High emissions

QI have a 1994 JDM import Toyota Celsior UCF11 with the 1UZ-FE engine (aka Lexus LS00) that was purchased cheap as a non-runner.

The main fault was a misfire and the last owner had spent hundreds of pounds in an effort to cure it at two different garages. The rotor arms, distributor caps, HT leads and spark plugs had already been replaced when I acquired it and I have rechecked all of these components.

Reading the OBDI codes suggested both pre-cat O2 sensors were faulty and I confirmed they had open circuit heaters, so they were replaced, the codes cleared and have been fine since. Nevertheless, the misfire remained and I diagnosed a defective passenger side ignition coil pack which, when replaced, cured the problem. The engine now starts fine and settles to the correct 650rpm tickover once it has fully warmed up.

The Celsior was then taken for an MOT and it failed on excessive CO emissions at fast idle (the reading was 1.1%). Following the OEM workshop manual, I have checked the operation of both O2 sensors (upstream) and confirmed they are producing the correct output both on an analogue voltmeter (needle flicks 8 times within 10 seconds) and oscilloscope waveform. I did suspect that the cats may have been damaged by the misfire but apparently these engines will still pass an MOT with them completely removed.

One other thing which may be relevant is that when holding the revs at a constant speed of, say, 2000-3000rpm you can hear a low rhythmic drone from the engine which appears for a few seconds, disappears then continues in the same cycle. My MOT tester even mentioned this, saying the engine sounded “unbalanced and not smooth as these engine should”. I have another vehicle of the same model and these engine are normally silky smooth.

All vacuum pipes have been checked and the air filter is a new replacement. The ‘check engine’ light is not illuminated and there are no stored codes present when E1 and TE1 are linked in diagnostics mode. The other MOT fails were the front LBJS and the front nearside lower suspension bush (inner) obviously these are bolt off/ bolt on replacements, but I don’t want to replace these for the retest and still have the emissions problem.

As a member of the LOC UK, it was suggested I take the car out on a good high speed run to clear out the engine, but having no MOT, RFL or insurance, this is not a good idea.

Any help or advice gratefully received. Steve Eardley

AHaving carried out the tests as listed, I would draw the same conclusion as you did: the cats have been damaged from the engine’s past misfire/running problems.

I can find no evidence to support the information that you received that the emissions levels can be obtained without the use of the cat, unless that comment was referring to the earlier levels of 3.5%. The reading you are getting of 1.1% would, I believe, be those expected of the exhaust pre-cat levels.

I would ensure that the vehicle was fully up to running temperature before taking it for the retest – the catalytic convertor will not operate otherwise. With this in mind, I would take a detour to enable a longer journey before the test. It is legal to drive the vehicle to the testing station without an MOT or road tax, but you do need insurance for the vehicle.

As you say, the fact that the engine feels unbalanced is unusual for this model, which is normally smooth running. Although listed as a four-hour job, it may be worth checking the valve clearance settings to ensure none are tight.