1992 Chevy 4.3 oscillates on idle.

[348tripower]

I have been all over this thing and I can't get it fixed. Pulled the intake and put new gaskets in twice. Has a new map sensor, new EGR valve and egr solenoid, new IAC, new cap, new rotor, new wires, new plugs, new coil, new temp sender. The only things that have not been changed are the fuel pump and O2 sensor. Any suggestions?

[wayne petty]

that would be a 4.3 TBI.. i would guess...

a few things..





first check for codes...  jump the A and B terminal with a short length of wire...   turn on the key.. you should get code 12 , 3 times..

check the vacuum hose to the power brake booster...   if the power brake booster has failed.. it will be a big enough vacuum leak to drive the computer crazy..   you can start the engine...    let the vacuum build.. step of the brake.. holding your foot with a medium pressure..   shut the engine off..  does the engine shutting off cause your brake pedal to instantly push up..


i would also check the vacuum hose to the map sensor .. a map sensor looks like a domino...  usually has a green 3 wire connector plugged in the bottom... i have found the hoses cracked.. and others clogged..  carefully remove both ends...  blow through it...


another place to check is the throttle shaft for play...

engine off.. open the throttle slightly... wiggle the throttle lever around...  if you have more than a tiny amount of play...  you will need a scan tool or to look at the live data stream from the TPS..
why..   when the throttle shaft bore wears...  the throttle blades close to a slightly different position.. this causes the TPS voltage to drop to a different level each time you close your throttle... this causes the ECM to think that you still have your foot part way on the gas.. so it does not drop the program to the idle program.. so it keeps changing the idle speed and the timing getting ready for you to accelerate...  it can also keep the fuel mixture extra rich...


the throttle position sensor can also be bad...

how is the engine coolant temp sensor...



how is the thermostat... if the thermostat is not a 192 thermostat.. the coolant will NOT reach the proper temp and the computer will keep it at fast idle...


with an OBD1  scan tool...

you can examine the coolant temp sensor ..
you can examine the throttle position sensor voltage
you can examine the idle air counts..



if you have the distributer cap off. you might want to look at the reluctor..


this is an reman distributer for a 4.3 TBI..  it is far superior than the OEM gm design..




you will also want to look at the ignition coil.....

[wayne petty]

if you have not read this post over at corvette fever...

gm up through 94 used OBD1 scan tools..    


it may not fix your car faster.. but with some experience.. you can find fuel injection problems easier...

http://forums.corvettefever.com/70/8585187/c4-corvettes/which-used-scan-tools-for-82-to-94-corvettes-with/index.html

[348tripower]

Here is what I found yesterday. If I fog over the throttle body with starting fluid the truck levels right out.  
In answer to your question Wayne it has a new temperature sensor.  I also noticed that I have a light on the dash now that says check gauges. This is a new one. It was on constant no it is intermittent.
I am leaning towards the fuel lines or pump because the owner told me the previous owner was replacing sections of rusty lines. But I may be full of Do Do.  
I am am getting the COE ready for the Hot Rod power Tour so I am limited on time to work on this. And it's been raining for 4 days again. Still need to cut the grass. We are pulling out for CoCo beach Wednesday morning.  

[wayne petty]

a few questions...

got a timing light?

hook the clamp to the coil wire.. fire the engine up...

aim the timing light at the cone spray pattern from the injector..

do you have a nice firm cone??  low fuel pressure will cause surging also...

do you have a digital volt meter that has a  MS setting... millisecond range..

if so...   hook to the TBI injectors..   at idle.. the injectors should be 1.3 to 1.6 ms..  and steady not moving around..



you can also disconnect the inlet line to the fuel filter inside the fuel rail.. remove the tiny oring.. shove some larger hose over the end of the tube and do a full flow test.. see if it really pours out.. i don't know what the flow rate should be..   you could also deadhead that line into a gauge..  pump only for a few seconds.. you should get close to 25 PSI... there is NO pressure relief for the pump.. so don't burn it out doing this..

you might also remove the fuel filter.. blow through it.. see how clogged it is...

replacement fuel filter orings are available in a filter oring kit from dorman help.. or the are the same as the larger gm fuel rail seals.. be sure not to use black .. they should be reddish or tan..

[348tripower]

Thanks Wayne and happy Memorial Day to all.