Intermittent Rough Idle, P0304

[Jumbojoe55]

I have been having this intermittent rough idle after starting, but it smooths out as soon as I exit the driveway. This only happens after the first start of the day and no issues after the car is warmed up.

It rained last week and the next morning it was cold, but sunny. It idled rough after starting and then I got a P0304, cylinder #4 misfire CEL. I never felt the misfire again on my 40-mile highway round trip.

Iridium spark plugs and Toyota ignition wires have about 20,000 miles on them.

I think there is a vacuum leak somewhere. I heard a momentary slurp when I checked some of the vacuum tubes under the driver’s seat when I squirt water on them. I changed 2-lines with tubing from Autozone. I also disconnected the battery to clear the CEL. It took a week for the rough idle to come back. But no cylinder misfire, but it has been dry and sunny in SoCal.

I’m not sure what to do next. Could it be my EGR Modulator or EGR Valve? I’m not losing coolant.



I changed the two lines on the left near the oil dipstick.

I wanted to take it to a mechanic, but he was hesitant on working on a Previa and told me he would only guess and replace parts.

1997 Previa DX S/C RWD
110,000 miles

[Previologist]

are you getting any white smoke on startup? This happened to me on my way to both blown head gaskets.

[Jumbojoe55]

No, but there is one vacuum switch that slowly bleeds off vacuum and I am wondering if that is my vacuum leak.

I don’t know what it does or what it is called.

I have a ‘97 Previa DX S/C and this vacuum switch is mounted on the rear of the radiator. It is next to the fan, below the power steering reservoir and in front of the supercharger pulley. It has three vacuum hoses and a electric plug.

Does anyone know what it is? I can’t find it on any parts diagrams.

[Jumbojoe55]

The part is called a Manifold Air Injection Control Valve, part #25740-76010. It is discontinued and I can’t source any new part. I found a used part, but it behaves the same as my original part. It slowly bleeds vacuum and if you apply 12v, I hear the solenoid click and it fully opens to hold no vacuum.

My original part bleeds vacuum to 0. The used part bleeds vacuum to 3-4in. of Hg, but the car idles with much higher vacuum then that.

I swapped the part out anyways, but I have a feeling my intermittent rough idle will come back. Makes me think my Head Gasket has a tiny leak in it. But I check under the car and under the passenger seat when I get the chance.

[Previologist]

I've never fiddled with that part. I hope its not a head gasket but if it is that makes 2 of us.