BubbaJJ

When this whole thing started, I went down and got myself a fuel pressure tester. I've been around cars long enough to know what fuel starvation sounds like. So I put the gauge on at the rail valve and it read 35# at idle and 45# while revving but not under load. This is where I screwed up, I noticed that it was running particularly well during the pressure test. I took off the gauge, slammed the hood and went for a drive to see if it was doing the same thing CRAP! runs like poopie. So at this point I had eliminated the fuel pressure as a problem (or so I thought). A friend of mine suggested that I test the pressure while driveing thinking that it might be different under load. Me, being the genius that I am, decided that this was silly as I had already tested the pressure and couldn't see how it would drop much under load. So after going round and round and not coming up with a solution, I figured I had nothing to lose to test fuel pressure again under load. So I hooked up the ol' gauge and went for a spin. Guess what, 35# at idle dropping off to 10# under load!!! WT..... So, now the question is, what the hell would cause it to have perfect fuel pressure one minute and then crappy the next. Would the regulator cause this? how can I test it? On one hand I'm really irritated that it has come back around to this. On the other hand, fuel pressure problems should be pretty easy to deal with. It's already got a new pump and filter, what do I do next?

CFI-EFI

Obviously, the engine uses more fuel, under load, which you've just learned. The systen is incapable of maintaining pressure under the increase volume demand. It could be a weak pump, a kink in a hard line or hose, a clogged filter, or a bad regulator. Now that you have discovered the problem, all you have to do is isolate it. Good luck, and...

RACE ON!!!

DON M

Dirty fuel filter or tank pickup filter. :yesnod:

Vic'89

A bad fuel pump does the samething.

Vic

vinnies87

Well,,, filters are cheap.... and checking the pump and screen are easy too, thru the filler door... that will take 2 out of the equasion... only leaves one to go.... good luck :cheers:

BubbaJJ

Well, I've already done a filter and pump. Is there a way to test the regulator. I think the testing under load might be deceptive because even now it wont rev at idle. Is it possible that when I relieved the pressure on the fuel rail to do the test it bumped the regulator loose temporarily?

NoWorries

How does the hose look between the regulator and the Throttle Body? If the Regulator isn't getting vacuumed, then it's not going to increase the fuel pressure under load.

That would be the cheap fix, but it's plausible that the diaphram has ripped in your regulator.

I'm trying to think of a way to test it. This may work, with the engine idling, remove the vacuum line from the regulator and see if the idle degrades. If it doesn't, then there is definitely a vacuum leak in the regulator.

If it does degrade, it doesn't mean the regulator is good by any means, here is a possible check for that. Remove the vacuum line from the TB and plug it with something...like a pencil or scribe or punch. Now, go drive the car and see if it drives the same. If it does, then you know that the regulator is not increasing flow to equalize pressure. If it runs exactly the same, I'd say your regulator is bad.

Good luck! :seeya


[Modified by NoWorries, 6:45 AM 1/31/2004]

BubbaJJ

I tried pulling the vacuum line off the regulator today. No change in idle quality at all. My brain was leaning toward a regulator problem. Problem with that is, if you have to take off the upper plenum might as well take the whole intake off and do a cam. If you got the cam out, heads are only another 50 or 60 bolts away. Crap.

corvettemaster

:iagree: mine done the samething. i hooked up a fuel pressure guage. it was good at idle, but under load it dropped to about 10. i couldn't get over about 2500 rpm's for nothing. it turned out it was the fuel pump. i had just bought it a month before, so autozone took it back under warranty and gave me a new one. have had no problems since :thumbs:


[Modified by corvettemaster, 5:44 AM 1/31/2004]

killain

A fuel pressure regulator holds pressure at a perdetermined level. Yours is not doing that. There could be someothing like a vent blocked or a rip in the vane. I know the idea hurts but go geta fuel pressure regulator (New) and put it on. I'm sure this will change the situation. You didn't believe it was a fuel problem before did ya?

Bruthish

:withstupid:

Same thing here too....sounds exactly like what I was having too. COuld have gotten a bad pump.

dlmeyers

BubbaJJ,

Interesting problem. Does the fuel pressure tester have the little side valve/port near the gauge? If it does, you can use that port and do a fuel volume flow test. Basically, you run the pump for around 15-20 seconds, and measure the amount of fuel accumulated in a container. (Engine is NOT running. This volume should equal the amount listed in the service manual (if applicable). Basically, you take the fuel pressure regulator out of the loop as you are running an open system. If the pump cannot provide sufficient volume, there will be insufficient pressure. When doing this test, keep a fire extinguisher handy, you will have to power the pump through the ALDL or fuel pump test lead likely located under the master cylinder. Not sure where it is on your car. Check the service manual schematics. It may say "Fuel Pump Prime Connector". Try around page 8a-20-7 Fuel control and fuel delivery, Section 8A or service manual for Code 54. Not sure if you can power this through the ALDL as I've not used that technique.

At any rate, you are in the ball park. If the fuel pump/supply system does a good job of filling up a can, plenty of volume, I suspect the regulator. If pump volume is sluggish, pump/supply system is supect. New pump too huh? Someone else with my luck.

Let us know.

dlmeyers 90 coupe zf6 3 speed shocks

BubbaJJ

I thought it was a fuel problem, I knew it was a fuel problem, but the stinking electronic fuel injection and computer outsmarted me. The new pump is awefully noisey, maybe I did get a bad one. My thought was to remove the fuel filter, splice the fuel pressure gauge in at that point and then see what the pressure reads. I believe that if the pressure holds in front of the regulator it should read constant under load or not. If the regulator is bad it should not affect the pressure generated in the line prior to it's location. If the pump is bad, i should have a pressure drop off in front of the regulator. Does this sound about right?


[Modified by BubbaJJ, 5:15 PM 1/31/2004]

SunCr

Pressure is volume and you should be able to check that out at the rail by disconnecting the inlet, attaching a hose to it and putting it in a coffee can. Crank the engine for 15 seconds and it should squirt at least a pint. If not, the filter is plugged or the pump isn't right. However, based on your tests, the volume restriction your experiencing is probably the regulator. As engine load increases, the vacuum applied to the diaphragm removes the restriction so there is greater flow to the rail and less to the return line. If it doesn't lift, fuel is returned to the tank and it won't maintain adequate pressure. A quick ckeck is to clamp off the return line. If the pressure goes up, it's the reglulator (or a stuck injector). The only other item you might want to check is the shrader. If it's leaking, you'll have good pressure with the gage hooked up - it might even run ok, but otherwise, fuel is flowing out of the rail and not to the injectors - an obvious fire hazard of course.

BBA

If your at idle pressure with no vacuum connected to the PR goes to 45 PSI but your pressure drops under load...then it is NOT the pressure regulator causing your problem.

It's either pump, filter or supply line restriction or old lifters that wont return to shut at max duty cycle.

Does your car put out black smoke at high RPM's while driving at WOT? If so, it's probably bad injectors. If not, it's probably a bad fuel pump.

no_radio

I had a Grand National with a similar problem. Check the filter, regulator, etc. Turned out to be a kink in the fuel supply line near the rear wheel. Seems a previous owner had gone on an "off-road excursion" and the frame pinched the line. Replaced that section of tubing and the problem was solved, but it took while to find the problem in the first place.
Suggest visually checking the entire length of your supply line.
Jeff

dlmeyers

BubbaJJ,

Jeff brings in another possible cause. Check that one out. It is downstream of pump and regulator.

OK, some pump points. Fuel pumps are designed with a head pressure greater than system pressrue. L98 fuel pump, say around 55 PSI max fuel pump pressure, ballpark. System pressure is say 42. Therefore, the pump can supply X amount of flow at 42 psi with the regulator keeping system pressure at 42. Now, if your regulator is failing, it COULD drop your system pressure down to say 15lbs, car doesn't run. Now suppose your pump develops sufficient pressure/volume at idle, however, when greater volume is required, the pump simply cannot supply enough volume to that L98 sucking in fuel, well, pressure will drop due to a failing pump. I would pick one or the other and perform the required tests. Noisy new pump is a little suspicious, but I've heard rattling electric pumps that keep on going.

If the pump is good, regulator good, then you may have other problems. Dropping pressure would be number one thing to fix.


Let us know.

dlmeyers 90 coupe zf6 3 speed shocks


[Modified by dlmeyers, 10:22 AM 2/1/2004]

BubbaJJ

Ok, I tried SunCr's suggestion of clamping off return line and then retrying pressure test. Same thing, pressure at idle and no pressure under load. I also tried it again with the vacuum to the PR off, but no change. While I was messing around with the fuel pump and had it up on jacks, I checked out the undercarriage and could find no signs that it had ever had anything strange done to it that would cause a fuel line pinch. So, given all this, would it be safe to say that the pump is probably bad? I cant immagine anything that would clog the line getting past the sock on the fuel pump. I also couldn't see anything clogging the lines after the fuel filter. The pressure really drops off fast and wont come back up until the throttle is almost completely released. On the injectors, they hold pressure after the engine is shut down and i've never noticed it belching black smoke under any circumstances. It idles fine so injectors probably arent the problem. My next step will be to replace the pump. It's under warrantee and it's an easy change. I should have been suspicious when it made so much noise. Wish me luck and thanks for all you tips. I'll keep y'all posted.

want2drivemy85

I had the same prob with mine. i replaced the pump with a new AC Delco unit. Well the new pump lasted for about a month and started doing the same thing. So, had to buy a new one, and do the pump change all over again. After that, I learned that just because a product says AC doesn't mean that its the best. Good luck!