Missfiring like its gone out of fashion.

Bluesman · April 1, 2016

I am not going down the M4 said my Lexie 1998 MK4 LS400

For the very first time since I bought Lexie 6/7 years ago today she let me down. I managed to limp her to the next junction misfiring all the way. No dash board lights on Called the RAC who put the computer fault finding thingamejig and it wouldn’t read the car. Mr RAC said that the car was to old but he knew a garage that could read it so off we limped. The reader said it was an oxygen sensor 1 or 2 so they get a couple of them and replace both, everything seemed OK then on the test run back to missfiring, the reader said Oxygen Sensor 1 or 2. They take all the plugs out that were only replaced with new ones last week they compression test all 8 cyl no problems then they look at the plugs all down one bank (left hand side as you look at the engine) the plugs were black where as down the other bank they are as clean as they should be. Any ideas?

mikeyv · April 1, 2016

Logically, it would seem to be something that splits two ways, lpg feed for example, and one side isn't working properly.

Sorry, but not familiar enough with the underbonnet area to make a more informed guess.

Good luck getting it fixed.

Not happy that RAC man didn't have equipment to read faults though.

steve2006 · April 1, 2016

The VVTI engine has combined coil/igniters over plug and these are all operated from the ECM from individual ports.

There are common connections to each coil with a blue wire/red stripe and then 2 outputs from the engine ECU which each feed 4 coils these being the plain yellow wire or the yellow wire with a green stripe, if you have lost one of these outputs it may explain why you have a complete bank of cylinders down.

It would be a case of checking the connections to the coils on the defective bank.

Also given the known problems with the engine ECU capacitors it may be worth investigating this also.

It does sound more ignition related as the black plugs indicate fuel is being injected but not being burnt.

Not sure if misfire monitoring is performed on the LS400 engine so the EML may not illuminate.

After the O2 sensors were replaced the car then drove OK for a short while then reverted back to misfiring, this could be another indication of a faulty ECU as those capacitors are affected by heat, could you leave the car to cool right down then restart and see if the engine behaves until warmed up?

The only other thing I can suggest is a blocked catalytic convertor on that bank, everything else like the AFM is shared across both banks so can be eliminated as a possible cause, may be an idea to get the garage to hook up a back pressure tester via the O2 sensor hole.

runsgrateasanut · April 1, 2016

On a single point system you'd need two Emulators for each four cylinders. Not sure on a sequential.

Perhaps a call to Profess Gas to find out. Could be one Emulator has failed, if you have two.

Just a thought that they may be able to give you some advice. Could be just a faulty Earth if you have Emulators?

Bluesman · April 1, 2016

Sorry should have said the problem is Petrol or LPG exactly the same.

Cheers for all that info Steve I will email it over to the garage tonight so they can try it all in the morning, What you say is very logical.

"After the O2 sensors were replaced the car then drove OK for a short while then reverted back to misfiring, this could be another indication of a faulty ECU as those capacitors are affected by heat, could you leave the car to cool right down then restart and see if the engine behaves until warmed up?" Yes exactly as you say.

Can the new Oxygen sensors, be blown, was the term used so that they don't work anymore. ?

I left them taking the crud off some terminals that go to the ECU.

Have a great weekend everyone Mike.

Bluesman · April 1, 2016

1 hour ago, mikeyv said:

Logically, it would seem to be something that splits two ways, lpg feed for example, and one side isn't working properly.

Sorry, but not familiar enough with the underbonnet area to make a more informed guess.

Good luck getting it fixed.

Not happy that RAC man didn't have equipment to read faults though.

I was a little concerned that there computers wouldn't work.. Steve has come up with some good ideas so have sent that across to the garage. Fingers crossed.

Bluesman · April 1, 2016

53 minutes ago, runsgrateasanut said:

On a single point system you'd need two Emulators for each four cylinders. Not sure on a sequential.

Perhaps a call to Profess Gas to find out. Could be one Emulator has failed, if you have two.

Just a thought that they may be able to give you some advice. Could be just a faulty Earth if you have Emulators?

The problem is on Petrol as well as LPG, thanks for the suggestion though. Mike.

steve2006 · April 1, 2016

I have the full OEM electrical manual for the series IV LS400, if you need a copy of the ignition circuits send me your email via PM Mike.

Bluesman · April 1, 2016

34 minutes ago, steve2006 said:

I have the full OEM electrical manual for the series IV LS400, if you need a copy of the ignition circuits send me your email via PM Mike.

Cheers Steve have PMd email. Mike.

runsgrateasanut · April 2, 2016

17 hours ago, Bluesman said:

The problem is on Petrol as well as LPG, thanks for the suggestion though. Mike.

Just a thought, (and I'll butt out after this) the emulator cuts the petrol feed when switching to gas. If it isn't doing its job it could it be that the side mentioned is overfuelling with gas and petrol hence not firing properly.

The Emulator may in a sequential be part of the LPG ECU. Worth a phone call to your friends at Profess gas possibly?

Bluesman · April 2, 2016

4 hours ago, runsgrateasanut said:

Just a thought, (and I'll butt out after this) the emulator cuts the petrol feed when switching to gas. If it isn't doing its job it could it be that the side mentioned is overfuelling with gas and petrol hence not firing properly.

The Emulator may in a sequential be part of the LPG ECU. Worth a phone call to your friends at Profess gas possibly?

Thanks Stuart, no need to butt out all ideas are gratefully received as one of them will be correct. Mike.

runsgrateasanut · April 3, 2016

Thinking about it in bed Mike, you could check that its not LPG related. Setting the switch to petrol and disconnecting the blue (live feed) wires to the solenoid on the Vaporiser and also on the seperate solenoid which feeds the Vaporiser. You've then cut off the automatic supply which come on with the ignition. If it then runs OK on petrol you've narrowed down the source to the LPG ECU.

The other thing which I stand to be coreected on, is the posibility that its the Coil pack. There is one for each side of the Engine and I've heard of them packing up on older Mercs.

Please use your own judgment if you want to try the test. I can't imagine it causing any problem but as the lawyers would insist I say, its "without responsibility or liability".

Bluesman · April 3, 2016

Cheers Stuart I will give it to the garage to try in the morning. All ideas are welcome. Mike.

The-Acre · April 3, 2016

5 hours ago, runsgrateasanut said:

Thinking about it in bed Mike, you could check that its not LPG related. Setting the switch to petrol and disconnecting the blue (live feed) wires to the solenoid on the Vaporiser and also on the seperate solenoid which feeds the Vaporiser. You've then cut off the automatic supply which come on with the ignition. If it then runs OK on petrol you've narrowed down the source to the LPG ECU.

The other thing which I stand to be coreected on, is the posibility that its the Coil pack. There is one for each side of the Engine and I've heard of them packing up on older Mercs.

Please use your own judgment if you want to try the test. I can't imagine it causing any problem but as the lawyers would insist I say, its "without responsibility or liability".

At least you weren't thinking about Mike in bed, just his car problem!!

On the MK4 each cylinder has its own coil pack, whereas the MK2 has as you say one for each side.

Titch LS · April 3, 2016

There should be an LPG fuse fitted that you can pull that will disable the entire system and allow the car to run only on petrol.

Spacewagon52 · April 3, 2016

Good luck Mike, I hope you manage to find the problem.

Bluesman · April 3, 2016

Thanks for all the support boys its much appreciated. Mike.

runsgrateasanut · April 4, 2016

19 hours ago, Titch LS said:

There should be an LPG fuse fitted that you can pull that will disable the entire system and allow the car to run only on petrol.

There are two on a single point system Brian, one to Battery and the other to switched ignition. Not sure if same on sequential? Fuse to switched ignition may be under the dash, but it would be another route to temporarily disable gas to check there is no duel fuelling going on.

Bluesman · April 4, 2016

Well that garage has given up so its up to Lexus tomorrow sometime unless I can find an ex Lexus engineer that can do the business. The Lexus garage is only about a mile from where the car is. Mike.

Scribe · April 4, 2016

Fingers crossed Mike. I'm sure you'll let us know what the problem was, once it's fixed.

I wonder when that RAC man last attended an LS400?

messi · April 4, 2016

I don't lexus will touch it as it's lpg converted

Bluesman · April 4, 2016

19 minutes ago, messi said:

I don't lexus will touch it as it's lpg converted

Some of them will but UK Teknics Ltd are looking best too sort poor Lexie.

cleverdick · April 4, 2016

I don't know if the ECU capacitors in the Mk4 are surface-mount - but as a general rule they do not age as well as good quality through-hole types. Certainly, due to their lower, wider profile (and hence greater unrolled length), their impedance is often slightly higher for a given C value.

However, a failing electrolytic capacitor will likely show abnormally high impedance (several Ohms at 100kHz) and much-reduced capacitance. But note that this is a chronic condition - caused by prolonged heat yes, but cooling won't improve matters once the damage is done. So I would suggest that failing ECU capacitors are not the cause of the trouble in this case.

Bluesman · April 5, 2016

She is off to UKTechnics in Maidenhead who are snowed under with work but they will do their best to get onto it as fast as possible. Updates to follow. Mike.