hard starting 1997 Tahoe

[harascho01]

I’ve got a 1997 Tahoe with the Vortec 5700. A while ago it the engine developed a bad starting manner: After a short cranking it starts but after parts of a second the idle goes down and sometimes it almost stalls. If it doesn’t stalls the revs go up again and I have a fine idle, no matter if the engine is cold or hot. If the engine stalls I start it again and give it a little throttle to cover the moment where the idle drops too low it also works fine. The problem is worse after the car sits a longer time (over night..)

This strange behaviour started right after I mounted a cut out switch and an K&N airfilter. Believe me the cut out switch is wired properly, I’m an electronic engineer and checked all mods more than twice….(switch cuts main ignition and batt voltage wires right after the ignition switch…). One thing, I had to disconnect the battery for these works!!

Before this mods the Tahoe started fine. Some cranks and I had a slightly raised, stable idle which adjusted itself down to a smooth idle at around 700 – 800rpm after a few seconds.

I read a lot about the sticking poppet valve problem (hoe has CA emissions) but after removing the upper intake manifold I cleaned the poppet valves, they really looked good and clean. To be sure I had some injection system cleaner in the gas tank the next time.

Ok what I did until now is: new spark plugs/ wires, new distributor cap, new fuel pump (old was noisy) and a new fuel filter. Nothing changed. I had no DTC’s all this time.

Now I bought an Autotap scan tool and will go a little deeper into it. I read that minimum idle, set by the throttle stop screw in the throttle body is the starting point. The TPS should have good ground (30mV very good) and good reference voltage (5.05V strange it is 50mV to high but ok). The TPS output should be in the middle of 0.45V and 0.65V. Mine had 0.63V from the beginning. I tried to adjust it to 0.55V with the throttle stop screw but at 0.57V the plate is completely closed and there is no slack in the TPS mounting to adjust it a little.
Is there any idea about the initial opening of the throttle butterfly? With this I would be able to adjust the TPS (enlarge the mounting holes??). to the proper voltage. And the I could read the IAC settings and see if these cover some other engine problems. But without the minimum idle setting I can’t interpret the IAC counts.
Any ideas???

[harascho01]

I forgot to mention the following:
IAC motor, tps sensor, and throttle body where changed. I was lucky to have another 1998 Tahoe to interchange and test some parts. The good starter kept on starting good after the parts of the bad starter (IAC motor, TPS sensor....) where mounted. The bad starter still started bad .....
So now I know the TPS is acting as expected (slightly high voltage with throttle closed as described earlier..) WOT voltage is also in the range (about 4.75 V), the IAC motor, the passage in the throttle body is clean and working correct......
Ok that's all for now....what can I do????