Code 41 And 91 Won't Go Away

[Mstng93SSP]

I'll still take the beer.........

 

[dz01]

scld1354 did you ever fix your problem? I'm in the same boat and trying to trouble shoot.

 

[scld1354]

Update..... still have the codes. I am covninced I do not have any vacuum leaks. Ive been driving it around a little bit lately and noticed the CEL only comes on during highway type driving and only after accelerating. I think this could be a "not enough fuel" problem instead of a "too much air" problem. As soon as the tank is near empty i will drop the tank and inspect the fuel filter. I might as well check the pump for delivery / volume.

 

[Cobra42898]

I like the FPR explanation, but one thing needs correction. Under vacuum, the fuel pressure output will be lower than 39 psi, and in boost it will be higher than 39 psi. A car under 10psi boost with 29 psi fuel pressure = kaboom. The term is delta fuel pressure, and it means the difference between manifold pressure and fuel pressure. It doesn't matter what the pressures are, as long as the delta is correct. This is why we change fuel filters, because it might flow fine at idle/low rpms, but at WOT/high rpms the clogged line can't flow enough to keep the delta pressure up and leans out the mixture. This is also why it's bad to have a FPR without a vacuum line attached (unless you're testing the fuel system).

Replace the fuel filter. Seriously, it takes 5 minutes. Then hook up a fuel pressure guage with the vacuum still attached to the FPR and go for a drive if possible. The pressure should start around 25-30psi and climb to near 40 at WOT. I had our 86GT idle fine but the car fell on its face in gear. Turned out the fuel pump partially failed and was only putting out 12-15psi. Enough to run, but not enough to do much else. New fuel pump, primed the pump a few times with the key, and it started and ran great the moment I hit the starter.

Tom

 

[jrichker]

I like the FPR explanation, but one thing needs correction. Under vacuum, the fuel pressure output will be lower than 39 psi, and in boost it will be higher than 39 psi. A car under 10psi boost with 29 psi fuel pressure = kaboom. The term is delta fuel pressure, and it means the difference between manifold pressure and fuel pressure. It doesn't matter what the pressures are, as long as the delta is correct. This is why we change fuel filters, because it might flow fine at idle/low rpms, but at WOT/high rpms the clogged line can't flow enough to keep the delta pressure up and leans out the mixture. This is also why it's bad to have a FPR without a vacuum line attached (unless you're testing the fuel system). Tom

Go back and reread the Tech note. It seems that you saw the 29 PSI and the word boost in the same paragraph and quit reading. This is a lot more that you either didn’t read or understand. "The regulator bypasses fuel back to the tank to maintain a constant 39 PSI to the injector tips.... The 39 PSI pressure is measured at 29.92 inches {or 14.7 PSI} of atmospheric pressure to get the proper flow rate. "

The key thing to keep in mind is the term effective pressure

As the pressure inside the intake manifold changes so does the effective fuel pressure at the injector tips. Less than 14.7 PSI in the intake manifold increases the effective fuel pressure. Greater than 14.7 PSI the intake manifold decreases the effective fuel pressure. Now that we know that vacuum or pressure changes the effective pressure at the injector tips, let's proceed onward.

I'm going to introduce a new term PISD. That stands for Pounds per Square Inch Differential. The proper definition is:

"The difference in pressure between two points in a fluid-flow system, measured in pounds per square inch. Abbreviated PSID or psid" (McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.).

The term PSID perfectly describes the situation we have here. In order to preserve the injector flow characteristics, we need to maintain the 39 PSID at the injector tips. That is done by applying pressure or vacuum to the back of the fuel pressure regulator. Pressure applied to the fuel pressure regulator increases the fuel pressure. Vacuum applied to the fuel pressure regulator decreases the fuel pressure. Keep that in mind along with the idea of effective pressure or PSID and you will have a better understanding.

When you connect a pressure gauge to the fuel pressure regulator, you are seeing PSIG. That letter "G" needs some explanation, so I will borrow a very good explanation from What is the difference between the units psi and psig? - Quora
Kim Aaron, Spacecraft Mechanical Engineer writes
When you stick a pressure gage on the valve stem of a tire on a car or a bicycle, the needle is reading the pressure difference between the air inside the tire and the air outside the tire. That's called gage pressure. The units of psig (pounds per square inch, gage) remind us that we are talking about this *difference* in pressure. The absolute air pressure at sea level is about 14.7 psi. Suppose the air pressure in your car tire is 30 psig. Then the absolute pressure inside is 30+14.7 = 44.7 psi. Sometimes, we write that as 44.7 psia (pounds per square inch, absolute) to remind us we are talking about an absolute pressure. In either case, we could just use psi. It just helps avoid confusion to use the psig or psia as appropriate to be clear which we are talking about