High idle on each start

[Boog]

I've been working at a total tuneup on the '91 replacing most all electronics under hood since most everything on it was original 31 yr old with 82,500 miles. It's bone stock. The 350 is solid, no oil usage or leaks. When I got it it had some issues. It stalled all the time when stopped in gear and ran a bit rough so I started with a new dist, plugs, wires, CTS, IAC, 02 sensor, rebuilt throttle body with 2 new injectors. I used AC Delco parts where I could. Tested and retested for vacuum leaks. None. Cleaned all ground wires. I did a manual idle speed adjustment that cured the stalling issue. Driving down the street I could hear and feel the engine skipping but not enough to turn on the money light. Step on it hard and it would get up and go. Well lately when I start it up it idled well over 1000 rpm and would not come down while in park. Once I put it in gear it would slowly come down and wasn't too bad when stopped in gear with foot on the brake. Every time I started it it would idle high again regardless of warm or cold. After reading a thread where someone had it do just the opposite and he found a corrupted chip and mentioned corrosion in the ecm I thought I'd check mine. I removed the negative battery cable. I removed my ecm and opened it up and it looks brand new inside. All pins look fine so I just sprayed them good with electronics cleaner and put it back in.
On the next cold start it idled up more like it should, a good bit less than it did before. So I took it up town and drove around a while to let it relearn then headed out of town 30 miles or so. It runs much better with less noticeable skip. Pull up and stop and it idles smoother. Once home I shut it off and let it sit while I tended to something else for 30 minutes or so. When I restarted it again it idled up like the first start of the day. Once put in gear then back to park it comes down to a normal idle speed again. I have no tach so I cannot give rpms. I have no scan tool for OBD1. I'd like to have a cable to hook it up to my laptop and use Winaldl or TunerPro or similar to see just what it says.
I guess the battery disconnect reset the program so it started relearn clean may be responsible for the improved running. It's been a long time since I last had a TBI truck so I don't remember just how well they ran. It's certainly not as smooth as my LS trucks.
Anyone have a suggestion for a cable for a newish laptop with hdmi ports?

[Palf70Step]

There is a way to read codes in OBD1 using paper clip, I just don't remember how or wat pins you join together. Hopefully the smart folks will chime in.

Sounds like a sensor issue to me, but I don't know much on the compter controlled stuff.

[Boog]

Yes you can use a paper clip or an OBD1 tool to retrieve any stored codes. I have no CEL currently. I am looking to review the diagnostics via a Winaldl or TunerPro type program and need a cable to do so.
I'm no techy but this may be the ticket. Baud rates and 10K resistors are just confusing to me.

http://aldlcable.com/

[Just call me Sean]

There can still be codes even if the light isn't on.

[sntrym95]

I had the cable from aldlcable.com but it didn't last very long. I'm currently using this one: https://www.ebay.com/itm/36367815293...Bk9SR_6vg-rBYA and I love it. The resistor switching is done by jumper so it's really easy.

[ChevyTech]

If you get data and post it, I will look it over.

Get it up to operating temperature with all accessories off, and idling, and do a capture.

There have been a few fast idle problems lately that have ended up being a bad PROM (chip).

Over the years a common problem has been poor connection at the connector that plugs onto the IAC. The terminals lose their tight fit and then they make and break connection.

You may want to double check the minimum air rate (base idle speed) and make sure it is not to fast. If it is to fast the data will show an IAC count of zero.

[Boog]

I'll check it for codes again today.
I have seen that Red Devil deal and wondered what it is like.
CT, I considered a bad connection at the IAC so I used a pick to spread those pins to tighten it up just last week. It didn't seem to make any difference though.
I know even new parts can be bad right out of the box too.
Thanks guys. I'll get something coming shortly.

[61_FL_Apache]

I had to replace my IAC for stalling on my 97 Tahoe. I made the mistake and bought a cheap one.. then replaced it with a "better" one and that resolved the issue.

[Boog]

There are no codes stored presently. I haven't driven it since saturday.

[Boog]

This my first attempt to use my cable so forgive my noob presentation.
This is today fully warmed up with AC off and in park.
I see the glare is terrible. I’ll get better.
CT please review this data.

[ChevyTech]

The big thing I see as it shows 27.3 kpa on the right which is only 8 inches of vacuum.

The MAP voltage does not look way off. Am I seeing .9 volt?

Screwed up MAP data will mess with the idle control.

Can you toggle the MAP data and look at it another way?

Other things I see:
It is a little on the lean side but in the acceptable range.

The coolant temp is a little low. Too low to run some self tests but as soon as you drive it I bet it gets a little warmer.

The battery voltage is a little lower then I would expect, but it is charging.

The ECM knows the engine speed but is not trying to slow it down. If it were trying to slow it down you would see the IAC count going down lower and lower as it tries to slow the RPM. If it was trying to slow the RPM, and you had a vacuum leak so it could not slow it down, the count would go to zero.

As for the low battery voltage, if you bring the RPM up does the voltage get to at least the high thirteens?
Another thing I would try is turning on some electrical loads, like headlights, blower motor and radio, and see what happens to the voltage.
Try testing the running voltage with a meter and see if it matches what the data shows.

The ECM has a battery voltage correction mode that increases the RPM and will richen the fuel mixture. I just have never seen specific criteria on this.

Somewhere in the data there should be a indication for the Park/Neutral switch that tells the ECM if it is in park or in gear. This make a difference for idle control. See if you can find that.

What speed does the idle settle to when in gear?

[Boog]

Hi CT. This is my first time using the new cable and Winaldl. I don't yet know what all I can do with it. More class work tomorrow.
8" of vacuum is puzzling because I have hooked up a manual vac gauge and it held a solid 19.5 ".
Map voltage does show .9 volts
So far, I have not replaced the map sensor. I did test it to see if it held vacuum.
It has a new 195* thermostat in it as called for and new fan clutch.
Battery charging rate does get up to 13.7 at times, tested with a multimeter.
I have sprayed carb cleaner all around looking for vacuum leaks and none found. I also figured a steady 19.5" vacuum ruled out a leak.
I will spend more time with it tomorrow and get back to you.
I had no idea this early OBD system did so much.
I have to learn what all those values mean and the preferred readings of each.
Thank you sir.
Also, I picked up a couple of proms at the junk yard from the same ECM number as mine to try if it comes down to that possibility.

[ChevyTech]

When you start the truck make sure you do not touch the accelerator and see if you can get a data reading of 0% on the TPS at idle.

The .9 volt on the MAP is ok. My guess is the WinALDL has quirks. See if you can toggle how it displays the MAP reading.

Glad you have PROMs. That seems to be getting to be a more common failure as these trucks age.