Fuel problem saga continues...

Discussion in 'Performance & Fuel' started by bmass2844, Jul 28, 2009.

  1. bmass2844

    bmass2844 Member

    So some of you reading this might have responded to some of my old posts on this same situation. But I now have new parts to add to the story. So the basic break down, my truck idles bad. Feels like its loping, not quite missing, but just a shake every few seconds. So far ive been throwing parts at it, o2 sensors, spark plugs, wires, air cleaner, most recently a fuel pressure regulator and tonight a fuel filter. Cleaned TB, IAC valve, MAF, seafoam through fuel system. About a week ago before i changed the regulator i measured the fuel pressure at 52 psi. which is too low (range is 55-62 psi static measurement). So i replaced the regulator, no change at all. Measured pressure tonight at 52 psi static (with just ignition on engine off) When i start the engine the fuel pressure drops to 48 psi. which blows my mind all together. Idle is sitting at a constant 500 rpm's. The thing that stumps me, is when I accelerate she clears up and runs just fine. No hesitation, stumble or surging. Only at idle in park, reverse, neutral, or drive. She drives down the highway at 55mph just beautifully, stomp it, she downshifts and goes. I recently had a flowmaster 40 series welded on, and while idling, you can hear the hesitation in the bad idle from the rumble of the muffler. Anyone have suggestions? I seriously doubt fuel pump. oh yeah btw 1999 silverado 5.3 w/ 86,000 miles.
  2. bmass2844

    bmass2844 Member

    Almost forgot, replaced pcv valve, and checked for intake gasket leaks with the carb cleaner trick. Spray around the intake manifold, if engine bogs down and tries to stall, it means carb cleaner was sucked into engine through leak. No leaks, and all hoses and connections double checked. For the idle problem i have been leaning towards IAC valve or TPS. But with IAC, symptoms are high or low idle, not just a rough idle. And with a bad TPS, i would think you have no acceleration because the sensor is not telling the ECM how much fuel to send when the throttle is opened. I dunno. What yall think?
  3. MrShorty

    MrShorty Epic Member 5+ Years 1000 Posts

    What happens to the fuel pressure if you find a rubber spot in the return line and clamp it off? The pressure should jump well over the spec of 60 psi. If there is no change in pressure, then that indicates that the fuel supply isn't generating enough pressure to even open the FPR.

    In addition to what you said, I would expect a bad TPS to throw a code. Even if the SES light hasn't been coming on, I might hook it up to a scanner just to see if there are any codes/pending codes that might help troubleshoot. I would also maybe use the PID monitoring feature of a scanner to see if any of the sensors (like the TPS) looked severely out of line.
  4. bmass2844

    bmass2844 Member

    I havent tried clamping the return line. Im not too fond of pinching or clamping a 10 year old hose. Thats just asking for a leak. As far as the TPS not throwing a code, i have found multiple threads online of people having, rough idle, surging while accelerating, they replace the TPS and it clears everything up. For the rough idle, im leaning more towards, the IAC, TPS, or EGR valve. But i think the IAC is fine, since the other day i forgot to plug it back in while working on it, i started the truck, and the truck idled very high around 2000 rpm. Plug it back in, idle went down to normal. I think IAC is working fine. Correct me if im wrong, but the TPS tells the ECM how far the throttle is open so it knows how much fuel to send to the engine. So if it is not working correctly, it can be sending too much, or too little fuel.

    Which makes sense with my new symptom i found this morning. Instead of using A/C, i put my windows down, every stop i came to, i smelled a strong odor of gas from the exhaust. I double checked and i have no leaks from fuel system. Though, that could also be EGR, as the EGR recirculates gas from the exhaust back into the intake. Which that gas is non-combustible, so if too much is sent to intake, the engine will run rough at idle.

    Anyone have experience with a failing EGR, TPS, or IAC, and it not throwing a CEL. Havent checked DTC's. Can the OBDII scanner from autoparts store read stored DTC's? Or will I have to go to the dealership? Also i found online, GM put out 9 ecm updates for 99-03 silverados. For TSB's like hard shifts, rough idle and a few other ones. Anyone know if dealerships have these updates and if they will update your ECM?
  5. MrShorty

    MrShorty Epic Member 5+ Years 1000 Posts

    I can appreciate not wanting to damage the fuel line. The idea is basically to see if the pump really is only capable of generating 50 psi, or if the regulator is opening too early. On other vehicles, I have disconnected the return line, then run the pump to see if there is any fuel being returned to the tank.

    While checking the fuel pressure reading, any change your pressure gauge reads low?

    The auto parts stores scanners/code readers should do just fine at reading stored codes. I'm not sure which models (if any) can't read pending codes, but I would be sure to check those, too. The real test would be if they will use a true scanner that would allow you to monitor the sensors directly. If the TPS were at fault, I would expect a scanner to show something other than a reasonable baseline value for the TPS parameter at idle.

    A strong odor of gas from the exhaust suggests the engine is running rich. What do the spark plugs look like? What do the fuel trims look like (again, you would need a proper scanner to see this)? Does the system hold pressure when the pump is off? If it doesn't, that might point to a leak regulator or injector that would cause excess fuel to get into the cylinders, which could easily be more noticeable at idle than at higher engine speeds/loads.

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