I have a 1985 Ford F150 Lariat, and I post frequently over in the 1980 - 1986 forum often. The problem I have is with my wife's 2001 Ford Mustang V6. The car has 90,000 easy miles on it, and the problem is mostly during acceleration. Lately it "stumbles" whenever I give it gas, usually in 2nd gear or when passing. If I keep the gas floored, it seems to straighten out of it until I try again. The car never stalls, just "stumbles." Occasionally it will "stumble" when it is first driven off, like its a fuel problem. Its been doing this for about a month now.

There is no "Check Engine Light" or anything else that I know of to indicate an electrical problem. I ran a can of Seafoam through the gas tank and the engine, but it doesn't seem to help. I sprayed out the throttle body, but it looked fairly clean to begin with. Fuel filter was changed last year, along with the spark plugs. Air Idle Control valve went bad last year and I replaced it with a Motorcraft part.

I just don't know what else to look for to get this otherwise dependable car running as it should again. I worry it is going to leave my wife stranded if I don't find out whatever is wrong with it. Does anyone have any ideas?

 

check the fuel filter.

 

Go to AutoZone or whatever chain auto store is near you and get them to check for any codes that might have come up. If there are no codes, then that helps narrow it down.

 

Closely inspect all the flexible air intake pipes for cracks/loose clips. Crack may be opening up when engine torques under acceleration.

 

Just a guess, but maybe the throttle position sensor. It kinda sounds like what my truck was doing and it turned out to be the tps.
Also check for vacuum leaks, those crappy plastic lines are known for cracking and breaking.

 

Thank you gentlemen.

Fuel filter? You mean I have to drop the fuel tank AGAIN? I replaced the fuel filter just last year!

The CEL has never been on, so pulling codes would be a waste of time, right?

I checked and re-checked all the vacuum lines I could find, including the air duct, and found nothing.

The Throttle Position Sensor sounds like a good guess, but wouldn't the CEL come on? Do you happen to know where this sensor is usually located?

 

The TPS is right on the the throttle body, it's the only thing there with wires on it.
i have no idea if the cel would be on or not.

 

I wouldn't assume that just because the check engine light isn't lit, that there are no codes. Rule them out, and you won't have to worry about them anymore. They'll read them for free, so a small investment of time is all you will be out.

 

A bad EGR valve or possible a bad coil might do it too.Allot of times if the car surges during idle a bad TPS would do that.

 

Pulled the codes Saturday, and NOTHING!

I sure would hate to just throw parts at it without knowing what the problem really is.

Is there a way to check the TPS before buying a new one?

 

Double post...sorry.

 

Warning -- following are just my comments -- I'm by no means an expert.

You can meter the TPS and look for dead spots or jumps. It's just a resistor, and the resistance varies with the throttle position.

Would probably be best to use an analog meter because it's easier to see small areas of bad contact or whatever.

But, I understand that they can be bad and not really show it.
- Good news is that tps parts are usually cheap and easy to change.

You might try the old vacuum gauge tests just to see if something odd shows up. I have to search the net for a reference all of the time, but there are a lot of them out there. Could show something.

It does sound like a fuel problem, but it's hard to say. Most Fords have a fuel pressure regulator that has a rubber diaphraghm controlled by a vacuuum. You could pull the vacuum line to check for any gas ( leaky diaphraghm).


Good Luck

 

In the 99+ v6 motors, the fuel pressure is controlled a little oddly, as there's only one fuel line running from the tank up to the motor. Instead of having a regulator with a return line, the motor varies the fuel pressure based off of a small sensor mounted on the fuel rail, the RAPS - rail absolute pressure sensor.

What it does is measure manifold vacuum on one side, and fuel pressure in the rail on the other. It adds them together, and sends the signal to the car's computer, and the computer reacts by varying the current to send to the fuel pump.

Basically, if you're pulling 10psi vacuum, and the rail has 30psi pressure, it reports a signal of 40psi absolute to the computer. It reads 40 (NOT 20!!!) because the vacuum (negative pressure) is effectively helping to suck the fuel out of the rails, while the pressure in the rails is pushing it out.

At WOT, when the manifold has 0psi, and the rail still has 30psi, it reports 30psi to the computer, and the computer reacts by increasing voltage and current until it sees 40psi absolute again. (Which in this case would be 40psi rail pressure).

If the car has 10psi boost (supercharged) and 30psi in the rails, it reports 20psi absolute to the computer, because it has to overcome manifold pressure to squirt fuel, instead of vacuum. In this case, the rail pressure would increase to 50psi to compensate.

The RAPS are prone to go bad/intermittent, especially if the car is super/turbocharged. It's located on the driver's side fuel rail at the very front - looks like a little black box mounted on a small flange with an electrical connection (2 wires?) and vacuum line plugging into it. I'd check that line first, make sure it's all connected properly.

~Brad

P.S. - Cliffnotes - check for vacuum leaks!

 

***UPDATE***

The "Service Engine Soon" light finally came on this week. Along with it more consistent engine hesitation. I pulled the codes again, and this time, code PO301 showed up, or "misfire at cylinder #1."

I pulled and checked the spark plug at #1, and it was fine, as all of them were replaced a few months ago. I reset the codes and the problem remains. I pulled the spark plug wire at the bundle(pack? rail?), and there is no spark there at all, and the engine didn't notice the difference.

Before I go out and buy a whole new set of plug wires, could it be anything other than a bad spark plug wire, like a bad fuel injector?

 

Could be a bad coil pack or the wire itself. Check the pack and see if it is firing. I'm assuming this has multiple coil packs and not a distribitor. Might also check the wiring in to the pack aswell. It may be an elctrical bug you're after now. I'm not an expert on this but you should have one wire with constant 12vdc. Best thing to do now is get a haynes or chiltons manual for all the wiring colors and troubleshooting.