Increased Cranking Time/Changed Cold Idle

Discussion in 'Engine Performance + Maintenance' started by 98gmc2dr, Feb 14, 2020.

Car Parts
  1. 98gmc2dr

    98gmc2dr Newbie

    Messages:
    5
    Likes Received:
    0
    Joined:
    Feb 14, 2020
    Location:
    Oregon
    Hi all, I have a 1998 GMC K2500 with the Vortec 5700 which the previous owner rebuilt about 65k miles ago. About a month ago I noticed that it took a bit longer to start than usual (typically less than a second even when cold), and today it was the worst -- it took about 3 seconds of cranking to fire up cold. It will fire right up like normal if I start it again right away, but after sitting a few hours it will take more cranking than it used to. Also, two weeks ago it began to have a different idle on start up, now when it starts it will idle up to maybe 1200 when cold, and 1000 when hot. It used to go up to 1600 and 1800 respectively. Not sure if the two issues are related, but I figured I'd post this here to get some feedback and opinions before I tried to throw parts at it.
    Thanks.
     
  2. SAATR

    SAATR /\___/\___/\___/\___/\

    Messages:
    2,272
    Likes Received:
    197
    Joined:
    Dec 16, 2010
    Location:
    Loo E Z an uh
    When you say "idle up", do you mean the rpm it spikes to when it initially fires off, or does it maintain those engine speeds?
     
  3. 98gmc2dr

    98gmc2dr Newbie

    Messages:
    5
    Likes Received:
    0
    Joined:
    Feb 14, 2020
    Location:
    Oregon
    My bad, I should have specified. I meant when it first starts it goes up to that rpm, then goes down. When hot it goes down to about 650, but when it's cold it will gradually go down as it warms up. So it spikes.
     
  4. stutaeng

    stutaeng I'm Awesome

    Messages:
    140
    Likes Received:
    21
    Joined:
    Aug 7, 2019
    Location:
    Dallas, TX
    Extended crank time before firing may be a weak fuel pump. Can you check for fuel pressure? Amazon has an inexpensive pressure guage. Should be 55-65 psi static.

    Do you have any SES codes?

    I'm not following you on the idling RPM though...
     
    kennythewelder likes this.
  5. 98gmc2dr

    98gmc2dr Newbie

    Messages:
    5
    Likes Received:
    0
    Joined:
    Feb 14, 2020
    Location:
    Oregon
    I have a buddy with an OBD reader, and so I was going to check the fuel pressure in a couple days.
    The CEL is not on.
    What I meant with the idle thing is that the 'peak' RPM right after starting has gone down. When the engine is first started it will go up in RPM then go down to an idle. The idle it goes down to is fine. What concerns me is that in the last couple weeks the 'peak' RPM that the engine goes up to before it goes down to an idle is about 500 RPM lower than it used to be. It used to be that I would start the engine and it would go up to 1800 RPM right after firing then go to an idle. Hopefully that makes sense.
     
  6. SAATR

    SAATR /\___/\___/\___/\___/\

    Messages:
    2,272
    Likes Received:
    197
    Joined:
    Dec 16, 2010
    Location:
    Loo E Z an uh
    A fuel pressure gauge and a scan tool that can read live data would be real assets here, and should be considered mandatory equipment if you plan to diagnose and troubleshoot EFI. As suggested, you need to check cranking fuel pressure. The injectors poppets require a minimum pressure (around 30psi, I think) to fire, so you will get no start until that much pressure builds in the fuel system. Cranking the truck while watching a fuel pressure gauge will tell you if the system is having trouble priming. One way to check this without a gauge is to turn the key on and allow the fuel pump to run for several seconds before trying to start the truck. If it cranks immediately after being given a chance to prime, then you have fuel system leakoff through the pump, one or more poppets, or the regulator. You can test the pump by running the pump with the line deadheaded to a gauge, and then recording how long it takes for the pressure to drop off after the pump stops running. The injectors and fpr can be checked by removing the upper intake, pulling the injectors out of their ports, and then cycling the key on to pressurize the system. If the FPR or any poppets drip fuel, then you have found a leak path. Dirty poppets can sometimes be cured with judicious application of fuel system cleaner, like techron. If the regulator is leaking back to tank, then it can only be replaced.

    Going back to the scanner, you can also watch IAC steps and make sure that it is being commanded to open during startup, and pulling the IAC valve and cleaning it and the passage doesn't hurt. Looking at all sensor data like intake air temp, coolant temp, throttle position, and manifold pressure may also point to a problem. If the coolant and intake temp aren't within a couple of degrees of each other with the truck dead cold, then one of the sensors is suspect and could cause a problem. Manifold pressure should be atmospheric pressure with the engine off. Throttle position should be zero. Things like this are important to check before digging too far into a bigger mechanical issue.
     
  7. 98gmc2dr

    98gmc2dr Newbie

    Messages:
    5
    Likes Received:
    0
    Joined:
    Feb 14, 2020
    Location:
    Oregon
    Thank you, I will try checking those various things with an OBD reader, though I'm not sure if the one my buddy has reads the TPS. Having said that, do you know where I could get an OBD reader that view all of that without breaking the bank?
    Would one just use throttle body cleaner for cleaning the IAC?
    Thanks again for the tips.
     
  8. SAATR

    SAATR /\___/\___/\___/\___/\

    Messages:
    2,272
    Likes Received:
    197
    Joined:
    Dec 16, 2010
    Location:
    Loo E Z an uh
    Screenshot_20200214-232114.png Something like this should work well. Reads codes and also performs factory tests and procedures. Fairly reasonably priced, given what it can do. I know people love to spend $20 on an OBD2 dongle and an app, but you have a hard time beating a dedicated scanner.

    Throttle body cleaner, brake cleaner, any will work. You'd be better served pulling the throttle body and cleaning the passage and valve, if it seems like it is a problem.
     
    HotWheelsBurban and 98gmc2dr like this.
  9. gearheadE30

    gearheadE30 OBS Enthusiast

    Messages:
    75
    Likes Received:
    29
    Joined:
    Oct 6, 2016
    Location:
    Columbus, IN
    1600-1800 is a lot for that after-start peak. It's possible that the ECM has just re-learned crank idle control valve position over time. Both of the Vortec 5.7 GMT400s (a 1998 and a 2000) never really jump over 1100 rpm.

    You might check your spark plugs and cap and rotor. Sounds weird, but I have had some cold start issues this winter and plugs fixed it. The ones I pulled out were pretty worn with a big gap, and apparently were struggling to ignite all the fuel being dumped in during cold starts.
     
  10. 98gmc2dr

    98gmc2dr Newbie

    Messages:
    5
    Likes Received:
    0
    Joined:
    Feb 14, 2020
    Location:
    Oregon
    My assumption was that the higher idle is from the cold weather package the truck has, RPO code V10, giving it a higher idle or something like that. It was originally a fleet vehicle from Canada.
     

Share This Page