V6 engine fixed at last !

[New S Wales 2010]

Thanks to all who gave advice on my recent V6 problems (mis-fires / rough idle / high emissions). I should have listened to frequent posts that say the most common problem on the V6 is the HT ignition. The “holy trinity” is the distributor cap, rotor arm and HT leads including the coil lead, so if you have any doubts it's best to change all three. Even if they haven't failed yet it will give you less chance of developing a problem in future. Be aware that symptoms can be strange (my van was fine from cold for the first 1-2 minutes, then strated sporadic mis-fires on all cylinders, but would always smooth out above 3000 revs, which I still can't explain). At any rate I changed all three of the above and cured the problem. If only I had known that a month ago !

After fixing the mis-fires I had very bad emissions as the ECU was running open loop. This turned out to have two causes:

Garage did a smoke test and found vacuum leaks on the air intake system. One was just a clip on the duct downstream from the air mass flow meter, so simple to fix. The other was from around the throttle position sensor on the drivers side of the throttle body. This was strange as there doesn't seem to be a seal in the original Mazda design here. Considered fitting an O-ring, but the collar on the sensor is not flat, so resorted to a sealing compound applied between the sensor and the collar on the throttle body.

The lambda probe had also failed. This is not surprising as the life of older probes can be around 50-60 k miles. Modern cars would give an alarm for this, but due to the age of the Bongo design you don't get an alarm or even a fault code. The first you are likely to know if your lambda goes is when you fail the emissions test at MOT. My garage fitted an NGK replacement and all now runs fine and in spec.

Finally for the real tech-heads, I had mentioned a VRIS solenoid #1 failure code which is a very common fault on the V6 engine. There is a vacuum solenoid that sits on top of the inlet manifold, connected to a diaphragm on the front corner. This is supposed to operate a butterfly damper which adjusts the “resonance” in the manifold above 3200 revs. To test, with the drivers seat up put your hand round the front of the manifold and rev the engine. You should feel the damper move as you go between idle and higer revs if all is working properly. On most cars the solenoid will have failed so nothing will happen. Advice from my garage was not to bother fixing it, as he doesn't think it will make any difference to the engine running, and disturbing the vacuum connections to the solenoid could even introduce more vacuum leaks ! Comments from experts are welcome.

Lastly, if you are in South Wales area I would recommend the Fairwater Garage in Cardiff for diagnostic work. They are always busy with the cars others cannot fix, so service was very slow (had my van for a week, although they only used four hours labour in total). However, they do seem to know their stuff, and had enough diagnostic equipment and expertise to get the V6 back to the beautiful running machine it should be.

[mikeWalsall]

Excellent post .. I have printed it off and put it in my V6 Bongo info folder ...!!