Intermitent idle/vaccum issues

[phazedphoton]

Hello, I have a new to me 02 rx300 with 170k on the clock. Im trying to figure out some issues Ive discovered and haven't found anything conclusive enough to fix. Thus, Im reaching out for help, all input would be appreciated.

The issue is partially intermittent. On idle when car is in gear and rpm is low ~600 there is a dead miss on 1-2 cylinders. It clears up as the rpm is increased. Additionally, the car suffers a low vacuum condition on occasion (largely after rough idling for awhile, or driving with the engine at low rpm). This loss of vacuum impacts the brake booster, which I find unacceptable and have been driving my other car (a 90' v12 jaguar xjs) instead. Not terribly practical.

I have generally looked for vacuum leaks without success. Have tried using smoke and noticed a flow of smoke to the booster area but haven't seen anything conclusive. Im also not convinced its a vacuum leak. It seems to happen intermittently, and like a switch if its good the car operates properly and other times, its rough and I ended up easing the car home without sufficient boosted brakes. Additionally, just by feel sometimes it seems the valve timing is not quite where it should be. Which has me suspecting the vvt system, which could potentially impact intake vacuum. I dont have all the codes with me but they were just general misses not cylinder specific.

I know almost nothing about the maintenance of the car, someones been here before. It has new plugs (didnt actually check back cylinders) and doesn't seem like an ignition issue. One of the screws on a vvt solenoid was replaced but the solenoid is original. I know bench testing solenoids isnt always an accurate way of determining if they are ok.

Im tired of having this issue and im ready to dig in to fix it. Let me know what things I should look at or address, Id love to hear your input. Im leaning towards replacing vvt solenoids, filters, and cleaning the iac valve (should I consider replacing the actual actuator?)

Best,

[RX400h]

Quote

I understand your frustration with the issues you’re experiencing with your Lexus RX300. Here are some suggestions based on the symptoms you’ve described:

1. Intermittent Misfire: This could be due to a variety of reasons including faulty spark plugs, ignition coils, or fuel injectors. It’s recommended to check each coil one by one to see if there is a change in the sound or rpm of the engine. If there is no change, that means the coil is bad and needs to be replaced. You can also swap the bad coil with a good one to confirm that the problem is not the spark plug, the injector, or the wiring.

2. Low Vacuum Condition: A low vacuum condition can be caused by a variety of issues including a clogged air filter, vacuum leaks, or issues with the valve covers. It’s recommended to check for vacuum leaks as a first step. If you suspect it’s not a vacuum leak, it could be related to the valve timing as you mentioned.

3. Valve Timing/VVT System: Issues with the VVT system can cause a variety of symptoms including diminished engine performance and poor fuel economy. If you suspect the VVT system, it’s recommended to check the VVT solenoids⁷. You can also consider replacing the VVT solenoids as you mentioned.

4. Idle Air Control Valve (IACV): If the car idles roughly or stalls, it could be due to a dirty or faulty IACV. Cleaning the IACV can often resolve these issues. You can consider cleaning the IACV as you mentioned.