[dmarty]

I’m new here and have reviewed old threads, but still can’t solve my problem. Any help would be greatly appreciated. I have a 99 Silverado 1500 4x4 with 5.3 and 227K miles. I’ve owned it since new and maintained it well. Recently it requires a little more cranking and stumbles briefly on cold (Wisconsin winter) starts. It has always started immediately with very little cranking required. Now I have to crank it 1 or 2 seconds before it starts and then it stumbles and runs rough for 1 or 2 seconds before it smoothes out. I sometimes have to try 2 or 3 times before it catches, other times it starts fine. Not a big deal, except that it will probably get worse, so I would like to fix it now. Idles and runs good after it starts and smoothes out with no error codes and mileage is good at 17 mpg.

The engine is bone stock. I use only OEM parts (air filter, plugs, etc.). The engine sensors and controls (MAP, MAF, IAC, TPS, injectors, O2 Sensors, EGR, etc) are all original. I have replaced the plugs and wires every 100K miles. Intake gaskets were changed at 100K when I replaced a noisy lifter. Fuel pump and sender were replaced at 150K.

I first noticed the problem after I cleaned the throttle body with TB cleaner. I do this about once a year as deposits cause it to be a little sticky when the throttle is first depressed. Can’t say if it started right after I cleaned the throttle, but soon after. It was also getting pretty cold about that time, so it may just be a coincidence.

So far I've tried doing a water spray test on the intake right after a cold start. Tried it several mornings. No steam out the tail pipe, so intake gaskets seem good.

I thought the problem may be caused by low fuel pressure and that after the pump ran a few seconds, the pressure built up and it smoothed things out. I tried turning the key directly from OFF to Start without pausing in the ON position and compared that with keeping the key in the ON position for 5 seconds before cranking and starting. No difference detected. I don’t think it’s low fuel pressure due to the test results and the fact that the pump was replaced at 150K. I haven’t measured fuel pressure yet, but could do it.

I tried several tankfulls of mid-grade gas (usually use regular) and bought gas at different stations. No difference, so I don't think it's bad fuel.

The injectors have 227K on them, but the MPG is good and it runs fine and idles good after it starts. I’m thinking injectors are ok.

I’ve noticed that it seems to start and run better if I depress the accelerator slightly before I turn the key. I’m thinking that the IAC may be bad or the throttle stop, set screw may need to be adjusted. I have a GM manual and it doesn’t give a spec for throttle plate to throttle body clearance. I know it comes from the factory preset and it’s not to be changed. Can the clearance change over time? Can the IAC stick and not work right at startup, but work fine once the engine is running? How about the TPS?

Any thoughts, comments or suggestions would be greatly appreciated.

Thanks