Okay... background first.

I'm trying to figure out why my L98 mainly stock coupe has a slight idle problem. I replaced a few items this morning (including an idler pulley that was shot), and took it for a drive. Idle problem still remained. I was told by a few people that I may have a vacuum leak. I've gone over all the lines, and cannot find anything wrong, in addition to that: I hooked up a vacuum gauge and got a steady 20+ inches of vacuum. I'm sure that my vacuum system is good.

So for sh*ts and giggles I hooked up a fuel pressure gauge to the shreader valve to see what thats doing. Turn the key to the *on* position, look at it says 15 PSI... WTF?! For what its worth, that number held for a while... But I'm supposed to be at 35PSI (1985 spec).

Does this mean my fuel pump is shot?

Fuel pump, strainer, and in line filter are all 2 years old. The age of everything else is unknown, but I assume it's 1985 if I haven't changed it.

Help?

 

Make sure the guage is connected good (if not you should have seen the pressure drop and seen gas leak). Make sure there is plenty of fuel. Any restriction in the lines will cause a low pressure along with a faulty FPR or weak fuel pump. There should be 12v at the fuel pump connector with the engine not running and terminal G hot wired...14v with the engine running. Pull the vac off the FPR and look for signs of gas, if no gas then change the fuel filter and pull the pump and look at the sock and strainer. If all check out good then the pump would be suspect.

If you have put fuel/injector cleaner in it....it will sometimes release trash into the fuel filter causing low pressure. Also the fuel pressure guage could also be faulty, but not likely.

 

I saw no signs of a gas leak anywhere, and I rechecked the shraeder connection a few times because I couldn't believe it was only 15 psi.

The fuel pump sounds bad when I energize it, but it sounds as though there could be a restriction in the lines. I have used fuel cleaner in it a few times (seafoam), and I know the filter in it is a cheapy one, so I think I'll try that out.

because of the sound, i dont think the fuel gauge is bad...

 

Whoa! Don't jump the gun, here. If a fuel pump is weak, the higher speed running is affected sooner more severely. If your only problem is at idle, it isn't likely to be a fuel pump or even fuel pressure problem. I would expect to see more tha 15 psi after the 2 second priming period, but is that the only point at which you checked the pressure? A leaky injector or a faulty pressure regulator could have bled off all the fuel pressure before you checked it at priming, and the 2 seconds may not have been long enough to establish proper pressure. Or, through leakage, they may have prevented the proper pressure from being attained.

*I* don't think the idle is caused by a fuel pressure problem. The 15 psi after the priming period, needs investigating, but I wouldn't be too quick to condenm the pump.

RACE ON!!!

 

I agree with that, I'm not sure what is causing my idle problem, and I don't think its my fuel pressure (I'm actually leaning towards 02 sensor now, but that's for another thread)- however, in checking for issues, I came across this, and it seems as though I should give this some attention. Once I'm comfortable with my fuel pressure then I'll get back to the idle problem.

I only checked it during priming, I didn't have the car running. Also, I didn't see the gauge immediately upon priming, Only after I turned off the key and walked around to look (5 second lapse?).

I definitely don't want to throw a fuel pump at it without being sure it will cure the problem.

What I do know, is that I don't experience any symptons of leaking injectors (such as a warm start problem).

This is a possibility, I suppose. I'm not at all familiar with FPR's. But my fuel pressure was holding at 15 psi for a while, and I had good vacuum as the regulator (about 22"). Also, I would expect that if the regulator bled all the way down to 15 psi, it would continue to bleed quickly down to zero.

Oh, and I don't smell gas fumes anywhere that would indicate external leaks...

So what I just bought was an inline fuel filter (an AC Delco this time). I don't mind throwing a fuel filter at it, since its been 2 years since I changed the original anyway. Plus it was only $5.

What is leaning me towards the pump is the noise that it makes during priming.

Thanks for the advice all... I'll report back after the fuel filter install- and I'll leave the FP gauge on while the car is running to get a few different readings.

EDIT::: I need to buy a new FSM... I no longer have access to my old one

 

I found my Fuel Pump Reciept.

It's got a lifetime warranty on it...

maybe I will just through a new fuel pump at it since all it will cost me is my time..........

 

Connect the gauge and monitor it, from before you touch the key, through the priming period, cranking, running, including revving it up, and after shut down. If you can get a free pump, great, but it would pay to trouble shoot what is going on before hand. If the new pump masks or hides another problem, the newly hidden condition may jump up and bite you when you least expect it.

RACE ON!!!

 

I'd probably replace the inline filter, check the fuel line for kinks etc. and then recheck the pressure...turn the key to the ON position, then I'd check at idle, and also take it for a drive with the guage taped to the windshield. I'm leaning toward the fuel pump for a few reasons,first the pressure.if this is correct,(barring any leaky injectors or a bad FPR), second the noise from the pump you're hearing, and lastly because of the experience Da mail man posted a couple weeks ago about his fuel pump.

 

First - thanks for any input. This is the first I've ever monitored fuel pressure on a car, so I'm learning... slowly.

Okay... just back from the garage with a little more info. This data is with a brand new AC-Delco inline filter.

Well... I had just let out all the pressure because I changed the fuel filter, so that was 0

Slighty above 15 PSI.... held for about half an hour then slowly worked its way down to 10 psi

Steady at exactly 30 PSI...

no difference from idle... throughout the RPM range it says right at 30 PSI.

Dropped to 20 PSI, then it held just as before (when only priming).

I'll have to search for that thread... You and CFI-EFI must have been posting at the same time. I think I hit all of your points.

I haven't gone and gotten another fuel pump yet... Even if I can get a free one, I don't want to if it's not the problem... But what do you all think? Is there something else I need to check?

 

When you replaced the fuel pump last did you also replace the sock?

 

Yep... see my first post. Strainer and a new pump two years ago.

 

My Haynes manual (yeah... I know), says:

"If nothing is leaking- block the fuel return line and energize the pump... If the pressure is still less then 34 psi, replace the fuel pump."

This is the very last step in that manual's pressure check troubleshooter. I've done every step before it except this one.

How do I 'block the fuel return line' ? That seems like my final check before a new pump.

 

Easiest to remove the gas lid, gas cap, then the rubber boot. From there you will see three lines leaving the tank. I believe the one on the left is the EECS (vapor) line, the one on the top right is the feed line and the one on the bottom right is the return line. Use a pair of vise-grips and some material, so not to tear the rubber.

 

You know you want to change that pump, just do it! #1 its noisy. #2 its free. #3 Piece of mind!

 

I found a good picture of that in my crappy manual and blocked off the line...

Pressure (from energizing the pump) read 15 psi, like always...

The pump wasn't that old, I couldn't believe it was bad. But after troubleshooting far enough, now I will. I shall return...

 

Did you try to pinch off the line while the engine was running? If so the pressure should shoot way up fast.

Pinching the return line off after energizing the pump is how I have checked for leaky injectors.

Waiting to hear the results after the swap...

 

new pump installed (man that's an easy job). And the results were okay, though not as good as I was hoping.

Priming pressure is at 30, and running is at a steady 32. This pump sounds much better priming, so I'm sure the old one was shot... however....

I guess from here I need to ohm test my injectors... But that's a topic for another thread, on another day.

Thanks all

 

Mikey, do you have an adjustable fuel pressure regulator?

 

As far as I know it's still the original 1985 delco part.

 

My original fpr gave me 40 psi most of the time, vac on. I have read normal pressure should be in the mid 40's and I have read pressure should be in the mid/upper 30's. Can't say which is really true and I am running 34 psi with 24 lb injectors and there is no miss, stumble, ect...

Good luck with the other projects and let us know how they go..