How did you clean the IAC valve? I would try to spray it a few times with the seafoam deep creep. Then I would check your vacuum hoses for leaks/cracks.

Here are two videos to help you with that.

Regards, JC.

 

The IAC valve was cleaned with Deep Creep. One thing I noticed was I didn't attach the stabilizer arm to the back of the throttle body when I put it back in, so I took care of that. Took me 45 minutes to get that damn bolt in, I was hoping that would solve all the problems, but it didn't. A brief visual inspection of the vacuum hoses didn't reveal anything, but I'll try to use one of the methods above and report back.

Thank you.

 

You don't still have any DTCs (check-engine codes), do you, either pending or current?

You now mentioned the #2 spark plug looked bad. That's unusual for Siennas. Typically, plugs look pretty good, even if left in too long.

Sure, it could be a vacuum issue, but there haven't been many of those reported over the years on Siennas. More typically, a component fails. In this case, I would still suspect the ignition coil. A faulty coil could produce the rough running and the spark plug damage (Google this). Plus, you had a specific diagnostic code for one cylinder--wouldn't a vacuum leak produce a less specific code? If it were my van, I'd do a quick vacuum check, and if I didn't find anything, put in a new coil on #2 for about $60, if you can swing the funds. That would be cheaper than a diagnostic visit.

Do you use your front window defogger? Remember, that setting engages the compressor, too. The compressor "compartment" condensation drain is close to behind the passenger wheel well. There was a recent thread about cleaning these.

Good luck!

 

You don't still have any DTCs (check-engine codes), do you, either pending or current?

You now mentioned the #2 spark plug looked bad. That's unusual for Siennas. Typically, plugs look pretty good, even if left in too long.

Sure, it could be a vacuum issue, but there haven't been many of those reported over the years on Siennas. More typically, a component fails. In this case, I would still suspect the ignition coil. A faulty coil could produce the rough running and the spark plug damage (Google this). Plus, you had a specific diagnostic code for one cylinder--wouldn't a vacuum leak produce a less specific code? If it were my van, I'd do a quick vacuum check, and if I didn't find anything, put in a new coil on #2 for about $60, if you can swing the funds. That would be cheaper than a diagnostic visit.

Do you use your front window defogger? Remember, that setting engages the compressor, too. The compressor "compartment" condensation drain is close to behind the passenger wheel well. There was a recent thread about cleaning these.

Good luck!

No, no error codes since I changed the plugs. I also just checked the vacuum lines using the cigar method. Got nothing but headspins. Don't use the defogger much, but I have had it on a couple of times in the last week - nothing unusually long.

I've noticed that the engine doesn't shudder on a cold start, until after running for about 5 minutes. Then it's very noticeable, but doesn't stall and doesn't hesitate when I hit the gas.

Wouldn't a bad coil re-throw a code?

 

You'd think it would throw a code...

Perhaps also, it could be a bad sensor, like an a/f ratio sensor or the knock sensor, or ect sensor. If the misfire code returns, do the coil. I don't recall, had you changed your a/f ratio sensors?

 

No, no sensors replaced, but I wondered about the a/f sensor. Again, wouldn't that throw a code? We took a 50 mile drive today, acceleration fine, cruising at speed fine, but when idling, it shudders like crazy.

 

Very frustrating! It started getting worse today, acceleration was getting sluggish, and at one point the CEL began blinking. Great, I thought, diagnostic information! But it went out and didn't leave an error code.

Could this tank of premium be the cause? I'm nearly empty so I'll fill it up with Reg. Unleaded again and see if it makes a difference. Could this be the sludge monster I've read about? I'm about ready to pull the valve covers off.

 

Very frustrating! It started getting worse today, acceleration was getting sluggish, and at one point the CEL began blinking. Great, I thought, diagnostic information! But it went out and didn't leave an error code.

Could this tank of premium be the cause? I'm nearly empty so I'll fill it up with Reg. Unleaded again and see if it makes a difference. Could this be the sludge monster I've read about? I'm about ready to pull the valve covers off.

What about something like water in the fuel or a clogged fuel filter? If you can find it, use a can of berryman's B12 (which you usually can get in Walmart in the US for around $3 a can). If the gas tank is around a 1/4 of a tank full, I would put the whole bottle in there and use it as a fuel system cleaner and see if that does anything for you.

Regards, JC.

 

What about something like water in the fuel or a clogged fuel filter? If you can find it, use a can of berryman's B12 (which you usually can get in Walmart in the US for around $3 a can). If the gas tank is around a 1/4 of a tank full, I would put the whole bottle in there and use it as a fuel system cleaner and see if that does anything for you.

Regards, JC.

I'll look for the Berryman's, thanks for the suggestion. As far as I know, this van doesn't have a serviceable fuel filter, correct?

The idle drops about 100 RPM when it shudders. I'm wondering if it's the IAC, even though I cleaned it. When I first had the IAC problem I cleaned, didn't replace, the IAC, and the problem showed up again in about 6 months. So then I replaced the IAC. That was about 6 years ago, and I've cleaned the new one once since then a couple of years ago when it started dying on startup again. Makes me think my van is chewing them up.

 

I'll look for the Berryman's, thanks for the suggestion. As far as I know, this van doesn't have a serviceable fuel filter, correct?

The idle drops about 100 RPM when it shudders. I'm wondering if it's the IAC, even though I cleaned it. When I first had the IAC problem I cleaned, didn't replace, the IAC, and the problem showed up again in about 6 months. So then I replaced the IAC. That was about 6 years ago, and I've cleaned the new one once since then a couple of years ago when it started dying on startup again. Makes me think my van is chewing them up.

The fuel filter is located under the air cleaner box. Yes it's replacable. Search youtube for videos on how to do it. The box needs to be removed to access the fuel filter.

If you don't regularly clean the IAC valve then yes it could get all clogged up. I normally clean my throttle body and IAC valve at the same time 2-3 times a year. It only takes about 10 mins to do for me from start to finish. Even cleanly it once a year would be wise.

Regards, JC.

 

The fuel filter was moved to the fuel tank in the 2001 Sienna update...

 

The fuel filter was moved to the fuel tank in the 2001 Sienna update...

Thanks for the clarification. I guess you're not going to change the fuel filter.

Regards, JC.

 

Ok, so I thoroughly cleaned the IAC today. It was not especially dirty, but I was able to put on a new gasket, which I shined off last time. I also changed the oil. I noticed for the first time an accumulation of grime along the valve cover seam. I checked the valve cover bolts, and tightened the ones I could reach. The two next to the pulley were surprisingly loose, needing 3/4 turn to tighten.
The result is a noticeable improvement, for whatever reason, but I'm not yet sure the shakes are completely gone.

Are the valve cover gaskets known to need replacing over time, and is this something that should happen to both at once? I'll definitely need a week off to do the odd-cylinders cover.

 

Reading some other posts turns up that many times, a faulty ignition coil can cause rough idling. This problem did originate with a couple of misfire codes, so I thought I would try swapping a good coil into the front cylinders, which includes the one that gave the first misfire (#2).

So I'm thinking of picking up a super-cheap coil just to do the swap-around. Are the aftermarket coils as good as OEM, or at least will they be good enough to run this test? I figure if swapping one out makes all the difference, I can spring for a good, used OEM. The cheapo brands are running about $15 on the auction site. (By the way, is the OEM Denso?)

 

If you just put in a new Denso (OEM) coil on #2, you would eliminate the coil as a possible cause. Not sure I'd trust another brand, especially one selling for $15. Being the #2 cylinder has thrown the misfire code, you should also be looking at the condition of that spark plug, which is very easy to reach.

Related question: what plugs did you use as replacements (manufacturer and part #)?

Since you mentioned the rough running only occurs after warm-up, that could implicate another sensor, like the ECT sensor, or even the A/F ratio sensor.

 

: what plugs did you use as replacements (manufacturer and part #)?

Since you mentioned the rough running only occurs after warm-up, that could implicate another sensor, like the ECT sensor, or even the A/F ratio sensor.

I replaced all the plugs with NGK Iridium IX about 3 weeks ago. This was the type that was in there, at least for the past 8 years.

The symptoms are frustratingly inconsistent. This morning, for example, the engine shuddered strongly as soon as I started up, but lessened after warming up.

I'll look into getting a good used Denso coil and see how that goes.

 

Quick thought: how's your exhaust? A restriction in the exhaust system could cause rough running. Doubt that's the problem in SoCal, though. Also, any smell of fuel in the exhaust, which could indicate incomplete combustion?

 

Nothing unusual with the exhaust. Smells normal, looks normal, and seems to have a strong flow. A replacement coil is on the way, so I'll let you know how that goes in a week or so.

Thanks for the continued suggestions.

 

Dropped in the replacement coil this morning, and the shuddering has vanished. It blows my mind how this solid-state piece of plastic-coated metal, which I thought was just a conducting medium, could be so touchy, and also how it could have such a noticeable problem that doesn't trigger the CEL.

Thanks to all who contributed advice.

 

Glad it worked!

 

Dropped in the replacement coil this morning, and the shuddering has vanished. It blows my mind how this solid-state piece of plastic-coated metal, which I thought was just a conducting medium, could be so touchy, and also how it could have such a noticeable problem that doesn't trigger the CEL.

Thanks to all who contributed advice.

Glad you got it resolved. Now you're the running rough expert!

Regards, JC.