'77 fuel pressure issue

[siteunseen]

Hello everyone. 1st post so I hope it's done right. I bought a '77 in pretty good shape. Hadn't been driven in awhile but would crank and run. Drained the tank, cleaned out cone shaped filter in fuel line from tank. Seemed to rev up better but when I took it for a drive fell flat after 3,000 rpm. Put fuel gauge in between filter and fuel rail and it shows 25psi at idle but when I give it gas it goes down to 20psi. Should I put new fuel pump on? Found Bosch at Advanced for $240.00 after 20% off $300.00. Does the fuel dampner/regulator ever get out of adjustment? Could it be that? Still holding at 22psi after not being cranked for 12 hours. Also have been cleaning all grounds and bullet connectors. Clear Fram G-3 filter out of gas tank and its still nice and clean. Advise me please. Second opinions are great. Thank you all.

[FastWoman]

Check your throttle position sensor (black plastic box on the side of the throttle) to see if it's clean and adjusted properly. If I'm understanding correctly, the engine won't rev beyond 3k. That could be because your TPS is stuck in the idle position, so that fuel is cut above 3k.

Fuel pressure: Might be a bit low. While the engine is running, pull the vacuum line off of the fuel pressure regulator. The pressure should then be about 36 psi, as I recall. Be aware that your fuel pump can test good when cool, but become inadequate when it gets warm/hot, so test it after running it for a while too.

It is normal for the fuel pressure to drop off as engine vacuum increases. In fact the fuel pressure is held constant with respect to intake manifold pressure, if that makes sense.

Welcome, BTW!

[SteveJ]

One of the best resources for answers is the factory service manual (FSM). Download a copy at the URL listed below. Read it, and you'll have most of your answers and diagnostic techniques straight from the factory engineers. While you're at the XenonS30 link, also grab a copy of the fuel injection bible.

I looked at a copy of the 76 FSM since I already have it on my computer. The EF section indicates the fuel rail should have a pressure of at least 36.3 PSI. Your fuel pressure is low.

[FastWoman]

Well, the 36.3 psi reading would be without compensation for intake vacuum (i.e. with the vacuum hose disconnected from the FPR. It still sounds low, though. Assuming the engine is pulling a really healthy vacuum of 19 in Hg, there should be almost a 10 psi decriment in fuel pressure, which would put the correct pressure at maybe 27 psi. Realistically, our old engines don't pull the vacuum they did when they were fresh (unless they've been rebuilt), so a pressure of 29 is more likely. (That's where mine runs.) Is that far enough from 25 to be low? Dunno... How accurate is the gauge? It still sounds a bit low to me, but it's not EXCEEDINGLY low.

If the fuel pressure IS low, it could be the fault of the pump, the regulator, the damper (rare problem), the fuel pickup screen, the fuel filter, or the fuel lines (clogged). To rule out a number of these items, I'd suggest finding a length of clear vinyl hose and hooking it to the outlet of your fuel filter. Then hot-wire your fuel pump (+ to the + connector on the pump), and see how the fuel flows. You can either pour the fuel directly back into your tank or spill it into a gas can. You should see a healthy flow. If the flow isn't healthy, then something's probably blocked up. The next test would be to reconnect your fuel rail to the fuel filter and hook the vinyl tube to the fuel return outlet of the fuel rail. As your engine runs, you should see a healthy return flow. If not, your pump's output might be inadequate. (That's how my own pump failed, BTW.)

[siteunseen]

I unhooked the FPR vaccum from the manifold and it jumped up to 32psi but as it got warmer it fell a few psi's and idled terribly rough. The suction is from the manifold side so do I need to replace FPR valve as well or is it supposed to be that way? Thinking pretty much about going ahead and putting the Bosch pump on, might worry too much any distance away from the barn, because I also put a glass inline fuel filter ahead of the stock filter and before fuel rail. It would sort of surge every few seconds with air bubbles so I put in the fuel pressure gauge. What do y'all think about Pacesetter 6-2-1 headers? I have to replace the exhaust gasket and stud closest to windshield. Thought about the headers with glass pack in place of cat converter. Thanks for all the help.

[siteunseen]

I think the clear hose test is going to be my answer for weak pump. I'll do that as soon as I can. Thanks. Sorry about header question while y'all are helping me with fuel pressure, I get excited with all the info on here, I'll ask about exhaust when I get the car fine tuned. Thanks again for help.

[FastWoman]

Well, there's another test you should do for your fuel pump. Just disconnect the hose from the fuel filter to the fuel rail, and hook a gauge up to the outlet of your fuel filter. That will give you a static reading. It should be approx 45 psi or better. You should probably do this test after your system gets warmed up enough for you to see that small pressure drop you mentioned. FAIW, before I replaced my pump, it woiuld give me about 50 psi when cold, but as I ran the thing for 5 or 10 min, the pressure would sag down to about 25 psi. I think it would really sag much farther than that, as my immediate problem was that the engine had died and left me stranded on the side of a highway. (Grrrrrrrr....)

You're right... better to have one issue per thread. There's an exhaust section, and lots of people have expertise in exhaust. However, they might be carb people and might have no interest in checking the fuel injection section. ;-)

[siteunseen]

I went ahead and replaced the original fuel pump (had some sort of body filler type goop on terminals so I'm assuming it's factory) fuel lines and fuel injection clamps, never knew there was a difference, been broken down 2 too many times because of bad fuel pumps. So now I'm unplugging vaccum line from manifold to FPR. The suction is really strong from manifold but it runs rough so I clamped off vaccum and drove around neighborhood. Pulls real hard in 1st & 2nd then I run out of road and come to a stop and the motor is revving really high, 2,000-2,500, then it drops back down after 5 to 10 seconds. Hooked the vaccum line back to FPR after it was good and hot and it starts "burping" through air box upon acceleration. Read about FPR in the "bible" and it makes me think it needs to be replaced. I'm running out of paint job money. Any thoughts?

[FastWoman]

Well, with that hose disconnected, you were running with more fuel, which made the engine run stronger. With the vacuum line connected, you get backfiring through the intake ("burps"). That confirms a lean condition. Your FPR might indeed be bad. Or maybe you have an inaccurate gauge, and other stuff is causing you to run lean. However, I think replacing the FPR would be a good move. Just the same, it would be great to have a static fuel pressure reading from that pump, just to make certain it can put out the needed 45+ psi.

[siteunseen]

I have an oil-filled 0-100psi 1.5" from Autobahn88 I got off the internet installed with brass "T" fitting between fuel filter and hard fuel line in front of valve cover. It reads 25psi at idle then when I unhook vaccum at FPR it goes to 35 or 36psi. Can I just run hot wire from battery directly to fuel pump without damaging it ($260 pump) it has 1 large post terminal and 1 a little smaller. Would the larger be the power side? and the smaller the switch? Put the hot wire to the larger terminal? Then it would run constantly and I could get a static reading? I'm so sore this a.m it may be Friday before I could get under there again. I need a floor lift. Thank you for all your help FastWoman.

[FastWoman]

You're welcome! No prob...

To get your static reading, just hook the gauge directly to the outlet of the fuel filter (i.e. with no "T" fitting and no connection to the fuel rail). Disconnect the starter wire from your starter (the little one). Watch the gauge as you turn the key to "start." You should have 5 sec of pumping, and your fuel pressure should go up to 45 psi or better.

IF YOU NEED TO, you can hot-wire your fuel pump directly without damaging it. I don't know which terminal is which. With the engine not running, just put a multimeter on it, ohms mode. One terminal will have zero ohms to ground, and the other will have infinite ohms. You can connect +12 to the side with infinite ohms to run the fuel pump.

If you're getting 36 (or 35) psi fuel pressure, that's fine. I'm suspicious of your measurements, though, as the 25 seems a bit low. I wouldn't pin that on a bad FPR, as it's a pretty simple device and could only give you inch-for-inch compensation in fuel pressure as vacuum is applied. I suspect your gauge might be sticking a bit somehow -- or might be inaccurate.

[siteunseen]

I am happy to say that without the "T" fitting, and the gauge straight on fuel line before engine bay filter and after filter(had to be sure filter hadn't already clogged) was 48psi. So glad to read advice from Fastwoman about static reading, thank you, now I know that I needed a new fuel pump because now I need a set of wheelie-bars. Car runs great. All those fittings don't do well on fuel injection. I was going back to my fast boat days when you needed that fuel pressure gauge always in eye sight. No fuel would seize a piston in a second. But those days are gone. Still have a problem with brief high idle but that's another post. Thanks again.