Fuel pressure at idle too high.

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SS_Sean

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Jul 10, 2006
Hey guys. I have fuel pressure at idle, line on at 40 psi, and line off is 45 psi (which is what the extender chip calls for). The trans plus settings don't have any provision for idle fuel pressure settings, only WOT.

Essentially I'm not sure why the fuel pressure at idle is that high. What's happening is the car is running pig rich at idle and part throttle. The plugs are competely fouled, and got so fouled I had to be towed back on a test run. The car is crapping out black smoke at idle, as well. It's RICH.

Any help or suggestions are appreciated.
 
What is the MAF BASE setting in the Translator? What are the 4 little dip switches set to ?
 
Sorry, missed that part....

MAF base - 0
MAF WOT - 4

Spark base - 0
Spark WOT - 0

We checked the fuel press reg today, and it's line off is 45, and line on is currently 36 or so. The 'line on' dropped because I got all the vacuum lines replaced recently and actually have some manifold vacuum. The car is still running pig rich at idle and part throttle. We're getting code 41, 42, and a whole bunch of others, 33, 34.
 
With all those codes it sounds like a chip or ecm issue. Do you have access to another ecm. Try to pull the chip and reinstall it. How old is the chip?
 
Gotta go with Rick on this one (chip/ECM)--45 psi FP is not really all that high and is certainly not high enuf to cause your rich idle issues...
 
"and it's line off is 45, and line on is currently 36 or so".
9 psi drop w/ vac added, is a bit much. Usually looking for 4-6psi drop.
 
Chuck Leeper said:
"and it's line off is 45, and line on is currently 36 or so".
9 psi drop w/ vac added, is a bit much. Usually looking for 4-6psi drop.
Click to expand...

9psi drop indicates that there's 18 InHg of vacuum as the reference. A little high but not unheard of.
 
It might have been 38, his gauge is hard to read with the incrementals...it was 40-45 prior to all the vacuum line replacement. I was thinking ECM, as well. We will try to reseat the chip and if that's not working I'll purchase another ECM and go from there.
 
I don't have times yet. I busted my rocker arm shaft last time I ran it, and haven't been back since.. :mad:
 
Interesting... Problem sounds familiar!

How low can you go? When you are adjusting the FPR with the line off, can you dial it down to say, 20 PSI? (Only as a test!)

If the return line is restricted, the FPR will not be able to lower the Fuel Pressure... Especially with a high-volume fuel pump! Either that, or the FPR is bad.

I forget what the Idle fuel pressure should be with the line on... Knowing this might help diagnose the problem... Maybe someone that knows will post what the FP should be with the line on!
 
6=8 said:
How low can you go? When you are adjusting the FPR with the line off, can you dial it down to say, 20 PSI? (Only as a test!)

If the return line is restricted, the FPR will not be able to lower the Fuel Pressure... Especially with a high-volume fuel pump! Either that, or the FPR is bad.

I forget what the Idle fuel pressure should be with the line on... Knowing this might help diagnose the problem... Maybe someone that knows will post what the FP should be with the line on!
Click to expand...

It really depends. There is no set amount because the line on pressure is dictated by the vacuum your engine is able to pull at idle.
As an example suppose you have an average engine and at idle it's pulling about 15InHg of vacuum. That would be aprox 7.5psi down from your desired line off pressure. If you want a line off pressure of 43, then you'd shoot for a line on pressure of 35-36.
Hope that helps.
But you need to know what your vacuum at idle is to set it this way (not a reliable method).
 
Wow... Thanks Dave! Your answer brings up a point I had not considered... The vacuum signal really needs to be checked on the same line going to the FPR, because if one were to have a vacuum leak on this line, it would cause all kinds of fuel pressure and driveability issues and may not even register as a "vacuum leak" if readings were taken on another port/line, such as one for the boost gage. No wonder this issue is so difficult to chase down!

Thinking toward the Modified (Ie- High-lift cam) engines, a low vacuum signal would make idle and part throttle fuel pressure difficult to manage, if not impossible... Even with an adjustable regulator, low vacuum would be very close to the "Line Off" setting, regulating only for boost conditions.
 
6=8 said:
Wow... Thanks Dave! Your answer brings up a point I had not considered... The vacuum signal really needs to be checked on the same line going to the FPR, because if one were to have a vacuum leak on this line, it would cause all kinds of fuel pressure and driveability issues and may not even register as a "vacuum leak" if readings were taken on another port/line, such as one for the boost gage. No wonder this issue is so difficult to chase down!

Thinking toward the Modified (Ie- High-lift cam) engines, a low vacuum signal would make idle and part throttle fuel pressure difficult to manage, if not impossible... Even with an adjustable regulator, low vacuum would be very close to the "Line Off" setting, regulating only for boost conditions.
Click to expand...


Well, it's really not all that complex, assuming there are no vacuum leaks. The reason we shoot for 43# line off, is because that's where most injectors are flow tested at, that is to say, around 3 bars pressure, which roughly equates to about 43#, closer to 45 actually but there are variences.

At zero boost/vacuum (line off) you can be assured that your injectors will be flowing thier optimum value with a proper operating FPR because as boost rises, it raises the fuel pressure on a one to one basis, thereby maintaining a differential pressure across the injector of about 43# (or whatever you set static) so even if you go up to 25# of boost, your fuel pressure should rise to 68#. So boost on ones side of the injector=25#, fuel pressure on the other side=68..... 68-25=43
So you see the differential pressure across the injector should never change.

Again, assuming no vacuum leaks.

Even at idle, suppose your engine can only pull 10InHg vacuum.
10InHg on the tip of the injector= -5#
-5# - +43#(line off) = 38# of fuel pressure indicated on dial.
The math all works out.
 
Do you have any exhaust leaks? A good exhaust leak pre oxygen sensor will get your car to run rich real quick. Just a thought. james
 
Odd, but I believe I have two or three issues that have cropped up all at once. I'm working with my mechanic (and good friend) to figure this stuff out. Presently we've replaced the cam sensor and most of the codes have cleared up but code 42. The car stopped running absolutely pig rich after the sensor replacement. I was able to make 15 pounds of boost again, and continued to drive it to see if I could get the exhaust cleared out. I then had some sort of failure and the car shut down (by luck in his parking lot). At this point we have no spark, and the car won't start. I'm thinking whatever fault was intermittent is now a hard failure. :)

We have a replacement ecm coming from Kirban's, which should be here tomorrow. We're going to swap that and see what happens. We're suspecting ECM or Ignition module at this point.

Don't you just love working on 21 yo electronically controlled cars? :rolleyes:
 
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