Runs bad off idle

[Flash]

'97 LTD SOHC V6.

I just had the engine replaced, it idles dead smooth and runs good with a lot of throttle but stumble very badly coming off idle especially if you're gentle with the throttle.
It won't cruise either.

I replaced the IAT sensor and the TPS switch and cleared the codes, it now shows no codes but is driving just as badly.
Well, it drives a little better I don't have to mash the throttle as much to get it going.

When it stumbles the exhaust blows black smoke so it's running rich.

I just had a thought; is the fuel pressure regulator vacuum controlled?

 

[Flash]

If anyone's still listening or cares, I found the problem.

The vacuum line that goes from one side of the bulkhead to the other had about 6 inches missing from it.
I replaced that and repaired a thin vac line going to some vac pot at the front of the engine and now it runs fine.

I would have thought that that much of a vacuum leak would throw a code.

 

[2000StreetRod]

rail mounted FPR

The fuel pressure regulator (FPR) on your vehicle is mounted on the fuel rail and connected to a vacuum hose. There is a fuel return hose to the fuel tank.

If the hose to the FPR is disconnected/broken then the FPR raises the fuel pressure to maximum as if the throttle were at WOT. The high fuel pressure from the FPR may have compensated for the unmetered air that entered the intake manifold preventing the PCM from setting a DTC. The PCM in your model has no sensors for manifold vacuum or fuel pressure.

 

[Flash]

The fuel pressure regulator (FPR) on your vehicle is mounted on the fuel rail and connected to a vacuum hose. There is a fuel return hose to the fuel tank.
View attachment 78409
If the hose to the FPR is disconnected/broken then the FPR raises the fuel pressure to maximum as if the throttle were at WOT. The high fuel pressure from the FPR may have compensated for the unmetered air that entered the intake manifold preventing the PCM from setting a DTC. The PCM in your model has no sensors for manifold vacuum or fuel pressure.

I remember seeing the FPR all bolted up snug with the vac line going to it.

What is that vacuum pot at the front of the engine on the passenger side for a LHD vehicle?
It has a thin, grey vac line coming around from the back of the engine.

 

[2000StreetRod]

EGR valve?

I'm not familiar with your driving configuration but I suspect you're referring to the EGR valve.

 

[Flash]

You know what else causes a hesitation/stumble/lurch at odd times, not at all predictable?

A bloody great tear in the side of the number 1 plug lead!