Hi,
We are hoping we can get some help from either a chevy technician or someone who has experienced the same problem we are.  We have tried everything we can think of to fix this low idle problem.  

We have a 1994 Chevy Silverado, TBI, 5.7L, 4X4, that has a periodic very low idle (100-200 RPM)while in gear (D) and sometimes when in reverse (R).  

In order to fix this problem, we have replaced PCV, spark plugs, wires, rotor cap, fuel filter, MAP, IAC, TPS, and O2 Sensor.  We have no trouble codes showing, and we used the code reader to check to see if the code was working, and we get a code 12 showing that it is working correctly.  We also cleaned the throttle body assembly and checked all vacuum lines and wire connections.  Everything checks fine.  When replacing the IAC we also cleaned the bore for the IAC. We have done all of this and the truck still has the same low idle problem.

The voltage regulator and alternator is working correctly.  The oil pressure is low only when idle is running low, other than that oil pressure is good.  

The truck runs good except when we stop at red lights and running reverse.  When coming to a stop, right before stopping, the RPM drop almost to zero, the truck shudders, then the RPMS go up to 800 then drops down to 200 - 300.  The truck does not always do this when coming to a stop, just periodically about half the time.  If we put the truck in neutral it will idle right at 650 - 700 RPM.  

We are hoping someone can help us with this.  Help is greatly appreciated. Thanks

Alaska_Chevy    

it really sounds like a bad ground wire.. clean the ones from the battery to the body, to the engine and to the frame...

the ecm grounds things to several points on the motor...

usually to the thermostat housing area... or the back of the throttle body area... these are sometimes twisted off or  broken .. each one leads back to the ecm...  they are all important..

it could also have twisted pair wires insulation falling off somewere in the harness...  i have seen that on other cars and trucks...

and the most likely villian..  the throttle shaft bore...

with the engine off... open the throttle with you hand and wiggle it.. if it moves.. around this is the problem...

what goes on if the throttle shaft bore is worn is the tps voltages vary when the throttle closes..  every time it goes back to a differnt spot... the ecm learns what the voltage was when the truck was first started.. if it drops below that voltage it causes strange things to happen....

since you are way up there... i have rebushed the throttle shafts with 3/8 thin will valve guide liners.. for .375" stems...   a letter Y drill .404 is also needed.. the biggest problem is the tips of the throttle blade screws are deformed so they have to had the deformed ends ground off to get the screws out...  and they are a weird metric size... like 2.5mm .060 pitch...  dont quote me on that...

the shaft is removed ... either end of the bore is drilled out  to the letter y size...  short legnths of the guide liners are inserted and the throttle shaft lubed and worked back in...
shaft size .372..
thinwall for .375 stems.. .015+ .015
drill size..404...    k-line does make a special reamer to do this...  i just drill them...


i think this truck needs to be on a scanner first.. watching various voltages...  and verifying stuff with a digital volt meter...watch the voltages the computer see while the problem is happening... this is only done with a scan tool...

here is a link to an earlier chevy truck online manual

http://www.autozone.com/addVehicleId,1095202/initialAction,repairGuide/shopping/vehicleSelected.htm


csk has scanners for 199 from innova model 3120 ..that do old and new stuff...  

Check the vacuum hose going from the MAP sensor to the throttle body for signs of trouble. When you replaed the TPS, did you set it correctly? There is a slight adjustment on mounting the TPS to the throttle body (I've got a '92 Silverado with the same setup) if it's not adjusted correctly this could also affect idle.

Another possibility, the ignition module or magnetic pickup in the distributor could be going bad. If the motor's not getting enough timing under load, it'll idle too low.

Let's also look more towards the obvious. When was the last time you changed the fuel filter? Fuel pump? And issue with either of these can cause the problem you're having, because at idle under load if the pump isn't making anouhg pressure, or the filter's partially blocked, the motor will idle real low because it's starving for fuel.