Rev hang?

[Slipperz]

Hi! I have a question about rev hang. My truck when in gear just driving around town will rev hang. Like when I pull my foot of the throttle the revs stay high until I hit the brake. If I push the clutch in the revs still stay high. Makes smooth upshifts a lil harder, but if I just take my times with shifts the revs come done normally but they will hang for a good 3-4 secs instead of coming done right as no throttle is being applied. I have checked my Throttle position sensor with a scanner and drove around with it plugged in and the TPS never hung or read anything incorrect, and checked PCV valve just to see if maybe it was stuck? Not sure it would change anything. If I am coasting in neutral the revs also hang. I am not sure what it would be. I have not really found any info on forums about it so any help would be helpful! My truck is a 97 with a 5.0l and a nv3500. Thanks!

Edit. Thought it might help to add that it has been doing it the whole time I have owned the truck. About 5Kish miles

 

[EJ_74]

You could start by operating the throttle arm by hand. Feel for any sticking, look for excessive carbon buildup. Make sure cruise and throttle cables are not holding it open. Cruise should have some slack and throttle should have a light preload (just the weight of the pedal)

 

[Schurkey]

when I pull my foot of the throttle the revs stay high until I hit the brake... ...I have checked my Throttle position sensor with a scanner and drove around with it plugged in and the TPS never hung or read anything incorrect

My truck is a 97 with a 5.0l and a nv3500.

So the TPS shows the throttle going back to idle, but the RPM stays high for awhile--but not forever?

Gotta be in the IAC or the IAC circuitry.

Connect a scan tool, verify the ability of the computer to control the idle speed.

In the Bad Old Days, manual-transmission vehicles would have deliberate interference with the throttle closing. They'd hold the throttle partly open, closing it VERY slowly, to reduce excess tailpipe emissions.

I haven't worked on a manual-trans GMT400, so I don't know if the GM engineers intend the "hang idle" on these vehicles.

 

[GoToGuy]

You could operate the throttle shaft by hand under hood, does it return to idle smoothly. Full open , release , snap back to idle? Remove throttle cable , operate cable by hand , smooth , no hard spots, hang up areas. On throttle body , shaft is smooth opening closing , high resistance to movement, rough operation. Same for cruise cable. No obstructions on floor or foot pedal shaft cable at firewall , linkage ? Off hand that's what I would normally check first. Then get more complex. Start with simple. Don't ignore a problem for for 5K plus miles , some simple things could go into bigger problems down the road.

 

[95burban]

Vacuum leak?

 

[1998_K1500_Sub]

If I push the clutch in the revs still stay high …. the revs come done normally but they will hang for a good 3-4 secs instead of coming done right as no throttle is being applied … If I am coasting in neutral the revs also hang.

My 1995 4.3L S10 5spd manual did the same thing.

The manual transmission ECUs have had that behavior for some time, it’s by design. The ECU opens the IAC valve to maintain an higher engine idle while the vehicle is in motion.

IIRC the behavior provides an anti-stall feature; it also facilitates shifting.

This behavior was discussed in detail earlier this year, I’ll point you to that thread if I find it.

 

[Slipperz]

Thank you all for the replies. To all of you that said to check if it hangs by hand under hood, I have done this, and it always seemed to return just fine. I can't check cruise cable as my truck is a no cruise truck.

@GoToGuy
I have not intentionally ignored I just have no clue what it was, and I mostly chalked it up to what @1998_K1500_Sub said about his S10.

In neutral completely stopped it will rev and return just fine. Just only when driving does it happen, so wouldn't a throttle cable related issue happen all the time? Maybe not. I am not sure

@Schurkey I could pull the IAC and see if it is dirty or needing of replacement. Also, yes like I stated if I do my shifts slowly the revs do come down almost like the rev hang was intentional as the timing at what RPM it drops to is nearly perfect.

Thank you all!

 

[1998_K1500_Sub]

In neutral completely stopped it will rev and return just fine. Just only when driving does it happen,

^^^Yup, that’s how it functions.

Put a scan tool on it and watch the IAC counts as the vehicle is driven

 

[Slipperz]

@1998_K1500_Sub
I will have to do that! Also sorry for my own confusion lol, but when you said that's how it functions did you meaning how your S10 acted, or you mean in general in neutral it will rev fine? If so, I meant like it only acted up when driving if that made sense.
Thanks!

 

[1998_K1500_Sub]

I was wondering if it is speed based would unplugging the speed sensor from the trans and go for a test drive tell me anything?

Unplugging the speed sensor may change this and, perhaps undesirably, other behaviors.

Like if it is an intended feature dependent on what speed you are driving at? Also sorry for my own confusion lol, but when you said that's how it functions did you meaning how your S10 acted, or you mean in general in neutral it will rev fine? If so, I meant like it only acted up when driving if that made sense.
Thanks!

The behavior you described jibes with my personal observation and discussions I’ve read on GMT400.

This is a behavior that’s common across years and (manual) powertrains.

If you Google “GM ECU” and include “throttle follower” and/or “throttle cr@cker" (I had to use an "@" sign because with an "a" the resulting word gets flagged) you’ll see much discussion about how the ECU manages the engine behavior for various transmissions (auto / manual) and operating scenarios (changes in throttle position, AC on/off, power steering load, etc.), e.g.,

Throttle Cr@cker and Follower for Dummies​

https://forum.hptuners.com/showthread.php?5664-Throttle-*******-and-Follower-for-dummies-sticky