My Bonneville is sorta jumping at idle speeds or coasting if you will.

You can power brake it and touch the gas pedal and it will almost die has a sense of missing if you will.

on a cold start if you leave hard on the throttle the valves are rattling bad ! But once warmed up no longer rattles.

Sometimes on cold starts leaving driveway brakes go to floor board but by the end of the drive have full pedal of brakes.

On cold starts when you gas on it you can hear a whistle under the hood or either air sucking but sounds more like a whistle to me .

Just replaced the mass air flow because it was under warranty.

Service engine light keeps coming on , once it was the temp sensor , now it is coding something about exhaust gas temp to high , which I was told would mean the EGR .

Evidently I am losing Vacuum because of the brake issue , and could possibly be the whistle im hearing .

Now on older cars you had vacuum advanced timing , which would explain the valves rattling but I think the timing on my 03 would be electronic ?

So do I go for a new EGR motor or the EGR Vacuum block or could I possibly have a cracked intake ?

Car idles fine it is not missing at all !

Car is not super charged.

When you say "valves rattling" are you talking about lifter noise or ping otherwise known as pre-ignition? Sounds like you have a vacuum leak for sure, the high exhaust temperature I suppose could be caused by the egr or possibly a lean run condition from the vacuum leak, although I would think the PCM would try to compensate. There are members here much more knowledgeable about modern computer controlled cars with their myriad of sensor laden feedback loops than I am, perhaps if you could be more specfic on the noises you hear and provide information on vehicle mileage and such, someone can help you.

2004 Bonne SLE
1971 Chevelle SS

I think you are on the correct path Roger , Like last week I put 3 gallons of 118 octane (airplane gas) in it a finished filling it up with 93 oc and they didn't rattle at all lol but on 89 they rattle bad when it is cold , but I will go along with pre detonation , and it doesn't do this unless im over half throttle when it is cold , but once at operating temp even with Wide open Throttle it has no rattle.

The car has 250210 miles on it , 64 pounds of oil pressure , Even with the issues going on it seems as though it has more power cold than it does hot, and not that it is relevant but it has what I think to be a exhaust heat shield rattle as well

Just went out and started car , while idling i unplugged EGR ,no change in idle , I unplugged egr vacuum up by the injectors , no change in idle , then I unplugged the 2 sensors under the mass air flow which i believe to be the Throttle pos. sensor the top one did nothing the bottom one made the car idle faster , then I unsnapped the air-box and revved the motor and it sucked the air filter up against the housing , is it possible my problems are directly related to the catalytic convertors ? drove car around block seemed as though it had limited power.

Thinking seriously about gutting the Convertors , what are your thoughts on this ?

By the way my fuel mileage has went from 29mpg to 19 mpg on the highway and in town from 21mpg to 13 mpg and when I set and idle my friends say they can smell gas in my exhaust like it is running rich for some reason

Think I would start by trying to track down the vacuum leak, starting fluid sprayed around suspected areas will help you track that down as the idle will temporarily bump up if it is sucked into the intake. Not for sure, but it'* possible that a vacuum leak has "confused" the PCM because no way should you get ping on 89 octane gas. My 04 SLE (N/A) runs just fine on the 87 octane ethanol containing fuel that passes for gasoline these days.

2004 Bonne SLE
1971 Chevelle SS

so if it is in fact a cracked intake by spraying starting fluid around it , then it too should increase idle ?

thanks for your help

I suppose it'* possible that you have an over-rich condition if the PCM is over compensating for a vaccum leak and the extra carbon generated has somewhat clogged the converter. If your 03 SLE has an exhaust system similar to my 04, you have one converter, then a small, cylindrical straight thru muffler/resonater until the exhaust system Y'* off with 2 mufflers, one ahead of each exhaust tip. I know this may sound silly, but before you replace the converter, give it a few whacks with a rubber mallet, not hard enough to dent it of course. Good luck.

2004 Bonne SLE
1971 Chevelle SS

if you have egr codes that could be your issue, if it sticks open its like a big vacuum leak. whats the exact code, i dont remember any egr codes that have anything about temp mentioned?

The code was p0404 egr control circuit range/performance........ I put a new ignition control module and 3 new coils on it because it had weak sparks but that didnt fix it , so ordered a new EGR valve put it on a wallah it is fixed , everytime i mentioned egr mechanics was saying no way egr can do that its not the egr guess I should have listened to the ECM to start with