Fuel pressure unstable

Tiny
MARIOC68
  • MEMBER
  • 1995 TOYOTA COROLLA
  • 200,000 MILES
Hi, make a pressure measurement on my toyota, 5A-FE engine I see a tool reading. This situation is normal? When I accelerate the reading should I increase? In my case it is maintained. I send a couple of videos that show the exposed.
Do you
have the same problem?
Yes
No
Sunday, December 18th, 2016 AT 12:07 PM

5 Replies

Tiny
CARADIODOC
  • EXPERT
The fuel pressure regulator is adjusted by intake manifold vacuum. You have fuel pressure pushing fuel through the injector, and manifold vacuum pulling it. During coasting, vacuum goes up, and that would cause an excessively-rich mixture. Instead, the regulator responds by reducing fuel pressure. That lower pressure offsets the higher vacuum, so the net force working on the fuel molecules remains constant.

The opposite happens when accelerating. Vacuum goes down, so the regulator increases fuel pressure to again, maintain a constant force on the fuel. If your engine runs near fifty pounds of fuel pressure, you will typically see that pressure go up and down roughly five pounds as vacuum changes.
Was this
answer
helpful?
Yes
No
Sunday, December 18th, 2016 AT 12:53 PM
Tiny
MARIOC68
  • MEMBER
Here are the links of the videos on youtube:
https://www.youtube.com/watch?v=GIiLATpZJ_I&feature=youtu.be
Was this
answer
helpful?
Yes
No
Sunday, December 18th, 2016 AT 2:24 PM
Tiny
MARIOC68
  • MEMBER
At idle the reading is erratic, as seen in the following video.
https://www.youtube.com/watch?v=p-bRmkdxMLs&feature=youtu.be
Was this
answer
helpful?
Yes
No
Sunday, December 18th, 2016 AT 2:33 PM
Tiny
CARADIODOC
  • EXPERT
That vibrating needle is normal. The fuel pressure regulator uses a spring-loaded metal valve to block fuel from flowing into the return line that goes back to the tank. The stronger the spring, the higher the fuel pressure. Manifold vacuum pulls against that spring to make the valve open easier. As vacuum goes up, it helps the valve open, so it does not keep the fuel under as much pressure. The pressure drops, and the volume of fuel going back into the tank increases. It is normal for that valve to vibrate a little, as you are seeing on the gauge.

For the larger oscillations about two times per second, pull the vacuum hose off the regulator and plug it. If those pulses stop, they are due to pulsing manifold vacuum. That can be from erratic idle speed or emissions systems pulsing on and off. One suspect is the charcoal canister purge solenoid. Those do not stay open when activated. They cycle on and off, about two times per second.

The changes you are seeing on the gauge should not cause an engine running problem.
Was this
answer
helpful?
Yes
No
Sunday, December 18th, 2016 AT 2:50 PM
Tiny
MARIOC68
  • MEMBER
Thank you
Was this
answer
helpful?
Yes
No
Thursday, December 22nd, 2016 AT 8:28 PM

Please login or register to post a reply.

Recommended Guides