97 Chevy C2500 silverado

chevrolet
2500

#1

Ok, so when i go to turn my truck on she’ll sit there and chug as it starts. usually a few seconds and then roars up just fine. However I’ve noticed a couple of distinct things.

  1. when it does start i see a second or three of black exhaust come from the tail pipe.
  2. if its been started recently. IE within 24 hrs it has problems. but if i leave it over a weekend then the first start has no problems.
  3. Last weekend we replaced the Fuel filter and the first few starts there was significantly less problems starting but after 5 starts its back to doing it again.

I checked and the fuel pump does work, I’ve tried changing the fuel filter ( #2 above), and put sea foam into the gas tank hoping to help clean out any gunk in the lines somewhere. It does turn on and when it does it runs great so i know its not having electrical problems as well as the alternator is charging like normal.

Im about to take it in for an oil change and see if that does anything, and I’m praying its not a clog in the Catylitic converter.
Im in the Air Force so i don’t get a ton of time to work on it and I’m relatively auto dumb but i have a friend that is much more auto savvy. That can help me if i can figure out whats going on.

Thanks for any help!


#2

To me, it sounds like a fuel pressure problem

Black smoke upon startup leads me in that direction

Have you measured fuel pressure

Key on engine off
Key on engine running
Does it hold after shutting off the engine

Does your friend have access to a fuel pressure gauge set and an injector balance tool?
That will quickly give you those answers

By the way, is the check engine light on?
If your friend has a code reader or scan tool, check it out


#3

When they change the oil ask them to check it for signs of excess fuel. One possibility is that your fuel injectors are leaking fuel down while the truck sits, and you are thus starting with a flooded condition. The excess fuel in the oil would come more from when it sits a couple of days. In these cases it would start up ok because the leaked fuel would disappear from some combination of leaking down to the crank case and evaporation.

The only way to find out if the fuel pump is working correctly is to put a gauge on it and find out if the pressure is with specs. If you did put a gauge on it and run it until its warmed up and then watch the pressure after shutting down, my guess is that pressure wouldn’t hold long. And furthermore, that if you pulled a spark plug or two you’d smell a lot of fuel in there.

Other than that - how’s the oil consumption? Do you have to add between oil changes? If so how much per how many miles?


#4

Does your base have an auto shop? Perhaps your friend and you could spend some ti9me there.

Black smoke is carbon, from too much gas in the mix. Since it’s happening upon start up, but not when it sits all weekend, I’d suspect that an injector or two is leaking fuel into a cylinder when shut down, and it’s evaporating over the weekend. That’d give you a black cloud on startup.

But that’s just another guess, which is what you’ve been doing. Injectors can be bench tested. See if your base shop has the equipment to do so or make a few phone calls and find someone who can.

A cat converter will not cause this. And, since it’s only happening on start up, you might not be storing a fault code… but check anyway. You might get lucky. A leaky injector won’t store one unless it begins to interfere with regular ignition, so don’t let the lack of a code (if there is none) mislead you into believing your injectors aren’t leaking.

The oil change is unlikely to do anything either, but of you need one do it.

By the way, you have the sincere thanks off a grateful nation, myself included, for your service.


#5

@Soundchief10‌

What engine do you have?

7.4
5.7


#6

Thanks everyone for the quick responses.
Im getting the general consensus is that its leaking fuel via the injectors somehow. i’ll have my buddy look into that when he gets back. Put leaking fuel out cause it never showed signs of missing fuel. burns fuel and efficient as i could ever ask for. ( i was surprised for its year coming off a 92 jeep cherokee which got worse ) but i will defiantly look into it. I just bought this truck so I’m going to reset all the timers for maintenance.

@db4690 i think its the 7.4 3/4 ton V8 and no check engine lights and doesn’t seem to burn oil.
@the same mountainbike We do and thats usually where i do all the work we have so far. i’ll look into what you suggested and you’re very welcome.

Again thanks all. i’ll see what i can find and post on here what it ends up being when i finally fix it.


#7

Leaking injectors won’t really take enough fuel for you to notice. All that can leak down is what is present in the fuel lines when you shut it down. That’s not a lot - but enough to cause the starting issues. And if it doesn’t burn much oil, then I’d say that increases confidence in excess fuel as the culprit. Another thing that needs to be checked is the fuel pressure regulator, but a problem with that would show up with checking the fuel pressure, and it would likely cause other symptoms while running.


#8

The fuel pressure regulator may be leaking. Check that first.


#9

Another cause of black smoke lasting a few seconds at start up is leaking valve stem seals. What is your oil usage?


#10

This truck has vortec spider injector system? Maybe you need refurb intake assy.


#11

I believe OP said the truck had the 7.4 big block

That would mean a conventional EFI system . . . no spider