Greetings,

My 85 has never idled smoothly, and finally had it narrowed down to the fuel pressure not holding. When switching on, I get the pressure reading between 40.5 and 47 at the rail, but when the pump stops, the pressure drops like a rock almost to zero (maybe sits at 1-3 psi). I went thru the troubleshooting steps, pinching the fuel feed line and fuel return lines in the back, with no change. Supposedly that means a leaking injector. So I proceeded with a complete TPI teardown and rebuild from the intake manifold gaskets up. I put in new FMS 24# injectors, along with a new AFPR and diaphram, and all new vacuum lines underneath the plenum- what the hell, it has 186K on the ticker and I figured do it right while it was all apart. Also had a new fuel pump put in about 2 or 3 years ago, no more than 8K miles on it.

She definitely runs better, but still doesn't idle smooth, lurching up and down, between 520 and 610 in D. Sometimes it will be "smooth" and only surge between 520 and 550. It also feels very rough, almost like a fouled plug when idling, but runs quite smooth while driving. Fuel pressure is same story, I have the pressure set at 42, but drops immediately to 2-3 after the pump stops running. WTF!!! I pulled the fuel rail and CSV, ran the fuel pump... no leaks. I even changed the CSV out with one that Jeb sent me (used), no leak, same symptoms. It also seems MPG is suffering by at least 5mpg or so, maybe more (haven't gone through a full tank yet).

Ideas anyone???

 

With the drop in fuel pressure so FAST.....I would look at the fuel pump check valve. It isn't likely that an injector would leak down so fast.

I had a similar problem where the pressure would drop to zero with a minute. Changed out the fuel pump and that solved the problem. You can change out your pump, a simple job that takes 30 minutes or less. You can run a search and get detailed instructions.

For the idle problem start by checking the setting on the TPS. This should be at about .54 . Also check to see that throttle body butterfly is not sticking. Next would be to clean and set the IAC.

 

if you ever used a product that is supposed to clean injectors,be careful, some have MEK or other solvents in them, and it can eat away at the vanes in the fuel pump, and soon, the pump will be able to produce fuel pressure, and be ok at idle, but it cannot maintain volume. I would recomment a pump change if the fuel pressure gauge needle fluctuates a lot at idle. It will be barely able to keep up the idle pressure.

 

My pressure would drop like a rock, turns out it was loosing pressure at the pulse damper above fuel pump. Piece of fuel injector hose and 2 clamps and my pressure is good and doesn't bleed off for hours.
Have you re-set timing, min idle and TPS ??

 

Don't replace that 2 - 3 year old pump unless it tests bad.



His problem is with his idle, not drivability. If the fuel pump couldn't supply enough volume for idle, the car would be undriveable.

c4_4ya, it appears that you have two, unrelated, problems. Fast fuel pressure bleed down and a rough idle. The fuel pressure bleed off isn't causing the rough idle. Since you have replaced the injectors, that leaves two places you fuel pressure can bleed off. One is through the fuel pressure regulator and the other is inside of the fuel tank. It could be the check valve in the fuel pump, the pulsator, or a hose inside the tank on the output side of the pump. You can do the hose pinching tests a little more carefully this time to determine which it is.

Idle problems are usually caused by vacuum leaks, but you couldn't have much of one, as slow as your engine idles. I would start with checking and cleaning the IAC valve and passages. Verify that the IAC is working. Then redo the minimum air adjustment and set the TPS. That may do it.

RACE ON!!!

 

I have a fairly new AFPR and diaphram, so I doubt that's it. How do I test the check valve, pulsator, and/or hose inside the fuel tank? I've done the pinch tests many times.

My TPS is set at .54, I've verified it many times. However, it is adjusted as far (counter clockwise) as it will physically move. I plan to clean the IAC, but how do I verify it's working? Also, any quick links to adjusting the min air adj.?

Thanks to each of you for all the input!

 

Just to clarify. You CAN drive the car, right? The fuel pressure holds steady while idling, right? If yes to both, while you have a pressure bleed off problem you would like to cure, it isn't the cause of your bad idle.

This should be in your FSM. With the return line hose pinched off, power up the pump. If the pressure holds when you remove the power from the pump, both the check valve and the injectors are not leaking. The regulator becomes suspect. If the pressure drops, either (some of) the injectors are leaking or fuel is returning to the tank. Now, power up the pump and while the system is pressurized, clamp off the tank output line. If the pressure holds, both the injectors and the regulator pass. If the pressure leaks down it is either the injectors or the regulator. By the process of elimination, the results of the two tests tell you where the problem is. "Leaking back into the tank" usually means a bad check valve in the fuel pump, but since yours is so new, it could be the hose or pulsator. If the tests send you in that direction, you will have to pull the pump and physically check the pulsator and the hose. If it really is the check valve in the fuel pump, it really isn't necessary to buy a pump, as all it can cause is slightly extended cranking for starting.

You will probably find the IAC test and the minimum air adjustment through a search, but your FSM, directly, is your best source.

RACE ON!!!

 

Yes, the car is driveable, and runs pretty smooth as long as it's not idling. Unfortunately, I don't have the FSM, just a pdf of the fuel system diagnostic flowchart. I have tested pinching the fuel return line several times but pressure still drops the same. Same with pinching the feed line, although I haven't pinched it while the system was pressurized- should I try this at idle? But even with the feed line pinched before running the pump, the pressure rises as normal, just bleeds down quickly to near zero- same as all other tests.

One of the tests (I think it was fuel return, I'll have to verify) I did with the line pinched resulted in the pressure going WAY higher than 42-43 psi, more like 60 or 70 which I believe is normal, but still bleeds off.

When warm, it takes about 2 or 3 seconds to start, instead of half a second when cold. I wouldn't really worry about it if not for the major reduction in mpg. Is it possible I have a bad shraeder valve or even a bad fuel pressure gauge giving me false readings?

I appreciate the input, maybe I should get a new fp gage and try the tests again. Also, after cleaning the IAC, how do I verify it is functioning correctly?

 

40 - 47 psi is way too high for a stock '85.

Was that the pressure before or after your work? The new 24 lb injectors should match your old set as those were 24 lb, so that shouldn't effect it...

Fuel pressure for priming a stock '85 should be between 34 and 39 psi. While idling it should be around 27 to 34 psi.... I would think that pressures in the upper 40's would foul plugs while idling... but that is just a guess. I never had that situation.

Anyway... I agree with everyone that has said check the pulsator.


EDIT::: Well now I see you have an AFPR. So you probably set your pressure there, so you can ignore everything I wrote. Sorry.

 

Your AFPR wouldn't happen to be one of the Holley's would it? I have the same problem as you, and after a long process of testing each of the above items it turned out to be the Holley regulator. I followed the FSM, as CFI-EFI suggested, replaced the pulsator with a short piece of hose, pulled the fuel rail, and inspected the injectors (no leaks there), and finally called Holley Technical Support, in hopes they could provide any assistance, since the only place the fuel could be going, was back thru the regulator. Spoke with a tech there by the name of Jim (as I remember, could be another name, as I'm forgetful at this age), and as he put it " Our regulators were not engineered to hold fuel pressure in the rail." I can't speak to any other brand, but i do seem to remember someone with an Aeromotive regulator having the same problem, and being told the same thing by his respective product tech.....

 

Idling requires the least fuel volume of any driving. If the car drives as you say, low fuel pressure or volume are not the cause of your poor idle. Get a FSM.



Either you are messing up the tests or you have leak-down in several places. BUT, this is not the cause of your idle problem.



[QUOTE=c4_4ya]When warm, it takes about 2 or 3 seconds to start, instead of half a second when cold. I wouldn't really worry about it if not for the major reduction in mpg. Is it possible I have a bad shraeder valve or even a bad fuel pressure gauge giving me false readings?The extended cranking is due to the fuel system leak-down.

RACE ON!!!