SamGrant951

Whats "within spec" for voltage at the fuel pump?

Some of you may remember I have been chasing a low fuel pressure issue when the car is driven hard on street/track. Usually easier to duplicate at the track.

It had been fine all year until recently and it started acting up again. It basically wont boost and hesitates. AFRS are lean/screwy under cruise when it does this and goes super lean when you try and roll into boost. It offers NO warning that its going to do this - totally random. Alan (vt951) drove it last weekend without issue and then a few hours later on the way back to Indy it was doing it. Did it the next day but its been fine since then.

I finally wired up some leads and ran them into the cabin so I can see what its doing under load.

Although I didnt get it to do its hesitanting/stumbling today and it ran great here are the numbers I got:

~12.05 at 70mph cruise, radio on/detector on.
~11.60-11.75 under full boost

When I first fired it up yesterday in the garage it read about 12.3-12.4 at idle.

Ill just keep driving it I guess until it starts doing it and then flip the voltmeter on.

Ive replaced/tested the: regulator, pump, tank screen, damper, evap system, alternator, 02 sensor. Injectors are a few years old (rebuilt/flow matched).

When the car does its goofy hesitating the fuel pressure gauge reads low and is very bouncy. Once you shut the car off its drops to zero instantly. When I just drove it earlier it slowly bleeds off pressure in the rail.

Very strange. Im sure it could use a new fuel pump check valve but I doubt that has anything to do with the hesistating. Im thinking of running a direct 30amp power relay to the pump to get rid of the old factory wiring but obviously would like to recreate the problem first with the voltmeter on board.

Thanks for any comments/suggestions...

Chris Prack

You don't have any where near enough voltage! It appears you have a charging system problem. What is the voltage at the batter?

SamGrant951

13.64 at idle, just tested it. Car failed a load test last year (alternator bad) replaced with reman 130amp unit.

Chris Prack

Ok. Put the positive lead on the negative battery post and the negative on the engine. Tell me what you get with the engine running. If you can do it while cranking the engine that would be good.

SamGrant951

Thanks much Chris.

What do mean by "negative on the engine"

Chris Prack

Your voltmeter has two leads, the red one (pos.) and a black one (neg.). Put the positive lead on the lug of the battery, not the cable end, and the negative lead on the engine somewhere like the intake or cam housing.

Tell me what you get. You need to set your voltmeter on a low DC voltage scale like 2 volts. You are going to be reading mV.

SamGrant951

Ok thanks Chris - most of my motor is powdercoated so I tried a few different bolts (fuel rail bolt, cam housing bolt) and got .005 or .006 with the motor running.

SamGrant951

When cranking the first time it went down to -.082, then quickly -.064, and quickly came around .015 and then slowly leveled out around .007 to .006. Tried it a few times and it didnt seem to spike as low as it did the first time. Hopefully I did it right - those numbers make any sense? Thanks again-

Chris Prack

That is good. I would use a jumper wire and jump terminals 30 and 87b at the DME relay to run the fuel pump and measure the voltage at the jumper with the pump running.

Just out of curiosity, have you tried a new DME relay already?

SamGrant951

I have tried a different relay...but since the problem is intermittent never noticed a difference (and wasnt looking at voltages then)

Should I plug in another relay and go out and measure the volts at the pump again or just jump those two pins and tell you what the voltage is (at the jumper) at idle?

Stupid question: How do I measure the voltage at the jumper? Red lead to jumper wire and black to engine or opposite? Other way? Sorry!

Chris Prack

Red lead to jumper and black to battery neg. term.

Chris Prack

Something that I would like to see is the charging system voltage while you are having low voltage to the fuel pump. Am I correct in saying that you have a Jap alternator on your car? I thought I saw that somewhere.

Intermittent problems are the worst. I would also be interested in seeing what the connector for the fuel pump looks like. Make sure there is no corrosion there.

I don't have my wiring diagram book with me at home and I don't recall where the fuel pump grounds. IF you have good charging system voltage and low fuel pump voltage you are either losing voltage to the pump or there is a ground issue for the pump. Every electrical consumer in the car needs to operate off of the charging system voltage. So if that is 13.8v then the fuel pump should be 13.8 volts or very close. You will lose a little voltage due to voltage drop but you should see - .5v maximum.

Chris Prack

Just remembered something else. Check the 14 pin connector under the black cover in the engine compartment. Behind the brake booster. They are known to corrode and the DME wires run through it.

SamGrant951

At the jumper line 30->87b = 12.25 when you first make the connection - stabilizes around 12.15.

No corrosion at the fuel pump wiring but the wiring is hard and old. So if my battery is 13.6 I would need to see no lower than 13-ish at the pump.

OEM alternator but its remanufactured 130amp, just installed earlier this year.

SamGrant951

Pins look OK, no corrosion. The black cover itself was pinching the green wire probably since 1985. Moved it a bit and snugged it back down.