Hard start after it sits a couple days?

10X

MEMBER
1998 CHEVROLET SILVERADO
7.4L
V8
4WD
AUTOMATIC
135,000 MILES

Hard starting after sitting a couple days. I have to turn the key on and crank multiple times before it starts. I have checked fuel pressure and it's good, about 55 PSI even before I crank it. Eventually it starts slowly and rough once it starts it smoothens put and runs great all day. Scanned it for codes, none. Also, I checked the coolant temperature sensor first think before starting 80f which is ambient temperature here in Florida. I watch the temperature reading once it starts and warms up and sensor is reading fine going up to about 198F. Plugs and wires are new.
I checked for misfires while running and all is good. I was thinking maybe the MAF might be bad, but I never have come across one causing a hard start, usually they run bad. I have no codes tripped; I am lost.

1 Reply

AL514

MECHANIC
4,572 POSTS

Hello, it sounds like you have a good idea of what you are doing. Do you have a scan tool that can monitor live data PIDs? Also, this truck has 2 fuel pumps, the one in the fuel tank and then there's a balance fuel pump on the inside of the left frame rail. Fuel pressure spec is 56-62, you might be having a fuel volume issue. Have you done a fuel filter in the last 20k miles or so? With this size engine and at a cold start, one concern might be fuel volume, I know your pressure is staying okay.

If you can pull up live data, I would graph the front Oxygen sensors and the rear ones, see if the signals look like after its warmed up and running. If the rear oxygen sensors mirror the front sensors, Id suspect a Catalytic Converter at that point. Another thing to look at is the Long Term and Short-Term Fuel trims, to get an idea of how much fuel correction is happening with the ECM. If you notice positive fuel trim numbers at idle and the numbers, come down after you start to drive and the throttle plates are open, that will point to a vacuum leak. Fuel trim numbers can tell you a lot about the running condition of the engine. So can the oxygen sensor readings, a wide-open throttle test, the oxygen sensors should go rich, if they lean out (low voltage) then suspect a fueling issue.
Do you have good strong spark too. There's a few things to check.
One other thing is checking to see if the throttle plates have any carbon build up on them.
Something I forgot to mention, the Fuel Trim with Chevy/GM can also be called the Block Learn. That would be a long-term fuel trim in memory.

Here's a couple TSBs I came across as well:

https://www.2carpros.com/articles/engine-cranks-excessively-before-starting

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

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