Toyota Avensis '99 - engine performance problem

Question

My girlfriend and I drive an old Toyota Avensis that was produced in 2000. Its engine is Toyota's 3S-FE. Let me spare you my theories and tell you what symptoms arise:

1. When the engine is cold, it can barely rev beyond 2,000 RPM, and there is noticeable shaking of the exhaust pipe. The shaking stops when the engine gets to operational temperatures.
2. After the engine has heated up, it can rev up to 2,900 RPM, then starts getting very unstable -- it speeds up to a certain point, and it seems to "hit the ceiling" and revs down, this effect is exacerbated with increased throttle position.
3. Opening the throttle completely starts this violent revulsion and then quite quickly, the ignition seems to turn off and it just stops dead. Restarting is no problem. This happens with and without load -- the ignition will turn off on the highway, and you have to put the clutch pedal down so the engine stops, only then can it be restarted!
4. The engine is very weak, going up hills and such you can "feel it struggling" like anybody who drives stick does, you start getting the feeling that you need to downshift because the engine's power output isn't great enough. Accelerating on highways is dangerously slow because of this, both because you can only drive in the 1,000-2,000 RPM range which has quite poor torque, and because if you accelerate hard, you will get this same "hitting the ceiling" effect.

5. It wasn't always like this, this happened on a long trip through Europe (and a fair amount of time on the autobahn) -- it started out with the engine misfiring in the range 3,600-4,000 RPM (but not above!) Needless to say, we kept this misfiring to a minimum so as to avoid permanent engine damage.

6. Loud rattling noise from the generator in the range of 3,400-4,000 RPM

7. There is a bolt loose on the engine. We're unsure what it does. See picture:

   

The spark plugs have since been changed, along with their wiring, and I don't know how well it runs now. The spark plugs were previously all different brands.

My theories:

1. Exhaust pipe clogged somewhere. Engine unable to expel exhaust at the correct rate beyond a certain RPM.
2. Broken oxygen sensor/vacuum line leak/faulty MAF sensor. This would lead to a lean fuel mixture and explain the misfiring. Also vacuum line leaks could cause poor engine pressure.
3. Faulty spark plugs, or faulty timing belt. Could cause misfiring if the fuel isn't ignited properly or at all.
4. Faulty fuel pump. Could explain why the engine felt weak to begin with.

Answer 1

Possible problems:

• Bad catalytic converter. They tend to plug up and block the exhaust. This would explain why the engine will not rev. Test it by removing the bolts that hold it in place and run the engine. The difference will be noticed immediately.

• Bad ignition distributor. The inner winding could have shorted. Remove the cap and rotor and inspect for small pieces of winding.

Answer 2

Let's summarize the symptoms

1. When cold, engine struggles to reach 2,000 RPM, exhaust shakes
2. When hot, engine stuggles to go past 2,900 RPM
3. Snap-open throttle causes engine to die
4. Engine struggles with high loads
5. Issues started after the engine began misfiring at cruising speeds between 3,600 - 4,000 RPM
6. Loud rattling noise from the generator in the range of 3,400-4,000 RPM
7. Loose bolt near the serpentine belt

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What these symptoms could mean

• 1,2 & 4 are classical symptoms associated with an exhaust restriction, quite likely a clogged catalytic converter.

  The high backpressure induced by the exhaust restriction makes it difficult for the engine to pump gases through, which is the reason it feels like the engine's "bogging down" or "hitting the ceiling".

  The fact that the engine hits the ceiling with the engine cold further strengthens this theory; under cold-start enrichment the fuel management pumps in a richer air-fuel mixture, which means that more unburnt hydrocarbons would make their way to the exhaust plumbing, making the flow through the cats more restrictive.

• 3 & 5 indicate issues with air-fuel mixture.

  If AFR is too lean, the likelihood of detonation increases.

  If AFR is too rich, you will end up with unburnt hydrocarbons.

  My take is that there are multiple issues here, which will be difficult to pinpoint due to the lack of information. What follows is my best guess as to what's going on.

  The clogged catalytic converters tell me that the engine spends a lot of time running rich, yet you have experienced misfiring which I assume indicates the presence of a lean mixture.

  Symptom 3 seems to indicate that the lambda sensors before the cats are not reacting quickly enough. The ideal thing would be to monitor the trace of the sensor output to see if it is functioning as expected. If the sensors are good, the next paragraph may explain what's happened.

  The fact that symptom 5 has me wondering about something. It is quite possible that something like a bad batch of fuel or weak fuel pump running very hot (was the weather hot that day?) caused the lean condition which resulted in physical damage to the catalytic converters that resulted in the exhaust restriction.

  The easiest way to confirm if the cats are damaged this is visually inspect the cats to see if they have deformed due to high temperatures.

• Can't comment on 6 & 7

  They may be related to one another. Difficult to say from just that picture.