The motor is a brand new 383 and has a random miss. I suspect there may be a combination of fuel and ignition problems. I have my Aeromotive regulator set at 45 psi at idle. Just revving the motor with the car parked, the pressure drops to 40 psi then picks back up. Under load while driving, pressure will drop down between 30 and 35 psi. I can't go to WOT due to wheelspin (posi is shot).

When I twist the key (but don't crank the motor) and hear the pump prime, pressure hits 45 psi, but then it drops down to zero immediately. I don't think this is normal. If the motor has not been started for a few days, unless I give it about 1/2 throttle, the motor will not start. However, after the motor has been running, I can shut the car off and fire it right back up.

I'm running a brand new Walbro intank pump so I doubt the pump itself is a problem. However, it is possible that I may have botched the install and didn't tighten up the corrugated piece of plastic onto the pump tight enough. I also kinked the feed line while removing the tank, but thought I "unkinked" it pretty well. Maybe it's still kinked.

Any thoughts?

My car also runs much worse when it gets warm which I think is a separate problem that's ignition related described in this thread. Maybe the two problems are related?

 

The check valve in the pump very well could be bad, or you could have not tightened the connections on the pump enough. I had a walbro fail on me within the first week on installing mine.

Shoebox has a good page for checking your Pump...I used the vice grip method and it kinda permanently bends your hoses, but didnt hurt anything.

http://shbox.com/1/4th_gen_tech2.htm...essure_testing

 

What exactly does the check valve do? Where is it on the pump? Also, I do not have any vacuum hose attached to the regulator. I take it I should attach one to it?

 

The check ball is internal to the pump. It's purpose is to retain some pressure in the fuel feed line to make starting easier.

If you want it to regulate pressure, yes.

Did you leave the vaccuum hose that is supposed to go to the regulator open to the atmosphere, creating a vacuum leak?

 

Rob,

There is no hose attached to the regulator. I plugged up the nipple that the hose is supposed to attach to on the intake manifold with a vacuum cap. There is no vacuum cap on the regulator.

So would not having the hose attached to the regulator cause the pressure to drop under load? That's an easy enough fix.

I've read several posts that say that if your car only starts if you give it 1/2 throttle or so while the motor's cranking, that the injectors are leaking. However, I have a hard time believing they are leaking that badly. I mean, fuel pressure instantly drops down to zero after the pump primes. So my question is, if the check valve is bad or there's a loose connection on the pump, would that also give the same symptom of only starting when throttle is applied while cranking the motor?

 

No. A missing vacuum hose will not cause fuel pressure to drop at high loads. The vacuum compensation is there to maintain the correct DIFFERENTIAL pressure (43.5psi) between the fuel rails and the intake manifold. If you don't have vacuum compensation, the fuel rail pressure will always stay at ~43.5psi and your injector will spray more fuel than it is supposed to at idle and part load (high vacuum/low MAP) conditions, forcing the long term fuel corrections to work overtime to correct the rich condition.

Dropping fuel pressure under load is a sign that your pump is not pumping an adequate amount of fuel, or the fuel filter is plugged.

Yes. There are three ways you can lose pressure when the pump shuts down - leaking injectors (pull the rails up and see if they are dripping), leaking FPR diaphragm (look for fuel at the vacuum port) or faulty FPR, and weak check valve in the fuel pump. I believe some people also have problems with the Walbro in-tank install, because they do not make the connections inside the tank correctly.

I've seen an awful lot of posts complaining about Aeromotive AFPR's. Hook the vacuum line up, and do the correct pressure check. 41-47psi at idle with the vacuum line disconnected. Reconnect the vacuum line at idle, and the pressure should dropin proportion to manifold vacuum.... 6-10psi drop with a sotck cam, maybe 1/2 that with something more radical. Then take it up to max load/rpm, and the fuel pressure should stay within about 2 psi of the "no vacuum" value you measured.

 

I get it now. So the vacuum line is there to reduce fuel pressure at idle, and whatever I set the fuel pressure at without the hose should remain fairly constant even under load. Correct?

Going through the process of elimination, the injectors have just been serviced, so a leak is unlikely, although I should check them anyways. The FPR vacuum port is not leaking. The fuel filter is relatively new, and this problem did not exist prior to replacing the pump. That isolates the problem to either the pump or the FPR. I know Aeromotive doesn't have the best track record, but I think it's more likely that I botched the fuel pump install. I suck, and have a record of botching simple installs.

If I do this test and fuel pressure drops as it should with the vacuum hose attached, does this mean the FPR is good?

Another reason I suspect the fuel pump is the problem is that pressure drops from 45 to 40 psi just from revving the motor with the car parked.

 

This thought just crossed my mind. Since I have AN fittings on the rails, I can just cap them off one at a time to isolate the problem.