: rough idling at lights

06-17-06, 06:40 AM
hi,i have a 1998 seville sts,in the uk,cars been great but recently when starting up the idle revs go between 500 and 1000rpm up and down all the time,when about to move off in drive the car shakes but after pulling away the car runs smoothly,once at a stop at lights say she starts to shake as if about to stall,which never happens,
i changed the plugs a while back but with no sucuess,i have been told i can disconnect the battery then reconnect which i did but no change,also i have no engine codes on dash,as if everything is normal............thanks...kev

06-17-06, 09:39 AM
Surging or "hunting" idle like you mentioned is a sign of a vacuum leak. Check the hoses & the ISC (Idle Speed Control) valve.

06-17-06, 10:52 AM
thanks very much i will try that

06-17-06, 10:54 AM
[any ideas whereabouts the idle speed control valve is on the northstar 4.6 v8...kev

06-17-06, 03:30 PM
ISC valve is on the forward side of the TB. Don't confuse it with the TPS which is there also but is inline with the throttle plate pivot. It's basically a throttle plate bypass to control idle.

06-18-06, 12:07 PM
Thanks 4 That,also i should have mentioned the rough idling is not only when in gear but also in neutral or park,the revs just go up and down beteween 100rom and 500rpm....kev

08-29-06, 03:56 PM
sorry to bring back an old thread but this problem appears to be the exact problem i am currently having.

I have yet to check the ISC valve and/or hoses but is there a way to check for vacuum leaks?

Also, i have cleaned the tb and the egr valve and it did not fix it.

08-29-06, 04:38 PM
You can listen with a stethescope for a leak, or you can spray potential leak areas with water, Brakleen, WD40, etc, all will have an effect on the idle speed when you spray the leak area.

08-29-06, 04:58 PM
hmmm i think i am having a little trouble locating the ISC valve. I see the two connections on the tb (upper and lower) but im not sure where the isc valve would be.

Any hints?

Thank you

09-02-06, 04:13 AM
My 98 Eldorado that I just purchased had a problem which surfaced the day after I bought it went away after fueling it with premium, dimming lights every 1-1.5 seconds constant. I filled it up for the first time and it seemed to stop completly, I was told it was probably the alternator but now I'm not sure, it still starts hard and has quit once when I put it into park. I'm not sure what to do because other than the symptom of stalling while in park it runs perfectly in fact better than any of the other three cadillacs I've driven in the past. It seems VERY solid, other than the hard start, it's quiet and calm(and growls when I hit it hard). Just looking for some advice as well. Should I get it into the shop or is there something that I can do myself? (I'm a computer tech support specialist not an auto mechanic :))


09-02-06, 09:00 AM
One connection should be about in the center of the TB bore, inline with the throttle plate pivot point. That should be the TPS (Throttle Position Sensor). The one above that should be the ISC valve.

Check the FPR. See the Tech Tips section. If that does not work, start a new thread.

09-02-06, 02:01 PM
i just posted something similar i should have read yours first, my 95 eldorado does the same thing only not as severe, when im stopped it revs from 500 to 700-800 rpm and makes the headlights pulsate, like get dim and bright again but when i step on the gas it stops and runs smoothly

09-02-06, 10:07 PM
Mine actually continues to pulsate like that, it went away for the last two days but is back, taking it to the shop on tuesday.

09-02-06, 11:21 PM
I took my car to the shop to let them take a look.

They found that the car was indeed misfiring and the car was actually running on 7 cylinders. Cylinder number 5 was the one misfiring. I checked for codes i believe 1 or 2 days after the symptoms started happening and i got nothing new, but at the shop they checked the codes and got a misfire code and as the guy was pulling out to give it a test drive the check engine light flicked on. This is very odd becuase usually the check engine light will go on immediately when a misfire is seen.

Anyways, after new plugs, wires, a pcv valve, and a large hole in my wallet, the car runs great.

Let me note that my lights never dimmed or flickered.

09-04-06, 10:38 PM
Mine was doing that when I had a bad battery, it would charge and do rough starts but got worse and worse, and idle was funky and when my brakes lights came on it made the symptoms worse. When I had it the shop it turned out that the battery wasn't holding a charge under load.

To fix my stutter and misfire problem new plugs solved the problem.

09-09-06, 12:20 AM
Yeah but did the battery ever go dead enough that you couldn't get it turn over? My starter turns fine and starts the car every time and doesn't sound like it's draining the battery. Mine PULSES almost like a strobe light while in gear, it doesn't seem to do it much if at all in park, only while in gear.