fuel problem

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Old 04-21-14, 09:34 PM
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fuel problem

98' Pontiac Bonneville. starts fine but will not run unless foot is on gas. will not idle. runs like a dream as long as you gas it. any ideas?
 
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Old 04-22-14, 04:36 AM
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TPS (throttle position sensor) or IAC (idle air control) possibly. I would at least get it checked for codes before proceeding any further.
 
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Old 04-22-14, 05:50 AM
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Maybe EGR could be stuck open too. You can/should test all of the above first.

IAC can be checked for voltage with a multimeter. With key on engine off if you backprobe the harness connector to the IAC (straighten a couple paperclips and stick them into the back side of the connector, following the wire) you should show 10-13V to indicate power. Then if you unplug the connector, check the resistance (in ohms) across the terminals of the IAC valve. There should be 6 to 13 ohms. Then check for an internal short by testing the resistance from either terminal to one of the IAC bolts, and there should be 10,000 ohms or greater showing that neither terminal is grounded. Then finally remove the IAC valve to determine if it is stuck open. Blow on the front port to see if the valve is allowing air to pass. (It should be closed). If it's leaking, you can try cleaning the carbon from it, or just replace the valve. If it tests out okay, its not the IAC.

The TPS has 3 wires to the sensor, a signal, a ground, and a reference. I can't tell you which is which on your vehicle... but at key on engine off, the reference wire should have 5V, while the signal wire should only show .5 to 1V at closed throttle. (backprobe the harness connector with 3 straightened paperclips, but don't let any of them touch the other.) As you move the throttle manually you should see the voltage on the signal wire increase to 5V at full throttle. If your voltage is inline with the above, it's not the TPS.

The EGR is a little more difficult to diagnose because there are a couple different sensors that signal the EGR to turn on. If the EGR is stuck open it will act like your engine has a vacuum leak, and will run rough at idle.

And that's the other thing it could be if it doesn't want to idle... a simple vacuum leak. You could check ALL your vacuum hoses to see if you can find any that are dry and brittle, cracked, loose, etc. The PCV hose is often a culprit. Or sometimes EVAP hose connectors under the car will spring a leak, since they are often subject to more wear from salts and road spray. I don't know what engine you have or even what it looks like but there are usually several main vacuum ports on the intake manifold, and you "could" unplug them one at a time, (engine off) then cap that port with a short piece of hose that is plugged/capped, and start the engine to see if the vacuum leak is originating from a vacuum line that is attached to that port. Or you could take it to a shop to have it smoke tested, where they pressurize the manifold and see where the smoke comes out.

If the above check out, then you could also- as you suspect- check the fuel system. Hook a fuel pressure gauge up and see what the pressure is (at the lowest idle that you can maintain) then see if that pressure drops as you let the foot off the gas and the engine dies. Your fuel rail should maintain operating pressure even after the car dies. If the pressure is too low, there could be a couple things going on. Easiest to do is replace the fuel filter(s). If, while checking the fuel pressure, you disconnect the vacuum hose from the fuel pressure regulator, your fuel pressure should jump up about 10 lbs. If it doesn't, and you have verified there is vacuum present at the vacuum hose, replace the fuel pressure regulator. If you have fuel hoses, and can pinch the return fuel line shut, the fuel pressure should rise... if it doesn't it is likely a defective fuel pump. If the fuel pressure rises "sharply" replace the fuel pressure regulator. Low fuel pressure can cause the car to die at idle. If, after the engine has been turned off for 5 minutes, the fuel pressure gauge has dropped below specifications (not sure of the specs for your engine) then it is likely that one of 4 things is occurring. Leaking fuel line, leaking fuel pressure regulator, leaking fuel injector, defective fuel pump.

Take the car to an auto parts store and ask them to read the codes for you, along with a list of possible causes. Don't buy any parts just yet, but take the info and use it to help diagnose the problem.

Hope some of that helps.
 
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