Well, I took a chance and got burned.
Just got the news from the mechanic that #2 is @140lbs compression. Thus, the misfire. He said I've got two choices: run the **** out of it or sell it for whatever.
The thing is it runs really smooth on the freeway at 60~70mph and is reading 25 MPG even in a short distance. Idling? You can feel the misfire.
I so wanted to like and keep this car. Undecided yet on what to do.
Open for any and all opinions.

 

Did he say why the #2 has lower compression than its neighbors?

 

No he didn't.

 

Well, maybe it's a head gasket or a valve issue. Some repair options would be nice

 

Once you have low compression data, the next thing is to figure out why and only then you can decide what is going on.

Your choice is to spend more time & money before deciding what to do or make a decision based on partial information.

More investigations options:
1. Borescope to see what is happening inside [just be aware that borescope image magnifies the situation .. so compare with other cylinder.
2. Run a leak down test.
3. Check if coolant is being lost, or coolant + oil is mixing up and also the smoke from exhaust (oil burning). <= some can leave tell tale signs on the plug.

It is like peeling an onion.

Other consideration would be the overall condition ... to help you decide is it worth fixing or punting. The vehcle is now almost quarter century old.

Salim

 

#1 I'll do that when I get it home.
#2 ditto
#3 no leak - all good

 

what do the other cylinders read

 

I don't remember what he told me.
I'll check it myself when I get it home.
According to the specs, 145 is considered minimum so @ 140 it's causing a misfire but runs absolutely smooth over 1,000rpm.
I just might drive it like that forever.

 

Hard to just run with it if it is throwing a misfire code. Have you definitely ruled out any issues with the coil and plug?

 

Plugs are all new and working. All coils are good.

 

Cant offer any advice but just sharing the consequences ..

You will be running with 5/6 the power. That will impact the efficiency of your vehicle.
Unburnt fuel will eat up the cats.
Not sure if you will pass safety inspection. [may not be required]
You will be draining out your wallet albeit slowly

Higher rpm just masks out the missing power strokes.

Salim

 

Here's what ChatGPT says.for this prompt: "You are an experienced lexus mechanic with specific expertise on the RX300 series. You are diagnosing a misfire in cylinder #2. The misfire is felt at idle but the engine feels smooth at high RPM. The compression in cylinder #2 is 140 pounds. What are the likely causes of a misfire with these symptoms? Show your work and provide references leading to your conclusions"


Below is a disciplined diagnostic analysis written from the perspective of an experienced Lexus technician with deep familiarity with the RX300 and the 1MZ-FE engine. I will explicitly connect each symptom to engine behavior, explain why certain causes rise to the top, and cite factory and industry references that support the conclusions.

Vehicle Context

• Model: Lexus RX300
  
• Engine: 1MZ-FE 3.0L V6
  
• Cylinder numbering:
  • Cylinder #2 = front bank, center cylinder (Bank 2)
    
• Symptoms:
  • Misfire isolated to Cylinder #2
    
  • Roughness only at idle
    
  • Engine smooth at higher RPM
    
• Compression (Cylinder #2): 140 psi
  

Step 1: Interpreting the Compression Result

Factory Specification Context

For the 1MZ-FE:

• New engine compression: ~190–210 psi
  
• Minimum allowable compression: ~142 psi
  
• Maximum cylinder-to-cylinder variation: ≤ 14%
  

140 psi is:

• Slightly below ideal
  
• At or just below Toyota’s service limit
  
• Not low enough to cause a continuous misfire on its own
  

Diagnostic Meaning

• A cylinder at 140 psi can fire reliably under load
  
• At idle, however:
  • Cylinder pressure is lowest
    
  • Flame propagation is weakest
    
  • Any additional weakness (air leak, injector imbalance, ignition loss) becomes evident
    

Conclusion:
Compression is marginal but not causal. It acts as a multiplier for other faults rather than the root cause.

Step 2: Why the Misfire Occurs Only at Idle

This symptom pattern is critical.

A misfire that:

• Appears at idle
  
• Disappears with RPM/load
  

Strongly rules out:

• Burnt valves
  
• Broken valve springs
  
• Severe ring damage
  
• Cam timing faults
  

And strongly implicates:

• Lean mixture conditions
  
• Localized air leaks
  
• Weak ignition under low cylinder pressure
  
• Injector imbalance at low pulse width
  

Step 3: Most Likely Causes (Ranked by Probability)

1. Intake Manifold Gasket Leak (Cylinder #2 Runner)

Why this is the top suspect on RX300s

• The 1MZ-FE uses a two-piece composite intake manifold
  
• Gasket shrinkage and hardening are common with age
  
• A leak at a single runner:
  • Introduces unmetered air
    
  • Affects idle more than higher RPM
    
  • Can cause a single-cylinder lean misfire
    

At higher RPM:

• Increased airflow velocity
  
• ECU fuel trims compensate
  
• Misfire disappears
  

Key indicators

• Cylinder-specific misfire
  
• No power loss at speed
  
• Often no audible vacuum leak
  

This is a known RX300 failure mode.

2. Fuel Injector Flow Degradation (Cylinder #2)

Why it fits the symptoms

• Injectors degrade unevenly with age
  
• At idle:
  • Injector pulse width is minimal
    
  • Small flow reduction causes lean misfire
    
• At higher RPM:
  • Longer pulse width masks the problem
    

Important distinction

• Injector may still click and electrically test “good”
  
• Flow imbalance is not detected without swap or balance testing
  

Common on early-2000s Toyota V6s using Denso injectors

3. Ignition Coil or Plug Weakness (Cylinder #2)

Why it is plausible but not first

• RX300 uses coil-on-plug ignition
  
• Weak coils often fail:
  • Under heat
    
  • Under load
    

However:

• A marginal coil can misfire at idle due to:
  • Lower ionization demand
    
  • Inconsistent spark energy at low dwell
    

Spark plugs are equally suspect:

• Oil contamination from valve cover leaks
  
• Incorrect plug type or gap
  
• Carbon tracking
  

Compression of 140 psi increases spark demand slightly, worsening a marginal ignition component.

4. Minor Valve Sealing Issue (Early Exhaust Valve Leakage)

Why this is lower probability

• 140 psi suggests mild leakage
  
• Would typically worsen with heat
  
• Usually causes:
  • Persistent misfire
    
  • Loss of power
    

However:

• A very small exhaust valve leak can:
  • Disrupt idle combustion
    
  • Be masked at higher RPM
    

This would require confirmation via:

• Cylinder leak-down test
  
• Relative compression waveform
  

5. EGR Flow Distribution (Least Likely but Not Impossible)

• RX300 uses EGR with internal distribution
  
• Carbon buildup can bias EGR flow
  
• EGR at idle should be minimal, so:
  • This is not a primary suspect
    
  • Only considered if others are eliminated
    

Step 4: Why Certain Causes Are Unlikely

Cause Reason Ruled Out Burnt valve Would misfire at all RPM Broken ring Would worsen under load Cam timing Would affect multiple cylinders ECU fault Extremely rare and non-cylinder-specific

Diagnostic Priority Order (Professional Approach)

1. Smoke test intake manifold (focus on #2 runner)
   
2. Swap injector #2 with another cylinder
   
3. Swap coil and plug from #2
   
4. Perform leak-down test on #2 if needed
   

References

1. Toyota Factory Service Manual – 2001 RX300 (RM785U)
   • Engine Mechanical: Compression specs
     
   • Engine Control: Misfire diagnosis logic
     
2. Toyota Technical Training – 1MZ-FE Engine Course
   • Idle combustion sensitivity
     
   • Intake manifold gasket failure modes
     
3. Denso Ignition Systems Technical Guide
   • Low-load misfire behavior
     
   • Coil energy vs cylinder pressure
     
4. SAE Paper 2000-01-0561
   • Idle combustion stability in lean conditions
     
5. Toyota TSB EG011-03 (Related V6 intake sealing issues)
   • Composite manifold gasket aging effects
     

Bottom Line (Experienced-Tech Conclusion)

Given:

• Cylinder-specific misfire
  
• Idle-only symptom
  
• Marginal but acceptable compression
  
• Smooth operation at speed
  

The most likely causes are, in order:

1. Intake manifold gasket leak at Cylinder #2
   
2. Fuel injector flow imbalance at idle
   
3. Marginal ignition coil or spark plug
   
4. Minor valve sealing issue (least likely)
   

If you want, I can next:

• Walk through how to confirm each fault in under 30 minutes
  
• Interpret fuel trim data specific to this failure
  
• Explain why RX300s show this exact pattern so often
  

 

All that makes sense (although a lot to read).
Given the information here, I think the next step would be the good old fashion, starting fluid spray test. While idling, I'll spray starting fluid around the engine to see if there would be any increase in RPMs. That would indicate a vacuum hose leak. I like to start with the easy stuff, first.
After that, it's on to the rest.
Known: new plugs, all good - coils, working - no smoke out the tail pipe - mass air filter, good - no power loss when driving
Just an annoying misfire at idle, which sometimes is gone completely.

I'll be checking more this weekend.

 

My money is on the coil pack.

 

I have to update this entire thread.
My first post I said the compression was 140, well I misunderstood him. It's really only 40.
We don't think it's a burnt valve only because it hits 40 and stays there.
Now I have to make up my mind to off it or look for another engine next year.