Intermittent High Idle and Occasional Hesitation on Acceleration

[chris3353]

Tons of people on this forumn have been trying to get cables,only supplier I've been able to find that actually has,is still in business,and will ship ALDL cables is moates and their cable is 80.00 Do you have a secret source for 20.00 cables or instructions in how to make one. Instructions that are not 20 years old that require obtaining components that were easy to get 20 years ago but not now, If so that would be really helpful to some members. Only instructions I've seen include what radio shack part numbers to buy. Radio Shack is gone so their part numbers do no good. Thank you sir.

You are right,,, Mine is Moates and it came with my autoprom.. back then i saw a ton for sale but times have changed.

 

[Sgt Gus]

Interesting, same issues w 87 R20, 454, rebuilt. Issues seem after warm up with high idle. Did put a 180° Stat in it.
Actually, going to try cable and data logging this weekend, because I haven't done yet.


I got mine from:

OBD Diagnostics, Inc. - OBD2 All-In-One Scan Tool w/ USB

The OBD2allinone pc scan tool to turn off the check engine light on OBD2 cars and trucks is made and sold by OBD Diagnostics Inc. Our automotive diagnostic tools can read and log any obd check engine code data.

obd2allinone.com

It's more $ now, but one piece.
Also,

Red Devil River, LLC.

Scan your 82-95 GM vehicle at home!

reddevilriver.com

 

[chris3353]

Interesting, same issues w 87 R20, 454, rebuilt. Issues seem after warm up with high idle. Did put a 180° Stat in it.
Actually, going to try cable and data logging this weekend, because I haven't done yet.


I got mine from:

OBD Diagnostics, Inc. - OBD2 All-In-One Scan Tool w/ USB

The OBD2allinone pc scan tool to turn off the check engine light on OBD2 cars and trucks is made and sold by OBD Diagnostics Inc. Our automotive diagnostic tools can read and log any obd check engine code data.

obd2allinone.com

It's more $ now, but one piece.
Also,

Red Devil River, LLC.

Scan your 82-95 GM vehicle at home!

reddevilriver.com

there is a sender for your gauge and a sender for the TBI computer. Without the data log, it is a guess.

Another overlooked idle issue is gear selection. There is a primitive plastic tab at the base of the steering column. when you shift into gear. it slides a switch to tell the computer. It adds ~25 IAC (maybe 25RPM i forget) to compensate for convertor drag. If it is broke, the engine might think it is in gear and always adding RPM.

 

[Sgt Gus]

there is a sender for your gauge and a sender for the TBI computer. Without the data log, it is a guess.

Another overlooked idle issue is gear selection. There is a primitive plastic tab at the base of the steering column. when you shift into gear. it slides a switch to tell the computer. It adds ~25 IAC (maybe 25RPM i forget) to compensate for convertor drag. If it is broke, the engine might think it is in gear and always adding RPM.

Cool, yes knew about senders, and a 3rd if you count aux fan...

Did NOT know about gear, idle switch. I did replace the turn signal lever and assembly.
I'll look into, thx. So many things...

 

[Sgt Gus]

there is a sender for your gauge and a sender for the TBI computer. Without the data log, it is a guess.

Another overlooked idle issue is gear selection. There is a primitive plastic tab at the base of the steering column. when you shift into gear. it slides a switch to tell the computer. It adds ~25 IAC (maybe 25RPM i forget) to compensate for convertor drag. If it is broke, the engine might think it is in gear and always adding RPM.

Do you have to check boxes to get the flag data? I was going to connect with winaldl to start, then maybe move to tunerpro.

 

[chris3353]

Do you have to check boxes to get the flag data? I was going to connect with winaldl to start, then maybe move to tunerpro.

When the data is live, the box will check itself if the flag is active.

Example, once the o2 sensor warms and gets a reading, the closed loop flag should check indicating you are in closed loop. If you are not getting into closed loop the ECM is running off a preprogrammed fueling and timing table. You need closed loop for the computer to make any changes. Many TBI issues come from not getting into a closed loop. You 100% need a heated o2 sensor. without it, the sender will constantly go open loop and default to bassline program.

If you need help, reply, or PM me your phone number. Once you get the TBI jargon, it is amazingly simple and bullet proof


the gear selector will check uncheck as you go from park to drive

 

[YakkoWarner]

Tunerpro RT can also show you the flags as well as the data stream, as long as you have the right ADS or ADX file. On mine I can see when the gear select goes from PARK-NEUTRAL to DRIVE (I don't think it knows which gear you are in, just that you are in a gear). It also shows when I'm in open or closed loop - but there is a second IDLE CLOSED LOOP thing which I am not sure what that is.

I am pretty convinced my PROM is written with a 950-1000 target idle. The IAC will always open and close to set that speed no matter what is going on. I don't like how hard that is on the transmission though.

 

[chris3353]

Tunerpro RT can also show you the flags as well as the data stream, as long as you have the right ADS or ADX file. On mine I can see when the gear select goes from PARK-NEUTRAL to DRIVE (I don't think it knows which gear you are in, just that you are in a gear). It also shows when I'm in open or closed loop - but there is a second IDLE CLOSED LOOP thing which I am not sure what that is.

I am pretty convinced my PROM is written with a 950-1000 target idle. The IAC will always open and close to set that speed no matter what is going on. I don't like how hard that is on the transmission though.

Do you have an autoprom? can you burn a chip? I can give you my chip data, I am running bone stock. I think Stock idle is 550 rpm.

You are correct, it does not care what gear you are in, the park/neutral vs gear adds a few IAC counts to offset convertor dragging the RMP low.

main reasons for high idle;
Closed loop,
gear selector,
physical idle screw to high. (I think my IAC has ~40 steps at idle)
calling for AC when not present.
Vacuum leak of faulty IAC

 

[YakkoWarner]

I don't have any way to burn a chip - and I'd probably want to find a way to read my existing chip first to confirm what it has programmed before going thru all that to find out it wasn't the case.

I don't think its a vacuum leak or faulty IAC because when I do the procedure to reset the IAC (jumper A-B. let code 12 blink while IAC closes completely, unplug IAC to keep it closed then start up, set base idle with screw) I can set the base idle all the way down to 250 without it stalling out (I know thats way to low but a vac leak wouldn't let it get down that far) and the IAC is obviously closing all the way because if I back the screw out any more it dies, indicating no air coming thru the butterfly bypass. Adjusted the throttle position sensor to be at 0.45-0.47 with the pedal fully up.

I set the base idle at 500 with the screw and once I start up with the IAC connected it will immediately step open until the idle is ~1000. Put it in gear and the idle will drop for a moment and I can watch the IAC counts go up until its back at ~1000, likewise go to neutral or park and it will jump up to 1500 for a moment and then the IAC will count down until again sitting right around 1000. I can see the ECU flag changing when I go from in gear to park.

I thought about the AC signal - I even disconnected the connector from the compressor and the pressure switch on the A/C line so that no voltage would be sent from the HVAC controller to the ECU to signal the AC being on. No difference. AC flag does't show as set in the ALDL.

Sensor readings seem to be in believable range (MAP, TPS, CTS and such) according to the ALDL stream, so I don't think there is anything wrong on the sensor side.

I replaced the ECU about 18 months ago chasing another problem (which turned out not to be the ECU's fault but I didn't have a way to read the ALDL stream at that time), but I had to reuse my existing PROMs. I suppose its possible a previous owner could have swapped ECUs from a parts truck and just used whatever ROM was in it, but without anyway to actually read the contents of the PROM there is no way to know if its right or not, and I don't think you can dump the contents of the PROM thru the ALDL link.

 

[chris3353]

Mine used to idle up there also, the IAC is chasing something. No one thing is adding that many IACs. I have a new laptop, I will install tunerpro and take a look at you bin and txt data. Do you know what ECU you have? 7747 is common.

If you want to ditch the EGR, I did, i can turn off the flag/error for that. I change my ECU. Stock the ERG can not activate until ~35 mph, forgot the rpm and map. I changed the speed to 200 so it can never call for EGR.

I will jump to your other thread.

 

[mtbadbob]

Did any of you guys with high idle issues change or check the coolant temp sensor? Common failures and they will feed the engine fuel if they're telling ecm that engine temp is cold. I've NEVER seen the shift indicater connected to anything electrical EVER. You can move the clip that holds the indictor cable up or down on the column sleeve to make sure the shift indicator is pointing to the right gear, but there is nothing electronic about it. At least not on my '87. Please enlighten me on this...

 

[gmbellew]

Did any of you guys with high idle issues change or check the coolant temp sensor? Common failures and they will feed the engine fuel if they're telling ecm that engine temp is cold. I've NEVER seen the shift indicater connected to anything electrical EVER. You can move the clip that holds the indictor cable up or down on the column sleeve to make sure the shift indicator is pointing to the right gear, but there is nothing electronic about it. At least not on my '87. Please enlighten me on this...

on my 1990, there is definitely a gear indicator readout in the ECM data. park/neutral, drive, and overdrive. it knows the difference somehow....

 

[Ricko1966]

on my 1990, there is definitely a gear indicator readout in the ECM data. park/neutral, drive, and overdrive. it knows the difference somehow....

My thoery is even though the column itself locks you out of being able to start in gear they are using the neutral safety switch for ECM input.

 

[chris3353]

Moseph3,​

I forgot to add to this post. (involved in multiple TBI threads)

My intermittent TBI high idle was my TPS. i fought for a long time. everything tested, everything right ans still on occasion it would idle high until key cycle.

One day while datalogging, i caught the TSP junping around while at a traffic light. Once that happened my idle was high until i cycled the key. The TPS tested perfect. It was new but defective. replaced and not one problem since.

Chris

 

[YakkoWarner]

Did any of you guys with high idle issues change or check the coolant temp sensor? Common failures and they will feed the engine fuel if they're telling ecm that engine temp is cold. I've NEVER seen the shift indicater connected to anything electrical EVER. You can move the clip that holds the indictor cable up or down on the column sleeve to make sure the shift indicator is pointing to the right gear, but there is nothing electronic about it. At least not on my '87. Please enlighten me on this...

I did change the coolant temp sensor last year sometime thinking that could be the issue (that was before I got the ALDL cable so I had no real data to go on at the time) - I doubt my old one was defective since nothing changed and the ECU is getting proper CTS readings.

There certainly is some sort of input to the ECU on gear selection, at least on my 1989. It doesn't seem to know which gear you are in, but I can watch the ALDL data and it correctly changes the flag from PARK/NEUTRAL to DRIVE as I shift. I have a TH400 so no overdrive.