Need help to figure out why fuel pressure at wot drops off

[Chuck Leeper]

Here's a start......

Chuck, could you explain how to do a volt drop test and what will it show me?
Checked voltage with different meter. I was 14.09 at alternator and 13.98 at battery. Even with bad tester the numbers should be consistent at least at the plug in by gas tank. Why so low through scan master?

Voltage Drop Testing

The purpose of the testing is to determine at what point the system is losing voltage.
IE: From 1 side of the batt cut off sw, to the other.
Across the + and - sides of the starter.
ETC, ETC.

There are many more articles on the subject w/ a Google search.

Keep in mind, that a volt drop HAS to be done w/ the system under load. If it's not, it's just a continuity test.

 

[TurboBuRick]

the pump is run by the voltage applied to its 2 terminals. the negative connection needs to be a solid, low impedance path back to the engine block or battery.

High positive voltage will do little if the negative side of the 'loop' has issues.

Bob

Very well said Bob!

 

[Clark6]

Chuck, could you explain how to do a volt drop test and what will it show me?
Checked voltage with different meter. I was 14.09 at alternator and 13.98 at battery. Even with bad tester the numbers should be consistent at least at the plug in by gas tank. Why so low through scan master?

Two reasons I've found why so low a voltage reading at the scan master. 1 was the ignition switch, replaced it brought voltage reading up a .1 or .2.
2nd reason check and see what else you have pulling off your fuse panel. I couldn't keep my voltage dropping at the scanmaster for nothing.
Noticed that I had my boost a pump input voltage and the alcohol injection coming off the fuse panel. I left the Alky control like it was and hotwired my Boost a pump with the hot wire kit (took ALOT of load of the fuse panel). Problem solved on my low voltage problem at the Scan Master.

Your's seems Over voltage (19). That's too much. Check that out, Make sure you have GOOD connections at the pump and retest again. Yes the Denso requires more Amps /Votage.

 

[TurboBob]

Two reasons I've found why so low a voltage reading at the scan master. 1 was the ignition switch, replaced it brought voltage reading up a .1 or .2.
2nd reason check and see what else you have pulling off your fuse panel. I couldn't keep my voltage dropping at the scanmaster for nothing.
Noticed that I had my boost a pump input voltage and the alcohol injection coming off the fuse panel. I left the Alky control like it was and hotwired my Boost a pump with the hot wire kit (took ALOT of load of the fuse panel). Problem solved on my low voltage problem at the Scan Master.

Your's seems Over voltage (19). That's too much. Check that out, Make sure you have GOOD connections at the pump and retest again. Yes the Denso requires more Amps /Votage.

The 19 volts is certainly odd. and not a good number for sure. I'm not sure why it would be so high, so we need to figure out that also.

So, lets break this down a couple ways.

1: why would the voltage at the alternator be so high? Are you running any kind of volt booster? How are you measuring this voltage (where are you connecting the negative lead of the voltmeter?)

2: The scanmaster displays the datastream from the ECM, its not measuring voltage. The ECM reads the voltage on the ignition input, which is the same circuit that feeds the ignition system (coil pack and module). I have heard of failing ignition components (bad module) and a flaky fusebox connection cause similar issues. Like the previous poster said, if you have something else feeding from that circuit it can cause low readings as well.

3: Measuring the voltage at the pump under load, measure the voltage between the power feed wire and the ground wire at the fuel pump plug. The ground leg of this circuit could be causing some loss in pump capacity.

4: check the body grounds: measure the voltage between the engine the body, and between the battery negative and the body.....

B

 

[Mike70gtx]

I stated a little farther up the tread that I Checked voltage with different meter. It was 14.09 at alternator and 13.98 at battery. So my tester was bad. I am running a lot of stuff off of the fuse box. My dual alky injection is, so I will get that hot wired. I'm sure when boost is going up and alky starts spraying, it's starving for voltage. I've needed to rewire that anyhow. I lost a head gasket because the connection got bumped off the fuse block when I was fixing the a/c and hurrying to get to the track. Got to line and didn't slow down to go through my check list and had no alky." Proverbs 30:2 "Surely I am more stupid than any man," (at least it seems like that at times.)

 

[turbofrank]

morale of story never hurry at the track bad things will happen