OBDII 3.8's

MustangJake88

Registered User
Figured I would hop on here and see if maybe you guys would have an idea of what the hell is going on with my new DD. Picked up a '96 3.8 Mustang a few months back, very nice car, great gas mileage, seeing as there were NO Super Coupes in my area for sale, at least ones in good shape. I still kick myself for selling my '89, but anyways. I picked up this car, it ran well when I first got it. Then ran into some sputtering issues and a rough idle. Changed plugs, wires, coil pack, fuel filter, O2's, and swapped out to a K&N. Also checked around for vacuum leaks as well. Still doing the same crap. It does it off and on. Car will hesitate and sputter extremely badly, and even have intake backfire, until I put the pedal to the floor and it kicks out of it and spins the tires. I have no other ideas on what it could be besides maybe a clogged cat or injector. Any ideas? Been tracing this problem for months and still no luck.
 
Also did a fuel pressure check and it checked out at 30psi at the rails.

30psi when?

While it was sputtering? At idle?

30psi is the barest MINIMUM while engine is running according to the Ford Service Information DVD.

37-45 PSI is what you should have during KOEO.

Here's the table in textual form:


FUEL PRESSURE SPECIFICATIONS Description
3.8L
4.6L (2V)
4.6L (4V)

Engine Running
207-310 kPa (30-45 psi)
207-310 kPa (30-45 psi)
207-310 kPa (30-45 psi)

Key On, Engine Off
255-296 kPa (37-43 psi)
255-296 kPa (37-43 psi)
255-296 kPa (37-43 psi)

RwP
 
30psi when?

While it was sputtering? At idle?

30psi is the barest MINIMUM while engine is running according to the Ford Service Information DVD.

37-45 PSI is what you should have during KOEO.

Here's the table in textual form:


FUEL PRESSURE SPECIFICATIONS Description
3.8L
4.6L (2V)
4.6L (4V)

Engine Running
207-310 kPa (30-45 psi)
207-310 kPa (30-45 psi)
207-310 kPa (30-45 psi)

Key On, Engine Off
255-296 kPa (37-43 psi)
255-296 kPa (37-43 psi)
255-296 kPa (37-43 psi)

RwP
It was checked while idling. I'm starting to think maybe the fuel pump is having some troubles. It's starting to act up really badly when fuel is at quarter tank or lower.
 
30psi when?

While it was sputtering? At idle?

30psi is the barest MINIMUM while engine is running according to the Ford Service Information DVD.

37-45 PSI is what you should have during KOEO.

Here's the table in textual form:


FUEL PRESSURE SPECIFICATIONS Description
3.8L
4.6L (2V)
4.6L (4V)

Engine Running
207-310 kPa (30-45 psi)
207-310 kPa (30-45 psi)
207-310 kPa (30-45 psi)

Key On, Engine Off
255-296 kPa (37-43 psi)
255-296 kPa (37-43 psi)
255-296 kPa (37-43 psi)

RwP

I think 30 psi while idling is okay. The FPR is set to 39 psi on most Ford vehicles from this era. That would match up approximately with the KOEO pressure spec. 40 psi +/- 3 psi gives you 37-43 psi.

But the FPR is referenced to vacuum, so if you apply the idling vacuum:
18 in Hg ~ 9 psi.
39 psi - 9 psi = 30 psi.

So to me, 30 psi while idling sounds correct. (Assuming that the engine really is pulling about 18 inches of vacuum.)
 
I think 30 psi while idling is okay. The FPR is set to 39 psi on most Ford vehicles from this era. That would match up approximately with the KOEO pressure spec. 40 psi +/- 3 psi gives you 37-43 psi.

But the FPR is referenced to vacuum, so if you apply the idling vacuum:
18 in Hg ~ 9 psi.
39 psi - 9 psi = 30 psi.

So to me, 30 psi while idling sounds correct. (Assuming that the engine really is pulling about 18 inches of vacuum.)
But 30PSI being the bare minimum kinda makes me wonder. I know the car sat for a little while before I bought it. I've ran numerous cans of Seafoam through the fuel and vacuum system. I'm guessing either one of the injectors is not firing or is stuck open. Or could be a fuel pump issue. Either way, it's annoying!
 
But 30PSI being the bare minimum kinda makes me wonder. I know the car sat for a little while before I bought it. I've ran numerous cans of Seafoam through the fuel and vacuum system. I'm guessing either one of the injectors is not firing or is stuck open. Or could be a fuel pump issue. Either way, it's annoying!

I don't think the "engine running" pressure values should be looked at as a minimum to maximum for any running condition. Rather, that set of numbers covers ALL driving conditions, from idle to WOT. Whenever you have the most manifold vacuum is when you should have the lowest fuel pressure, by design. The time when you have the most vacuum is coast-down from speed in gear, and the time with the next most vacuum is at idle. So, as long as you have a good amount of vacuum at idle, like 18 inches, I think 30psi at idle is not "the minimum", but rather "correct". Now, if you had 30psi at cruise or at WOT, well, you would have big problems.

The other set of numbers ("key on, engine off") is a range for one specific condition. Now that, I think, should be read as minimum to maximum. Can you check the fuel pressure with the engine off and compare that to the range Ralph provided?

Please note that my conclusions are based on the assumption that the pressure differential across the FPR is 39 psi, which is correct for the cars featured on this site. If the Mustang FPR is set differently, then my numbers would be incorrect. But I think it was a standard used across the line of Ford vehicles until they switched to returnless fuel systems, and maybe even still in use since then.
 
I don't think the "engine running" pressure values should be looked at as a minimum to maximum for any running condition. Rather, that set of numbers covers ALL driving conditions, from idle to WOT. Whenever you have the most manifold vacuum is when you should have the lowest fuel pressure, by design. The time when you have the most vacuum is coast-down from speed in gear, and the time with the next most vacuum is at idle. So, as long as you have a good amount of vacuum at idle, like 18 inches, I think 30psi at idle is not "the minimum", but rather "correct". Now, if you had 30psi at cruise or at WOT, well, you would have big problems.

The other set of numbers ("key on, engine off") is a range for one specific condition. Now that, I think, should be read as minimum to maximum. Can you check the fuel pressure with the engine off and compare that to the range Ralph provided?

Please note that my conclusions are based on the assumption that the pressure differential across the FPR is 39 psi, which is correct for the cars featured on this site. If the Mustang FPR is set differently, then my numbers would be incorrect. But I think it was a standard used across the line of Ford vehicles until they switched to returnless fuel systems, and maybe even still in use since then.
I'm pretty sure the Mustang FPR is the same as most Fords. But when I get the chance I'll run another pressure check on the car. It's weird because like I said, it sputters and coughs only when the engine isn't to normal running temperature, and if I put the pedal to the floor it instantly kicks out of it's little hiccups.
 
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