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engine RPM fluctuation


joeniv

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A year ago, I bought my dream car - an '01 Cad ESC. It had 44k when I bought it and I just clicked over 50k. So far, so good, the car has been all I've wanted. The truck in the background has 130k and is still running great, I hope to get another 5 years of service from both.

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my question - and some background - when I first got the car - it would stall out on odd and relatively frequent occasions. it would just quit at a stop sign. it always started and I could drive away. took it to the dealer - under warrenty - and he replaced an engine sensor - that problem has gone away.

the issue at hand is that the engine will rev up and slow down - sounds like you are pumping the gas pedal even when just sitting there warming up in the driveway. it does this first thing in morning when in Park, also at a stop light when in drive. not a real problem, but it bugs me and indicates something must be going wrong. took it to the dealer to diagnose, he charged me for the diagnosis and recommended injector cleaning. I ran a can of injector cleaner thru it on the last tank o gas - the issue remains.

follow up question - how the heck do you change plugs? I took off the top cowling and couldn't even see them! Yikes! I'm an engineer by trade - have a hard time letting plugs run past 50k miles - even though they are supposed to last 100k...

thanks in advance - infrequent poster here - so I apologize if this topic has already been covered...

-niv

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Plugs are pretty easy just follow the wires (back plugs are a little tough but not if you take off the coil packs..2 bolts..easy) unless its missing they go 100K easy.

the idle is probably the thottle body blade needing a good cleaning, Scotty... er...Mike...er...BodybyFisher posted some good photos I'm sure he'll chime in with the link. Nice lookin Eldo!

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I seriously doubt plugs are your problem. Having said that, you may not have any wires to follow. I'm not sure what year they went to the COP (coil on plug) ignition but I want to say 2000. My '02 has it. If that is the case, you'll see a coil pack, or maybe it is called an ignition module which containes the coils, neatly fitted into each cam cover. You'll have to remove it first and the plugs should be under it.

I would do a good TB cleaning first though.

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Get the trouble codes first. They will guide you in right direction. I do not think plugs have anything to do with that kind of RPM fluctuations.

The saddest thing in life is wasted talent

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I knew I shouldn't have combined two questions in one thread. I'm changing plugs because they have 50k miles on them and are 5 yrs old. I know the manuals all say they are good for 100k, but the engineer in me knows what sort of service conditions they endure - I can't stand to leave them in that long...

I'll go look under the hood and see if I can even find a throttle body. Maybe I should go stay at a holiday inn express first...

thanks for the tips so far...

-niv

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I knew I shouldn't have combined two questions in one thread. I'm changing plugs because they have 50k miles on them and are 5 yrs old. I know the manuals all say they are good for 100k, but the engineer in me knows what sort of service conditions they endure - I can't stand to leave them in that long...

I'll go look under the hood and see if I can even find a throttle body. Maybe I should go stay at a holiday inn express first...

thanks for the tips so far...

-niv

I would not suspect the plugs - not at 50K miles. If it makes you feel better, pull them out and inspect them. If the platinum pad are present on the ground electrode, the plug is fine and will go another 50,000 miles.

Kevin
'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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I had the same surging conditions in my STS. After cleaning the throttle body, swapping plugs, and wires, and doing just about everything else you're likely going to try I isolated the problem to a dying battery and damaged battery terminal. Eveything else in the car worked fine, it didn't seem like anything was wrong with the elecrical system but the PCM wasn't able to set a stable idle due to the less than consistant spark current. The voltage wasn't the issue, the DIC still showed 14 volts, the ignition coils just wern't getting the current they needed. The problem goes away once you get your alternator spinning, but at idle the engine speed is inconsistant, and annoying.

This is how I fixed the probelm you are having, you may have a different problem, but I hope the info might be helpful.

Caddilac+Bad battery=Problems

--Ben

BTW, this will likely not throw a code and I payed two mechanics to not figure out something that simple.

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I had a problem like that on a Lincoln once. It was an engine vacuum problem. The EGR valvle was fine, but after a couple weeks of searching I finally found out it was a weathered hose poking out the back of the motor that was destroying my engine vacuum. Dunno, maybe that helps.

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I had checked everything else, and there was no loss of vacuum. I had a mechanic give me a second opinion on that. I even replaced the ICM. I took the battery to the auto parts store and had it tested. It was one of the last things I checked, should have been the first. After I installed the battery the problem seemed better so the terminals were the next step. Battery terminals oxidize extreemly easily and produce a pretty decent resistance. A few ohms of resistance can cost you alot of current at such low voltage.

I'm pretty sure I was just lucky.

--Ben

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