High idle on each start
 

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?

Re: High idle on each start
 

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.

Re: High idle on each start
 

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/

Re: High idle on each start
 

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

Re: High idle on each start
 

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.

Re: High idle on each start
 

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.

Re: High idle on each start
 

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.

Re: High idle on each start
 

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.

Re: High idle on each start
 

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

Re: High idle on each start
 

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.

Re: High idle on each start
 

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?

Re: High idle on each start
 

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.

Re: High idle on each start
 

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.

Re: High idle on each start
 

On cold start up or full warm?
Upon testing today, it never did idle down from 1,000 unless dropped in gear and I think it then idled around 700.

Re: High idle on each start
 

See where the idle settles when fully warm, in gear.

As you get familiar with the WinALDL look at the data several times and capture it or takes notes to see if the readings are consistent when you reach the same conditions. I would like to know if it runs on the cold side all the time and if the voltage is always hanging low. Look at the oxygen sensor voltage which should jump all over under one volt. I want to make sure it is not reading on the rich end consistently while giving a rich command.

Have you ever had the intake off and looked at the build up on the valve steams. Have you ever used SeaFoam to clean buildup from this engine.

Re: High idle on each start
 

Will do.
Brief notice of vehicle history:
1991 Chevy c1500
84,500 actual miles
350 with OD auto
my parents bought it in 1994 at low mileage
never any engine problems so no, the engine has never been opened up.
I took possession of it last Nov and put 2,000 miles on it so far
It doesn't leak and uses no oil
I've never used any additives
It doesn't trigger codes, SEL does work
It's all stock as the General built it.

When these thunderstorms pass, I'll get back out there.
Thanks

Re: High idle on each start
 

I spent the afternoon on this truck and can’t say I’ve accomplished anything.
Bat voltage is 13.4 - 13.7 at 1,000 rpm since it will not idle slower.
Up to temp at startup (no touching the pedal) it idles at 1,000-1025.
Foot on brake in gear is 850-900.
Put back in park it jumps to 1525 then very slowly falls down some to 1100 or so.
The aldl displays looked much like yesterday’s.
So I did a manual minimum idle speed adjustment (just enough to still idle in park) I estimate around 450-500 rpm, 6 times with disconnecting the battery each time to clear memory.
I also sprayed the topside of the engine with carb cleaner again and find no vacuum leaks.
The map is not new.
The IAC is new but I do not remember brand.
Here is the first data screen of today.
Winaldl does not let us convert any data.

Re: High idle on each start
 

Another display.

Re: High idle on each start
 

More

Re: High idle on each start
 

He is what I think.

As for the high idle I would try swapping the PROM and if that did not work I would swap the ECM. The system shows the high RPM but does not look to be sending a signal to close the IAC.

1450 RPM at closed throttle with coolant @186 is not a correction for low voltage. That being said the low voltage would bother me to the point I would be testing diodes in the alternator. Compare it to your other vehicles and see what you think.

As for the not running real smooth in the first post, I think there is a good chance this engine has heavy deposits on the valve steams. It is running cold now and my guess is the truck has not been driven hard most the years it has been around. I would use SeaFoam slowly pouring some down the TBI while holding the RPM up (do not liquid lock the engine) on a hot engine, let go of the throttle purposely stalling it and letting it soak 15 minutes. Then restart and pour some more down the TBI at while holding the RPMs up. But before that I would replace the thermostat with a new 195 degree STANT brand thermostat. These engines get heavy deposits in the head ports mostly on the valve streams and especially if they run cold.

I have written about SeaFoam before and it seems like the exact same product as General Motors Top Engine Cleaner we used at the dealer.

When you get it running at the correct RPM I would adjust the throttle stop screw (Minimum Air rate) using the IAC count after some drive time above 45 MPH so the system does a relearn on its own. Get it adjusted so the IAC count is about 20 with it idling nicely, with a fully warm engine, transmission in in park, all accessories off.

The BLM (Long term fuel trim) and INT (short term fuel trim) are not consistent. The relearn you did, and working with it could account for that.

Keep in mind the fact that you are connected to the ALDL can affect how it runs and the idle speed. Disconnect from the ALDL and drive it after you make changes. I have never used WinALDL and don't know if WinALDL will let you switch modes.

Here are the modes:
Normal (open) mode
Some (limited) data can be observed without changing the engine operating characteristics.

Data link (also called 10K or special) In this mode all obtainable data is readable.
10K mode affects:
Closed loop timers are bypassed
Distributor ignition (DI) is advanced
PNP restrictive functions are disabled
IAC controls engine idle to 1000 RPM + or – 50 RPM (if applicable)
On some engines the canister purge solenoid is enabled

Factory test Mode (backup or 3.9K)
In this mode the ECM is operating on the fuel back-up logic used to control fuel delivery if the ECM fails.

I don't want to drown you in to much information. Let me know what questions you have.

Re: High idle on each start
 

I went and look at the http://www.winaldl.joby.se/ web site and the cable you use controls which diagnostic mode it is in.

I looked at the link in your earlier post http://aldlcable.com/ and I did not find anywhere that said it has a 10K resistor or not. My guess is it does not. You can test it by disconnecting it from everything and testing between the terminals that plug into position “A” and “B” of the ALDL connector with an ohm meter and see if it measure 10,000 ohms, or open. If it is open using the WinALDL program will not alter the way it runs which is good.

Re: High idle on each start
 

As you mentioned, corrupt proms have been found lately. Maybe it's an age thing. I'm not big on buying used electronics but I did pick up one last week. Mostly because the ecm was already unhooked and laying there and it was the exact same ecm as mine so it sort of looked meant for me. ;)
If after some driving tomorrow the high idle issue doesn't improve, I will swap the proms.
This truck was my retired parents so no, it wasn't driven hard while they owned it. A long trip in that truck for them was an occasional 10 mile trip to town for most of the years they owned it.
I'm not big on additives and the like but I will try Seafoam.
The aldl was disconnected before I began adjusting the idle today. The laptop battery ran out.
I believe you once posted your version of the minimum idle speed adjustment procedure that differed from the repair manuals.
The Winaldl is only an easy way to get into OBD1 diagnostics that most likely falls short of what some other programs can do.
I'll post again with any improvements. Thank you CT

Re: High idle on each start
 

No joy today either.
The last thing I did yesterday was the minimum idle speed reset.
Today the first start was the usual high idle (around 1,000) then I took it on a good ride around town and out on the interstate and let it rip some. It settled down and was driving real good with normal idle at stops. It wasn't trying to push through an intersection.
When back home it idled much like it should in gear or in park. I shut it off for 5 minutes then restarted it (not touching the pedal) and it revd right back to 1,000 rpm and stayed there.
So I hooked up the cable and logged a bit. I cannot upload a video so I have this doc file of about 3 min 20 sec run time sitting here at home.
The column at the left is the seconds of the run.
As you can see it's at run temp and all is steady at about 1,000-1025 rpm.
Then at 33.7 sec I dropped it in gear w/foot on brake and the idle dropped to 750 then creeped back up to 975 still in gear on brake.
Then at 93 sec I put it back in park then the idle shot up to 1625 and on to 1700 before it began to slow down a bit.
At 156 sec it slowed to 1025 rpm where it pretty much remained until the end of this log.
This is insane...
I may just try the used prom I found at the junkyard next.

Re: High idle on each start
 

It was helpful to see that many line of data.
Next time if you can see how to export it as a excel spreadsheet it is easier to view.

First thing you need to do is fix the low voltage problem. The voltage was as low as 12.5 volts. That is low enough that the ECM will raise the RPM because of this. My guess is the truck has an alternator problem.

Re: High idle on each start
 

It is apparent it is charging less than just a few days ago. The truck has a fresh Delco battery on it too. I was unaware the ecm would raise idle speed in order to increase batt voltage. Something else I learned today. ;)
I'll see to getting the alternator swapped out.
So do all the other numbers look to be in line at this point? As a data logger noob, I don't know the preferred readings of each sensor.