Idles high after being in gear.

[1998_K1500_Sub]

Fighting it in traffic with brakes it starts to get hot. If you let your foot of the brake it idles up to speeds of 20 or so mph.

Yes, it puts heat into the transmission oil.

How is the IAC controlled from the ECU that year, does anyone know offhand (stepper motor with multiple drivers on the ECU?), and is there feedback to the ECU (so the ECU would know "target position" & "actual position")?

 

[PlayingWithTBI]

How is the IAC controlled from the ECU that year, does anyone know offhand (stepper motor with multiple drivers on the ECU?), and is there feedback to the ECU (so the ECU would know "target position" & "actual position")?

It's a 4 wire stepper motor without feedback (see bottom right of diagram). The ECM just sends pulses to "targeted" counts. This is for my 88 but s/b the same for an 89.

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[1998_K1500_Sub]

It's a 4 wire stepper motor without feedback (see bottom right of diagram). The ECM just sends pulses to "targeted" counts. This is for my 88 but s/b the same for an 89.

You must be registered for see images attach

Thank you.

I'm trying to envision if / how a stepper motor might respond uncharacteristically if, say, one or both of the two "phases" was weak...

- bad / weak driver on the ECU board
- ground problem (IAC passes all current through the ECU, so same problems might exist on "power" side, see next)
- power problem on the ECU (oxidized trace on the PCB, other current paths that may have been compromised)
- wire problem (oxidation / high resistance on one or more of the four wires)
- internal problem to the IAC (I think OP said it was new)

I was hoping there was position feedback.

The ECU's where... behind the glove box, passenger's side? Where's it grounded? What's the condition of its ground and power leads?

OP, have you tried disconnecting the ECU leads and inspecting the condition of the contacts? If look good, that's good. Still, "working" the connectors by removing / re-inserting them can "fix" poor connections; it's something that's easy to try.

Fact is, I don't yet have a sound position on the matter... any of these could be a problem, but I can't say whether any of them would cause the uncharacteristic behavior. I need to think about it more.

 

[BigRedOne]

Thank you.

I'm trying to envision if / how a stepper motor might respond uncharacteristically if, say, one or both of the two "phases" was weak...

- bad / weak driver on the ECU board
- ground problem (IAC passes all current through the ECU, so same problems might exist on "power" side, see next)
- power problem on the ECU (oxidized trace on the PCB, other current paths that may have been compromised)
- wire problem (oxidation / high resistance on one or more of the four wires)
- internal problem to the IAC (I think OP said it was new)

I was hoping there was position feedback.

The ECU's where... behind the glove box, passenger's side? Where's it grounded? What's the condition of its ground and power leads?

Fact is, I don't yet have a sound position on the matter... any of these could be a problem, but I can't say whether any of them would cause the uncharacteristic behavior. I need to think about it more

Thanx for the response. I wish it got feedback also. Might be easier to figure out why it is doing what it is doing.

So let me say that I have been going off of info that I'm starting to believe is not correct. Let me ask you if you have the actual factory info for what the base idle (with iac disabled)should be set at? What the tps should be set at? I played around with these today and got some interesting results
Got me thinking I may be working with incorrect info. If you or anyone following this could get me "actual" factory info (not I thinks or I have always done it this way info) actual factory specs for those 2 things for a 1989 454 that would be great and I could go back and start from scratch and see what I end up with.
Thanx in advanced for any help anyone can give towards that.

 

[1998_K1500_Sub]

If you or anyone following this could get me "actual" factory info (not I thinks or I have always done it this way info) actual factory specs for those 2 things for a 1989 454 that would be great and I could go back and start from scratch and see what I end up with.

Let's see what others say.

I thought that "base idle" was that which would give the IAC a reasonable count @ idle, engine hot closed loop, e.g., something like 32 counts on the IAC. I've seen information like that in *some* manuals. Didn't matter much the engine, it's was more a function of "we need this much margin on the IAC" (in terms of counts) to handle cold engine, A/C on off, etc. loads while maintaining RPM while engine at any temp. So, around 30 with engine hot, allows for much wider opening (what... up to 127? or 63? or 255?) when it's needed.

 

[PlayingWithTBI]

The ECU's where... behind the glove box, passenger's side? Where's it grounded? What's the condition of its ground and power leads?

Yes, behind the glove box and it's grounded at the T-Stat (2 wires).

I thought that "base idle" was that which would give the IAC a reasonable count @ idle, engine hot closed loop, e.g., something like 32 counts on the IAC. I've seen information like that in *some* manuals.

There's a couple ways to set base idle. One is to connect a scan tool to it and watch IAC counts while adjusting the min idle screw to get it down to ~10+. The other way is this, it's an old one but works for all TBIs. It also shows how to calibrate the TPS.

 

[1998_K1500_Sub]

Yes, behind the glove box and it's grounded at the T-Stat (2 wires).

Two wires, OK. One is "sensor ground" / "signal ground" and the other is for the digital ckts and "everything else from ECU ground", I would assume.

 

[BigRedOne]

Let's see what others say.

I thought that "base idle" was that which would give the IAC a reasonable count @ idle, engine hot closed loop, e.g., something like 32 counts on the IAC. I've seen information like that in *some* manuals. Didn't matter much the engine, it's was more a function of "we need this much margin on the IAC" (in terms of counts) to handle cold engine, A/C on off, etc. loads while maintaining RPM while engine at any temp. So, around 30 with engine hot, allows for much wider opening (what... up to 127? or 63? or 255?) when it's needed.

Ok so not an actual rpm? Well I have tried several rpms that were supposed to be from a manual such as 425. Then Was told 150 below target so 625. All found the target of 750. And still rev'd to 1700rpms when put in gear and back. I have been playing today just to see what will happen with the tps. Set it to .45 thinking maybe the evu was thinking the throttle was open and low and behold it idled even higher!?! Didn't expect that. So I went the other way set it to .70 and it only spiked to 1500 and seemed to want to idle down more but it was fighting it. So the last base idle I tried with the .70 tps was 700rpms. I'm wondering if a lower base idle and higher tps setting might be the answer. Tps has limited adjustability. Just would like to know the "actual" factory settings instead of guessing anymore.

 

[1998_K1500_Sub]

Tps has limited adjustability. Just would like to know the "actual" factory settings instead of guessing anymore.

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.

 

[PlayingWithTBI]

Two wires, OK. One is "sensor ground" / "signal ground" and the other is for the digital ckts and "everything else from ECU ground", I would assume.

The ECM has 2 wire connectors, each connector has a dedicated ground. IIRC G107 and G108.