"Fuel Trim Limit" code - Brill {P}
Volvo V70R

The 'engine management light' came on earlier in the week, had it investigated and the code came up as "Long term fuel trim limit". The light was extinguished by garage (two days ago).

Anyway, today the light has come on again, so I'll need to take it back.

Does anyone know what "Long term fuel trim limit" may mean in normal speak?

Do I need to start worrying?

Stu.

(just to add to the fun, the loan car they lent me broke down, ho hum).
"Fuel Trim Limit" code - Andrew Moorey (Tune-Up)
The joys of technology!! In simple terms The computer is programmed to adapt its fuelling and ignition strategies as time and miles clock up and components wear. What has happened to your car is that the adaptation limit for the fuel mixture has been reached and the computer is unable to correct the mixture further. This is where the diagnostics come in, this state has been caused by some fault in the engine management system which has caused the fuel mixture to become over rich or lean due to any number of reasons such as a faulty coolant temp sensor or fuel pressure regulator for example.
What we would need to do is to carefully check all the inputs to the ecu against the outputs to the actuators. My hunch would be either the coolant temp sensor or possibly the O2 sensor has gone lazy.
Andrew


Simplicate and add lightness!!
"Fuel Trim Limit" code - Brill {P}
Many thanks Andrew,

I got the car back today, apparently the code pointed to the catalytic converter but no problems found here or in fact anywhere else. (can a cat be checked, or is it only when it fails completely that you know is is U.S?)

They suspected dirty fuel, but this is the second time, and I've been using Optimax so not sure myself. I'll be using the new BP brew and see what happens over the next few days.

Cheers,
Stu.
"Fuel Trim Limit" code - pettaw
If its the same as the 850 T5s then round the turbo manifold there's two pieces of rubber tubing that perish and cause an air leak. That might well be it, but I would post on the Volvo Owners Club forum. They might be able to help.
"Fuel Trim Limit" code - Brill {P}
Needless to say the light has come back on for a third time (each time a day or so after being reset.) The last time apparently they reset the computer back to its default settings. Yawn, yawn, yawn.

Stu.
"Fuel Trim Limit" code - Brill {P}
Sorry to add further, but just heard:

The garage have just told me they want to replace the cat .

Any ideas on a fair price for this including labour? (London).

Also will this work affect the rest of the existing exhaust system at all, or does it cut and shut?

Stu.
"Fuel Trim Limit" code - Peter D
HAve the garage check the pre and post cat emmissions to ensure it is the cat that is a way. Regards Peter
"Fuel Trim Limit" code - John S
Brill

Given that the last bit of engine management control is the lamba(oxygen) probe, located BEFORE the cat, how do they know the cat's faulty?

Regards

john S
\"Fuel Trim Limit\" code - Brill {P}
John,

The trouble is that when it gets to this point, if you\'re not all that technically sharp (and they can spot you a mile off) garages can tell you whatever they like.

To be fair to them they have agreed to do further checks, but have ordered a new cat anyway. I don\'t feel sufficiently confident to argue the toss on this with them. I\'m sure others in this forum would be able to push for more answers, but they are the ones answerig the questions here, not asking them like me.

Sorry can\'t answer your lamda question, which seems perfectly legit., which has me thinking....

Thought:
Perhaps there is a opening for someone to create a new profession, as an intermediary agent for someone to handle the garage on behalf of an owner. Charge a fee to an owner, and require the garage to report to them instead of the owner. A garage is less likely to say \'it\'s the throttle bypass flange scuttle plate\' if they know the agent will say \'scuttle plates only appeared after 1967\' . You get the idea.

Of course I won\'t prejudge their (third) report, nor am I at this stage criticising them.

Stu.
\"Fuel Trim Limit\" code - John S
Brill

I\'m glad they are doing furthur checks, but I was raising a serious question.

The way the system works is that the catalyst must get the correct air fuel ratio for it to sucessfully convert the CO to CO 2 etc. The lambda probe sits before the cat and continually signals back to the engine what the exhaust gas oxygen content before the cat is and thus the engine management trims the fueling. Provided that\'s right, the cat will then work. Now, there\'s no check of the exhaust after the cat in normal operation. So a faulty cat will not show up on the car\'s instrumentation (Some american staes are pushing for this sort of post cat check, but your car won\'t have it), nor will it drive the engine management.

So, the only thing driving your car\'s engine to adjust itself is the exhaust gas before the cat. It may be that the garage has checked the tailpipe emissions and found them out of spec. However, even if the cat is faulty, then its replacement won\'t cure the engine management problem, as the tailpipe emissions are not fed back to the engine management. As you\'ll realise though, if the engine management is faulty, then it will give the cat the wrong air fuel ratio, so the cat won\'t work properly and may appear faulty.

In a nutshell even if the cat is faulty its replacement won\'t cure this problem! Their effort need to be concentrated on the system before the cat.

Regards

John S
"Fuel Trim Limit" code - Brill {P}
John,

Many thanks, very informative and clearly put.

I'll post back with the result from the workshop, (another week away) but this info will at least give the impression that I know one end of a car from the other!

Stu.
"Fuel Trim Limit" code - Tony N
Now, there\'s no check of the exhaust
after the cat in normal operation. So a faulty cat
will not show up on the car\'s instrumentation (Some american staes
are pushing for this sort of post cat check, but your
car won\'t have it


This is incorrect, OBD (as of Euro Stage III I think) requires monitoring of the catalyst with a post catalyst (or mid brick depending on catlyst architecture) oxygen sensor. Therefore modern cars have TWO oxygen sensors. If the system detects drop in cat efficiency under a preset threshold (by comparing the outputs of both sensors) then the engine malfunction lamp will imuminate as a result of a CATALYST malfunction.
"Fuel Trim Limit" code - John S
Tony

Ah! Didn't think that would be there! Many thanks.

However, even if this system is showing a faulty cat (and I don't think from Brill's post the a cat fault is indicated), that defect wouldn't adjust the air fuel ratio, I believe. It would surely just indicate a faulty cat - as you say. You can't compensate for a faulty cat by adjusting the fuel air ratio. The fuel air ratio has to be right before the cat, and then the cat works properly. The problem here is that the fuel trim limit has been reached, and I don't believe this is affected by the condition of the cat - I think it must be related to the signals being received before the cat, which are indicating that the fuel air ratio is wrong. What do you think?

Regards

John S
"Fuel Trim Limit" code - Tony N
Hmmm difficult to say, the fact its taking a day for the MIL to come back on points to the cat as it takes a little while for the ECM pick it cat problems up. Any dodgy sensors would most likley have been picked up by an associated diagostic code so its difficult to say whats going on. I know the downstream O2 Sensor does influence fuel trim so it may be that something going on with the cat is causing the ECM to exceesively tweek fuel trim as a result of the signal from the downstream O2 sensor - I'm not sure what could cause this though as my understanding of catalysts isn't great. Life was so much simpler with carbs and points....! (yeah, right!)
Update - Brill {P}
"My hunch would be either the coolant temp sensor or possibly the O2 sensor has gone lazy" (Andrew Moorey)

Well the news today is that the latest code was for the front O2 sensor, this has been replaced and it's fingers crossed time. The cat. was inspected, and described as 'mint' so was not replaced.

Stu.
Update (O2 sensor?) - Brill {P}
24 hrs since the new sensor, the light is back on.

Just looking for a bit of sympathy!

Stu.

ps. will it burn faster if I douse it with Optimax or ultimate?
Update (O2 sensor?) - Tony N
Probably a harness fault, normally is 9 times out of 10. It's always easier to blame sensors though, as there easy to change.
 

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