donandmax Posted April 23, 2012 Report Share Posted April 23, 2012 When I'm stopped for a red light the engine runs a little rough if I shift into neutral it s so smooth I cant hear or feel it run. No codes, temp 199. What could cause this ? Is the idle adjustable or computer controlled.. Link to comment Share on other sites More sharing options...
misloader Posted April 23, 2012 Report Share Posted April 23, 2012 I have been chasing a rough idle on a 2002 Seville SLS. 4.6 engine codeY. Thought it was a coil pack, EGR valve . Wasn't either component. Turns out it was a vacuum leak at the intake manifold plenum. Plenum is attatched to manifold and mounts up to the crossover housing. Rubber was rotted at bottom causing a vacuum leak (, rough idle). Before going on a wild goose chase with parts. Make sure that the engine is airtight( no vacuum leaks). Even if the service engine soon is on. Was getting codes of p171 , p174 lean condition bank 1 and 2. P300 random misfire. Had a air flow sensor performance code. A vacuum leak will cause a HOST problems, rough ile , knocking , inconsistent idle. Make sure that thing is air tight. Link to comment Share on other sites More sharing options...
donandmax Posted April 24, 2012 Author Report Share Posted April 24, 2012 I have been chasing a rough idle on a 2002 Seville SLS. 4.6 engine codeY. Thought it was a coil pack, EGR valve . Wasn't either component. Turns out it was a vacuum leak at the intake manifold plenum. Plenum is attatched to manifold and mounts up to the crossover housing. Rubber was rotted at bottom causing a vacuum leak (, rough idle). Before going on a wild goose chase with parts. Make sure that the engine is airtight( no vacuum leaks). Even if the service engine soon is on. Was getting codes of p171 , p174 lean condition bank 1 and 2. P300 random misfire. Had a air flow sensor performance code. A vacuum leak will cause a HOST problems, rough ile , knocking , inconsistent idle. Make sure that thing is air tight. Yea but it idles good when in neutral only rough idle in drive. and no codes Link to comment Share on other sites More sharing options...
adallak Posted April 24, 2012 Report Share Posted April 24, 2012 I have been chasing a rough idle on a 2002 Seville SLS. 4.6 engine codeY. Thought it was a coil pack, EGR valve . Wasn't either component. Turns out it was a vacuum leak at the intake manifold plenum. Plenum is attatched to manifold and mounts up to the crossover housing. Rubber was rotted at bottom causing a vacuum leak (, rough idle). Before going on a wild goose chase with parts. Make sure that the engine is airtight( no vacuum leaks). Even if the service engine soon is on. Was getting codes of p171 , p174 lean condition bank 1 and 2. P300 random misfire. Had a air flow sensor performance code. A vacuum leak will cause a HOST problems, rough ile , knocking , inconsistent idle. Make sure that thing is air tight. Yea but it idles good when in neutral only rough idle in drive. and no codes EGR is disabled in neutral and park. I know cleaning EGR valve and some passages is not fun on a Northstar. Can you drive for a while with disabled EGR to see if idle smooths out? You can either disconnect EGR solenoid or pull a vacuum line from the EGR valve (do not forget to plug that line with something like a screw). Some EGR related codes may be set if you disable it. The saddest thing in life is wasted talent Link to comment Share on other sites More sharing options...
adallak Posted April 24, 2012 Report Share Posted April 24, 2012 Before going on a wild goose chase with parts. The saddest thing in life is wasted talent Link to comment Share on other sites More sharing options...
Ranger Posted April 24, 2012 Report Share Posted April 24, 2012 Try cleaning the TB first. If that doesn't help you may be due for a set of plugs and wires. Idle speed is computer controled. Link to comment Share on other sites More sharing options...
donandmax Posted April 24, 2012 Author Report Share Posted April 24, 2012 I have been chasing a rough idle on a 2002 Seville SLS. 4.6 engine codeY. Thought it was a coil pack, EGR valve . Wasn't either component. Turns out it was a vacuum leak at the intake manifold plenum. Plenum is attatched to manifold and mounts up to the crossover housing. Rubber was rotted at bottom causing a vacuum leak (, rough idle). Before going on a wild goose chase with parts. Make sure that the engine is airtight( no vacuum leaks). Even if the service engine soon is on. Was getting codes of p171 , p174 lean condition bank 1 and 2. P300 random misfire. Had a air flow sensor performance code. A vacuum leak will cause a HOST problems, rough ile , knocking , inconsistent idle. Make sure that thing is air tight. Yea but it idles good when in neutral only rough idle in drive. acI'll tell ya this engine is all together different for previous engine over off the engine..after removing the nd no codes EGR is disabled in neutral and park. I know cleaning EGR valve and some passages is not fun on a Northstar. Can you drive for a while with disabled EGR to see if idle smooths out? You can either disconnect EGR solenoid or pull a vacuum line from the EGR valve (do not forget to plug that line with something like a screw). Some EGR related codes may be set if you disable it. I looked yesterday for the egr valve (as I know it) but couldnt find it. This engine sure is different from anything I have ever worked on before No wonder mechanics shy away from it. I cant even find a distributor on this thing. Link to comment Share on other sites More sharing options...
scott y Posted April 24, 2012 Report Share Posted April 24, 2012 Nor shall you Link to comment Share on other sites More sharing options...
donandmax Posted April 24, 2012 Author Report Share Posted April 24, 2012 I have been chasing a rough idle on a 2002 Seville SLS. 4.6 engine codeY. Thought it was a coil pack, EGR valve . Wasn't either component. Turns out it was a vacuum leak at the intake manifold plenum. Plenum is attatched to manifold and mounts up to the crossover housing. Rubber was rotted at bottom causing a vacuum leak (, rough idle). Before going on a wild goose chase with parts. Make sure that the engine is airtight( no vacuum leaks). Even if the service engine soon is on. Was getting codes of p171 , p174 lean condition bank 1 and 2. P300 random misfire. Had a air flow sensor performance code. A vacuum leak will cause a HOST problems, rough ile , knocking , inconsistent idle. Make sure that thing is air tight. Yea but it idles good when in neutral only rough idle in drive. acI'll tell ya this engine is all together different for previous engine over off the engine..after removing the nd no codes EGR is disabled in neutral and park. I know cleaning EGR valve and some passages is not fun on a Northstar. Can you drive for a while with disabled EGR to see if idle smooths out? You can either disconnect EGR solenoid or pull a vacuum line from the EGR valve (do not forget to plug that line with something like a screw). Some EGR related codes may be set if you disable it. I looked yesterday for the egr valve (as I know it) but couldnt find it. This engine sure is different from anything I have ever worked on before No wonder mechanics shy away from it. I cant even find a distributor on this thing. The egr valve on the 4.6 l is electronic and takes special diagnostic equipment to diagnose it. According to the Haines manual. I see no vacuum line going to it. I'm thinkin very expensive to buy.......The 4.9 Liter engine looks like the egr valve Im used to seeing.. Link to comment Share on other sites More sharing options...
adallak Posted April 25, 2012 Report Share Posted April 25, 2012 I have been chasing a rough idle on a 2002 Seville SLS. 4.6 engine codeY. Thought it was a coil pack, EGR valve . Wasn't either component. Turns out it was a vacuum leak at the intake manifold plenum. Plenum is attatched to manifold and mounts up to the crossover housing. Rubber was rotted at bottom causing a vacuum leak (, rough idle). Before going on a wild goose chase with parts. Make sure that the engine is airtight( no vacuum leaks). Even if the service engine soon is on. Was getting codes of p171 , p174 lean condition bank 1 and 2. P300 random misfire. Had a air flow sensor performance code. A vacuum leak will cause a HOST problems, rough ile , knocking , inconsistent idle. Make sure that thing is air tight. Yea but it idles good when in neutral only rough idle in drive. acI'll tell ya this engine is all together different for previous engine over off the engine..after removing the nd no codes EGR is disabled in neutral and park. I know cleaning EGR valve and some passages is not fun on a Northstar. Can you drive for a while with disabled EGR to see if idle smooths out? You can either disconnect EGR solenoid or pull a vacuum line from the EGR valve (do not forget to plug that line with something like a screw). Some EGR related codes may be set if you disable it. I looked yesterday for the egr valve (as I know it) but couldnt find it. This engine sure is different from anything I have ever worked on before No wonder mechanics shy away from it. I cant even find a distributor on this thing. Can you see the starter? LOL The saddest thing in life is wasted talent Link to comment Share on other sites More sharing options...
airmike Posted April 27, 2012 Report Share Posted April 27, 2012 if I didn't know where the starter is that wouldn't be funny Link to comment Share on other sites More sharing options...
bilysinger Posted May 5, 2012 Report Share Posted May 5, 2012 I have been chasing a rough idle on a 2002 Seville SLS. 4.6 engine codeY. Thought it was a coil pack, EGR valve . Wasn't either component. Turns out it was a vacuum leak at the intake manifold plenum. Plenum is attatched to manifold and mounts up to the crossover housing. Rubber was rotted at bottom causing a vacuum leak (, rough idle). Before going on a wild goose chase with parts. Make sure that the engine is airtight( no vacuum leaks). Even if the service engine soon is on. Was getting codes of p171 , p174 lean condition bank 1 and 2. P300 random misfire. Had a air flow sensor performance code. A vacuum leak will cause a HOST problems, rough ile , knocking , inconsistent idle. Make sure that thing is air tight. Yea but it idles good when in neutral only rough idle in drive. and no codes EGR is disabled in neutral and park. I know cleaning EGR valve and some passages is not fun on a Northstar. Can you drive for a while with disabled EGR to see if idle smooths out? You can either disconnect EGR solenoid or pull a vacuum line from the EGR valve (do not forget to plug that line with something like a screw). Some EGR related codes may be set if you disable it. I also went through this scenario. These rubber plenums are of poor design. They used a sub grade of rubber when manufactured. It took me 2 tries to find a 5" slit in the plenum. it was within one of the corrugations and hiding from my sitght. But, when I bought a new one from the dealer they had 5 more in stock. Must be that it's a common cause and to have that many in stock is not typical for this item meaning, THEY ALL GO BAD WITH AGE. Thanks Cadillac for using poor rubber to attach the throttle body to the intake manifold. Link to comment Share on other sites More sharing options...
Ranger Posted May 5, 2012 Report Share Posted May 5, 2012 The rubber plenum is only used on the 2000+ engines. The Donald has a '98. Link to comment Share on other sites More sharing options...
donandmax Posted May 19, 2012 Author Report Share Posted May 19, 2012 The problem was plugs and new wires. They fixed the problem idles like a new one. Thanks for all your suggestions Link to comment Share on other sites More sharing options...
Chris Posted November 8, 2013 Report Share Posted November 8, 2013 The problem was plugs and new wires. They fixed the problem idles like a new one. Thanks for all your su Please ignore/delete this post, I decided to make a new topic to better describe both of my symptoms. Thanks -Chris Link to comment Share on other sites More sharing options...
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