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Olle

STS 1994 severe idle problems, please have a look, need input

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Dear Cadillac friends.
Posting from Stockholm, Sweden.
Background: I inherited this car from my dad, he used to frequent this particular forum (and other Cadillac forums) quite alot before
he got ill and "moved on".

94 STS, Car is in excellent condition, many new parts, ran like a champ up until end of September...
Northstar engine. Rebuilt, (time sert) new headgaskets, new alternator etc, the works..Car ran great for a few years.

Here´s what going on since last couple of weeks:
Problem:
At idle it goes up and down 800-1500 rpms. misses and chugs, comes down to 800, revs up again, misses chugs etc etc….
When I rev it to 2000 rpm and hold the throttle, it runs somewhat more smoothly, better. I let go of throttle,..back to idle chugging, misses and raises up / comes down..

No codes ! but SES light came on and flickered on/off while driving car for a day, but, does not lit up immediately when I start the car now.
 yey, it is Not drivable at the moment, have it at my shop/garage,

Car is in exellent condition, many many spare parts laying around.( dad was a part time Cadillac mechanic)

People have mentioned Vacuum leaks, and I have searched around quite abit, nothing found..Yet..


What I've changed / tried so far, giving no change,
New Sparkplugs, wires, crank sensors, cam sensor, coil pack, fuel filter, tps sensor , egr valve. air filter, injectors. Map sensor.
searched for vacuum leaks.
unplugged brake booster, plugged hose, no change..Sprayed starting Fluid/brakecleen around hoses etc.., no noticeable change so far..
removed the whole intake manifold, installed new gaskets, reinstalled. cleaned Tbody.
Sparkplugs look ok.. visually at least,they are about 1 year old..
Absolutely no change whatsoever with all these changes/parts !!icon_rolleyes.gif

If any of you guys have some ideas, please help ! Im good with wrenching, at a dead end with the trouble shooting, need input..
No codes, this is the annoying part..
Vaccum related ? electical ? whadda you think?
Many thanks in advance, Olle

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Most often it is an Idle air control valve ( IAC ), Or Idle speed control motor, can't ever remember the change over year from ISC to IAC :(

Disconnect the battery cables and touch the 2 cable ends together, that will clear the modules, reconnect the battery and see if that helps. Look around the spare parts for one and change it if you have a spare one.

Could also be an EGR valve or EGR port problem making it idle rough, it could be stuck partially open because of a dirty seat. Pull it and take a look at the pintle seat.

Edited by OldCadTech

THERE IS ALWAYS ENOUGH TIME TO DO THE JOB RIGHT - THERE IS NEVER ENOUGH TIME TO DO THE JOB AGAIN !!!

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As @OldCadTech stated the ISC is usually the problem, see my thread from a few months ago, replacing the ISC motor solved the problem

You can purchase the ISC at rockauto, they ship to Europe, if you can not find it.  If you search rockauto we have 2 discount certificates for 5 percent that you can use.

 


Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

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Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Thanks a bunch guys!, I indeed do have a spare ISC motor in my stash already, could go that route later.

Also, I have already changed the EGR-valve and sealed it good, to no avail...exactly the same behavior..

It is not that it only raises the the rpm, it also jerks and bucks as it goes up down in rpm´s..like a miss?

So, it "appears" like fuel or spark related?, while it is doing it, but not if I hold the throttle around 2000 rpms..the it runs smoother.

Strange...I must have spent 20 hours searching the internet, and 30 hours on the car so far, no luck yet..

Someone mentioned "clean the injectors!" with STP injector cleaner via the gasoline (mixed in obviously) BUT-

I have already swapped out the injectors for another set (again, many spare-parts laying around..) that ran fine a few years ago

as we took em off another car, so, It shouldnt  be in injectors..or?

Possibly, a rotted out vacuum hose? (somewhere)

Question, Is there a vacuum hose going to the Fuel Pressure regulator ? or, is it

 run on the "pressurized manifold vacuum" itself? (this being a 94 Northstar model engine)

 

I really appreciate your ideas, I want this car to at least last this coming winter season..such a comfy ride and it works great in the snow believe it or not!

/Olle

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Why did you remove and seal the EGR?  This isn't a 1972 engine that might benefit from something like that.  You need to restore the EGR.  EGR flow is important for proper combustion on these engines, I believe it cools the incoming charge.  

The flickering check engine light indicates that you are getting a misfire.  I do not recall how the 94 handles a misfire, but I think it does not set a code.  Someone will confirm that for me.  @KHE.   You should check coils, coil connections, ignition wires with an ohm meter and plugs. 

I can not recall if this engine has egr phenolic spacers, if it does they need cleaning.  KHE?

 

 


Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Thanks BodybyF,

no no no, sorry, with "sealing it" I meant installed a new EGR valve and new gasket (it is sealed tight). made no change/improvement whatsoever, so, original

EGR valve is good.

I does indded "appear" and sound like a misfire, but only misses at idle..

I have already tried another coil pack, and wires. no improvement. the Spark-plugs are not the stock type, but something that is OEM/fits according

to parts-store. The plugs are no more then 2 years and maybe 3000 miles on them...so, but they have been great for 2 years, so? what gives?

maybe one of the plugs has decided to give up? (but only at idle, not when you rev it?

( Question: does the coils boost ignition-power to plugs at higher rpms? anybody knows?)

I did visually inspect the 4 plugs front of engine and they all look the same and OK to me, but what do I know? hehe...

I will check the other 4 plugs at rear of engine later on, perhaps purchase 8 new ones of the same brand, they have been great for 2 years, so?

Many thanks!

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Check the fuel pressure regulator. Pull the vacuum hose off and check for leaking fuel, if there is fuel replace the regulator. If you have a miti-vac apply vacuum and see if it holds.

Did you clean the EGR ports? There should NOT be EGR at idle. Check engine vacuum, post your results here plz.

Are you getting black exhaust smoke? Sounds like an over-rich condition.. You can pull a spark plug and see if they are wet from too much fuel, or a heavy black soot look.


THERE IS ALWAYS ENOUGH TIME TO DO THE JOB RIGHT - THERE IS NEVER ENOUGH TIME TO DO THE JOB AGAIN !!!

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Thanx, No black soot and dry (at least) on the front 4 sparkplugs, they are gray/whiteish..

And, No black smoke, but it sure smells of raw unburnt fuel when I start it up, whether that is due to misfire or due to FPR dumping copious amounts of

fuel into the engine I can not tell...?

Yes, cleaned the EGR ports in the intake (block side if I remember correcly?) when I had it off, engine had a total overhaul ca 5 years ago so it still looks nice

and clean inside. not much grime in the EGR passages, just a little, which I removed.

I will Check engine vacuum next time I get a chance working on it..

( I have been wrenching Ford 5.0 "Fox Mustangs" for 15 years and I know those inside out, so the technology/principle is somewhat similar, late 80´s early 90s technology)

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Do me a favor, find someone with an engine analyzer and have them hook it up and determine if #8 or #7 is missing at idle.  Its important.  See where the misfire counts are at idle and see if the misfire counts drop at highway speeds.  

You are using synthetic oil in europe correct?  Can you get 10w30 standard oil?

You have done a lot of work and diagnostics.

 


Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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Did you do an idle learn after replacing the TPS? Or did the symptoms start/continue before/after that.?

I'm back to disconnect battery and reset modules. Perform an idle learn procedure. Check the engine vacuum at the booster hose before the check valve. Watch the ISC motor when you move the throttle and ascertain if it is moving smoothly.

Cadillacs are a different breed and not all the "rules" are applicable esp when module communication is affected. I know it sounds silly from a tech point of view but Cadillacs are SPECIAL.


THERE IS ALWAYS ENOUGH TIME TO DO THE JOB RIGHT - THERE IS NEVER ENOUGH TIME TO DO THE JOB AGAIN !!!

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Thanx guys, OldCadT, have not done a "relearn" after the new TPS was installed, but the symptoms have been exactly

the same after as before/after...nothing´s changed with the missing erratic idle..not one iota after new parts or anything..go figure.

I have had the battery disconnected for days, many times, it acts the same after its been reset/off/back on when I start it up..

You could say (dont get me wrong here..) problem acts up as "hard ware" related..(like in modern computer language), meaning it has not much to 

do with relearning fuel tables or similar, it is like a part is broken or loose, something very obvious maybe that I have missed, since non of these changes has made a change, for better or worse..

I even had the computer module "ECU"? out yesterday, inspected, cleaned connections etc, and reinstalled it back up by the passenger foot area.

Then, fired it up after connecting the battery again, same thing...Something is wrong, but what? Im not about to give up..eventually we´ll catch it.

/Olle

 

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to BodybyF: yes, Ive run the same semi synthetic 10/40 weve used for 10 years in this car..

It gets cold here in the winter, 20 below sometimes..

we can get 10/30 if needed, no probs.

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Just do me a favor, check with an analyzer where the misfire is coming from.  Ill bet you its #8 or possibly #7.  

I have a theory. 


Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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"...No codes ! but SES light came on and flickered on/off while driving car for a day, but, does not lit up immediately when I start the car now ..."

That to me...is very odd behavior....

That said...it is a export car.

There is a service bulletin for bad '94 PCMs....shouldn't take 20 years to show up...but it sounds like a bad PCM to me.

You can drop it down and do some tap testing on it while running. Any stalling or goofy behavior while tapping would indicate the PCM has gone bad. Sometimes you can find a spot where it will stall or goof up everytime.

 

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Thanks Logan. I do believe I have a spare PCM somewhere, I have to look for it..

Car is originally from Germany. I took the PCM out and re-installed it, then again, I did not tap on it while running..

just put it back, no change, I inspected it for corrosion between "pcb board tracks" and such, looked nice and clean..

sprayed electronics cleaner/contact cleaner on the connectors, inspected the Electrolytic Capacitor, no leaks that I could see..

Who knows, PCM might have gone bust, only one way to find out.. have to dig out the spare one if I can find it.

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Also,thought I might add I do get some codes from the PCM, like battery has been disconnected, memory something, and

a few codes of "S" type, related to the suspension since I have Arnott´s replacement shocks/struts allround..(nice ride by the way)

Nothing related to engine failure, but the PCM/computer does communicate with sensors and whatnot, without a problem.

 

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Also, I do have the two GM Service Manuals related to the 94 Seville , the "phonebooks" for the car..And studied it a bit, but I am not

in possession of some of the diagnostic instruments/tools to go as deep as Id like to, unfortunately.

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I posted a long post that for some reason did not get posted, and was lost, I lost connection with the board.  Ill try again.  

You have done a lot of diagnostics and checking.  You even changed the intake seals.  Have you checked the injectors? 

I would like to see the misfire counts and which cylinders its coming from and what happens to the misfire counts at highway speeds.

Does this misfire go away at highway speeds and come back at a stop or idle?

Do me a favor and get the synthetic oil out and use standard dyno 10W30 oil (not synthetic) and see if it changes your symptoms.  This is important, I am diagnosing something.  Synthetic flows 7 times better than dyno oil, if the clearences at lifter become excessive the lifter will leak down as the oil will flow around the lifter rather than inflating it when the oil pressure drops low at idle.  Change to 10W30 DYNO if you can get it, and see how the engine responds, use an ac delco filter.

Please answer my questions directly you have not given direct answers to my questions above, thx


Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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hello, sorry bout your long post getting lost, that stinks, I know the feeling, happened to me also at one point.

Injectors are another set, off another car (94) that ran good before removed. it is not the injectors causing the symptoms..

The engine shakes and vibrates as it is ideling, hold the throttle at 2000 rpms, it smooths out, but prolly not

running right, just less noticeable, I guess.. more info to come..

I like your idea on the lifters, sounds possible.

I will try your diagnosing instruments when I get a hold of one, gotta run, 6 pm here, in Sweden going to the movies!

 

Many Thanx, Olle

 

 

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You say probably not, but it may well be that it is smoothing out because the lifter inflates because of increased oil pressure which would cause the misfire count to drop and misfire to stop.  Check to see if the check engine light flashes at 2000 rpm or at highway speeds, ill bet it stops.

Enjoy the movies


Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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#47-65-06: HARSH CHUGGLE/MISS/HESITATION DTC PO71 (CHECK/REPAIR DTC) - (May 12, 1994)

 
SUBJECT: HARSH CHUGGLE, SURGE, MISS, HESITATION ACCOMPANIED BY A HISTORY DIAGNOSTIC TROUBLE CODE PO71 (CHECK/REPAIR PER DTC)

MODELS: 1993 CADILLAC ALLANTE, ELDORADO COUPE AND SEVILLE SEDAN 1994 CADILLAC ELDORADO, SEVILLE AND CONCOURS WITH 4.6L ENGINE (VINS Y AND 9 - RPOS LD8 AND L37)

CONDITION:

SOME OWNERS MAY EXPERIENCE ENGINE ROUGHNESS, HARSH CHUGGLE DURING HIGHWAY DRIVING, SURGE, MISS AND HESITATION WITH NO MIL OR MESSAGE AND A HISTORY DIAGNOSTIC TROUBLE CODE PO71.

CAUSE:

CODE PO71 SETS WHEN THERE IS A RAPID CHANGE IN MAP WITH NO CORRESPONDING CHANGE IN THE TP SENSOR OR RPM. A DTC PO71 MAY BE A SYMPTOM OF A DRIVEABILITY CONDITION SUCH AS BACKFIRE, STALL, OR LOSS OF POWER. A FALSELY SET DTC PO71 MAY BE CAUSED BY INTERNAL "NOISE" IN SOME NORTHSTAR PCMS INDUCED INTO THE INTERNAL PCM MAP CIRCUIT.

NOTE: ENGINE OIL NEAR OR ON MAP SENSOR IS A NORMAL CONDITION. MAP SENSORS SHOULD NOT BE REPLACED IF ENGINE OIL IS PRESENT.

CORRECTION:

THE NORTHSTAR'S PCM HAS BEEN REDESIGNED TO CORRECT THIS INTERNAL CIRCUIT "NOISE" CONDITION. TO DIAGNOSE AND REPAIR THE ABOVE CONDITIONS, PERFORM THE FOLLOWING PROCEDURES:

1. IF THE VEHICLE HAS MULTIPLE DIAGNOSTIC TROUBLE CODES AND DTC PO71, DIAGNOSE AND REPAIR THE **OTHER** DIAGNOSTIC TROUBLE CODES FIRST.

2. IF THERE WAS A BACKFIRE OR STALL, DIAGNOSE THAT CONDITION AS OUTLINED IN SECTION 6E-B OF THE SERVICE MANUAL.

3. IF THE VEHICLE IS A 1993 AND HAD A LOSS OF POWER AFTER AN EXTENDED HIGHWAY DRIVE, PERFORM THE PROCEDURES OUTLINED IN CORPORATE BULLETIN 216306 (GM OF CANADA 93-6-118; CADILLAC T-93-49 IN GROUP 6E).

4. IF THE VEHICLE IS A 1993, PERFORM THE PROCEDURES OUTLINED IN CORPORATE BULLETIN 316005 (GM OF CANADA 93-6-118; CADILLAC T-93-101 IN GROUP 6E).

5. IF THE VEHICLE HAS A HARSH CHUGGLE DURING HIGHWAY DRIVING, SURGE, MISS OR HESITATION ACCOMPANIED BY A HISTORY DTC PO71, INSPECT THE SERVICE PART NUMBER ON THE PCM. IF IT IS ONE OF THE PRESENT SERVICE PART NUMBERS LISTED BELOW, REPLACE THE PCM USING ONE OF THE APPROPRIATE **NEW** SERVICE PART NUMBERS LISTED BELOW.

6. IF THE PCM HAS THE NEW SERVICE PART NUMBER, FOLLOW THE APPROPRIATE SERVICE INFORMATION MANUAL PROCEDURES FOR DTC PO71 AND INSPECT THE MAP SENSOR CONNECTIONS AND CIRCUITRY.

PARTS INFORMATION:

PRESENT NEW DTC PO71 DESCRIPTION APPLICATIONS SERVICE P/N SERVICE P/N ----------- ------------ ----------- ------------- POWERTRAIN CONTROL MODULE 1993 V WITHOUT V4J 16159277 16207768 POWERTRAIN CONTROL MODULE 1993 V,E,K WITH V4J 16180517 16197429 POWERTRAIN CONTROL MODULE 1994 E/K + KSP 16196402 16197429

PARTS ARE EXPECTED TO BE AVAILABLE ON MAY 23, 1994.

FIGURES: 0

 

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Where does.that come from @Logan

I need favor.

Years ago the guru mentioned that original 300 horsepower Northstar had a lope at idle and that owners complained about because it was too rough. GM issued a firmware update to the PCM that took the lope away or at least calmed it down. Can you do me a favor and search you are archives to see if you find any mention of this in the service bulletins I would be very appreciative if you have any information on this thank you


Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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GM service bulletins. Here are all the ones for 1994 STS when searching for 'PCM'.

 

Keyword Search Results:
Bulletins, Campaigns or Preliminary Information
#03-06-04-038: Info - Intermittent Light Surge, Chuggle or Bump at 45-60 MPH with TCC Engaged (Normal Condition) - (Aug 4, 2003)
#01-00-89-008: Info - Current CIPS Campaign Number to Previous DIvisional Campaign Number - (Apr 6, 2001)
#01-00-89-006: Info - February, 2001 Bulletin Summary - (Mar 2, 2001)
#00-00-89-024: Info - September, 2000 Bulletin Summary - (Oct 6, 2000)
#00-00-89-016: Information - June, 2000 Bulletin Summary - (Jul 6, 2000)
#00-00-89-007: Info - March, 2000 Bulletin Summary - (Apr 4, 2000)
#99-00-89-016: Warranty Admin - September, 1999 Labor Time Guide Updates - (Aug 10, 1999)
#40-05-01C: Warranty Parts Center/Corporate Parts Return Program - (Feb 19, 1999)
#87-65-17: DTC P039 SET WHEN DRIVING UP SLIGHT GRADE AFTER COLD START - (Jun 1, 1998)
#71-65-70: SMU-SEC, 6E - ENGINE CONTROLS REV. STEP 5 IN CHART - (Jan 13, 1998)
#96033: CAMPAIGN: 4.9 LITER ENGINE PCM MEM-CAL/PROM REPLACEMENT - (Sep 1, 1997)
#95-C-72: CAMPAIGN: 4.9 LITER ENGINE PCM MEM-CAL/PROM REPLACEMENT - (Aug 1, 1996)
#53-33-03: INTERMITTENT 'SERVICE RIDE CONTROL' MESSAGE (RSS DTC 44) - (Sep 7, 1995)
#51-65-47: REVISED TECH 1 MASS STORAGE CARTRIDGE (MSC) OXYGEN SENSOR - (Aug 10, 1995)
#47-65-13: INTERMITTENT MISS/SURGE/LACK OF POWER (REPAIR WIRING) - (Jun 6, 1994)
#47-65-06: HARSH CHUGGLE/MISS/HESITATION DTC PO71 (CHECK/REPAIR DTC) - (May 12, 1994)
#47-71-08: SMU - SEC. 7A-5A AND 7A-5B - SHIFT SPEED CHART REVISED - (Mar 25, 1994)
#31-66-02: EXHAUST ODOR AND OTHER FUEL RELATED CONCERNS - (Nov 18, 1993)

 

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I dont see anythng.  I .recently found out that the 300 hp engine was produced first in 1993.  This search was for PCM.  I trust if there was a rough idle complaint that was corrected by a  PCM firmware update that we would see it in this batch?    

I am going to be a pain in the butt, but can you check for a rough idle complaint back to inception.  I am sure bbobynski spoke of this.  I have been looking for evidence of it for awhile.

Thank you sir.


Pre-1995 - DTC codes OBD1  >>

1996 and newer - DTC codes OBD2 >> https://www.obd-codes.com/trouble_codes/gm/obd_codes.htm

How to check for codes Caddyinfo How To Technical Archive >> http://www.caddyinfo.com/wordpress/cadillac-how-to-faq/

Cadillac History & Specifications Year by Year  http://www.motorera.com/cadillac/index.htm

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I looked at '93...first year of Northstar....I don't see anything.

PROM / Memcals have gone the way of the buggy whip. I doubt you could even buy one today.

The older IBM GM computer systems you could look up info using the PROM ID and see updates etc....but that was 20 years ago. 

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