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Old 06-16-2010, 06:06 AM
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425 rough idle problem

My Caddy has a rough idle, mostly after it warms up to operating temperature. I'm going to give it a full tune-up either today or tomorrow, but for some reason, I don't think that will fix it.

Any suggestions?

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Old 06-16-2010, 07:50 AM
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Look for vacuum leaks.
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Old 06-16-2010, 02:20 PM
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I've tried the ol' "can of carb cleaner to find a vacuum leak" trick, and didn't find anything, any other easy ways to find a vacuum leak?
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Old 06-16-2010, 11:21 PM
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A vacuum gauge will help. Should have about 20 in. or more of vacuum at idle
on a manifold port.
When you say full tune up, what are you planning on doing? There is about
10 things that can cause a rough idle.

cam
lifters
valves
vacuum leak
spark plugs
cables
dist. cap
rotor button
O2 sensor
And so on.
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Old 06-17-2010, 05:36 AM
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Quote:
Originally Posted by sqzbox
A vacuum gauge will help. Should have about 20 in. or more of vacuum at idle
on a manifold port.
When you say full tune up, what are you planning on doing? There is about
10 things that can cause a rough idle.

cam
lifters
valves
vacuum leak
spark plugs
cables
dist. cap
rotor button
O2 sensor
And so on.

By full tune up I meant plugs, wires, dist. cap and button, all new filters (though I'm pretty sure that's not the issue, I know it needs them).

Talking to the guy down at my local Napa Auto, I'm going to replace the PCV valve tomorrow to start with, and a couple of the hoses are cracked a bit, so I'll replace those as well. Figure I may as well start with the cheapest things and work my way through.

I checked the wires today, and they all seem intact, but I never got around to checking the plugs or the distributor. I'll replace it all eventually, but money is tight, and at this point, I don't want to spend more than I need to. I'll have some spare cash in a month or so, so I'll be able to do whatever is needed, hopefully. Until then, I'll try it one step at a time and hopefully get it worked out without having to have the motor torn down.
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Old 06-17-2010, 07:09 AM
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A rough idle after warm up could very well be a vacuum leak. When the engine is first started, It is on a rich A/F ratio like when the choke is on and after warm up when the mixture leans out, the little gremlin starts to show itself.

What year caddy are you working on?
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Old 06-17-2010, 08:06 AM
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Before removing the plugs, with the engine idling, remove one spark plug wire at a time to see if there's a drop in RPM (there should be). Any cylinder(s) that doesn't change the idle will indicate a problem w/something to do w/that cylinder. Replace the wire after removing it, before going on to the next plug.

When you remove the spark plugs, keep them in order so you know what hole they came from. This can help narrow things down- like if you have 7 "normal" colored plugs, and one "white" plug, and that cylinder's intake runner just happens to have a vacuum fitting located on it. It would be a safe assumption that something the fitting feeds is leaking vacuum, or the hose itself is bad.

Look also to see if any plugs are abnormally oily, wet w/fuel, etc. This can help diagnose the problem.

As far as saving money, as long as the plugs aren't fouled, they can be cleaned and regapped. I use a brass wire wheel mounted on a bench grinder, but you can use a wire brush and some carb cleaner.

Unless the cap is cracked, I'd continue to use it as long as the center electrode still is good. The oxidation on the inside terminals doesn't do any harm for the most part, and can be left alone unless excessive. Same thing for the rotor- just clean the tip of the rotor and don't grind on it, you don't want it any shorter.
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Old 06-17-2010, 09:15 PM
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Quote:
Originally Posted by sqzbox
A rough idle after warm up could very well be a vacuum leak. When the engine is first started, It is on a rich A/F ratio like when the choke is on and after warm up when the mixture leans out, the little gremlin starts to show itself.

What year caddy are you working on?

It's a 78 Coupe DeVille.
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Old 06-17-2010, 09:34 PM
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Quote:
Originally Posted by cobalt327
Before removing the plugs, with the engine idling, remove one spark plug wire at a time to see if there's a drop in RPM (there should be). Any cylinder(s) that doesn't change the idle will indicate a problem w/something to do w/that cylinder. Replace the wire after removing it, before going on to the next plug.

When you remove the spark plugs, keep them in order so you know what hole they came from. This can help narrow things down- like if you have 7 "normal" colored plugs, and one "white" plug, and that cylinder's intake runner just happens to have a vacuum fitting located on it. It would be a safe assumption that something the fitting feeds is leaking vacuum, or the hose itself is bad.

Look also to see if any plugs are abnormally oily, wet w/fuel, etc. This can help diagnose the problem.

As far as saving money, as long as the plugs aren't fouled, they can be cleaned and regapped. I use a brass wire wheel mounted on a bench grinder, but you can use a wire brush and some carb cleaner.

Unless the cap is cracked, I'd continue to use it as long as the center electrode still is good. The oxidation on the inside terminals doesn't do any harm for the most part, and can be left alone unless excessive. Same thing for the rotor- just clean the tip of the rotor and don't grind on it, you don't want it any shorter.

I'm not too worried about saving pennies at this point, so putting out the $20 for new plugs isn't a big deal. When I get to that point in the troubleshooting, I'm just going to replace them. I found a box in the trunk that had the old spark plugs and wires in it with a receipt for buying the new ones in 2005 and marked about 5000 miles back from where the car is now, so that's new plugs and wires 5 years and 5000 miles ago. And the plugs aren't platinum, so I figure I may as well get some platinum plugs in there for good measure.

I still didn't get around to checking the dist. or plugs. The plug wires all look good, and aren't anywhere they can be rubbed or clipped by a fan. Have no cracks or gouges. so I figure the wires should be fine.

I did replace a couple of cracked vac hoses today, which seems to have made the problem worse... Now once it warms up, it will barely stay running at idle, and dies sometimes. Also it will die (or try to) any time I stop at a light or stop sign. It also has a HUGE hesitation when hitting the gas. It almost dies and then suddenly gets a rush of fuel and shoots forward. The car was on almost empty when I got it, so I figure the fuel filter needs to be changed. Any other suggestions on this? And where is the fuel pump on this car? (please don't tell me it's in the damned tank!)

Thanks for the help, guys. You've been awesome!
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Old 06-17-2010, 10:11 PM
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Did you put new plug wires on it? It may be a long shot, but I've had problems with old carbon core (TVRS) wires.
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Old 06-18-2010, 01:15 AM
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Quote:
Originally Posted by WDCreech
Did you put new plug wires on it? It may be a long shot, but I've had problems with old carbon core (TVRS) wires.

Not yet, once I replace the plugs and check out the dist. wires will be the next buy. I've looked them over good, and made sure the connections were tight, and the connections are all good, and the wires don't have any damage.

I went out tonight and messed with the idle screw. I don't think it was off, but it idles well enough now that it doesn't die anymore. However, it shifts into gear from park pretty hard now. So I'm sure it's too high, though it doesn't sound too high, really. But at least it's not dying all the damn time. It will work until I can fix it right.

I also replaced the PCV valve before adjusting the idle and got no change. The old PCV valve was still good, though VERY dirty. Should there be oil in the PCV?
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Old 06-18-2010, 05:13 AM
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To set the idle mixture screws, you would like to have a vacuum gage. Otherwise, turn the screws in and out to get the idle at the highest RPM as you can. If the idle gets too high (above about 850 RPM), lower it using the curb idle screw. Then resume balancing the idle mixture screws.

You will want to put a timing light on it to see where the initial and total timing is at, as well.
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Old 06-18-2010, 05:15 AM
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Quote:
Originally Posted by cobalt327
To set the idle mixture screws, you would like to have a vacuum gage. Otherwise, turn the screws in and out to get the idle at the highest RPM as you can. If the idle gets too high (above about 850 RPM), lower it using the curb idle screw. Then resume balancing the idle mixture screws.

You will want to put a timing light on it to see where the initial and total timing is at, as well.

I don't have either of those.
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Old 06-18-2010, 09:29 AM
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I had an 80 eldorado once. It was the bigest PITA I've ever owned. It had new paint, tinted windows, and looked awesome sitting in the driveway where it spent most of it's time while I owned it. I've hotrodded many cars, bought, traded, fixed, and sold them. To be honest, the caddy was the hardest one to fix with just garage tools because of the electronics and sensors are so different between years that even junk yard parts were different from one year to the next.
The day I traded it in for what ever I could get for it, it threw the belts off on the way to the car dealer and then wouldn't start after I got there! I'ts like it was possessed by the devil!!!! DA dealer gave me 700 bucks trade in which is about 600 bucks more that it was worth to me. Best day of my life was when I drove away with a 90 Dynasty which we drove for another 100K mi. till we sold it for 700 bucks 8 years later.
My best advice to you is do whatever you can to unload it because it will nickel and dime you to death. JMHO don't hate on me! SQZBOX
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Old 06-21-2010, 03:02 AM
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So I replaced all the rough looking vacuum hoses, the PCV valve, and the fuel filter... All without much change. Adjusted the idle screw and temporarily fixed the issue... But this is the amazing part... I run a bottle of STP injector/carb cleaner through the system, and now it idles WAY better, isn't trying to stall when I hit the gas, and has almost double the power it had before!

I always figured that stuff was just some more "snake oil" crap, but I'll be damned if I didn't see amazing results after about fifteen minutes of driving with the cleaner in the tank...
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