Got an odd one here, which looks like a PO hack.
To begin with, I have checked for codes, and I do not have any...

The MAP sensor is pretty simple, having a hose from the manifold directly to it to measure manifold vacuum. At some point, the PO or a shop added a T to the line, that goes to valve which adjusts the amount of vacuum the MAP picks up. The valve has a fuel filter on it to prevent dirt from getting into the engine.
If I close off this valve, then the engine idles roughly. If I open it up a bit, the idle smooths out. Basically, vacuum to the atmosphere. If I plug the end of the filter, the idle roughens up.

I'm guessing it was added when the air pump was removed, but I'm not sure why.

Here's the problem, starting is a bit of a pain; only starts on the second try. And, if I turn the AC on, the engine will die. I changed the IAC, but that made no difference. So now I am realizing that the air pump delete was probably not done correctly.
I want to get rid of this hack, and solve my start issues. Anyone seen this before, or have an idea why the PO did this? How would I fix it correctly?

Thanks

 

Can you post a pic of this "hack"?

I've never heard of this, maybe the wiser members here can interpret the PO's design as well.

 

What engine and year is the truck?

Sounds like the engine is running lean, and that valve is a vacuum leak to compensate(fools MAP into thinking engine is at load with lower vacuum to give more fuel).

I would get rid of that "T" in the MAP vacuum line and see how the engine runs. And diagnose from there.

Could be a MAP sensor out of tolerance and that vacuum leak brings it back(not correct way of fixing though). Check fuel pressure, it could be low and causing this. Also check for any vacuum leaks, that too can cause issues like this.

 

Truck is an 88 F350 with the 5.8 and manual trans.
Here's a picture of the hack:



I colored the vacuum lines in so they would be easier to see.

I did remove the T, and this caused the engine to sort of lope at idle. So I put it back on, and by adjusting the air flow (amount of vacuum leak), I can get a nice smooth idle. I did wonder if the MAP was bad, and this was to compensate, but it sure seems like the hard way to fix it...
While trying to figure the reason for the hack, I found two vacuum lines that were plugged with golf tees, which I think are part of the old thermactor system.

Just got a fuel pressure gauge, and will be checking that today.

Would a lean condition cause the engine to die when the AC is turned on, even though the IAC is new?

 

Yea I'd remove that hack job fix it right, removal of the air system the pump and related plumbing isn't a problem or reason for it. Just don't want any vacuum leaks, if removed and vac line/s are plugged that would be correct.

Does sound related to the IAC but couple questions, happen to be equipped with a tach, what is its idle speed?
When starting it does it help if holding throttle part way open? With engine sitting at idle, unplugging the IAC harness connector have any effect?

 

"adjustable manifold pressure"

Now THAT ^^ is funny. Best hack mod ever. That's even more comical than the resistor that replaces the IAT sensor to fool the PCM into running richer.

I would clear codes, get rid of that whole setup and see how she runs, and THEN read the fault codes again.

 

With the hack removed, the idle sits around 720rpm, and the engine runs rough.
With the the vacuum leak, the idle sits around 770-800, and the engine idles smoothly.

I am going remove the hack, and check the codes, and I'll try and test the MAP sensor as well. Going to check the fuel pressure at the same time.

For my clarification, the hack is forcing the computer to richen up the air/fuel mixture. So, the system must be running lean otherwise, which can be caused by another vacuum leak somewhere, like the intake manifold, low fuel pressure, or a bad MAP sensor. Is there any other common lean mixture causes to look for if I don't get any helpful codes?

I did look up new MAP sensors, and they run ~$100 for a Motorcraft brand, so I guess I can see someone adding the hack to avoid the expense of a new sensor.

 

What the hack is doing is giving the MAP sensor a slightly higher pressure, making the computer think the engine is under more load, which adds fuel.

This suggests you have a lean condition. This could be caused by a variety of things:

Low fuel pressure
Clogged (or even wrong) injectors
Bad fuel filter
Weak fuel pump
Bad O2 sensor
Bad MAP sensor.

I'd start by pulling the codes, changing out the O2 sensor and checking the fuel pressure.

 

Doesn't matter where a vac leak happens to be created to lower engine vacuum, the MAP reads it the same regardless.

Yea a drive cycle or two would help see what kinda codes it does generate, a starting point.

Be worth a look at the spark plugs too after drive cycle, now and post "hack" removal would likely shed little light on the subject and with any luck backup what the computer returns.

 

Exhaust leak is another cause. Anything letting excess air into the system before the o2 sensor. Which could also be the air pump system but since that was removed.. Are you sure that all the lines for that when it was removed were plugged and/or aren't leaking?

Also are you getting a lean code? If not, and you are sure its running lean, then check out the o2 sensor and its wiring. If you aren't sure its age, replace the o2 sensor anyway with a good Bosch or Motorcraft unit.

 

OK, got a bit of an update.

I changed the MAP sensor, and removed the "hack". The idle is now fine, so the MAP sensor was bad.

Next, I checked the manifold vacuum and fuel pressure.

First, manifold pressure is about 16 at idle, drops, then jumps to 24ish when I rev the engine. Maybe a little low at idle?

And the fuel pressure; static pressure starts at 20psi, but will drop down. Idle fuel pressure is 34psi. With the vacuum removed from the fuel pressure regulator, the fuel pressure jumps to 40psi. Rev or high idle pressure is 41. When I shut the engine off, the pressure drops fairly quickly down to zero. If I cycle the key to run a few times without starting it, the pressure jumps up to 40.

It appears that I have either a bad check valve in the high pressure pump, or really bad injectors.

Note, the exhaust does smell a bit rich, but I don't get black smoke when starting. Also, regardless of cycling the key or the new MAP sensor, the engine still starts on the second or third try.

Besides the fuel pump or injector possibilities, does anyone see any other possible issues?

Oh, I didn't check codes today, but I have in the past, and didn't get any issue codes.

Thanks