Random starting issues fixed by tickling her Schrader

Discussion in 'Performance & Fuel' started by subgyro, Jan 19, 2013.

  1. subgyro

    subgyro Member

    In the last couple years, I've had this happen twice; she'll crank and crank and crank but just never fire. Mess around with the Schrader valve, and voila! Vroom!

    Two years ago my mechanic neighbor used his fingernail to check for pressure and squirted gas all over himself. Last week, I put a fuel pressure gauge on it, suspecting the fuel pump. I did have 60 when the pump ran, then it dropped and held at 58psi. My Haynes says I should have 60-66, and I don't imagine 2psi would make the difference in whether the truck actually starts or not. But of course she started after this test.

    I assume I should do my fuel pump regardless, as the big ol' gurl has 230,000kms on her and her pump is certainly noisy. Sat beside a co-worker's unit and couldn't even hear his pump over mine, and his was done last year. I'd hate to be stranded out in the mountains hiking half-way home to get cell coverage to call a tow truck!

    But why does messing with the Schrader valve wake her up? Am I getting some kind of vapour lock? Or is my pump enjoying messing with my mind?

    My Bourbon: 1999 GMC K2500 SLT, 230Kk, no real mods, just slowly replacing old bits with brand new ones. Soon I'll have a brand new '99 Suburban!

    I thank you all in advance for your consideration!
  2. Skippy

    Skippy Member 2 Years 100 Posts

    Fuel pressure problems may be a result of an unpressurized line as a result of fuel pressure bleed-off while the vehicle sits (with engine off). Common with fuel pumps that are going out, basically, when the engine is off, instead of maintaining fuel pressure, the fuel pressure slowly drops off. A single prime of the pump may not be sufficient to pressurize the lines enough to start.

    Symptoms commonly include folks believing that 2 or 3 starts will make it work (then it magically works everytime after that for a while). instead of attempting to start the vehicle, turn it to the electrical ON position (allowing the pump to prime) for 3 seconds, turn it off, then do this again twice more. You'll find that the line pressurizes and bingo, the engine starts the first attempt.

    This will get worse over time. Eventually, you'll be able to see the fuel pressure dropping WHILE your gauge is connected. To do this, connect your gauge to the fuel line, engage the fuel pump (it should pop to 60+lbs) then turn off the vehicle. A good pump will hold the pressure and will drop only very slowly if at all. A bad one will allow the fuel to leak back into the tank at a very quick rate (visible on the gauge).

    This is called a "Leak Down Test."

    Please also note, the pressure problem may not actually be a fuel pump issue! Other leak areas include the fuel pressure regulator and bad fuel injectors. For those to have serious drops in line pressure, you'll have other issues. If you are having major pressure problems when the vehicle sits idle, the fuel pump is a likely culprit. If it's only very minor, you may want to check the regulator and monitor your O2 sensor readouts and fuel trims for what's going on in the engine (leaky injectors cause rich conditions, which then force the engine to "lean up" your fuel trims). Don't think the engine would throw a code at the first sign, though, the engine codes are built around emission compliance. It won't throw a code until it gets really bad. A good live feed scanner will be able to show you what's going on.

    If your fuel pump is already making some weird noises, I'd suspect that first. I just had one of these exact scenarios just 2 months ago come through my garage, it was the second pump in 6 months (it's been my experience that about 10% of all aftermarket pumps have problems that require new pumps within a year... save your receipt!)

  3. subgyro

    subgyro Member

    Wow, Skippy, thanks for all the info. Couple questions back at you though...

    It had been 1 week since I'd tried to start the ol' gurl when I put the pressure gauge on her. The previous time I let the pump prime (as always) before cranking her over, but to no avail. Off again, on, crank. Repeat numerous times. I tried starting her long enough for it to start sounding like my battery was getting low before I stopped.

    Would my cranking have negated any pressure the pump was trying to build up?

    Also, does 0psi sound right after sitting for a week?

    I found Delco pumps for a reasonable price online, probably better to get that than some other so-called OE brand, no?

    Thanks in advance,
  4. Skippy

    Skippy Member 2 Years 100 Posts


    Yes, cranking can negate pressure in the fuel line if it isn't restored as quickly as it's being used. Just a quick note of caution though, don't let your starter attempt to crank the engine for more than 3-5 seconds at a time without a cool down. You definitely don't want to burn out your starter on top of having fuel problems!

    I would suggest you turn the ignition key to the on position (WITHOUT cranking the engine) 3-5 times, each time listening for the fuel pump prime. Once you've done that, this would be a great time to check the fuel line pressure with a gauge (or better yet, have a buddy watching it while you're priming). The fuel pressure should hold in the line and drop no more than roughly 1 PSI per second. (you may see a little bump down right at the immediate time the prime stops, that's normal.)

    No fuel line pressure after a week is normal. A properly functioning Fuel system should pressurize in the first prime, though.

    As for "off brand" pumps. It's been my experience that about 10% of them fail or don't function well within a short time. I use them because most times the cost difference is significant enough that it's worth it to the customer (and me, when I've done it for my own engines) to use the less expensive part, and potentially have to do the job twice. I always let them know up front, so if they have problems, they know to bring it back. AC Delco pumps would definitely be a more reliable system.

    You may find, though, that if your leakdown test shows you're losing pressure like crazy, that you have problems elsewhere. This is where it gets to be needle in haystack... Leaking fuel injectors are notoriously hard to diagnose without an engine code telling you which cylinder is having issues. You can roughlyi figure it out on a V-8 by checking the fuel trims and O2 sensor output during live data, but you've got to have the equipment (a hand-held $300 scanner is usually sufficient), and even then you're looking at a bank of cylinders (half of a v-8)...

    One other thing you might check if you actually CAN'T start the vehicle. Double check the fuel safety cut-off switch hasn't been tripped. You'll have to check your owner's manual for specific locations. The fuel safety cut-off switch disconnects power to your fuel pump in the event of an accident. basically, if you can HEAR the pump priming, it's fine.

  5. kinson33

    kinson33 Member 1 Year 100 Posts

    To piggy back on this topic:

    I am having a similar problem, truck has to prime for a few seconds after sitting for more than 30 mins. Buddy at advanced looked at the trims and everything is looking right when the truck is running.

    Safe assumption it might be the valve in the pump that's letting it leak back to the tank?
  6. subgyro

    subgyro Member

    So I didn't have any issues for a while so didn't continue troubleshooting. Then, yesterday morning, cranked with no fire again. And I always listen for priming before cranking, which she did.

    I turned her completely off, heard the random noises she makes shutting down, turned her back on, and heard no priming. Tried once more, still no priming.

    My untrained mind says it is for sure the pump now. Comments, concerns, outbursts?
  7. Skippy

    Skippy Member 2 Years 100 Posts

    The only thing I'd suggest before dropping that kind of cash is to check the relay. Find the fuel pump relay and swap it with another relay in the fuse box. For example, the horn or a light relay will work. If you find the problem goes away, you likely have a relay issue. (cracked solder can create weird anomolies due to heat/cold expansion).

    If that's the case, then it cost you nothing for a diagnosis, and about $15 for a relay, and a WHOLE LOT less effort!


  8. subgyro

    subgyro Member

    And THAT's why I posted here first! I shall report my findings!
  9. subgyro

    subgyro Member

    Wow, look at that! It's been a year since I've updated this!

    So I wasn't able to swap any relays, as none of the others had the same pin configuration.

    At the beginning of the summer I decided to do some preventative maintenance in the manner of plugs, wires, cap and rotor. The rotor did have some corrosion on it... And, I haven't had the weird starting issue since! Does that make any sense?

    Admittedly, I have been using the truck more often, it is all I drive in winter. So she hasn't had the couple months to sit again.

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