Seville 93 4.6 N* idle slow to come down

[floods]

Hi all,

I am new here, i am from France, and always drive Cadillac (have a Seville 93 N*, a SRX 2005 N*, a CTS 2003 3.2 V6, Eldorado 1974 V8 8.2 and a Chevy Van 1980 5.7 V8).
After really long search i decide to ask here about my problem, i have a 93 Seville 4.6 with the Northstar engine Euro version (export) and i get a little issue with the idle that is slow to come down when i release the pedal, so when i release the pedal, RPM go down to about 1000 RPM, and just get up to 1200-1300 RPM and then slowly go down to normal (take about 3 to 4 sec.).

So, i check the ISC motor, because that was looking to be the problem, when i release the accelerator, i can clearly see the ISC get in action, he make the RPM go up to 1200-1300 RPM and i see it get slowly in to get the right idle speed.

I have a spare (but used) ISC motor, i install it, and that just do exactly the same....

The engine is fully rebuild from top to bottom with new piston 0.25mm over sized, all new valves, new valve seat, bearing, timing chain.....
I get this problem since i have the car, but 1st i was thinking that because the bad engine (was lot of vacuum leak, and really bad valve and seat, only get 75 PSI of 2 cylinder and other was not a lot better, that why i rebuilt the engine), but now that all is new, i get the same problem with the idle.
I already replace the PCM because of a really rough idle that i get too since i have the car, the idle was playing from 700 to 1500 RPM after 15 to 20min driving, so that problem was solved with the new PCM.

I check for vacuum leak, don't find any, i plug the EVAP system just to check if that the problem, and no, i change the ISC motor, and of course i adjust it according to the service manual and i do a IDLE learning like said in the service manual, to be sure, i reset the learning IDLE (override PS13 in diagnostic mode), so i get the P080 code to tell me to learn the idle and TPS, so i did it and the P080 go in history, but still exactly the same issue.
So i change the TPS and do a learning again, but no change.
And of course i check the pedal cable, that not the problem, he is moving freely and little loose when release, and seme for the cruise control, the cable is perfectly adjusted i little loose when ISC pin is retracted.

I check with the GM TEch all live data and all look good, don't see anything wrong, the switch in the ISC motor work nice too, all value of sensor is right, and all sensor are new.
When i play with the GM Tech IDLE testing, all is fine, the ISC motor can increase speed and go down fast with no issue, it is really that moment when the ISC get in action (when the switch is pressed on the ISC pin) that the moment all the problem happen...
I try to unplug the EGR valve, but no change, but maybe this EGR valve can be the problem ? i check it, and all is clean, of course the throttle body is clean like new too.
I try a power balance to see if there is not a bad injector, but all look perfect, and O2 sensor (the 2 are new) give the same value on the 2 bank.
The car run perfectly, i just finish the break-in and remove the break-in oil, all perfect, no idle variation, really stable when A/C is switch on, and go in P.... just slow down to idle issue.

Maybe is that normal on this car ? but i don't think.

Oh, and a other try i did, i let the car tun at idle and unplug the ISC, and try that way, the problem is gone, but can't run that way, that just tell me the ISC get a wrong info or is bad (but as i say, look strange to have 2 bad ISC motor that do exactly the same), plus a check in internally (clean is, put new grease, check the gear and switch).

I see there is a adjustment screw on the throttle body for the minimum air that should give a 450-500 rpm idle with ISC retracted, i don't think that the problem, because mine stall when the ISC motor is retracted manually and frozen bu unplug it, but i have no access to adjust it.

Thanks for help.

[cdgrinci]

I will certainly yield to anyone with better knowledge but how about the iac valve (idle air control).  Or are you referring to the iac when you say ISC motor?  Other than that, it does sound as you might have an evasive vacuum leak.  Best of luck in remedying this.

Edited July 9, 2020 by cdgrinci

[BodybyFisher]

The 93 had an ISC motor to control the idle, you can attempt to disassemble it and clean it up paying close attention to the closed throttle contacts which can cause the ISC to remain open if they are dirty.  I had this problem on a 95 recently and once I replaced the ISC the idle and return to idle was perfect.  Barry94 has a good writeup on the ISC motor, here is the link

 

 

[floods]

4 hours ago, cdgrinci said:

I will certainly yield to anyone with better knowledge but how about the iac valve (idle air control).  Or are you referring to the iac when you say ISC motor?  Other than that, it does sound as you might have an evasive vacuum leak.  Best of luck in remedying this.

Thanks for help, there is no iac on this engine, that a ISC motor, he directly acting on the throttle and move to keep the idle right.

1 hour ago, BodybyFisher said:

The 93 had an ISC motor to control the idle, you can attempt to disassemble it and clean it up paying close attention to the closed throttle contacts which can cause the ISC to remain open if they are dirty.  I had this problem on a 95 recently and once I replaced the ISC the idle and return to idle was perfect.  Barry94 has a good writeup on the ISC motor, here is the link

Thanks for help, but i already did it, i have 2 ISC motor, i clean the 2 and switch is working perfectly, one is really used and the second is in really good condition, and get exactly the same result.

On live data i can see the switch acting when i release and press the pedal, work right and really sensitive.

But to be really sure i order a new one today (a GM oem), so will see if that better, because honestly i don't see any other solution, if i unplug the ISC when engine running, i can get the RPM drop fast under 700 RPM, so that really mean there is no vacuum leak or other problem that don't let the rpm go down fast, it's that ISC motor that keep high and slowly go down, now question is to know if that the ISC itself, or it is the infos he receive, will be fixed when i get the new ISC, and in the same time i order a new TPS (who know if the 2 i have are bad, i already get that in the past on a other car)

Thanks

[BodybyFisher]

That is what I would do, install a new one and eliminate it as a source of the problem.  I cleaned one, replaced it with a used one, and couldn't solve the problem until I replaced it. 

Let us know how this turns out.  While you are waiting, check that the throttle plate closes and the cable isn't binding. Clean the TB throat and plate including the back of the plate really good. 

Your TPS can be tested with an ohm meter, sweep through its range and see how the meter reacts, it should sweep smoothly with no drop outs.

[floods]

Ok, i will let you know

About the TPS, yes, i check it with the ohm meter, all look really good, and same in the live data, pretty smooth on all the travel, so i think that nor the problem, but just in case, i order one in the same time (save me shipping).

[BodybyFisher]

4 hours ago, floods said:

Ok, i will let you know

About the TPS, yes, i check it with the ohm meter, all look really good, and same in the live data, pretty smooth on all the travel, so i think that nor the problem, but just in case, i order one in the same time (save me shipping).

I am very interested in all the work you have done to your engine.  I have not reviewed it in detail yet but will.  Good luck and keep us up to date.  Post some photos.  Let me know how I can help

[floods]

Check my youtube channel at this link : https://www.youtube.com/c/Floodsusinage/videos

I have already 12 parts video of the N* rebuilt (not finish the video editing of the rest, but soon) and all is in French, but if you have any question, don't hesitate

[BodybyFisher]

The first video I opened was you discussing and performing piston ring gap clearance!  Nice.  

[floods]

Ok, that must be the most recent video (part 12)

And i install the Northstar performance stud kit to be sure i never get head bolt issue 🙃

[floods]

Hi,

Receive the new ISC motor and TP sensor, but no luck, i install it and did the learning procedure, but still the exact same issue...

So i think i will check more on the EGR valve.

I can't really understand why the ISC motor to that !

[BodybyFisher]

OK, at least you know the ISC is not the issue and you can eliminate that as a cause.  

Just one thing however, is it retracting?

Are there any codes?

If you had the throttle body off, did you replace the gasket/o-ring?   I have a 94 factory service manual, I will open it tomorrow and look it over. 

As you know, a vacuum leak causing unmetered air will cause a fast idle, what are you idling at?, does the idle come down to normal?, or does it stay high?

Is the throttle plate closing?

Did you replace the intake manifold gaskets?   You have phenolic egr spacers on that engine, I believe there are gaskets on both sides of it.

There is an anti-backfire valve in the intake by the power steering pump, if that leaks you will get a high idle, listen near that to see what you can hear as far as vacuum leaking, it might whistle. 

On 96 and up, we can see a lean mixture caused by a vacuum leak by lean codes P0171 and P0174, I am not sure how a lean condition can be found on the OBD1 system

Ill look at my manual, and see what I can find

[BodybyFisher]

@Logan  @KHE  @Barry94

The guru used to have an easy method of adjusting the ISC motor, do any of you guys remember his trick?   Thanks guys

[floods]

9 hours ago, BodybyFisher said:

OK, at least you know the ISC is not the issue and you can eliminate that as a cause.  

Just one thing however, is it retracting?

Are there any codes?

If you had the throttle body off, did you replace the gasket/o-ring?   I have a 94 factory service manual, I will open it tomorrow and look it over. 

As you know, a vacuum leak causing unmetered air will cause a fast idle, what are you idling at?, does the idle come down to normal?, or does it stay high?

Is the throttle plate closing?

Did you replace the intake manifold gaskets?   You have phenolic egr spacers on that engine, I believe there are gaskets on both sides of it.

There is an anti-backfire valve in the intake by the power steering pump, if that leaks you will get a high idle, listen near that to see what you can hear as far as vacuum leaking, it might whistle. 

On 96 and up, we can see a lean mixture caused by a vacuum leak by lean codes P0171 and P0174, I am not sure how a lean condition can be found on the OBD1 system

Ill look at my manual, and see what I can find

Yes he retracting, i check in diagnostic mode with PO07, the ISC extract and retract perfectly.

No, i don't get any codes (no current and no history code).

Yes, i change the O-ring on the throttle body, and check for vacuum leak in this area too and pretty sure there is no leak (spraying brake cleaner all around when engine running)

Idle is about 650 - 700 RPM when hot, just perfect, and all time idle go down to normal, really stable in all condition (AC on/off, in gear or not), only too slowly to go down.

Yes throttle plate is closing right.

Yes, i change all the gasket on the manifold (both side of the phenolic spacer insulator), i check for leak too (spraying brake cleaner to see if RPM get higher, but all stay stable).

I don't have this anti-backfire valve, on the 93-94 engine, i have the one with the magnesium cover with the rubber on the bolt (that let the cover open a little when backfire), and have the injector in the manifold (the fully magnesium intake not the later plastic version)

I try to unplug the ISC when engine is running, this way the RPM go down fast and normal when i release the pedal (but of course no idle right because the ISC is unplug).

So, that tell me really that the ISC receive a wrong information, because i can clearly see that the ISC plunger go out when he go in idle mode and then slowly retract to go at the right idle, can't understand why he do that.

To adjust the plunger i use the service manual method, you must override PS03 to fully retract the ISC plunger with the cooler button then go check the value of PD01 (that is the throttle position) and note (take about 10 sec to 15 sec to get it retracted), then fully extract it with PS03 hot button and check again PD01, the difference between the 2 must be from 10 to 11.

So mine get -1.8 fully retracted and 8.3 fully extracted, to total 10.1 between the 2 value, look right to me.

Then i did the learning, first i reset all value with PS13 cooler button, that let the code P080 show up to tell that ECM need a idle/tps learning, from there i have to get in learning mode by :

- Ignition ON

- Go in diagnostic mode

- Ignition OFF

- Waiting 20 sec.

- And repeat 2 more time (so 3 time total).

- Start engine and let IDLE with A/C On when engine is at 80°C wait 5 more min.

- Press brake and get in Drive and let idle 30 sec then turn A/C Off and let idle 30 sec again.

- Go back to park let idle and turn all off.

The Service Engine Soon light go off in the procedure because P080 pass in history indicate the learning is complete.

Thanks a lot for help.

[BodybyFisher]

Oh yes, sorry, I forget the 93/94 has the magnesium enclosed intake.  The fuel pressure regulator is in there and vacuum directly acts on it.

So explain to me exactly what your problem is.  

You say it idles at 650-700 when hot that sounds perfect.

Is this an original unmodified ECM?

When does it slow down too slowly, warming up or coasting to a stop? Right after start up when the engine is cold?   As you know when the engine is cold it runs in open loop and it will have a higher idle and will run rich until the system goes into closed loop and the o2 sensors take over.  

 Explain your symptom in detail.  

 

[Logan]

Attached is a pic of the idle procedure right from the 1993 GM service manual. 

I think it idles ok.....just takes a 'long' time to get there. Perhaps a misconception that it does take a few seconds to get there rather than instantaneous.  

Sounds like he may be starting the car with his foot on the gas. He should not have to do that. 

Glad he checked the cruise and gas pedal cables for freedom. 

There is a return spring on the throttle body shaft that may be broken.  

[BodybyFisher]

Just a thought, I am reaching here.  Are the O2 sensors new?  Are they torqued to spec?  An O2 sensor slow to warm up to operating temp might delay closed loop operation.  

Have you observed real time O2 sensor and fuel trim data?

Does the exhaust smell rich while idling fast?

Can you confirm open and closed loop operation either by an analyzer or via the DIC?

[floods]

7 hours ago, BodybyFisher said:

Oh yes, sorry, I forget the 93/94 has the magnesium enclosed intake.  The fuel pressure regulator is in there and vacuum directly acts on it.

So explain to me exactly what your problem is.  

You say it idles at 650-700 when hot that sounds perfect.

Is this an original unmodified ECM?

When does it slow down too slowly, warming up or coasting to a stop? Right after start up when the engine is cold?   As you know when the engine is cold it runs in open loop and it will have a higher idle and will run rich until the system goes into closed loop and the o2 sensors take over.  

 Explain your symptom in detail.  

 

As say in my 1st post, the problem is that the idle is slow to come down when i release the pedal, so when i release the pedal, RPM go down to about 1000 RPM, and just get up to 1200-1300 RPM and then slowly go down to normal (take about 3 to 4 sec.), i can clearly see the plunger of the ISC motor extract (that why RPM go to 1200-1300) and then he slowly retract to get right IDLE.

ECM is new, but get the same issue with the original one, i have to change it because when engine was hot, the IDLE was completely wrong (from 600 to 1500 RPM), new ECM solve that problem, but still have this slow down issue.

Engine cold and hot do exactly the same, when it only happen when i release the accelerator (all time when the ISC motor is activated by his switch to go in idle mode), when the car in park en neutral and when i drive, just same all the time, so when i drive if i release the accelerator 3 to 4 sec before i have to stop the car, that ok, that give enough time to the ISC to get right, but when i have to accelerate just a little to move the car in hard traffic for sample, the time i get right IDLE the car wan to go forward too much, i can feel it push (normal because RPM is too high for 3-4 sec).

The most important, is when the ISC motor is unplug, then the rpm go fast down like it should.

4 hours ago, BodybyFisher said:

Just a thought, I am reaching here.  Are the O2 sensors new?  Are they torqued to spec?  An O2 sensor slow to warm up to operating temp might delay closed loop operation.  

Have you observed real time O2 sensor and fuel trim data?

Does the exhaust smell rich while idling fast?

Can you confirm open and closed loop operation either by an analyzer or via the DIC?

Yes 2 O2 sensor are new and torqued to spec exactly, i will check again for the open and closed loop operation (i have the Tech 2)

Yes i observed many time the O2 sensor and fuel trim data, all look in spec, and same on the 2 bank.

No exhaust never smell too rich, smell more when cold, but normal.

5 hours ago, Logan said:

Attached is a pic of the idle procedure right from the 1993 GM service manual. 

I think it idles ok.....just takes a 'long' time to get there. Perhaps a misconception that it does take a few seconds to get there rather than instantaneous.  

Sounds like he may be starting the car with his foot on the gas. He should not have to do that. 

Glad he checked the cruise and gas pedal cables for freedom. 

There is a return spring on the throttle body shaft that may be broken.  

Exactly, that what i did (i have the same manual) and in the same manual there is the plunger adjustment procedure i did too.

Maybe that normal, i don't have a other car to compare, but look long to me.

No, i don't touch the pedal when starting the car.

Already check the cruise and gas pedal, no issue here, the cruise cable is loose just right (i try to disconnect it and same), the pedal cable is right to, move freely, and little loose when release.

The throttle body spring is ok too, (throttle body is new).

Thanks a lot for help

Edited July 15, 2020 by floods

[floods]

Today i did some check on the open and closed loop with the GM Tech2, and that working, engine cold after start stay in open loop, and then switch to closed loop when O2 sensor are hot.

Plus i try manually with the Tech2 to switch in open loop when engine is hot, and have the same issue in open loop and closed loop, so no difference here.

I check again all parameter, and can't see anything work, MAP look right, all temperature sensor work and show right..... maybe a leaking injector ?? but i should be able to see it when i did a power balance, or ?

Thanks again

[floods]

Little update, so i did a full EVAP system checking with the vacuum pump, don't have any leak, the valve work right, i change the solenoid just in case and be sure hose are install right, so EVAP system is ok.

And just in case i install a new PCV valve.

So at this point no difference.

I did a fuel pressure test, holding perfectly 45 PSI with ignition ON, no drop, then i did a injector flow test, all of the 8 injectors did exactly the same pressure drop (13 PSI) when acting one by one the same time (with the flow testing mode in the diagnostic mode), in this test i must run the fuel pump manual for 5 sec to get full pressure, then go activate a injector (PS10 if i remember right) that activate the selected injector for small and precision amount of time, and check the pressure drop, make same for all 8 injectors.

So, i have no leaking injector and no bad pressure regulator, today i will check pressure in running condition (was too late yesterday).

Other possible way is the EGR valve, so i will get a new one monday, will try it, because when i first drive the car after rebuilt, i get a EGR code, can't remember the code, but never get it again, so maybe the problem is there.

And if needed i will try a other ignition module and coil (i have a spare one).

[floods]

So, i check the pressure in running condition, all fine... so not a fuel pressure issue.

Here some pictures of my engine

Edited July 18, 2020 by floods
Forget to add the picture

[BodybyFisher]

Try not to 'parts replace". 

Insert smile here >>slow down<< 😁

I can't help but think that your initial ISC setting is too high.  I only went through the factory manual setting procedure with my 91, since then I have used the 'static' setting method to set the ISC gap.  I asked the board to see if anyone remembers the procedure.  I will endeavor to recall or find the procedure.  

But as I get it, these are the facts, correct me if I am wrong:

1) When the engine is cold (open loop), the idle is too fast getting up to 1,200 rpm

2) When the engine is warmed up adequately (probably closed loop) the idle settles down and is good (650-700)

3) When you drive the car, you aren't experiencing any high idle or idle, surging at stop signs or stop lights or in traffic.  When this happens it can get scary in traffic as you need to hold the car back.

Is what I said in 1, 2 & 3 above correct?

The engine is going into closed loop

O2 and air-fuel data is good.  Be SURE, in open and closed loop you aren't getting a lean air-fuel ratio indicating a vacuum leak.  

The coolant temp sensor is good, TPS is good, EGR is good, TB o-ring replaced, linkage not binding, checked for vacuum leaks and the TB plate closes all the way.  You don't smell fuel at idle at the exhaust that would potentially indicate a rich mixture.  

You replaced the TB with a new unit?, is it the correct part number?  I don't think so, but is there a throttle body learn procedure?

Do you have a coolant thermostat installed?

>>What is your altitude?  What fuel are you using?  <<  not likely an issue, just to complete the entire picture here

I am in the middle of a huge home project right now and my time is limited and my physical health hurting, I need to pull the manual and do some research.  I know you gave a lot of information throughout this thread, but I want to focus it here.  Please answer any questions I have raised up above so we know the exact situation at this point and so other members don't need to read the entire thread.  

Be patient.  Ill get into the manual asap.  Stop parts replacing, remember the on board diagnostic system will alert you to problems.  This is something simple.  I need to find the static ISC setting.  

This is what I recall 1) retract the ISC plunger all the way using the DIC, unplug it.  2) using a wrench, turn the plunger to get a .060 inch gap (convert to metric 😉).  Plug it back in, drive car!  I need to confirm this procedure someplace else, this is from my memory, do not do this until I find confirmation of it.  

[BodybyFisher]

This is my thread from 2017.  Please look through it, I will also.  Did you have a minimum air adjustment and did you adjust minimum air?

 

 

[BodybyFisher]

By the way, your engine looks beautiful!!! WOW!

[floods]

Thanks for help.

Yes, you totally right on 1, 2 & 3, that exactly what happen.

Here all my reply in blue :

The engine is going into closed loop Yes

O2 and air-fuel data is good.  Be SURE, in open and closed loop you aren't getting a lean air-fuel ratio indicating a vacuum leak.  Yes, right, never get too lean

The coolant temp sensor is good yes, TPS is good yes, EGR is good that not 100% sure (that why i will try a other one), TB o-ring replaced yes, linkage not binding yes, checked for vacuum leaks yes and the TB plate closes all the way yes.  You don't smell fuel at idle at the exhaust that would potentially indicate a rich mixture no, never.

You replaced the TB with a new unit?, is it the correct part number?  I don't think so, but is there a throttle body learn procedure? yes, exactly the same part number and check it before install it, no there is nor learn procedure for it.

Do you have a coolant thermostat installed? yes a new OEM

>>What is your altitude? 280meter (pretty normal altitude)  What fuel are you using? i using 98 RON index, so in the USA that 93 AKI index (over premium as i know).  <<  not likely an issue, just to complete the entire picture here

I know that not the best way to replace parts, but since that the only part that not new on the engine and was not so expansive parts, i was thinking that not a bad idea to replace it and keep the old as spare, because here in France you don't find any parts, i have to order it in the US, i will just get a EGR valve to try, and i have already a spare ICM that i can try if needed.

I see the static method to adjust the ISC plunger, but as i know that not for the Northstar but the 4.9 engine, right ? according to the 93 service manual for the N* i have it right, but tomorrow i will try the static method, can not do anything bad to try it, because as you say, that look too high, the ISC push too much when engage, i will check in the 4.9 section of the manual what they said, because the 93 manual cover the N* and the 4.9.

About the minimum air adjustment, yes i have it, i don't touch it, in the manual they said just that at the minimum position, the engine must not run above 450 RPM if it stall that ok, my engine keep running for about 10 sec and stall, i can't really adjust it without remove the throttle body from the engine.

2 hours ago, BodybyFisher said:

By the way, your engine looks beautiful!!! WOW!

Thanks

Edited July 18, 2020 by floods