My 2000 S10 with a 4.3L has started doing some strange things. Of course it was during the time my girlfriend needed to borrow it but I don’t think it is anything she has done. The truck is over 20 years old. It is going into the shop whenever the parts arrive and the mechanic has time for another issue so might just have them take care of whatever is going on.
First thing, there is a strong gasoline smell around the truck when parked. It always stinks like gas. I cannot pinpoint where the smell is coming from. Codes and pending codes have been read MULTIPLE times and there is nothing showing an EVAP code or similar.
The only code that popped while she had the truck was a throttle position sensor code. I replaced, cleared the code, and that one never returned.
Soon after I replaced the TPS, another strange problem cropped up. The engine started revving. Just being in 4th gear it was wanting to go 45mph. I was busy but had her stop at an AutoZone to have the codes read. There was nothing. I figured it might be a vacuum leak as I had to disconnect the lines to replace the TPS so checked it that night. All lines were connected and in the proper place. Again, there was not a check engine light or codes of any kind.
I ran some seafoam through the intake and through the port for the IAC and it seems much better but still revvs. I am wondering if the IAC is gummed up but wouldn’t that trigger a code?
The truck is going in for a new upper intake, spider fuel injection, and intake manifold gasket. GM is notorious for intake gasket failures on these engines and others. The mechanics said you would never re-use a 20 year old plastic intake and fuel injectors so is best to do it all at once. I figure whatever is causing the revving is probably in the upper intake so I will tell them to clean and/or replace any sensor that might cause this.
I did disconnect the battery and turn on the headlights to reset the computer before using the Seafoam but that made no difference either. It probably has a couple hundred miles or more since I did that. Again, no check engine lights or codes have returned.
IF the gas smell is coming from the rear, I would check the gas fill pipe for cracks. also, I believe there is a rubber hose that connects the pipe to the tank. it could be dry rotted and cracked. just a thought.
I’ve seen bad IAC valves cause unintended acceleration.
And no, they don’t always set a code,
And it’s usually not because they’re dirty, (the engine stalls out when that happens), it’s usually because the windings short out in the IAC valve stepper motor causing it to move to the wrong postion.
The gas smell could be caused by a leak in the fuel system after the fuel pump.
Such as a leaking fuel pressure regulator, or a leaking poppet valve injector.
The OBDII system monitors fuel tank pressures closely, so if it were a fuel tank leak the CEL would be on.
I am going to look into the fuel pressure regulator. Something I forgot to mention is there is a pretty extended cranking while it builds fuel pressure before it fires up and starts so I was already wondering about that as well.
I am more suspect of the IAC now as well. I saw something about the cruise control acting up but this seems to be connected directly to the throttle and I am not seeing any signs of the throttle plate opening when it is revving. Here is how this goes. You give it a tad bit of gas in neutral and let off. Suddenly it starts revving even more. I have been standing there with the air filter removed, manually activating the throttle, then it revs even higher when the throttle plate is completely closed. I think this probably points to the IAC and is probably just a good idea to get one of these and install it if I have time. Otherwise I will just have the shop do it but it doesn’t look hard.
This truck isn’t driven daily and sometimes sits a lot which I suspect is part of the problem.
Agree I have the same type of problem with one of my trucks that has a carb. have to play around with it for a while after it sits before it will run right.
Check the book for the procedure to replace the iac when I replaced mine there was a specific adjustment procedure, not just screw a new one in. Yours could be different though.
Sometimes vacuum problems can exist in the dash climate controls. In other words, a certain setting (DEF, FLOOR, etc) might cause a leak to exist that goes away when the knob is turned to another position. That can also have an effect on engine idle/Idle Air valve.
Injector seal leaks can have the same effect.
To me anyway, the best way to check for below throttle plate vacuum leaks is with a vacuum gauge and that is an invaluable tool. It’s also cheap and easy to use.
