Misfires at about 40mph espcially uphill, runs ok otherwise

Tiny
DAVID ANDERSON
  • MEMBER
  • 1999 FORD ESCORT
  • 141,000 MILES
I replaced fuel filter.
I get no trouble codes.
I have checked all the grounds and battery voltage to the FPDM under load and all are good.
Putting a screw driver to the fuel injectors and listening to them, they all seem to be working ok. The resistance measured at the injectors are at specification.

I checked fuel pressure: koeo I turned key on and off several times, each time the pressure would rise - started at about 10psi and went up to about 40psi or so. The voltage across the fuel pump from the FPDM is about 10.8 V DC peak value on an oscope, with an average value of about 8.5 to 9V DC read on volt meter. Manual says that the pressure with engine not running should be 50 to 85 psi. If I put the battery across the pump motor I do get 53psi.

At idle I get about 33psi, and if I rev the engine this goes up. The voltage at the pump is about is about 8.5 to 9 VDC with meter. When driving on highway the voltage goes up over 10 sometimes. If I remove the vacuum from the pressure sensor the pressure rises.

Chilton manual says that if the KOEO voltage to the pump is over 10.5 volts or so and the pressure isn't at specification replace the pump. If not the problem is elsewhere.

So my problem is should I replace the fuel pump since the engine off pressure is not at specification unless I put battery right across the pump, or should I try replacing the FPDM since it doesn't seem to be putting out voltage specified by Chilton. Any advice or suggestions are appreciated.
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Tuesday, May 7th, 2013 AT 8:24 PM

9 Replies

Tiny
SATURNTECH9
  • EXPERT
Have you checked the plugs wires etc yet?
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Wednesday, May 8th, 2013 AT 7:02 AM
Tiny
SATURNTECH9
  • EXPERT
Does the miss fire get worse with the ac on if the ac works?
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Wednesday, May 8th, 2013 AT 7:03 AM
Tiny
DAVID ANDERSON
  • MEMBER
I checked the plug wires and they are within spec. I haven't really checked the plugs but I checked them last fall. I haven't run the AC in a long time, I'll try it and get back to you. I forgot to mention that I thought it might be the transmission since it often shifts about that point so I took it to a transmission shop and they told me that it wasn't the transmission and that was all they work with. They told me that I did have a leaking O ring on the 2nd gear servo that I should repair if I'm going to keep the car after I find out what is causing the misfire.
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Wednesday, May 8th, 2013 AT 8:53 AM
Tiny
DAVID ANDERSON
  • MEMBER
The A/C doesn't seem to effect it much. I forgot to mention that I checked the plug wires at night and all seem ok. What does help is if I give it a lot of gas when starting to misfire. It does misfire between 40 and 55 or so but seems worse at 40 to 45. I do have an actron auto scanner that allows me to view data and I can send the data at idle if you want.
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Wednesday, May 8th, 2013 AT 11:22 AM
Tiny
SATURNTECH9
  • EXPERT
I have ohmed out spark wires that were within spec and turned out to be bad. So I stopped even trying to test them that way. Have you looked at the outside of the spark plugs for carbon tracking?Also the inside of the spark plug wire boot where it goes onto the spark plug for carbon etc?
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Wednesday, May 8th, 2013 AT 5:38 PM
Tiny
DAVID ANDERSON
  • MEMBER
Ok i'll check the wires closely or get new one's in. Tomorrow. I take it that you aren't absolutely convinced this is necessarily a fuel problem with the engine off key on fuel pressure of only 40psi - unless I put the battery directly across the pump to get a pressure (58psi) which is barely within specification of 55psi?

Thanks for your help
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Wednesday, May 8th, 2013 AT 11:41 PM
Tiny
SATURNTECH9
  • EXPERT
What is the fuel pressure while it's running and acting up? That's what is important
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Thursday, May 9th, 2013 AT 12:29 PM
Tiny
DAVID ANDERSON
  • MEMBER
I replaced the spark plug wires. The car is fixed. Was almost positive that this was a fuel problem. LOL
Thanks a lot for your assistance.
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Friday, May 10th, 2013 AT 5:01 PM
Tiny
SATURNTECH9
  • EXPERT
Had a feeling that's what it was that's why I asked you to check them glad to hear you found the problem. Your welcome.
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Friday, May 10th, 2013 AT 6:08 PM

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