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'99 Runs Rough While Stopped In Drive, Less in Neutral

3628 Views 8 Replies 7 Participants Last post by  iRt
My wife's '99 Sienna runs rough when stopped in drive and runs LESS rough when put into neutral and when I turned the A/C off, it runs smooth as if it does has not a problem. Any suggestions from the Brain Trust?

All input is deeply appreciated.

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Update on the Rough Running '99. It started to idle rough a few months ago after my wife heard "a hard thump" when shifting from reverse to drive. The van was at a complete stop. It would idle rough when stopped in drive, but runs smoother when I shift to Neutral. When I turn the A/C off, it runs as smooth as if there is nothing wrong.

The Check Engine Light came on about a month ago. Diagnosis said it was running lean. I changed the O2 Sensor and it ran smoother but didn't solve the problem. Then I changed plugs and wires, making it run smoother, but it would still run rough in drive. Upon recommendation from the forum, I took the IAC valve off and Marine Corps cleaned it with Deep Creep. That took about 3 hours, but it looked about as close to new as I could get. It was clogged with the black gunk from the ethanol This took out a lot of the roughness, but it is still problematic.

My next step is to change the timing belt, water pump, idler pulley and tensioner. I need a brand recommendation. The dealer changed the belt ONLY over 109K milers ago, so I know it needs changing, and especially the other parts. I will also change the power steering pump this weekend, because it is howling like it needs changing. It is three years old and is either an Advance Auto or O'reilly brand with about 40K miles on it.

Any other suggestions? Again, it runs rough wile stopped, but runs really smooth when I turn the A/c off.

All help is deeply appreciated.

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Are you sure there isn't a vacuum leak somewhere?
Are you sure there isn't a vacuum leak somewhere?
Also change your PCV valve and spray some deep creep into that hose that connects the PCV valve to the engine. When were the spark plugs last changed?

I would open the front valve cover and check for sludge buildup as well.
Over the years, the IAC valve has been the most common cause of rough idling. There usually isn't a correlation with the A/C, though.

Can you describe your rough running more? Is it like a "missed firing" or skipping or knocking, is it like the engine is going to stall, and does pressing the gas pedal eliminate the roughness?

I'd make sure you're current on all your preventive maintenance--things like spark plugs--which you likely are. Maybe run a PEA-containing fuel system cleaner, like Chevron Techron, or Redline SI-1, at full recommended strength, to see if that helps.

Is your A/C cooling well, and cycling at nice, long intervals? Do the fans on the radiator come on and off, with the A/C, when your van is at operating temperature?

Vacuum leaks are possible, but have not been common on our generation Siennas. Aside from the IAC valve, some have had issues with their MAF sensor becoming dirty, affecting running; use appropriate care, if you clean yours, as they're delicate. Personally, I think IAC, until proven otherwise; people have been known to pristinely clean their IACs, then go through more diagnoses, only to later have their problem be a malfunctioning IAC valve.
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I had the similar symptoms as OP. It was the rear motor mount that needed replacement. The proof was putting it in reverse in garage with brake on. Revved it slightly and the rear of the engine raised 4-5 inches. Under $40 for the part at Auto Zone
I know this is an old post, but my 98 started idling rough when I had to stop for lights, etc. I bought some throttle body cleaner and used that myself to clean the throttle body. Was easy to do and that fixed the idle issue.
... I bought some throttle body cleaner and used that myself to clean the throttle body. Was easy to do and that fixed the idle issue.
I wonder if the cleaner found it's way down into the IAC and that was where the improvement came from.

I've seen YouTube vids where somebody sprays throttle body cleaner in an intake and they wipes it off with a rag, then shows the rag, now a bit brown and says "look at all those power robbing deposits" and I always shake my head. That's like thinking that if your house water main had 1/100 inch of slime on the inside it would affect your water pressure.

Is there really a difference between a) throttle body cleaner b) carb cleaner and c) Walgreens 90% isopropyl alcohol?
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