more like 15 mpg. Has a carb, had to add an electric fuel pump in series with the engine driven one to suck gas up from that 80 gallon fuel tank way in the back. Trying to sell it now, we hardly use it.
Went nuts last year and purchased a 20' Volvo IO, almost August and still sitting in the garage, had severe cold weather in June, wife is either on call, or raining on the weekends. Was nice last Sunday, but had to help my daughter move back home. Always something.
Least by having the motorhome, used that last Sunday to help my daughter move, saved money renting a truck, but recall the reason I sold my last boat, same reason, lucky to get to use it once a year. But then I was working full time, thought I would have more time being semi-retired. That was a joke.
Was trying to say in too little words, MAP may be either the culprit or the victim. Its the victim if receiving poor voltages or poor vacuum from the intake manifold that can be caused by several different problems. Its the culprit if receiving an accurate vacuum, has good electrical connections, but outputs either no pulse or one loaded with jiggers like mine did.
At idle, your engine should show 20"/Hg of vacuum at sea level, will drop about an inch for each 1,000 feet above sea level. When revving the engine it should smoothly drop as you gradually increase engine speed. Would check that first.
I am getting 155.1 HZ here in OK (1200 ft)...and the Hz's change with vacuum applied, according to the chart. BUT, I am doing this with a Multimeter and I guess there still could be "jiggers" in the movement.
I also tested the MAP following this procedure from the following PDF:
This approach allows you to test the MAP with the motor running, and according to this document the Hz's should drop to the 90's when you start the car and mine only dropped to 104-118 Hz...
I'm just trying to test this thing as thoroughly as I can before just going out and buying a new one. I have read about Ford MaAP sensor 'drift' and it sounds like what I have going on, but the the sensor "appears" to be testing out good!
The frequencies you found seem flakey to me, if I take the values in your link at face value. At 1200' elevation you should have less than 152 Hz with KOEO, not more (155). At idle it drops to 104 - 118 Hz? That's a pretty broad range. 104 would be about 18.8" Hg, OK for idle; but 118 would be 13.3" Hg, not good for idle in or out of gear. Lots of an engines operation is keyed to the manifold pressure; spark timing, EGR operation, fuel injector pulse-width, acceleration enrichments, fuel pressure, etc. That's why I think it would be useful to observe the manifold vacuum as the engine is stumbling. Yeah, a mechanical gauge will dampen out rapid fluctuations, but I'm pretty sure the PCM software does too.
OK, but that new chart contradicts your previous link; I really don't know what it is supposed to be. MAP will certainly not decrease 3 psi per 1000' of altitude; I think NickD gave the correct value earlier in this thread. Regardless of the calibration, idle should produce a consistent MAP reading if the idle running isn't erratic.
See the frequency goes down with a vacuum increase, I didn't delete the Kpa column, just ignore it. This is a different year than the vehicle I was working on, and using a hand vacuum pump, see where I said 95 Hz at 20"/Hg where they are saying 104 Hz and 159 Hz with no vacuum applied where I said 155 Hz. So close, wouldn't worry about it, PCM makes corrections anyway.
Also noted this guys graph contradicts his chart, graph shows an increase in frequency with vacuum, chart is just the opposite, but whatever it is, it is.
Key factor is, is the frequency breaking up, can see where if it does, will drive the timing advance, A/F ratio, and the EGR crazy. Barometric pressure in the USA is measured in inches of mercury, not psi. But what's the difference, just want to get rid of that stumble.
Would know if I had it on my bench and can test it, even have an environmental chamber to test it over a temperature range.
Did have a Ford shop manual, crazy, couldn't find the test specifications anywhere in that manual, assumed it outputted a linear voltage. Was one of those step by step hierarchical manuals. Like, your vehicle won' start. Do you have a battery? Yes, go to the next step. No, go to section A13 under subsection 2b. Was over two hundred pages of that kind of stuff with no page numbers, drove me nuts.
Also I only posted the specifications for one MAP sensor, when we would evaluate a competitors product, would get a hundred to establish a mean. Don't worry, we didn't pay 100 bucks each for these things, more like a couple of bucks each, we had connections. I don't have those connections anymore, so have to pay a hundred bucks.
I always start off with a compression test, normally followed by decarbonizing engine before digging into the electronics. Read in many of these library fix it yourself manuals, you are looking for one problem. These guys are nuts, typically a chain of problems. But once in a great while, its just one problem.
I don't mean to belabor a previous comment, but to prove the MAP sensor is OK you would have to observe a vacuum gauge while the engine is stumbling and observe an oscilloscope trace of the MAP output. If the two correlate, and the gauge is tee'd in close enoguh to the MAP sensor, and the gauge is fast-acting enough (which may mean using a second "known-good" electronic gauge and 'scope system!); then the MAP sensor could be considered OK. Get it? You will need a good (like new, OEM) MAP sensor and a dual-trace 'scope that you can take driving with you. It's probably more practical to just install that new MAP sensor. But first, take a look with a simple mechanical vacuum gauge (while driving!) to be sure there isn't something obviously wrong with vacuum as it stumbles. The MAP sensor can only react to what it sees.