What Causes A 99 Tahoe 5.7 Liter To Stall Out For Shifting Gears

Why does my car momentarily lose power after shifting gears?

You've asked several questions here...and also given some information that may have an impact on the answers you might get.   You mention that the clutch is about to "go".  If this is the case, it is typically manifested by "clutch slip" which is when you press the accelerator and the engine speeds up, but the car doesn't go any faster..i.e. the clutch is "slipping" against the flywheel/pressure plate without transferring the increased rpm of the engine to the transmission.  If the clutch is indeed slipping, then this phenomenon is most noticeable right after you change gears, because your putting more demands on the engine (i.e. accelerating) and attempting to put more torque from the engine into the transmission (via the clutch) which causes it to slip more than it might in steady-state driving at a constant speed.   If you are experiencing a slipping clutch you most likely need to replace it and afterward the car should accelerate smoothly and there will be a synchronicity between engine speed and car speed.Now, if your clutch is NOT slipping and your experiencing a perceived loss of power after shifting gears, it may just be that the engine rpm are dropping off too far before you reengage the clutch and resume acceleration.  If this is the case, you may be taking too much time between disengaging the clutch (by pressing the clutch pedal) and re-engaging it (by lifting off the clutch pedal)  When shifting gears, the clutch pedal should be pressed and released fairly quickly as part of a smooth, movement synchronized with moving the shift lever from one gear to the next. If you are pausing in the middle of the shift, or taking too long to reengage the clutch, the engine rpm will fall and the engine will be making less power than it was at the higher rpm level it was when you initiated your gear change.  The idea is to keep the engine in a range of rpm that produces enough power to smoothly take up the level of acceleration you're trying to achieve while changing up through the gears.

Why does my car idle at a really low RPM while stopped, and start shaking?

You may have a misfire, a vacuum leak, or a clogged/faulty idle air control valve. It could be a couple other things, too, but those are the most common and first things you should explore. This is because:If you have a misfire, one of your cylinders is not working and while that may be less noticeable at higher speeds when the engine is spinning really fast and the throttle is wide open, at lower speeds the difference is more apparent.If there is no misfire (and no codes / check engine light), the problem is most likely air related. This could mean a vacuum leak from a hose or the intake manifold itself. Or, commonly, it could be that your Idle Air Control Valve is clogged or not working well. The engine monitors via sensors how much air is coming in vs engine speed and uses that to control fuel injection and ignition timing.If it's a vacuum leak, the engine is getting more air than it thinks it is and that is causing a 'lean' fuel/air ratio.If the IAC valve (and its passages around the throttle plate) are clogged up, the engine thinks it's getting more air than it really is at idle with the throttle plate closed (foot off the gas), resulting in a 'rich' fuel/air ratio.All of these scenarios will probably cause some sort of engine code and checking them should be your first step. Note that an oxygen sensor code will be indicitave of one of the above 'air' situations because it detects how rich or lean the ratio is.

What are the main causes of high oil pressure?

Oil pressure in a car is measured as the resistance to flow between the oil pump in the oil pan and the bearings in the crankshaft. If it is actually too high, it means you are not getting enough oil through the bearings to keep your engine from self destructing.The higher your rpm, the higher your oil pressure, up to the point where an extra valve detours the excess oil back to the oil pan to recirculate. Typically, at idle you will have 10 to 15 psi, with 30 to 40 psi at driving speeds.The colder your engine, the higher your oil pressure, as the oil is still much thicker than it will be at operating temperatures. Typically, on startup, you will have 40 to 60 psi, which will drop to ranges listed above once the engine heats up.A high oil pressure reading on your gauge means:The oil is too viscous (thick). Most cars today are designed for 0W-20 to 5W-30 viscosity. If you are using 10W-40, 20W-50 or something like that, you will have high oil pressure and high wear.Your filter bypass valve on the filter or in the block is stuck closed, and the oil is too thick or dirty to pass through the filter media, trying to go through the bypass valve.If the high pressure is at high rpm, it may be the secondary bypass valve I mentioned at the beginning.Clogged oil passages to the bearings from sludge.Your thermostat is missing or stuck open, not letting the engine heat up.Your gauge is broken. Check it with a manual gauge.

Does a faulty knock sensor affect gear performances?

Yes, it does. When knock sensor doesn't work, meaning no signal is fed to the engine computer, the engine will use the standard advance curve according to the mode the engine is running. It may not do anything at off-idle situations, but at full power acceleration, it may cause engine damage. If the knock sensor send signal at the wrong time, the timing is retarded at 4 degree intervals until the signal stops. If the signal doesn't stop, the engine will loose power and the drivetrain computer will downshift due to the lack of power.

Why is my car shaking and the check engine light flashing?

A shaking vehicle AND flashing MIL ("check engine light") sound like the vehicle is misfiring on at least one cylinder. As @Anna Nguyen, @Brendan Sinclair, @Antoun Nabhan and others have said, it could be an ignition related problem. It could also be fuel-related. If one or more fuel injectors are clogged or dead, that would also cause a misfire in the affected cylinder(s). These are relatively cheap/easy things to check and fix.Other causes (which could be more complicated and more expensive) to fix include contaminated fuel, a bent intake or exhaust valve, a broken valve spring, a damaged piston/connecting rod, and on and on.I recommend getting a scan tool and checking the codes which are causing the MIL to light/flash and I also strongly recommend NOT driving the vehicle until the problem is fixed. If you continue to drive the vehicle as is, you will exacerbate the problem. Dumping unburned fuel into the exhaust due to a misfire will overheat and kill your catalytic converter. A more serious problem like a bent valve can eventually cause you to crater your engine (i.e.: completely destroy the engine).

What problems go along with a bad catalytic converter?

A clogged catalytic convertor causes the symptoms of a loss of power when accelerating or going up a hill. This is an often overlooked cause of loss of power, can cause check engine fault codes and lights, and mechanics will often change a few parts unnecessarily , especially if it is only partially plugged. A couple ways I check for plugged cats when a low power complaint is a symptom, and other obvious basic maintenance items have been checked for service is to have someone hold the rpm at about 1800 to 2000 steady. Then check for a good push of exhaust with my hand from behind the tailpipe. A plugged or partially plugged cat typically leaves a hot sickly feeling exhaust flow from the tailpipe. Compare to a known good working car nearby, known to be performing properly and with the same amount of cylinders. That slow , hot sickly flow at 2000 rpm compared to the hard push blowing your hand away of a not clogged cat is a dead giveaway, and I have NEVER misdiagnosed a plugged cat , or made false diagnosis with this method. Another way is watching a vacuum gauge connected to the intake manifold. When revving up, if the exhaust is plugged the gauge reading will climb then begin to slowly drop as the exhaust pressure builds up. Further testing will be needed to verify which part of the exhaust is plugged. Diagnostic by codes is unreliable and until now I had not heard of it. A code p420 only refers to catalyst efficiency. And can be set by faulty o2 sensors. Removing the exhaust and testing for engine power returning to normal works. But the hot sickly flow has been most accurate with my experience, in several cases where shops had missed it, were stumped and called me to come check. They are always embarrassed for missing the simplicity, but need not, as cars and their complicated electronics and even age old problems can lead to loss of power without any immediately obvious cause.