Luke

Any 4.9 experts? Idle issues

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Hey guys, I have a 92 Seville 4.9 with 107k miles. I bought it in September with 99k. It has gotten to where it will sometimes stall at idle or it will hunt for idle, like 600-1300+ rpm. Sometimes it revs all over the place. Then sometimes it’ll run perfectly smooth all day. It drives perfectly fine on the highway. 

Maintenance and repairs done since I’ve owned it:

Spark plugs, wires, cap, rotor, coil (then later replaced entire distributor), map sensor, throttle position sensor, idle speed control motor, coolant temp sensor, intake air temp sensor, cruise control servo, idler pulleys, belt, battery, radiator, heater core, fuel injectors, fuel filter, fuel pressure regulator, one trans cooler line, new receiver drier and charged AC with R12. 

Anyone have any Idea what causes the idle issues? The TPS and ISC motor were properly calibrated and idle learn procedure done. The only code is E062 which is a cruise control servo fault. I have a new warranty replacement I need to install. I have the factory service manual. Could the cruise control service cause these issues? Only other thing I can think is that the computer has gone bad. I actually have a new one but wanted to ask if anyone knows what could cause this before I put it in. Can anyone offer any ideas?

Edited by Luke
Omission

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Not an expert by any stretch, and will yield my input to any others, but has the throttle body/butterfly been cleaned (de-carbonized).  A recent post on another Cadillac forum had similar symptoms that were rectified after a throttle body cleaning.  Best of luck with your problem.


Chuck

'17 XT5, '04 Bravada........but still lusting for that '69 Z-28

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Thanks for the reply. It has been cleaned maybe 4K miles ago.

I forgot to mention that the EGR valve has been replaced too. 

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Hi Luke,

92 was a very troublesome year for the PCMs.  Drop the PCM down and with it idling do a tap test on it.  Using your knuckle tap solidly on the pcm and see if you can induce any odd behavior.

Also check and restore all engine grounds.

Do a search here on the 92 and you should find some info. 

Also, be sure the ISC motor is operating correctly.  There is a closed throttle switch in it that is suppose to retract the plunger.  

There is a lot of great info here on the ISC.  In the How to section you will find info by Growe3 on the ISC that has helped many.


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 replaced the ISC motor a few weeks ago. It moves very smoothly. 

I replaced the defective cruise control servo last night. It ran beautifully last night after changing it but chugged all the way to work today. Not sure I’ll have time tonight but tomorrow night, I’ll open the dash and start working with the computer. Hopefully that fixes it. If the computer isn’t the culprit, my last resort before deciding it’s some sort of wiring issue is the catalytic converter. I have a spare one on hand that fits this car. I’ve read quite a bit online where people have had similar issues with 4.9’s but rarely posted a solution. Occasional new distributor or computer is all I found. This is my 4th 4.9 but the first that didn’t run right. It ran right when I picked it up in Las Vegas and the whole trip back to Texas then slowly developed over the past few months. 

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Explain chugged?

I had a 91 seville 4.9

Check fuel pressure at idle and at 2000 rpm

I did have a cat go bad but it only rattled, it didnt hurt performance, but on my 01 monte carlo a clogged cat sapped it of power badly.  

You say NO codes? No P codes at all?

How did you set up the ISC?, how did you adjust it.


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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Check engine vacuum at idle 18-21” with a gauge 

check the main ground stud and bracket next to the starter motor . Make sure it’s tight and clean 

isc retracted the tps should be .57 running and set min air with alternator connector grounded to turn alt off.

full extended I think the isc is 1.13 I think can’t remember and you adjust the isc plunger  screw for full extended adjustment. Engine off on this setting .

i have had a harmonic balancer spin and when the mechanic went to set timing it was way off so maybe verify tdc#1 and mark on water pump . 

The idle learn has to be started I think when the car is fully warmed up . 15 min running and 1 min in drive and 1 min in drive with ac on .

Edited by barczy01

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