Rough idle when hot or under load

I have a '93 Eldo 4.9 with 59K on it. Sitting at idle I get a stumble which can be felt as a vibration inside the car. The rear view mirror actually shakes. It gets progressively worse when the load is
increased by putting it into "Drive" and turning on the A/C. Neither can be isolated as the sole cause of the stumble. Each item in turn simply exacerbates the symptom which is just barely present in neutral with the A/C off. The hotter the engine temp, the more noticeable is the stumble/vibration. I've been chasing this for a couple of years. I've eliminated ignition and fuel as potential causes by the simple expedient of throwing parts at it. EGR system is clean, valve is new. ISC motor is new. Vacuum hoses are all new. Fuel pressure is to spec. The fooler is that it worsens the hotter the engine gets such as when sitting in traffic on a hot day. It's hardly noticeable when first started in the AM. No computer codes are present ever. Other than this the car runs very well now, but this problem is getting steadily worse and I envision a time when it will begin to stall when brought to a stop. Any thoughts about what avenues to pursue now would be most welcome.
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First of all I think you need to determine if there is in fact an engine idle roughness such as a suttle misfire at idle as opposed to a drivetrain vibration being transmitted through a bad engine or trans mount. Keep in mind the mount doen's have to be broken to do this, the isolator can become hardenened or in some cases softened by oil contaimantion and or heat, then the ability to dampen normal drivetrain harmonics is lost. A deteriorating harmonic balancer on the crank can cause low speed vibrations. Without actually feeling this I can only throw out these off the cuff/past experience ideas. Possibly taking a vacuum reading at idle may help confirm or deny whether engine performance is an issue. These engine were famous for carbon deposits forming on the back of the intake valves, this is another possibility.

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Shep, Thanks for the fast and detailed follow-up. It definitely is a misfire which gets worse the more the engine warms up. I observed it this morning, idling in the driveway, starting at dead cold up to full temp when the fans went on. Engine shake was progressively more pronounced that warmer it got. I took a vac reading from two unported sources with an old gauge I've had since Ford Y block days. It was a steady 15" until the A/C kicked on then dropped to about 14 1/2" and the shake was more pronounced. I had my wife sit in the car with her foot on the brake. No change so I don't believe it is a booster leak. Had her put it into drive and observed yet more engine shake and vac went to 14". The gauge may be tired now and not as responsive as it could be. The marks on the face of the gauge indicate 15" as late timing but I don't know how relevant that is to a fuel injected engine. The timing is on the button at 10 degrees in timing mode with RPMs under 800 as per spec. The "carbon deposits forming on the back of the intake valves" theory is interesting. Would symptoms of that be heat sensitive? Are there any products or procedures that you like to remove the carbon without taking out the oxygen sensors ?
Regards and TIA
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GM makes a top engine cleaner that will clean up carbon, follow the instructions exactly, but your vacuum is low, you need to look for a throttle body vacuum leak or try to isolate the cylinder, if you can get into obd short out one cylinder at a time and watch the rpm drop, the one with the least drop in rpms, may be the culprit. If you do not know how to use the obd thru the ac control head go to caddyinfo.com

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One BIG tip about that top end engine cleaner. DO NOT USE IT IN A GARAGE. Don't use it around your neighbors house if you want to avoid problems either. And DO NOT HAVE THE EXHAUST POINTING AT ANYTHING YOU WANT TO KEEP CLEAN. You won't believe how much carbon and crud will blow out of that pipe. And the smoke generated will make you think the world just exploded. I usually take the vehicle out into the woods or up on the state land (unused dirt roads mainly) and run the cleaner through it there. Still looks like a forest fire but it is out of the way. I buy the stuff by the case and run it through the fire departments gas rigs once a year and my own once every two-three years. Then change the oil and filter and do a tune up.
--
Steve Williams

"Shep" < snipped-for-privacy@capital.net> wrote in message
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Good advice and true, Steve

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I know of a couple folks who didn't believe me when they decided to use some on a vehicle they had. They used it in a garage next to a nice convertible with a white paint job and top. Took a LONG time to get all the stink and crud off that car and out of the top. I went over and asked him why he didn't listen when I told him about what it does. He said " I didn't think you meant it! "
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Steve Williams
Near Cooperstown, New York
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west.superfeed.net:

SeaFoam is a smilar product. about 5 bucks a can here...
--
1984 RZ350

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west.superfeed.net:

Shit, heres the URL
http://www.seafoamsales.com/motorTuneUp.htm
--
1984 RZ350

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Thanks for the heads-up on the procedure.
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Again, many thanks. Because the vac is rock steady even though low, am I correct in assuming it is more likely the TB gasket or intake manifold rather than any one cylinder? I've done the power balance test as outlined in the FSM and no cylinder stood out as the culprit. I've sprayed around the TB with carb cleaner and also used an unlit propane torch to try to find any vacuum leak with no success. Can a TB have an internal vacuum leak? What are the most likely areas for a vacuum leak? I've replaced all of the small vac hoses on the engine. What has me puzzled is why the situation is the most pronounced when the engine is hottest. Am I correct in assuming this is because the fuel trim is then at its leanest and, any vac leak being constant, the net impact of such a leak is more noticeable?
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Anyone?
On Sun, 07 Aug 2005 21:10:15 -0400, MisterSkippy

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