I have a '96 GMC K1500 Suburban with a 5.7l.
Shockingly, the local GMC shop techs can’t answer this question for me, just as they can’t answer any other questions without a service appt or, heaven forbid, offer a part number for any component ever made by GMC. Making a service appt to ask this question seems absolutely absurd to me so I’m posing it here.
Does anyone know which component meters the fuel at start-up, based on engine temp (i.e. dumps more fuel in for cold start and less for hot start)?
Only at start-up. It’s the only time I have a certain issue. Not looking for injectors or check valve or fuel pump, they are fine. I’m trying to figure out which component actually takes the reading from the engine coolant temp sensor and meters the fuel at start-up based on that reading.
Thanks in advance.
the amount of fuel on a cold start is a fixed value and the pcm does start to adj. the air:fuel ratio untill the o2 sensers are up to temp. and the coolant temp senser reaches a unknow to me temp.also known as open loop then it goes into closed loop and the pcm adj. the air fuel ratio based on the imput from the o2 sensors and other sensors for load and needs at that moment. if the fuel pressure is off even by a few psi that can couse a cold start problem
Like @Big Marc said, the ECU does that. It basically guesses what the fuel needs are based on other sensor inputs, leaning towards the rich side,until the O2 sensors warm up.
BTW, what specifically is that cold-run issue?
The computer determines the air/fuel ratio from the information it receives from the coolant temperature sensor.
When the engine is started cold the computer is in the open loop mode. The computer at this time only takes information from crankshaft position sensor, throttle position sensor, and the mass air flow sensor to determine the air/fuel ratio. Once the engine reaches operating temperature or goes into the closed loop mode, the computer then uses the oxygen sensors to determine the air/fuel ratio.
The collective insights from this site never cease to amaze me. I am so appreciative for the help, folks.
Hey BustedKnuckles. I won’t run on with this one here because I already have a thread opened for the issue. The cold start issue in a nutshell is motor fires immediately when cold, cranks for 3-5 seconds before firing up and running fine when engine compartment hot. As this issue has continued, I have started seeing a slight bit of black smoke at hot start-up now, suggesting the issue is caused by too rich of a mixture only at hot start-up. I’ll copy this response over to my thread entitled “Acts like vapor lock when hot” (or that’s close to the title) if you want to throw in any opinions on it.
My answer is that as you turn the key to start the car the engine computer looks at coolant temp sensor, intake air temp sensor, MAP sensor (to take a barometer reading), and throttle position sensor (to verify you’re not holding the throttle wide open), to come up with a fuel injector pulse width to get the engine started. Once it’s running, the ecm looks at those things plus air flow sensor and engine rpm to meter fuel.
From your description in your other post I’d suspect a leaky fuel pressure regulator. Have you installed a fuel pressure gauge and monitored for pressure drop off after a hot stop?
Which component meters fuel at startup? The ECU (the car’s computer).
It does it based on engine temperature, engine speed, throttle position, mass airflow, and on some cars manifold absolute pressure. It ignores the oxygen sensor and uses a preset signal instead until the engine warms up.
If you’re getting black smoke at a hot start and extended crank times, I would suspect a leaking fuel pressure regulator. That’s the most common problem with your description. The regulator is located inside the intake manifold. the throttle body can be removed to view the reg.
If the regulator tests bad, might I suggest to just install the newer and upgraded fuel injection assembly?
This unit comes with the regulator. Plus, the injectors are true EFI injectors, rather than those poppets that you have. It’s a plug and play replacement. All you need to is remove the upper intake to get to the fuel body. You can find aftermarket units for less than $300