Seville 93 4.6 N* idle slow to come down

[BodybyFisher]

Have you checked the ground at G104?

It looks to me like the ISC motor is controlled via pulsed width modulation (PWM). In this instance, the 2 parameters would be voltage (most likely 5 volts) and duty cycle.  A missing or bad ground would possibly reduce the voltage.  The duty cycle would be set and determined by the ECM.  

Find G104 and restore it.  The manual electrical grounds section index will show you exactly where G104 is.

What is odd however is why does this only occurs when the system is cold but works fine when the system is up to operating temp.  It is possible that a bad ground improves when the connection is hot?, I think I have seen that.

If there were a slow reacting ISC, why is it not slow to react all the time?  You should be experiencing problems coasting where the ISC isn't allowing the throttle to close fast enough when the throttle linkage hits the closed throttle contacts.  It should surge and feel like you can't hold it back at that point, but this slow retraction is only happening on start up when cold 🤔

[BodybyFisher]

The closed throttle contacts are on terminals A and B, G104 is in that circuit.  

Just thinking here, no point to my ramblings.  

I have read a bunch a articles by Larry Carley, he actually does a nice article regarding the Northstar.  In the following article he makes this statement, the key being temperature:

The circuit isn’t complete so current doesn’t flow. Road vibration, or changes in temperature and/or moisture, may cause resistance in the connection to change, creating an intermittent drop in voltage.

https://www.brakeandfrontend.com/tech-feature-solving-intermittent-electrical-problems

Keep in mind this vehicle is 27 years old.  Check the pins and connections on the ISC circuit at the ECM specifically the blue connector 3E

 

[floods]

@BodybyFisher Ok, so i will check this ground, that can be the issue.

No, don't work when hot, as i say the issue is the same engine hot and cold, all the time i have the same issue, when cold you don't feel it that much because IDLE cold is higher, it is really at operating temp that it is the worse, i think that was a misunderstand when you ask me this question : 2) When the engine is warmed up adequately (probably closed loop) the idle settles down and is good (650-700), i say yes, but i mean the the RPM is always right 650 to 700, but too slow to go down to this RPM

I try the old PCM, he is really wrong, i was not able to make a IDLE learning before he get the bad IDLE issue, but during the warm-up i get the same issue, so i don't think that the PCM.

By the way, my engine calibration ID is 1104 and transaxle is 1305.

I will check this ground, and i just did a video, i will put the link when i get it online, so it will be a lot more easy for you to see the problem.

Thanks a lot

[floods]

Video is online, here the link : https://www.youtube.com/watch?v=fH2I4wBJHFg

That show exactly the issue, and when engine cold, that the same.

Thanks again

Edited July 21, 2020 by floods

[floods]

Just back from the car, i checked all the ground on the PCM and all the power supply, all is fine, i check the 3 wire from the PCM to the ISC Motor, all is ok (0 Ohm), check between each wire and ground, all right too, the pin A on the ISC Motor is a good ground, no issue, get to G102 (according to the 93 manual) that the ground point on the back passenger side of the engine, who is perfect too.

That issue make me crazy 🙄

[BodybyFisher]

2 hours ago, floods said:

Just back from the car, i checked all the ground on the PCM and all the power supply, all is fine, i check the 3 wire from the PCM to the ISC Motor, all is ok (0 Ohm), check between each wire and ground, all right too, the pin A on the ISC Motor is a good ground, no issue, get to G102 (according to the 93 manual) that the ground point on the back passenger side of the engine, who is perfect too.

That issue make me crazy 🙄

Its driving me crazy also! Good night!

[BodybyFisher]

Here are the conditions required to allow closed loop to occur

[floods]

Exactly that what i see too, but mine don't wait to be at 80°C as i know ! and ECT show right, that strange.

Did you see my video ?

Thanks a lot

[BodybyFisher]

Have you tested the ECT for correct resistance?

I will watch it when I get up. 

[floods]

No i don't, since the temp is showing right, but i will check that to be sure.

Thanks

[BodybyFisher]

1 hour ago, floods said:

No i don't, since the temp is showing right, but i will check that to be sure.

Thanks

What I am concerned with is an ECT that responds slowly.  Do you know if it is an OEM AC Delco ECT?

Years ago, an engineer from the Northstar design team visited this forum daily.  He was adamant about using OEM parts, because in his words the validation had been done at the factory, and to use aftermarket components meant that you the user was doing the validation.   For that reason, I tend to use OEM in critical areas.  He was especially adamant about items like the EGR valve and thermostat being OEM.  

Consider what might happen if your ECT changed slowly with temperature it would take longer to go into closed loop which might explain your cold engine high idle.

I am going to look at the video now

[BodybyFisher]

I just watched the video.   Please take a photo of the plunger and post it.   I may be wrong but the plunger seems to be out quite far as I think I see threads.

I noticed something odd, the plunger turns side to side as it operates I don't think I have ever observed that action before.  And believe me, I have watched many a plunger.

What make is this ISC?  I have used an aftermarket ISC made by Standard Products with success in a 95.  What I worry about here is us assuming the ISC is new, when it is functioning sluggishly.  Assuming a new part is good is something that can create this kind of headache.   Companies re-box stuff returned with little or no inspection or testing.

That said, does this create any issues while driving it? Does it feel like you need to hold it back in traffic?  I do agree that the idle appears to slow down too slowly, but maybe its not that it slows down too slowly, but its that it is extending too far and the retraction begins at too high of an RPM.  If this slow retraction was at a lower RPM it wouldn't be an issue.

You've done a beautiful job on this engine

[floods]

Yes, it is a brand new OEM AC Delco ECT (ref 213928) i change it when i rebuild the engine, i always try to get OEM parts when possible.

Actually he get faster in closed loop, not slower, because closed loop is before he get 80°C on the temp, cold the RPM is higher, but normal, not too high, so nothing look wrong, what is wrong is exactly what i show on the video, and that same when engine is cold, but RPM is a bit higher when start the engine cold (normal).

Ok, i will do a picture of the plunger, how it is now, is with the right gap and right according to the IDLE cehck in the manual (10 to 11°)

The plunger just rotate a little when changing direction (little play, really not much, that the play in the hexagonal hole in the body), the 3 ISC motor i have did the same, the one on the video is a new OEM AC Delco (Ref 217425)

Yes, in traffic i have to hold it back, is not that much, it is not dramatic, if really we can't find anything i can drive that way, not a big issue, it was worse with the old ISC motor, because it was getting about 1200-1300 RPM before slow down, with this new one, you can see it is about 1000 RPM, that way much better.

It would be nice to find anybody with the same car to compare.

Thanks a lot for help.

[BodybyFisher]

@Logan do me a favor and watch the YouTube video linked to a few posts above and tell us what you think.  Thanks

[floods]

I just finish checking the ECT, all right, this working with a dual curve, below 50°C and above 50°C i check the voltage directly on the PCM and all is right, i check resistance too, but don't see the value in the book, only give the voltage curve, and i check the real temp with the infrared thermometer (a quality one from Fluke), and no issue with the ETC as i see, showing the right temp all time and right voltage.

Here a picture of the ISC plunger, the screw is more than halfway in.

Thanks

[BodybyFisher]

I do not recall, the screw being out that far.  And I know you have the correct 10° to 11°

Have you attempted throwing the book out the window and experimenting with the plunger length?  Do me a favor turn the screw in 1 full turn and see what you get.  Mark the flat on the plunger with a marker so you get exactly 1 turn.  Lets experiment

You may reduce the overall rpm range and improve driveability

If one turn is too much back it out a 1/2 turn.

By the way, it is also possible that a long ride might cause the system to learn/adapt

[floods]

Yes, i did, turn by turn in, and i do the same out, not getting better, i just notice when it is a lot more out (4 turn more out), it is a bit more smooth, there is not that little RPM increase before getting down.

And when the plunger is more in, i notice that when i just touch a little bit the accelerator and release it, the ISC has no time to correct the RPM fast enough and the engine is close from stalling for 1 or 2 sec before RPM get up again.

I try about 2 turn in and 4 turn out from the actual position if i recall right.

We speak about the PROM, so i did some looking, so the most important is the BCC code (Broadcast Code) mine is the BBSX, that the right according of what i found, and i am lucky because i found one with the BBSX calibration for about $30 i think i will give it a try.

Thanks for help

Edited July 22, 2020 by floods

[BodybyFisher]

I did a little research and found that

BBSX, is superceded by BHMU according to info at this link

https://forum.tunerpro.net/viewtopic.php?p=13643

 

[floods]

Yes, i see this link, but how to be sure, because on the BBC lookup list i found, the BHMU is for the 16180220 PCM, i have the 16180517 PCM so not sure, and is there any advantage ? i don't know, would be great to have the BIN file to read the parameter and see exactly how the ISC work, i did it already long time ago for a Fenix ECU, was a lot of work to find the parameter in the hex file.

Thanks.

[Logan]

Early GM Techline terminals used to have a PROM charts back in the DOS and IBM PS2 days. It would have specific PROM details for every GM car. You could search each and every PROM for details.

Haven't seen those charts in 15 years. There is more to it than this PDF chart.....details like engine....FED, Canada, export emissions....trans....final drive ratio....maybe 20 Proms per car.

Did find some info... page 63

http://www.scotthansen.net/BCCLookup.pdf

 

Some eBay Prom and ECM BBSX details..

https://www.ebay.com/itm/Prom-Chip-BBSX-1993-Cadillac-Eldorado-4-6L-PCM-ECM-16180517-Memcal-16200887-/223903910936

 

To me....it seems likely his PROM is ok......it seems like a ECM quad driver issue......if he sets it on the low side....it's too slow to catch up....it wants to stall......if he sets it high....its slow coming dowm.

It is interesting the the ECM has been replaced before for a idle issue.

There were some goofy 1994 ECMs.....there is a TSB about those.

25 year old car....could be something like a worn throttle shaft...weak spring...etc.

 

Snapshot is from page 63 PDF chart.

 

[floods]

@Logan I am confused, because the PCM is a remanufactured one, and the old PCM just look to do the exact same issue, but what you say about the PCM has sense for sure.

About throttle shaft i don't think, the throttle body is new.

Did you see my video ? if yes, can you confirm this is not normal in this car ?

About the PROM, that exactly the one i order finally if that not the issue, can only be good to keep it as spare just in case since i plan to keep this car for long.

Thanks for help.

Edited July 23, 2020 by floods

[Logan]

I stated 'it could be something like'......a 25 year old car....it really could be anything along that type of problem. 

Reman ECMs can sometimes cause more problems than they fix. 

1993 service manual......page 6E-A-46 and 47 shows the grounds for the ECM. G100, G102, G401. You need to inspect all 3.....and not just a visual....you will need to take each one apart and clean. They can 'look' ok but actually be causing a problem.  

There is also a splice.....S234 that should be inspected very closely. 

[floods]

Ok, thanks, i will check S234.

I already check all ground on ECM, not just visual, i check with ohmmeter each one, clean them, try to move it and see what ohmmeter show.... i check already G100 and G102, i will look more in detail G401

About the ECM, ok, but strange that the old ECM did the same.

Thanks again

[Logan]

You really need to take them apart and clean.....they can look perfect just sitting there. 

I should have asked earlier......any aftermarket stereos /tvs ....amps....alarm system.....remote starter....pocket remote unlock systems installed on the car?

 

I prefer GM IAC motors over ISCs. GM had IACs around for over 10 years before Cadillac started using them with OBDII. ISC motors are like using a broomstick to turn on a wall switch. 

 

Here is another lengthy post about ISC motors. Maybe you'll see a clue... 

https://caddyinfo.ipbhost.com/topic/49651-1995-eldorado-racing-idle-isc-motor/#comments

 

 

 

 

[floods]

Yes, that what i did, except for S234, i will take it apart this weekend.

No, no any aftermarket on the car, all is original including the stereo (just the CD reader who is not working), no after market alarm system, amps is the OEM Bose gold, i just see the anybody cut the wire right in the rear of the stereo system and put it back together, i suspected anybody who try a put a aftermarket stereo and don't know that more complicated ten that on the bose system

I agree about IAC, over ISC, i install some of it on some old car i converted with EFI with the Megasquirt ECU, i learn a lot with this ECU, and get lot a trouble with IDLE, using a IAC over the Bosch 2 ways air valve was the right solution and work perfect all time.

As you say, maybe the re-manufactured PCM is the issue, since the original is not working long enough to be sure he did the same, i contact the seller and will send the PCM back and get a other one.

I will read the post of BodybyFisher again, maybe i miss a detail that can help.

Thanks again for help.