TPS issue and Dealer

So, no more codes, no CEL. Cold start is a hard start, revs to 1500, drops to 400 and then goes to normal idle speed?
Engine coolant temp reads properly?

Maybe leaky injectors?

Someone here has been mentioning the flood clearing procedure..
Pedal to floor, crank engine over (blowing excess fuel out).
Then re-crank with foot off gas. ? I think that was the gist of it...

Sundancer said:

The last 2 days our temp has been in the teens here and the truck give me an extreme hard time to crank up. I have owned it since new and it never has done this before or given me any major issues like this. It is NOT showing Any trouble or Engine light and on the repair sheet it says NO TROUBLE CODES STORED.

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Go on Amazon and buy an OBDII adapter that works with Torque Pro for Android or Car Scanner for IPhone. I use a Foseal Wifi adapter for the Androids and a Bluetooth one for the IPhones.

Anyhow, get that, set up your phone and find out what your alcohol content is. As the original sensors age, or you use aftermarket sensors, they begin to not calculate the alcohol content in the fuel correctly and this creates hard starting issues.

This gives you the ability to see engine functions and clear codes but not reset the alcohol content to my knowledge. For that you need a bi-directional scan tool. Since you just came from the dealer and paid for repairs, and if your alcohol content is indeed the problem, they ought to reset it for you on their dime, since they replaced something that didn't fix the problem.

Oh boy, Whuddah mess. For the record what typically comes up that shows no code is …

Carbon canister leaking its pellets into the fuel lines and tank. If you had the fuel pump changed they should have changed that too. The fix, if they find pellets in the fuel lines is …

Blow out the fuel lines, drop the tank again, swab the pellets out of the tank and ask them kindly to warranty the fuel pump with an OEM gm unit if they used a generic one because generic typically last 1 mo longer than the stated warranty.

RockAuto & PartsGeek have OEM f/p w/ lifetime warranty for 2-$3h.

The crank position sensor / cps can die and not throw a code. Long starts are the telltale.

Bottom line is that you need someone to diagnose it, someone competent. You can collect all sorts of data but need to understand what that data is telling you. If you want to dig into it, do what @swathdiver recommends and start logging some live data. Check the codes again with a good scanner to see if any are Pending or History codes; not all code readers will see these. Keep a notebook and document the conditions when it happens (after a hot soak, on a cold start, after re-fueling, ambient air temp, etc.) and what you did to get it started.

If changing parts, stick with GM OE ones for the critical sensors. Less expensive aftermarket sensors tend to cause issues and it's worth the $$ to use OE for reliability. Buy from trusted vendors, not Amazon.

Are you describing a slow crank? Or a normal fast crank but it takes a long time for the engine to start?

What kind of fuel is being used? Anything over 10% or 15% alcohol? Any use of E85?

Look a the MAF reading in g/sec at hot idle; it should be very close to the engine displacement in liters (e.g., around 5.3 g/s).
How's the air filter? Is the tubing from the air filter sealed all the way to the throttle body, with no cracks or disconnected air hoses?
What do the hot short- and long-term fuel trims look like, both at idle and at 1800 rpm or so?

There are some other good suggestions given, too, like tryng the clear flood mode when you have a good crank but no start.

Long crank time can be caused by many things (and this likely isn't all of them):
Weak fuel pressure or pressure leaking down with engine off (leaky injector, etc.)
Incorrect calculation of the alcohol content of the fuel (there was an updated GM 'calibration' issued for some engine computers to cure this)
Purge valve on the engine incorrectly allowing fuel vapors to enter the intake, causing a flood condition when attempting to start
MAF sensor mis-reporting
Engine coolant temperature mis-reporting
Defective crank position sensor, or sensor not synced with the cam position sensor (CASE relearn)
Defective fuel pump control module

That's quite the list, but some good live data and knowing the fault conditions would go a long way to get a focus on the cause.

Sundancer said:

Folks, I am back with some much needed advise. In my original posting, I described the issue of that my Check Engine light went on after a long drive from NY to Richmond, Va and was diagnose at Auto Zone as a P0121 - Throttle Position Sensor., which was replaced. Came back home worked perfectly for about a week, then the truck started giving me issue when starting. Under advisement from my long time mechanic, I took it to the Chevy dealer and got it back 7 days later because they were so backlogged. They replaced the battery, Sensor, performed throttle relearn procedure, cleaned the throttle body, performed the G.M. multi point inspection and still the RPM's go from 1500 to 400 and then levels out on initial start up. The last 2 days our temp has been in the teens here and the truck give me an extreme hard time to crank up. I have owned it since new and it never has done this before or given me any major issues like this. It is NOT showing Any trouble or Engine light and on the repair sheet it says NO TROUBLE CODES STORED. I called up the dealer and they suggested that I change the Mass Air flow Sensor since they were back ordered on it, which I then did. but no noticeable change in idle and start up.
Truth of the matter is that since I am not mechanically versed, I am looking to repair this issue and sell it/trade it and move on with a newer Truck.......................ANYONE have any ideas on why the dealer cannot come up with a solution.

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this was the post your issue made me think of.

Thread 'Update on Idle dipping and fluctuation problem' https://www.tahoeyukonforum.com/threads/update-on-idle-dipping-and-fluctuation-problem.144229/


I don't know for sure it would help. Just a thought.

I liked the 15% alcohol part. Cheers!