Quote:
Originally Posted by ChevyTech
Hi Rocky, and welcome to the message board.
It is true that low fuel pressure affects the idle on TBI engines. Oldertruckfan has a valid point.
Don't replace parts unless they test out to be bad. The quality is so bad on replacement parts that you don't want to try to fix it by throwing parts at it.
The coolant sensor readings you got were good so don't replace it.
It sounds to me like the IAC (Idle Air Control) is not working. If the TPS (Throttle Position Sensor) does not give the ECM a correct voltage for idle the ECM will not send pulses to the IAC to control the idle speed.
TPS testing with a meter:
With the wiring connected, test the voltage on the wire that connects to Terminal “C” of the TPS (possibly dark blue wire).
Engine off with key on for this test. It should be between .5 volt and 1.25 volts at idle and increase smoothly as you slowly open the throttle to at least 4 volts at wide open throttle. Some spots give a different allowable voltage reading at idle in GM manuals, but this is what I go by.
If the TPS test out good, I would be suspicious of the IAC. They get old and stick. Carb cleaner can damage them.
If there is a part I would say to try if you want to try parts, it would be the IAC.
If the EGR is sticking open it can cause it to stall.
You could try disconnecting and plugging the hose to the EGR valve and taking it for a test drive.
You could try to lift the EGR diaphragm with your fingers, with the engine idling, (being careful not to burn yourself or get your fingers caught in the EGR valve) and see if it closes well or wants to stick open.
NOTE: Your 1 ton truck may not have a negative backpressure EGR valve like the smaller trucks and there might not be big enough openings to lift the diaphragm by hand.
If you find it needs a EGR valve, lets talk it over before you pick one to buy.
Does your truck have a fast idle when you start it and then the RPMs slow back down as it runs?
Do you have to push, or hold down, the gas pedal to get it to start?
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Hi ChevyTech, thank you for your response and thank you to everyone else as well.
In response to Oldertruckfan, the truck has about 160k miles on it and I should have mentioned that the fuel filter was replaced about 5k miles ago.
ChevyTech, I tested the TPS as per your method and got a value of 0.68 volts at idle and it swept smoothly up to 4.20 volts at full throttle. I removed the IAC valve and inspected it. The internal surfaces are all clean but it does appear to be the original factory valve. I will order one and replace it as seems like a good place to start as you also confirmed.
I checked the part number on the back of the EGR valve and it read 'DBR17087185 118 72'. The EGR solenoid also looks similar to the one you posted a link to. Based on these 2 items, I believe you are correct in saying that this 1 ton truck has the Port EGR system. There is enough room to get my fingers on the diaphragm but I did not try any tests with the engine running as I don't know what to look for to see if the EGR valve is working as expected. Should I replace the IAC valve first before looking at replacing any of these EGR components? I have to order OEM parts online as the local shops all seem to carry aftermarket brands, and I'm wondering if I should just order both at the same time.
In the mornings, the truck has a fast idle when first started and will slow down a bit as the truck is driven. I never need to touch the gas pedal when starting the truck cold or warm as it will fire up quickly by just turning the key.