Guys this is will be a long post but trying to provide as much info as possible. I rebuilt a farm truck for my granddaughter that had about 200,000 miles on it. I went over the other day and it was running rough, ran compression tests and found one exhaust valve with a chip out on number 8 and number 7 exhaust was not sealing the best.
I took the block and heads to a machine shop and had all new valves and springs put in, exhaust manifolds planed and new pistons pressed onto the rods. I installed all new bearings- cam-rods-mains, high volume oil pump. Since the injectors were 30+ years old I installed them as well in addition to a new Accel coil to replace the original, new plugs, a new 3 slot timing chain set up timed to the 0 degree slot.
The engine fired right up when I got it back together, everything looks and sounded good. It idles good, it seem to be responsive and pretty smooth in the shop. When I drive it under a load it runs rough with a miss, not sure if its one or two cylinders or a general miss.
I rechecked the timing, pulled the SPOUT off, set to 12 BTDC plugged it back in and it advanced and advanced more when increasing RPM's. Put a new distributor cap on, put the old coil on no change, reinstalled the new coil.
Checked the fuel pressure 30psi with vacuum and 40 psi without, good fuel flow from filter. Ran on both front tank and back tank with similar results.
Vacuum is 18 inches at idle and pretty steady.
Spark plug wires are about a year old, I did pull them and cylinders 1,3 and 6 did not seem to make much of a difference at idle, weird because the idle is pretty smooth. I pulled number one and it was arcing great across the gap.
I pulled the MAP vacuum line and the engine stumbled so its sending a signal back. I did not check voltages, which I probably need to.
Had anyone had a similar issue, if so what was the fix. I appreciate any input.
Thanks,

 

Have you checked for codes? If not, start there.

 

Long post (good) but no reference to what year it is
Check the caps in the computer, put a spark tester on it and check the spark
See if it sparks constantly with no breaks in the arcing from the spark tester
Like the above post mentions, do a code test and I'll add to check the fuel pressure too
I'd also check the compression on the cylinders that make no difference when pulling a plug wire off

 

The truck in a 1991 XLT Lariat 4x4 and was a farm truck most of it's life. The air pump, EGR and Cat convertor have been removed years ago. I will have to read up on getting codes, I'm sure by the removal of the items mentioned there will be several codes from that.
Since it had set in a barn for several years mice were a problem as well, but it has ran very well for about a couple of years after getting it back on the road. Due to its age and idle time I replaced both fuel tanks and pumps/filter/pressure regulator, IAC when we were getting it back on the road two years ago.
I have a neon spark tester i can put inline at each plug, but that may be different than what you are talking about. Will check the compression and get back on that and will need to do some research on getting the computer out and checking the capacitors.
Checked the fuel rail pressure earlier and it was 30 psi and 40 psi with vacuum / no vacuum.
Appreciate you guys responding.

 

A Innova 3145($35) will come in handy if the plan is keeping it, or see below how to count flashes.

 

Great video on codes, I think I will order the Innova 3145, would make life easier. I did find out some things today. I checked the MAP and the signal voltage was constant at 2.5VDC no matter what the vacuum was. Put a new one in "Standard" but it read the same thing... I have another one ordered should be in tomorrow. With the connector unplugged I had 4.97 VDC across both the supply and signal wires and approximately 1 mVDC on ground. I briefly touched the signal to supply and it pulled to zero which should prove the path to the PCM. I did find a small crack in the vacuum hose feed to the MAP so I replaced that as well. Used a hand vacuum tester to fluctuate the vacuum but no voltage change on signal wire. This may not be all of my problem, but I think it's part of it.
I pulled all the plugs and all looked good, nice and white with only a few miles on them since overhaul except #1 it was black, compression on this cylinder was 150, which I'm thinking should be pretty good for a fresh overhaul, would expect this to improve a bit as it gets broken in. I may have an injector issue here. New wires and cap, rotor looks good...
Will confirm a good spark on this cylinder tomorrow...
Thanks again for the input.

 

The map sensor puts out Hertz on a 1991 Ford so measuring voltage is futile
Also
The processor only checks the map at initial startup on your 1991
No map sensor code basically means leave it alone, but do check the Hz and verify it is correct for your altitude if you want
* They sell Ford Hickok star testers online occasionally for cheap (look for a Superstar 2)

 

I pulled the codes using the check engine flash method, my codes were 22 ,31, 85 repeating. If I understand this correctly the 22 relates to MAP, the 31 to EGR and 85 to purge canister. The EGR is blocked off and purge tank removed, solenoid unhooked.
I was thinking on some of the earlier models they used a 3 wire MAP with a 5VDC analog input signal, I know the later ones were frequency/digital. I could sure be wrong on this, but the Standard MAP sensor has a lifetime warranty so I won't be out anything but my time changing it again.
Thanks for the info, I have a Klein meter that I may be able to check Hz on, will give that a shot later as well. Do you know what Hz I should expect to see at 900 feet elevation at KOEO and at idle @ 18" of vacuum?

 

159 HZ is seal level
89 is 15,000 feet
Yours at 900 feet will be around 150-154

 

Just checked the Hz on my MAP replacement with a hand vacuum pump with KOEO it was 153 Hz, with 10" 121 Hz and 20" 103Hz. Looks like this sensor is doing its job. Thanks for the frequency info, I was going down a rabbit hole thinking it was manipulating a voltage signal.

 

Fuel Injection Technical Library » Manifold Absolute Pressure Sensor (MAP) Test MAP

 

Hey guys good news, I had put all new injectors in when I did the overhaul thinking they would be an upgrade over the 30 year old ones. Bad assumption... I replaced one of the new ones on the number 1 cylinder with an old one and it helped quite a bit on power and smoothness under load, but still not perfect. I decided to make up a hose and air chuck dialed in at 50psi and wired a push button circuit up to manually cycle the injectors and run some solution through them. I got all of them working great with good patterns and installed them in the truck.
It's running great now, what a hassle. I hate to admit it but they were Amazon injectors, couldn't bring myself to pay 800 dollars at the parts store for them.
Thanks to everyone for their input always good to learn something new. If anything changes on this I'll keep you up to date.