Idles high after being in gear.

[1998_K1500_Sub]

You said "When I put it into gear the rpms drop to around 625 and it slow increases to 750rpms and stays there.(good right?) The IAC counts are at full tilt 145!! Now I go back to park and it rpms race up to 2000rpms and hang there for about a minute then they start to come down and settle at ...... "1000RPMS!!!!"

This really sounds like the IAC isn't moving as it should. The ECU thinks it's moving it, but it's not IMHO. The IAC's moving some, yes, but the IAC's not moving as far as the ECU believes. The evidently SLOW response of the IAC to changes would be a consequence of "missed" steps; the ECU's driving it (say) 100 steps but the IAC's only moving some fraction, (say) 50.

If / while you're checking your IAC wiring, measure the resistance on the four wires between the IAC and the ECU. Remember, you need to disconnect them from the ECU and the IAC to do this.

If the IAC isn't responding b/c the wiring is compromised, the ECU is likely "driving" it excessively high steps in its attempt to manage the RPM... but the IAC's not responding completely (maybe high resistance in the wiring is preventing the IAC from making every "step" commanded to it by the ECU). I'm VERY curious about this. Please take a look, and look hard at the connector(s) (on the wires to the ECU, and the ECU itself, and also near the IAC too) to check for corrosion.

 

[1998_K1500_Sub]

Yes the 7747 has the memcal

For reference, I (and we) need to align our terminology. I've been mistaken, as I assumed this vehicle used a MEMCAL. It's evidently got a PROM and a CalPak.


According to the service manual:

ELECTRONIC CONTROL MODULE

There are two types of ECM’s used in light duty
trucks.

A vehicle equipped with a 4 cylinder engine has an
ECM (referred to as GMP4) with two parts for service.
A controller (an ECM without a Mem-Cal) and a Mem-
Cal (Memory and Calibration unit).

All vehicles with a V6 or V8 cylinder engine have
an ECM (referred to as GMCM) with three parts for
service. A controller (an ECM without a PROM), a
PROM with specific program information for an
engine and vehicle and a CalPak with specific
calibration information.

Mem-Cal (L4)
(Figure 3-3)

This assembly contains both the functions of the
PROM and CalPak. Like the PROM, it contains the
calibrations needed for a specific vehicle as well as the
back-up fuel control circuitry required if the rest of the
ECM becomes damaged or faulty.

PROM (V6 & V8)
(Figure 3-5)

Information for specific engine and vehicle is
programmed using an integrated circuit called a
PROM, (Programmable Read-Only Memory). In the
parts book, it is listed as a calibrator. This allows one
model of controller to be used for many different
vehicles. The PROM is located inside the ECM and
has information on the vehicle’s weight, engine,
transmission, axle ratio, and several others. While
one ECM part number can be used by many vehicle
lines, a PROM is very specific and must be used for the
right vehicle. For this reason, it is very important to
check the latest parts book and Service Bulletin
information for the correct part number when
replacing a PROM.

CalPak (V6 & V8)
(Figure 3-5)

A CalPak is used to allow fuel delivery if other
parts of the ECM are damaged. If the CalPak is
missing, it will result in a no start and run condition.

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[PlayingWithTBI]

Doesn't Moates have a daughter board for sale that enables this?

I'm not familiar with the AutoProm or G2 so, IDK if they totally bypass the EPROM. What I was getting at is IF the EPROM is bad, theirs way to eliminate it. Your idea may be one too.

 

[PlayingWithTBI]

For reference, I (and we) need to align our terminology. I've been mistaken, as I assumed this vehicle used a MEMCAL. It's evidently got a PROM and a CalPak.

Yeah, I mixed the MEMCAL with the CalPak, my bad.

 

[BigRedOne]

So here is a question. The service manual I just read says to set base idle to 700+-25 IN DRIVE! I've been doing it in park. I've never set idle on any vehicle in drive. Always park and make sure it don't die in drive. So I went to try it. Well if you are setting it with the IAC parked and unhooked that means when you go into Park the base idle is like 1000 or more. So do I set the base idle with IAC connected and make sure the counts are 5-30 in drive? This scanner can't do it when hooked up cause of the RPM variance. I tried it iac unhooked with my timing light on rpm mode and its like at 1100rpms in park to get it in drive to 600rpms. I guess I can try it with the IAC hooked up when my buddy gets here and see if I set it in drive will the IAC counts come down at all even in the scanner it should show it moving. Right now With the scanner it settles at 1000rpms in drive but as it comes up past 750 its already at 128-140 counts. but will it idle down to 750 in park after? That is the real question. I'll let you know after tonight. Fingers crossed its that simple.

 

[BigRedOne]

For reference, I (and we) need to align our terminology. I've been mistaken, as I assumed this vehicle used a MEMCAL. It's evidently got a PROM and a CalPak.


According to the service manual:

ELECTRONIC CONTROL MODULE

There are two types of ECM’s used in light duty
trucks.

A vehicle equipped with a 4 cylinder engine has an
ECM (referred to as GMP4) with two parts for service.
A controller (an ECM without a Mem-Cal) and a Mem-
Cal (Memory and Calibration unit).

All vehicles with a V6 or V8 cylinder engine have
an ECM (referred to as GMCM) with three parts for
service. A controller (an ECM without a PROM), a
PROM with specific program information for an
engine and vehicle and a CalPak with specific
calibration information.

Mem-Cal (L4)
(Figure 3-3)

This assembly contains both the functions of the
PROM and CalPak. Like the PROM, it contains the
calibrations needed for a specific vehicle as well as the
back-up fuel control circuitry required if the rest of the
ECM becomes damaged or faulty.

PROM (V6 & V8)
(Figure 3-5)

Information for specific engine and vehicle is
programmed using an integrated circuit called a
PROM, (Programmable Read-Only Memory). In the
parts book, it is listed as a calibrator. This allows one
model of controller to be used for many different
vehicles. The PROM is located inside the ECM and
has information on the vehicle’s weight, engine,
transmission, axle ratio, and several others. While
one ECM part number can be used by many vehicle
lines, a PROM is very specific and must be used for the
right vehicle. For this reason, it is very important to
check the latest parts book and Service Bulletin
information for the correct part number when
replacing a PROM.

CalPak (V6 & V8)
(Figure 3-5)

A CalPak is used to allow fuel delivery if other
parts of the ECM are damaged. If the CalPak is
missing, it will result in a no start and run condition.

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Just like figure 3-5.

We are on the same wave length. After looking over the service manuals I found info on location and testing of the IAC wiring. So I went out and did it all and It is all doing as it should and the resistance tested 0.

 

[1998_K1500_Sub]

So here is a question. The service manual I just read says to set base idle to 700+-25 IN DRIVE! I've been doing it in park.

(edit) See my next post, I see where you got this information. What follows below is my unedited post from earlier... still relevant. (/edit)

(edit) Also see pg. 4-5, bottom-left in the bullet list, where they talk about "controlled idle speed". ALSO see the instructions for checking Controlled Idle Speed on pg. 4-17 (/edit)

I'm looking on pg. 4-44. Where did you look? They make a statement there about "open loop". They didn't say anything about having the IAC unhooked.

I suspect one leaves the IAC connected and trims the throttle blade screw until the IAC counts are as-specified on pg. 4-44, i.e., "5-30", although I would likely shoot for "20-30" to leave allowance for the engine to idle down properly in P/N (you can always trim this later by repeating the process). So, with the scan tool connected and while watching the IAC counts, closing the throttle blades will open the IAC (higher counts) and vice versa. All the while the ECU will be trying to keep the RPM at 750RPM, moving the IAC accordingly; you needn't try to set it to any particular value yourself, you'll only be fighting the IAC.

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[1998_K1500_Sub]

ALSO, see pg. 13-2, it details the significance of that 10k resister and the idle speed changes you have (or should) see. SEE ALSO the explanation of "Open (Normal) Mode" on pg. 13-1, because it's relevant.

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[1998_K1500_Sub]

So here is a question. The service manual I just read says to set base idle to 700+-25 IN DRIVE!

(edit) I've attached the relevant pages from the service manual which I've been quoting recently (/edit)

I see this, on the bottom of pg. 4-44 (attached).

One needs to also read the paragraph on pg. 4-5, bottom-left:

• The minimum idle air rate is set at the factory
with a stop screw. This setting allows enough air
flow by the throttle valves to cause the IAC valve
pintle to be positioned a calibrated number of
steps (counts) from the seat during normal
controlled idle operation. The IAC counts will be
higher than normal on an engine with less than
500 miles, or an engine operating at high altitude
or an engine with an accessory load such as the
alternator, A/C, power steering or hydra-boost
brakes activated.

Also see pg. 4-17, "Minimum Idle Air Rate Check", steps 1 to 18 (I will not copy them here).

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[PlayingWithTBI]

I haven't found specific instructions for this. It almost sounds in conflict with the "Controlled Idle Speed" in the table above (my earlier post).

I would *guess* this step requires one to set the IAC fully closed with the engine off (through whatever means... or block the air passage), disconnect the IAC, then start the engine, put it in "D", and trim the throttle blades.

This seems to be what I'm seeing in TunerPro RT "Parameter Tree Comments". The ECM targets 750 RPM with it in "D". If it doesn't see "D" it won't try to adjust idle to 750.

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In this post of yours, P/N restrict functions will be disabled with the 10K resistor.

ALSO, see pg. 13-2, it details the significance of that 10k resister and the idle speed changes you have (or should) see. SEE ALSO the explanation of "Open (Normal) Mode" on pg. 13-1, because it's relevant.

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I don't remember, does the "D" wire work or is it shorted out? I think OP needs to go back to setting min idle screw with IAC closed as per my previous attachment (post 26), and verify P/N and D work properly. Then let the ECM do its thing without the 10K scanner. This may not fix it but maybe we can eliminate something.