avalon99 engine vibrates when decelerates

[roryhonor]

99 avalon, 68k mi., problem every 2-4 weeks for the past 20 months.
when i decelerate below 40 mph, the tachometer will widely oscillate at low rpm and the car will shake terribly as if it is going to stall. it has stalled around 4 times. there is often a strange smell to the exhaust afterwards. the next time that i start the car, it will hesitate before starting normally( extra second or two) then the car will function perfectly for the next few weeks. once it triggered a code. an egr sensor was replaced (at my cost). no improvement. 8 other times no computer codes were triggered. fuel injector cleaner tried 3 times. local mechanic and local toyota dealer tell me that they can't diagnose and fix without a diagnostic code, or unless it dies. i would appreciate any assistance.

[tfeld]

Rory-

Not much to go on here; as your mech told you, a code would be helpful.

BUT...you did say one thing that would move me to investigate in a particular direction: "the tachometer will widely oscillate at low rpm and the car will shake terribly as if it is going to stall. it has stalled around 4 times. there is often a strange smell to the exhaust afterwards."

If what you describe as a "strange smell to the exhaust" could also be called an acrid smell, then I would suspect a possible head gasket problem. What can happen is that coolant will/can sometimes leak from the cooling circuts across the broken gasket and enter the cylinder directly. When this coolant is burned, it gives off a very nasty and unforgetable smell. If there's enough coolant leaking, it will make the car shudder as you describe. You might not trip a CEL since most of the codes that would be effected would have two-trip logic which you did not meet.

I would ask your mechanic or dealer to check the oil for coolant contaimination. If there's water leaking into your oil, you can sometimes see it as a milky-brown coating on the inside of the oil cap. But if there's any contamination (the oil test is the most sensitive), then you'll have to have the head gasket replaced. The test is relatively inexpensive (about $25) plus whatever your mechanic will add to mail it off.

[Guru_37]

It could be spark plugs that need to be replaced, becuase they can cause misfiring in the engine and you can feel it. Also the car will shake or stutter. Have the spark plugs check and replace them if you need too, it can also cause a burning smell in your exhaust.