Hot Start, Idles rough at stop

I have a 88 Corvette that is hard to start when the engine is hot. It starts on the first try when cold.

Also, the tachometer will fluctuate from 500 to 700 rpm in drive position while at a stop.

Can anyone throw me some suggestions on what I can do to solve these problems?  

a few things...  

engine off... grab the throttle lever...  open the throttle slightly and wiggle the throttle lever while observing the throttle shaft to housing bore play..   there has been a lot of L98 throttle bodies found with throttle shaft bore wear. this allows the throttle lever to return to the stop but the shaft being in a slightly different position.. this effects the CLOSED throttle angle and the Minimum TPS voltage..  0.53 volts is where it HAS to be every time the engine comes back to IDLE.. or the computer will think you have your foot on the gas keeping the fuel control program out of the IDLE program and in enrichment as its waiting for you to drop the right foot to the floor...   you can tell with an obd1 scan tool...  

the idle speed at normal operating temp in DRIVE with your foot on the brake should be after about 10 seconds.. 450 to 475 RPMs..


engine warmed up and idling in drive.. foot on brake..

TPS voltage  0.53 volts DC exactly...

IAC counts..  20 to 50...

MAF  6 or .6 grams per second on the MAF.. i forgot where the decimal point goes right now..

injector pulse width... 1.3 or 1.4 Ms  milliseconds..

block learn close to 128

integrator close to 128...

oxygen sensor showing some cross counts.. but not fixed..

this is just a start...

i would like to know if it has overheated lately and how long the coolant temp sensor has been in the front face of the lower intake manifold.. they tend to go bad and the pigtail connectors fail.. if you buy them in a kit.. with the sensor and the pigtail they can be had for under 20 bucks. AZ wells SU102..

look in the active posts or the C4 forums for the post i put up about scan tools for the C4 corvettes..
do you have access to a scan tool that will read live  

L98 throttle bodies can be rebushed by companies who do that..

replacements can probably be found..  

this is just the tip of the puzzle..

please post any live data readings you get with a scan tool... i can decipher them...  

SpeedyKev42:
the tachometer will fluctuate from 500 to 700 rpm in drive position while at a stop.


Do the engine revs change up and down  with the tacho readings?

90 /91 tachos are well documented as to problems with erroneous readout  

vettoz

Electronic tachs really suck... If you have a way to verify the engine RPM is doing it then there is another problem. I'm wondering if you have an RF interference. If so it can be corrected by installing a filter in the tach line.
  Just try this first for a Hah Hah. Don't have the engine running, and remove the tach wire from the HEI.Then put it back on, but first check it for corrosion. Or a loose connection. Do the simple things first....
  I remember when the early Vettes ran a Mechanical Tach from the generator to the distributor.  
Bob  

vetteoz

Check to see how much draw that starter takes cranking? when the engine is hot. That year I believe they still had the big starter in them. The bushing in the nose of those old starters bind when the engines hot.

Bob