1993 Pontiac Grand Prix fuel pump or regulator or vaccume l

Tiny
JKASENO
  • MEMBER
  • 1993 PONTIAC GRAND PRIX
  • 6 CYL
  • FWD
  • AUTOMATIC
  • 210,000 MILES
My 93 grand prix started idling erratically. I changed the plugs and wires, that didn't help. Then I checked the fuel pressure: I hooked up the pressure gauge and turned the ignition switch on. The pressure went up to around 45 psi (which is where it is supposed to be) but then dropped slowly all the way down to zero after the fuel pump stops running. It is supposed to hold steady. Does that mean I need a new fuel pump
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Sunday, September 19th, 2010 AT 8:27 AM

13 Replies

Tiny
KHLOW2008
  • EXPERT
Hi jkaseno,

Thank you for the donation.

Is vehicle equipped with Throttle Body Injection (TBI) or Programmed Fuel Injection (PFI)?

For TBI system the pressure would dissipate after a short while.

Do you mean the idling speed is surging?
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Sunday, September 19th, 2010 AT 9:01 AM
Tiny
JKASENO
  • MEMBER
Well im not sure what you mean by surging it goes rrRRRrrr. RrrRRRRrrrr. RrrRRRrrr. And shakes, and stinky exhaust cloud when you rev the engine. It barely idles
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Sunday, September 19th, 2010 AT 9:14 AM
Tiny
JKASENO
  • MEMBER
Yea, the idling speed seems like its gonna die, it goes down almost to zero then like its trying to fix it self it revs up a little, then drops again at a constant pace rrRRrr. RrrRRRrrrr. RrrRRRrrrr
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Sunday, September 19th, 2010 AT 9:20 AM
Tiny
KHLOW2008
  • EXPERT
Yes, surging is the term we normally use for such conditions.

Problem is due to erratic idling speed. When this occurs, the PCM would try to compensate and the idle would surge.

Possible causes are
1. Low idling speed. Air passage restriction would reduce the idling speed. Cleaning the throttle body and idle valve should help if there are no other faults.
2. Misfiring. If the engine is not running correctly due to bad spark plugs or related parts, the the idling speed would drop.
3. Vacumn leakages. If any vacumn leakage occurs between the MAF and throttle body, idle would be erratic.

Is the Check Engine Lamp indicating?
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Sunday, September 19th, 2010 AT 9:30 AM
Tiny
JKASENO
  • MEMBER
No the check engine light is not on when its running. I did replace the plugs and wires right away and that didn't help much
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Sunday, September 19th, 2010 AT 9:55 AM
Tiny
KHLOW2008
  • EXPERT
As the idling speed is low, get the throttle body cleaned and retest.
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Sunday, September 19th, 2010 AT 10:08 AM
Tiny
JKASENO
  • MEMBER
Im not sure how to tell if my car is throttle body injection or port fuel injection, it does have a schrader valve, and in my haynes manual that suggests it is port fuel injection
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Sunday, September 19th, 2010 AT 10:09 AM
Tiny
KHLOW2008
  • EXPERT
TBI means there are no individual injectors. You have part that looks like a carburettor.

PFI has individual injectors for each cylinder.

From the description, it should be PFI.
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Sunday, September 19th, 2010 AT 10:44 AM
Tiny
JKASENO
  • MEMBER
So that brings us back to the beginning, the fuel pressure dropped after the pump stopped running when I did a fuel pressure test. On a PFI that is not supposed to happen. What does that mean?
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Sunday, September 19th, 2010 AT 11:02 AM
Tiny
KHLOW2008
  • EXPERT
That could mean a faulty pressure regulator, fuel pump or leakage in the fuel delivery lines.

Clamp the return hose and test if the pressure drops after fuel pump stops running. If yes, check the fuellines for leakges. If none found, you need to remove the fuel pump to check the hoses between fuel pump and housing out of tank.
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Sunday, September 19th, 2010 AT 12:10 PM
Tiny
JKASENO
  • MEMBER
I inspected the lines and there is no smell of gas or any signs of a leak. So its probably the regulator or the fuel pump. One last question, is there a way I can eliminate one of the possibilities?
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Sunday, September 19th, 2010 AT 1:24 PM
Tiny
KHLOW2008
  • EXPERT
The pressure test outlined in previous post would eliminate the pressure regulator if the pressure drops with the return fuel line clamped.
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Sunday, September 19th, 2010 AT 1:35 PM
Tiny
NICKHEADZ34
  • MEMBER
When you figure it out email me nickhead882@yahoo. Com. I got a z34 does that when u start it up initially. Only after certain amount of hours
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Saturday, October 2nd, 2010 AT 10:09 PM

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