Idles high after being in gear.

[BigRedOne]

I would focus on the IAC first, just because that's where I've had trouble in the past. Get those counts to where they belong at warm idle, if not there already.

My 1998 3.0L Honda Accord's idle stability is quite sensitive to the setting of the air bleed screw ("base idle"), so it's something I've seen. It doesn't act like yours, but it notably changes how well stable the idle RPM is with changing AC and alternator loads. It's as though the performance of the idle RPM management algorithm in the ECU is a function of the current IAC position in some non-linear way.

Oh it's definitely something to do with the iac. The question is WHAT is telling the iac to do what it is doing? The iac seats fully when commanded to in diagnostic mode, it moves out when put into gear, it returns when the ignition is cycled. It just don't go back to the same position in park after being in gear. WHY? Something is telling or not telling it to. That's why I want to start from the beginning making sure everything is properly set to factory recommend spec's and go from there with the diagnostics if need be. I don’t want to be chasing a ghost if the improper setting on something else gives misleading info. This way I'll have a better chance of finding why the iac is doing what it is doing. Eliminate all variables.

 

[1998_K1500_Sub]

Oh it's definitely something to do with the iac. The question is WHAT is telling the iac to do what it is doing? The iac seats fully when commanded to in diagnostic mode, it moves out when put into gear, it returns when the ignition is cycled. It just don't go back to the same position in park after being in gear. WHY? Something is telling or not telling it to. That's why I want to start from the beginning making sure everything is properly set to factory recommend spec's and go from there with the diagnostics if need be. I don’t want to be chasing a ghost if the improper setting on something else gives misleading info. This way I'll have a better chance of finding why the iac is doing what it is doing. Eliminate all variables.

Agreed.

It *is* a quick process to trim the idle air bleed screw so as to dial the IAC into a reading of ~10 (as was noted earlier, see document submitted prior) when warm. I would chase that down because it's quick and easy, and possibly the reason for the wacky response. None of us know the ECU's IAC algorithms (well, likely none us), and from what I've learned (from the Honda) the ECU seems to care about where the IAC is *now* when it's deciding where to move it to *later*. So, the decision as to "how far to move it out when going into gear" might be based on "where it is now", and if "where it is now" ain't where it's supposed to be (e.g., ~10 when engine warm, no AC, etc.) then confusion may result.

I also don't know if or how the "throttle follower" function's being affected and / or reacting to the IAC's (possible mis)position at idle.

Notably, the IAC seats fully when commanded; that's just the ECU driving the stepper for a whole bunch of (excess) steps, with the assumption that once the IAC bottoms it simply won't move anymore... a "reset the position" sort of action. So that's good.

Good luck!

 

[BigRedOne]

Agreed.

It *is* a quick process to trim the idle air bleed screw so as to dial the IAC into a reading of ~10 (as was noted earlier, see document submitted prior) when warm. I would chase that down because it's quick and easy, and possibly the reason for the wacky response. None of us know the ECU's IAC algorithms (well, likely none us), and from what I've learned (from the Honda) the ECU seems to care about where the IAC is *now* when it's deciding where to move it to *later*. So, the decision as to "how far to move it out when going into gear" might be based on "where it is now", and if "where it is now" ain't where it's supposed to be (e.g., ~10 when engine warm, no AC, etc.) then confusion may result.

I also don't know if or how the "throttle follower" function's being affected and / or reacting to the IAC's (possible mis)position at idle.

Notably, the IAC seats fully when commanded; that's just the ECU driving the stepper for a whole bunch of (excess) steps, with the assumption that once the IAC bottoms it simply won't move anymore... a "reset the position" sort of action. So that's good.

Good luck!

What do you mean by "idle air bleed screw?" The only screw I'm aware of on my tbi is the idle screw that adjusts the throttle plates. Is that what you mean or something else that I'm unaware of?

 

[1998_K1500_Sub]

What do you mean by "idle air bleed screw?" The only screw I'm aware of on my tbi is the idle screw that adjusts the throttle plates. Is that what you mean or something else that I'm unaware of?

That’s the one, yes.

On the Honda there’s a separate idle air passage around the blades, with a separate screw controlling the bleed, so I used that descriptive reference. Sorry for confusion.

 

[BigRedOne]

That’s the one, yes.

On the Honda there’s a separate idle air passage around the blades, with a separate screw controlling the bleed, so I used that descriptive reference. Sorry for confusion.

No Problem. Thanx for clearing that up.

 

[1998_K1500_Sub]

Random thought:

Is the pintle on the new IAC identical to the former’s?

Is the thread pitch on the pintle shaft identical? Yes, likely hard to tell, but…

Both of these could be significant factors in the relationship between “steps of the stepper motor” and “engine RPM change”. Call this relationship “gain”.

The ECU’s RPM management algorithm is a feedback loop, and this “gain” is within the loop. Feedback loops generally don’t respond well to changes in loop gain that differ markedly from the original design. That’s enough on that topic for now.

 

[BigRedOne]

Random thought:

Is the pintle on the new IAC identical to the former’s?

Is the thread pitch on the pintle shaft identical? Yes, likely hard to tell, but…

Both of these could be significant factors in the relationship between “steps of the stepper motor” and “engine RPM change”. Call this relationship “gain”.

The ECU’s RPM management algorithm is a feedback loop, and this “gain” is within the loop. Feedback loops generally don’t respond well to changes in loop gain that differ markedly from the original design. That’s enough on that topic for now.

No threads on IAC that I can see. Yes it is exactly like the one removed. Changed it because the old one was acting the same way. It's as if it doesn't realize you shifted from drive back to park. But I can see it in the scan tool that it changes.

Can you tell me why I have 3 coolant temp senders? 1 in each head. Single wire each. If I unplug the drivers side the gauge stops working so I know what that one does. If I unplug the pass side one nothing happens. And 1 two wire in the intake. If I unplug that the rpms race up. So I'm assuming that's the one signaling the ecu. I think for good measure I'm going to change all 3 (what the hell it's only money...lol) And I guess the EGR valve as well. It tests fine but it looks old and who knows what its actually doing.

 

[PlayingWithTBI]

Can you tell me why I have 3 coolant temp senders? 1 in each head. Single wire each. If I unplug the drivers side the gauge stops working so I know what that one does. If I unplug the pass side one nothing happens. And 1 two wire in the intake. If I unplug that the rpms race up. So I'm assuming that's the one signaling the ecu

Did or does your truck have an electric fan? The sensor on the right head would feed that, yes the one on the left head feeds your gauge and the 2 wire feeds the ECM. When you disconnect that one, the ECM thinks it's -32° so it'll raise idle and richen the AFR.

 

[BigRedOne]

Did or does your truck have an electric fan? The sensor on the right head would feed that, yes the one on the left head feeds your gauge and the 2 wire feeds the ECM. When you disconnect that one, the ECM thinks it's -32° so it'll raise idle and richen the AFR.

It does have an auxiliary electric fan. I didn't know it was stock. There's a switch on the dash that's not hooked up. Thought maybe that was for the fan. Learn something new everyday. So far it doesn't have an overheating problem except sitting in stand still traffic for a long time in gear. I just throw it in neutral and cycle the key and it cools down with the lower idle.

 

[PlayingWithTBI]

It does have an auxiliary electric fan. I didn't know it was stock

It may or may not be stock, a lot of folks use that port for aux fans. Depending on the year/PCM it may be controlled by the computer for temp and A/C on.