Can a badly mis-firing plug cause a fuel pressure drop?

I have a 2000 E-150 with a 4.6 with coil on plug ignition. There is a bad stumble / misfire when accelerating. The problem is most noticeable when the throttle is slightly open as when driving in city traffic. If
the engine downshifts the stumble is gone or at least much less noticeable. There is never a check engine light.
I took it into one of the major tune-up chains in my area and they diagnosed the problem to be a bad fuel pump because they are seeing that the fuel pressure is low (35 PSI) and drops off to almost 0 when the engine stumbles.
This van has only 36,500 miles on it and drives fine except for the miss under acceleration. The van has a new fuel filter and I have always kept the tank above 1/4 tank since I bought it new, so I am skeptical that the fuel pump is bad already.
My question is - if I had a bad spark plug or coil causing the misfire, would the ECM toggle the fuel pump to eliminate damage to the catalytic converter?
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no, but if the fuel pressure is insufficient under acceleration, you would have a stumble...............

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Oh, come now... it can't be that simple, can it? A tech said the fuel pump is bad so it MUST be something else... never mind that fuel pressure leaves home when the problem occurs....
Sarcasm mode = <OFF>.
I have aboslutely why the original poster is refuting the diagnosis.... looks pretty cut and dried to my weary old eyes. Original poster... look at the FACTS. If there is no fuel pressure when the problem occurs, there must be a problem with the fuel system. There's a very good chance that it is fuel pump but there are other factors that come into play... most will require the removal of the pump to verify.
You have a problem with it being a pump and I have a problem with it being a plug - especially in the abscence of a misfire code.....

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Jim Warman wrote:

information here. I am only looking to get an unbiased opinion.

had a problem with an electric fuel pump at such an early age (I bought it new on 2001) and low mileage. I was hoping that somebody out there knew if the ECM would shutdown the fuel pump on a sever misfire. A friend on mine's catalytic converter started on fire a few minutes after his SAAB started running badly.
I know that the Ford ignition is quite complex. If the ECM can shutdown the fuel pump, replacing it would be an attempt fix the symptom and not fix the real cause of the dropped fuel pressure. It just seems that if the pump was bad, it would cut out at high seed too, not only at city driving speeds.

pump and strainer yesterday and the drive home was proof that the problem is not fixed. I will be going again back today...

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Can't say for sure-monitor fuel pump voltage to check your theory...
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"Mike J" wrote

The fuel pump provides pressure, unless the pump is bad, the inlet is plugged, or the power is cut off, either by the ECM or a poor connection somewhere.
The fuel pressure regulator maintains that pressure, unless it's faulty.
As "Rick" suggested, monitor the voltage at the pump.
Also check the fuel lines (plastic?). If there's one out of place, the rocking motion of the engine as you apply power could squish it and cut the flow.
Check the wiring too. I've read other posts of a loose battery cable causing problems as the engine motion makes/breaks the connection.
The problem is figuring out the proper question: - Does the stumble cause the loss of pressure? - Does the loss of pressure cause the stumble? - Does something else cause the stumble *and* the loss of pressure?
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Good information, thanks.
MasterBlaster wrote:

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I should also have mentioned to monitor current just in case the fuel pump and/or wiring is faulty...
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Mike -
Just went through the exact same thing on my 99 F150 4.6, I didn't think it could be the plugs because they had only 30K miles (if that) on them...However, that's where I was going to start one thing at a time to narrow down (next were wires, fuel filter, fuel problems). I must have had a plug go bad, because as soon as I changed it was fixed. Plugs are cheap, so I'd change them out to start....Again, this is my opinion anyway....There are a lot more knowledgeable people here....

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Fuel pressure gages are cheap, too. At least Autozone carries one that only costs about $30. Handy thing, it is.
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No need to shut down the fuel pump, the PCM controls the individual injectors. Are you sure you had a bad coil pack? Ford recently issued a TSB on coil pack diagnosis and mentioned that over 50% of the coil packs returned under warranty as bad were not actually bad.
Ed
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the coil boots are a way more common problem than the coils them selves (Standard Ignition carries them cheap) At my shop we replace them every time we do plugs. The computer only turns the fuel pump on and off and does not turn the fuel pump off for a misfire. The vans newer than yours Ford actually turns the injector off to a confirmed misfiring cylinder, logs a code, but because no unburned fuel is entering the exhaust (hydrocarbons) emitions are not effected and the Cat is in no danger it does not turn on the check engine light for the sole purpose of an unsuspecting person not noting the vibration and power loss and not returning for warranty as explained in a class I went to. fuel pump makes fuel flow, fuel pressure regulator restricts the flow to create pressure, and the computer controls how long and when the injectors are open. beg/barrow a fuel pressure gauge. must be a little since I hope to why they though fuel pump, I hope

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What!????!!!!!!
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wrote in message A misfiring plug will cause the oxygen sensor to detect a too lean condition because too much oxygen would be in the exhaust gases. This would turn on the Check Engine light. The oxygen sensor only measures oxygen in the exhaust so the cause of the condition could be not enough fuel or no ignition at all.
--
Earl F. Parrish



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To put it another way, the fuel pressure in the line would not be drastically affected even if the PCM DID momentarily shut off the fuel pump when it saw a misfire. And, as noted, this would be readily apparent by looking at the pump relay coil voltage.
I cant think of any reason for this OTHER than the pressure dropping first.
Unless it was actually backfiring into intake, in which case the FP gauge needle might bounce from very high to zero for an instant. Highly unlikely.
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