4.3 Vortec idle issue

[Figuarus]

Hey guys.

Newbie here. Was told this is the place to get help with my 96 C1500.

It's a 4.3L with 5spd. Reg cab, Longbed, 2wd.

Engine is bone stock.

I bought this truck about 2 years ago. It's been great, but have noticed a few issues that I have never gotten around to trying to diagnose. That being said, i figured I should trying fixing them one at a time.

SO, First this return to idle/high idle mystery.

When I drive this truck to work, (everyday) I have an issue where the engine rpms will not return to idle unless the truck is stopped. I know that sometimes happens in situations where the engine is cold and what not, but this happens no matter the outside temp, or the operating temp.

I should take a video to help visualize this, but I dont have a safe way of doing this, so text explanation will have to suffice for now.

When shifting gears, up, the rpms, stay at roughly the same rate as they were before I move to another gear. Not really a problem here, but when I downshift, it's more of an issue.
I might downshift to third or second, and the engine rpm hovers around 1400 to 1600. This happens even though i'm coasting in neutral, and whether the clutch pedal is pressed or released.
Only when I come to a near stop, do the rpms finally drop under 1k.

Hopefully that made sense.

I have checked for damaged cables, cleaned the thottle body, checked for vacuum leaks (using the old spray can method) and nothing has really helped.

So, I come to you guys. What should I check or do next?

And, yes, I have looked for CEL codes, and there isnt anything in memory or pending.

 

[HotWheelsBurban]

Throttle position sensor may be going bad. Most of the time it'll affect the idle, where it'll fluctuate a lot(without the gas pedal being depressed any). Basically this sensor tells the main computer where the throttle is in relation to air and fuel flow. These trucks, particularly the Vortec ones from 96- up, have a more sophisticated fuel injection system,and it works best when the sensors that govern its operation are clean and in good working order. Some of these parts are not things GM expected to last as long as they have! I've had to replace the TPS and IACV (idle air control valve) on my '97 and '99 trucks. You also may have a situation on your truck, where the throttle linkage or cable are dirty and need cleaning because they are binding up....

 

[Schurkey]

Verify fuel pressure.

Connect a scan tool, verify EVERY sensor and computer output. Codes can be useful, the real diagnostic power is the data stream and bi-directional control.

How old are the usual "tune-up" parts--plugs, plug wires, cap, rotor, fuel filter, air filter, PCV system, heated air intake system, etc.?

 

[Figuarus]

Verify fuel pressure.

Connect a scan tool, verify EVERY sensor and computer output. Codes can be useful, the real diagnostic power is the data stream and bi-directional control.

How old are the usual "tune-up" parts--plugs, plug wires, cap, rotor, fuel filter, air filter, PCV system, heated air intake system, etc.?

I'll have to find a port to measure fuel pressure. I didn't think about that. As regards the other wear components, they are all relatively "new". The previous owner had done some work on it, and I looked at the obvious things like cap and rotor, and the wires were still new (at the time. The box for them was in the cab and the receipt in the box)

Air filter I changed out about 3 months ago. Cleaned MAF and verified it was good as I suspected it was on the fritz.
(Installed new one and ran the same as the old one)

The only other thing I haven't don't is swap out the fuel filter since it had been done about 6 months prior to me buying it. It's due again, but merely as a precaution

 

[1997]

When shifting gears, up, the rpms, stay at roughly the same rate as they were before I move to another gear. Not really a problem here, but when I downshift, it's more of an issue.
I might downshift to third or second, and the engine rpm hovers around 1400 to 1600. This happens even though i'm coasting in neutral, and whether the clutch pedal is pressed or released.
Only when I come to a near stop, do the rpms finally drop under 1k.

since my 97 does the same thing from your description, i suspect GM engineers figured they would fix those "rough downshift issues" for you, like it or not .......

probably can get ECU re-programmed, if it really bothers you.

 

[Figuarus]

I managed to record a video although I will warn that it's a little shaky. Hopefully Its not nausea-inducing.
I did a poor job of explaining previously. (partially because I recounted from memory)

So here is a video that will hopefully shed some more light on the situation...

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[95 Tahoe]

I just went through the high idle issue. I plugged in a scan tool and the ECU was actually commanding the IAC to open up to allow more air. I spent months trying to figure out what it was. Also, I had a knock starting to get worse. I rebuilt the engine and TBI. Now it idles fine.

 

[Figuarus]

I just went through the high idle issue. I plugged in a scan tool and the ECU was actually commanding the IAC to open up to allow more air. I spent months trying to figure out what it was. Also, I had a knock starting to get worse. I rebuilt the engine and TBI. Now it idles fine.

So how did you fix the idle issue? If the ECM was commanding the IAC, then changing the IAC wouln't fix the problem.

Is this a case of flashing the ECU?

 

[Schurkey]

Connect a scan tool, FIND OUT what's going on with the IAC commanded position. Could be faulty computer. Faulty IAC. Faulty wire harness. Or something weird, which might be pointed-out based on the info from all the other sensors you should be verifying.

 

[95 Tahoe]

So how did you fix the idle issue? If the ECM was commanding the IAC, then changing the IAC wouln't fix the problem.

Is this a case of flashing the ECU?

I have 2 possible ides. I think the the engine was compensating for excessive blow by on one of the cylinders. The PCV valve was sending this back into the intake. Second, the fuel pressure regulator was bad. Mine appeared to be leaking fuel out the backside. Rebuild the throttle body and the fuel pressure regulator. t's a cheap try and it probably needs it anyhow.