40 psi fuel

[efnfast]

Finally put a fuel gauge on. Ran about 40 psi at idle, about 50 when I goosed the throttle. Impending doom? Old age? Not to worry? TIA- Steve

[tomtom72]

Steve, the idle PSI number is at normal engine temp? If it is then it's about 5lbs low for a PSI result at hot idle. Also, the 50 PSI number is within 5 lbs of the max number on a "blip" than the FSM says it should be, 55 PSI is what the FSM says.

My OEM pumps at 17 yrs of age were giving me almost the same numbers, maybe a bit lower. I replaced them just because "how long could they last?" was something I didn't know the answer for. Well that and how easy would it be to do a road side swap of the pumps if the primary failed due to age.

I used chart A-7 in the FSM to do my diagnostics. I did that chart because I didn't have a 'cranks but will not run' situation. I just wanted to test the output to see what was up with my OE pumps. I decided that my results were telling me that it could be a good time to replace the OE pumps with some new ones.


Tom

[scottfab]

40psi is not just low. It is way low. Given a stock LT5 you'll see problems with starting the car even at 45psi. (got long cranking time?)
Sure sounds like new fuel pumps to me.

[efnfast]

Tom, I checked the pressure when the car was cold, didn't think it would make a difference.

Scott, I love it when I start the car when it's dead cold. Hit the key and it roars right to life. When it's warm on a restart, it does crank quit a bit.

[tomtom72]

Sorry Steve I have been busy lately at work & haven't had much internet time!

In the FSM Chart A7, the reason that it uses a cold start is that both pumps run on cold start till 140* (?) and then the second pump shuts off. Therefore it is an important fact in diagnosing a "cranks, but no start condition." On a hot restart both pumps run for seconds only, and then the secondary pumps takes an early exit.

Therefore when testing the PSI output if you do it cold you get both pumps in aggregate, and when hot you get a look at the primary pump only. Hot restart long cranking time could be a weak primary pump or a leaking injector or a combination of the two.

When I did my diagnosis of my pumps I ran Chart A7 from top to bottom, and from stone cold to normal coolant temp just to be sure I was getting valid test results. It's a real PIA to get to the fuse for the 2* pump. In the end I learned that my pumps were asking to be allowed to put in for retirement.


Tom

[efnfast]

So in short it sounds like it's time to do fuel pumps and injectors. To the best of my knowledge, the injectors are original. I'm suppose to do a 1600 mile trip next month. Am I worried?

[VetteMed]

It depends -- how much do you trust Old Man Murphy?

Changing fuel pumps and filter can be done in an hour -- I think that's a no-brainer to do before your trip.

Injectors aren't "likely" to suddenly worsen, but the can and do stick open or closed suddenly in some cases, so I don't think I'd be comfortable taking that chance, when far from home and at the mercy of a mechanic who may have never even seen an LT5.

You may want to try a different fuel pressure gauge - I have one that reads kind of low, you may not have a problem at all.

[efnfast]

Keep in mind, I'm one of those mechanics with very little experiance. So an hour job will take me longer.

Injectors will have to wait for now. Been a bad $ year.

Price on fuel pumps? Do they come out as a single unit?

[VetteMed]

I've discussed some of the procedure for fuel pump R&R in this thread:

http://zr1.net/forum/showthread.php?t=9778

The pumps come out as a single unit, along with the fuel level sender assembly.

Even if you're going to use the stock replacement type pump, and not modify the housing as I've described, the R&R procedure is the same. You can get a stock-sized fuel pump for a '96 Suburban with 454 (7.4L), probably for 50-60 bucks each.

[tomtom72]

Originally Posted by efnfast

So in short it sounds like it's time to do fuel pumps and injectors. To the best of my knowledge, the injectors are original. I'm suppose to do a 1600 mile trip next month. Am I worried?

Next month as in September? If so when? I can come up on a weekend if I know in advance so I can cover my side job.

Not to be one to throw your money around so I'll just say this about injectors, specially the OEM ones. Mine quit on me after a brief period of a mild miss, almost had to say "was that a miss?"....then all heck broke loose. Four primaries on the right bank took a vacation all at once with me about 200 miles from the barn. Needless to say I'll know the true extent of any damage from an excessively lean condition on that bank when my headers finally go on. I suspect that I could have a few valves that maybe in need of attention. JMHO on the real cost of me ignoring that very slight miss. I knew about the injector issue on the early cars and for fear of never having worked on an LT5 chose to check the plugs instead of changing injectors.


Tom

You have my phone number still? Call me so we can set something up Steve! Road Trip!