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MAP / fuel pressure typical delay times

Murray_Calavera

By Murray_Calavera,
in Engines & Forced Induction

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Murray_Calavera Mentor

Murray_Calavera

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Well, good news. The fuel pressure situation is a million times better. Please see attached dataz. 

@Dose Pipe Sutututu what battery voltage would you expect to be seeing? When I'm puttering around it floats between 12.8 to 13 volts and when punching through the hills its about 13 to 13.2 volts. Are these normal values?

But yeah, I'm so glad I don't have to dig this fuel pump out again lol. 

Fuel pressure.jpg

Dose Pipe Sutututu Grand Master

Dose Pipe Sutututu

Posted
Posted

Much better graph!

Battery voltage wise, seems a bit on the low ish side but not terrible.

Would be nicer of it had a delta of 0.2V instead of 0.5V

Maybe try add in an earth strap between the motor and chassis and check to see if the power cable from alternator to fuse/relay box hasn't degraded 

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MBS206 Grand Master

MBS206

Posted
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If it's an original "dumb" style alternator, you should be seeing your voltage running, in the 14s

A healthy lead acid, charged, is 12.8V, so if your engine is running, and you're seeing 12.8V, you have issues.

 

If you're at 13V, you have very little power going back into the battery.

I'm going to guess you probably need better wiring to the fuel pump too, as I bet you have a voltage drop on your wiring to the pump too, and it's probably seeing sub 12V

 

Murray_Calavera Mentor

Murray_Calavera

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6 minutes ago, MBS206 said:

If it's an original "dumb" style alternator, you should be seeing your voltage running, in the 14s

A healthy lead acid, charged, is 12.8V, so if your engine is running, and you're seeing 12.8V, you have issues.

 

If you're at 13V, you have very little power going back into the battery.

I'm going to guess you probably need better wiring to the fuel pump too, as I bet you have a voltage drop on your wiring to the pump too, and it's probably seeing sub 12V

 

Yep, factory alternator. 

Fuel pump wiring isn't bad. Not perfect, but isn't bad. The gauge of wire has no issues supplying the 20 amps, the only issue is I'm using the factory connector in the fuel pump hanger which I'd be amazed if they are rated for 20 amps... but nothing has melted so far lol. It's also got the typical relay direct power from battery etc. 

TurboTapin Experienced

TurboTapin

Posted
Posted (edited)

Voltage read at the ECU and battery will not be identical, especially on 90's cars. If the delta is excessive (~ >0.8VDC) verify your wiring to ecu, ECCS relay and ignition switch.

If your battery voltage (Measured at the battery) while running is under ~13.8VDC, verify your existing grounding, exciter circuit and alternator. 

Do not add any further grounding.

Edited by TurboTapin
Added more details.
MBS206 Grand Master

MBS206

Posted
Posted

Seconding Taipan.

 

Either alternator is worn out, and dieing, or you've added some hectic load and it can't keep up.

 

Also, you say "the fuel pump wiring is handling 20amp", how do you know? Have you put a current meter inline?

 

The fuel pump will be wanting 20amp at a set load, at a set voltage, as that load, is what pushes the resistance electrically for the fuel pump (an unloaded electric motor, in theory draws zero amps).

 

Now if you've got the load, but less volts, you've got less amps, which means less torque, so the pump won't run as quick.

Add in, all wire has resistance, if you get 0.25ohm resistance in your wire in total, and run 20amp through it, you have 4v drop in the wiring alone. That means on a 12V supply, your pump now only sees an 8V potential across it, not the preferred 12V.

If your pump is meant to draw 20a at 12V, that gives it an impedance of 0.6ohms, add in another 0.25ohms, in wiring, and realistically, your pump is now only actually getting 14amps, at about 8.4v...

 

Let's say wiring is perfect, and we expect 14.4V output from the alt, and that is giving you your 20amp pull. Then your pump has an impedance of 0.72ohm. now let's say your alt is cactus, and only giving 13V out, now we've dropped to 18amps. If we claim flow to be linear as we alter voltage, then we've lost 10% of your flow.

The above is to indicate, unless you've measured current, you've no idea what it's really getting, only what it's rated for at a given voltage.

 

Shove a multimeter at the battery positive with one prob, and then on the positive feed into the fuel tank with he other probe, check out what the DC voltage is. That alone is giving you your voltage drop from battery to fuel tank lid.

Murray_Calavera Mentor

Murray_Calavera

Posted
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1 hour ago, MBS206 said:

"the fuel pump wiring is handling 20amp"

It's 12 gauge Tefzel wire. Should be right for 20 amps. 

1 hour ago, MBS206 said:

unless you've measured current

I've clamped the fuel pump wiring, it measured around 20 amps. I can't remember the exact figure, I did it a while ago now, but it was around 20 amps. 

Still, lots of good info in this thread now. I'll keep investigating and post back with what ends up solving the voltage problem. 

MBS206 Grand Master

MBS206

Posted
Posted
1 hour ago, Murray_Calavera said:

It's 12 gauge Tefzel wire. Should be right for 20 amps. 

I've clamped the fuel pump wiring, it measured around 20 amps. I can't remember the exact figure, I did it a while ago now, but it was around 20 amps. 

Still, lots of good info in this thread now. I'll keep investigating and post back with what ends up solving the voltage problem. 

On a 20amp run, on "12V", if you're getting power for the pump direct from battery in the boot, you should be good on 12AWG.

If the wiring is from up front, you want to get to a 10awg.

  • 2 months later...
Murray_Calavera Mentor

Murray_Calavera

Posted
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Posted

Ok, update time. 

So while solving my fuel pump / melted connector issue, I thought I'd see what voltage I was getting with the factory connector bottleneck removed and the EFI Solutions bulkhead terminals installed.

Started the car, battery voltage was bouncing between 13.6 and 13.7 while at fast idle (cold start). Huge improvement over the 12.8 to 13.0 previously. 

I then installed the ARD 150 amp alternator, on cold start it was bouncing between 13.9 and 14.0. 

Here is the mystery, I went for a drive and now the battery voltage is 13.5 with the ARD alternator setup. I suppose I'm happy with this, it's rock solid at 13.5 but I can't help but wonder if the factory alternator would have performed just as well.

If anyone can explain the mystery difference in voltage on the first cold start 14 volts vs 13.5 I'm now getting while driving, it would be much appreciated! 

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