I've had a hard starting condition for a while and am trying to diagnose it.

With key on, the system pressurizes to about 38psi, but on cranking it falls fast to about 28psi, then recovers when the car is started.

Does this mean my fuel pump is bad or going out? Or is that impossible because it seems to run fine once its started?

I'd just like to make sure before I drop that ol' tank.

 

TTT

 

Maybe the starter is sucking to much power when your starting. Causing the pump to not get enough power to run at max?

 

Ok so would then I'd be looking for a drop in supply voltage, right?

How about this scenario: Let's assume the pump is going bad. I'm not an electrican, but I would assume that this would result in a higher resistance (correct me if I'm wrong). So then, should I expect to see a drop in current because the battery can only supply a certain voltage, or would I expect a spike in voltage because the pump will draw the same amps with a higher resistance?

I'm really just trying to understand the nature of behavior of the fuel pump electrical circuit.

 

A spike in voltage would be impossible. If the fp draws an assload of current as the pump first turns on, that could mean it's going bad. Drawing a little extra current is normal though.

Is this stock wiring? You're sure the thing is actually getting power when the key is in the crank position, right?

 

Yup. Relay clicks and system pressurizes. Like I said, the FP drops 10 psi or so under crank load.

I'm testing the pressure drop after engine shut off. Right now I'm at 20 minutes and it's dropped 14psi. What sort of results should I be looking for?

Thanks!

 

Take a good look at your distributor. When the engine starts to crank the fuel pump pressurizes the system. If it continues to crank and the ecm does not get a signal from the crank sensor on the distributor, it will cut the fuel pump off. Once it gets that signal that the engine has turned over, the fuel pump resumes its duties.

 

To elaborate,


The ECM primes the pump by running it for 2 seconds. It also purges the lines by opening the injectors for a short time (6ms in my install, unsure of the stock number)

Then, the ECM shuts down the fuel pump and waits for reference pulses from the distributor. When it sees an rpm, the fuel pump is turned on again. If your car has weak/sporadic pulses in the cranking range <300 rpm, the fuel pump would not ber turned on.

Is the car hard to start?

 

Yeah, and the longer it's sat the harder it is to start. I'm starting to think that points to a leaky injector, but I may also have a pump problem since the pressure drops while cranking.

So I should be looking up the crank sensor and checking its output during crank? Where is it located specifically?