Random idling issues

CLAYTON TEMPLE

MEMBER
1991 CHEVROLET S-10
4.3L
6 CYL
2WD
AUTOMATIC
111,800 MILES

My truck has been giving me trouble with random surges while in gear mostly when compressor is running. I started by replacing the IAC valve, TPS sensor, fuel filter, and completely rebuilding the throttle body(including replacing the fuel pressure diaphragm and spring. Sometimes I’ll crank it after it sits several hours and it starts completely fine and when I put it in gear and let off the break it will spit and Sputter then level off. Then the other day I was backing out of the driveway when I went to put it in gear It idled real low and died. I’ve checked probed the TPS sensor and got a good ground and a 5 volt signal from the computer. The strange thing is when I checked the voltage from the TPS back to the computer I was getting around 0.26-0.28 volt with a fixed sensor non adjustable so I bought an adjustable one and still can’t get it within specification.

3 Replies

ASEMASTER6371

MECHANIC
52,797 POSTS

Good morning,

The voltage should be 5. to .75 at closed throttle.

Remove the adjustable one and install the OEM back.

Did you set the base idle RPM? It should be 500-550 RPM.

I would start by performing the base idle procedure and reset the IAC.

The EGR valve is stuck open is also a common cause of this issue as well. I would remove the valve and clean the end as carbon can hold the valve open causing a vacuum leak and lowering the idle and stalling.

https://www.2carpros.com/articles/stall-at-idle

Roy

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Thursday, February 6th, 2020 AT 5:38 AM

CLAYTON TEMPLE

MEMBER
5 POSTS

I have replaced IAC valve, new EGR, and fully cleaned and rebuilt entire throttle body. The IAC port was also cleaned very well on throttle body. I haven’t set or checked base idle due to it being set from the factory and by modifying it I would be masking the underlying issue. I was leaning more towards maybe a fuel pump issue. A lot of forums are saying if the rubber hose that connect the pump to the top of the tank is cracked it may cause an idling issue but I have no way to check the fuel pressure. I’m going to pull the bed and going to install a new pump and sending unit which I have ordered because if I have to pull the bed off I’m going to go ahead and do the pump and sending unit and also because it’s the original pump. I have checked the PCM to TPS sensor for 5 volts and it’s correct but the TPS to computer isn’t within specifications I’m getting between 0.26 and 0.28 volts with a non adjustable TPS sensor what would be causing it to read low and how can I get it back within specifications with the OEM sensor?

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Thursday, February 6th, 2020 AT 9:18 AM

ASEMASTER6371

MECHANIC
52,797 POSTS

Okay, over time the throttle body wears. You need to adjust the base idle to get the TPS in spec. You are not covering any issue by doing that at all.

You can rent a tester from a parts store for the fuel pressure. You will need to T into the fuel filter connection to read the pressure. You need 9-13 pounds.

Roy

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