I received a check engine message a few days ago on my 1998 XJR. Codes were P0172 & P0175 indicating rich fuel. There was a restricted performance light on for about 10 seconds, which then went out. I monitored the fuel trims tonight while running some errands. I cleared the codes after the first few minutes. The light did not come back on. When driving steady at about 50 Km/hr, Bank 1 was showing long term at around-8.6 and Bank 2 long term at around -6.3. Short Term trims were generally close to 0, moving into + & - territory occasionally. As I drove longer, the long term trims improved to Bank 1 -6.3 and Bank 2 -4.7 During fairly hard acceleration the Bank 1 LT would go as high as -12.5, Bank 2 about -9.5. That improved a bit too after about 30 minutes of driving.

The car seems to run just fine, but I am not wanting to take out the cat. converters. Not knowing much about what fuel trims should be, I am hoping someone who has the knowledge will share it. I cleaned the K&N air filter tonight, but have not re-tested trims yet. If someone can advise on what numbers are considered normal, I would appreciate it.

Best Regards

 

Just a suggestion - I'd start with some very simplest of repairs first. Given the age of your car, and without knowing the quality of fuel that has gone into it over the years, I would:

1. Relpace the fuel filter
2. Add some high quality injector cleaner such as Chevron Techron - PER BOTTLE INSTRUCTIONS ONLY - DO NOT over concentrate it.
3. Clean the part and full load breathers.
4. Clean the MAFS - ONLY if it is visibly dirty.

Try these and going through a tankful of gas first. You could have some injectors stuck open.

Next - If it were my car - I'd ditch the oiled air cleaner for a factory specified paper element. I'm not a big fan of thes oiled filters for reasons that are typically too controversial for the forums, so last I'll say about it.

If all of these fail, I'd probably next do an injector clean and a plug change. If these fail to bring it in alignment, then I'd begin looking at more serious problems/solutions, but I think you're starting too serious.

 

The ECU is calculating these values based on the MAF input and the feedback from the O2 sensors. So based on the MAF input it assumes a certain amount of fuel to be injected, however when the engine is running in closed loop the fuel trim is adjusted according to the feedback it gets from the O2 sensors.

This basically means that either the ECU gets wrong readings from the O2 sensors, or wrong readings from the MAF, or a (or more) injectors do not squirt the amount of fuel the ECU thinks, or a bad combustion. This all assuming the ECU is working fine.

In your case the ECU corrects the fueling downwards compared to what it thinks the MAF sensor is providing.

Usually MAF sensors give lower readings when they become dirty, or when they are cleaned (too often), and have another effect in that the fuel trims go into the positive numbers, ie getting leaner.

What would help is to know is when the car is running in closed loop the exhaust has indeed the right lambda value by measuring it externally. This will then guarantee that O2 sensors work correctly and also that the ECU is working right.

Might be worth also checking your plugs, and check their color so you can see how each cylinders combustion is. If they are old then worth renewing, if not then at least you will see if there is an imbalance from the plugs, i.e. when in 1 or more cylinders richer combustions have taken place, it could mean leaking injectors causing the richer condition.

Then it might also be worth checking the fuel pressure itself, as when that is higher than normal, the ECU also needs to reduce the fuel trim.

Hope that this gives some thoughts as to where the culprit could be, lots to check…

 

Thanks Gents,

I am not overly concerned at this point, but frankly I was curious to know if the readings I am getting should make me concerned. Unfortunately, my ignorance is not bliss! I have heard that anything under +/- 10% is OK. If someone can tell me if readings like -6 to -8 are significant, and what I should expect to see ideally, I would appreciate it. Should all the trims be near zero and at what point does one take action? I cleared the codes & they did not come back after about 30 minutes of sustained city driving. Will take another look, as it did appear to be improving without any intervention. If the code returns, obviously I will need to solve it, but maybe it was a bit of crud in the fuel passing through.I only burn premium fuel, but one never really knows when a bad tank gets pumped in.

This could be a case of a technically ignorant user with a new tool in his hand. I would like to learn though, and my searching the web has not yielded much to give me any comfort on the fuel trim topic.

To follow up, I have run injector cleaner occasionally. About 5000 Km ago, I used GM injector cleaner, which was recommended highly by a mechanic. I changed the fuel filter after that. That issue turned out to be tensioners failing and the rough engine was cleared through my DIY of changing all the chains, tensioners, etc. A few times since, I put a dose of Lucas in but only as a preventative measure. I am not really sure if Lucas does any good.

I have cleaned the MAF sensor once, probably 10,000 Km ago. I have also cleaned the throttle body without removing it. Plugs were changed about 25K ago. The breathers appeared to be clear at my last look about 4K ago.

When I said I did not want to take out the cat converters, I did not mean to physically remove them, I meant I did not want them to be wrecked through running too rich.

I will start simple, and perhaps try a regular air filter, then some injector cleaner. If the symptoms persist, then perhaps having the exhaust tested is a good idea to verify the readings. Again, if the CIL does not return, should readings under 10% be worth much effort?

I had an earlier CIL indicating EGR flow malfunction, so that is what I expected to see yesterday, as I had cleared it twice over a couple of months. I had not yet done anything about that but did purchase new gaskets for the EGR and Throttle Body, thinking I would take them both off & try cleaning clean them... if I saw that message again. Again, I am not sure how effective that would be, but perhaps worth a try. The EGR CIL;s were about 1000 km apart. I am kind of wondering if there is any connection between the EGR & Rich codes. Forgot to mention that in my first post on this.

The car has 153,000 Km (95,000 Miles) on it. Is changing the 02 sensors as a general maintenance item a good idea? They obviously have a part in the readings, but does one simply change them at this mileage as a matter of course?

Again, the car is running quite strong and smooth and aside from the CIL, I would be none the wiser.

I suppose I could ask endless questions. Any further guidance would be appreciated.

 

egr not working can cause rich mixtures in 2 different ways. first of all it is an extra bit of inert gas injected into the motor at cruise, not injecting it can cause a rich mixture. Also no egr can cause nox and cause the ecm to richen the mixture to correct.

 

Thanks Brian

I will move the EGR cleaning up on the agenda. Hoping cleaning will be a fix, rather than a new valve, etc.

Still kind of curious about the point where a fuel trim reading becomes a concern. Are readings less than + or - 10% considered OK?

The EGR message was certainly intermittent once every 3-4 weeks. The trims have not returned to readings above -6.5 at cruising speed of 30 - 40 MPH. The rich mixture CIL initially came on at highway speed as I was coasting down a fairly long & steep hill.

 

10% would be on the border imo, in the sense that the tolerances are stretched, close to the ECU going to throw errors (not necessarily), at least I would start searching from where it comes, to be ahead of potential issues, but that is me.

Not sure if the EGR could be related, what was the DTC code you could for this?

On deceleration the EGR is active (ie open state), but thinking from logic ( so not from experience), the EGR would only add exhaust gasses with a very tiny amount of unmetered oxygen coming back, and that I would associate with a more lean condition (i.e. more oxygen is passing unmetered, so the O2 sensors would actually need to add a little fuel. But as said, this is just from thinking, not from experience. As I have blanked my EGR, I can check if in the condition where the ECU is opening the EGR (left it working), would also get a richer condition, maybe this weekend, will let you know.

My previous thoughts still count to aid the search for the root cause of the higher then normal fuel enrichment.

You should be able to find a shop that can measure exhaust gases, I personally would think this is relatively cheap and you can get at least a better idea of what parts are working as should and what not to aid the search for where hte culprit lies.

Hope this helps

 

Thanks again Avos.

I think you are right. I will take my car in to have the exhaust gas analyzed before digging into the the Throttle Body & EGR. The EGR message was P0400, so thought cleaning it up might help. I could clean out the pipe as well, if it will come off the manifold without a huge fight. I have replaced the EGR valve recently on my daughter's Ford Escape, along with the front & inline cat converter. The Escape's EGR is much easier to access, and it was less than half the price. (it also had a 1/2 " hole blown out the side of it! cleaning was not an option). An intermittently faulty coil was the root cause there. I put the original O2 sensors back on, but wondered about the wisdom of that too.

Did you say you removed your EGR valve? I assume that would require some ECM mods too. It sounds like you have seriously modified your engine. 600 HP has to be fun.

Thanks and Best Regards

 

I think I was just lucky with my EGR blockage; it seems that at least the 2000MY AJ27 SC engines do not have the MAP sensor in some regions, as that checks the flow of the EGR. In the North American market they do have the MAP sensor. I am not sure how the EGR flow is checked with your AJ26 SC engine as it doesn’t have the MAP sensor, so all I can think of it would check if there is a change in airflow based on the MAF sensor, so that is difficult to tweak. However I am still trying to get the EGR blanked on a STR with the MAP sensor, and if that works, I am sure it can be made to work on other engines as well.
Just speculating here: If the EGR isn’t blocked, nor the pipe (be careful with that one not to move it too much, the harmonica part is fragile to crack and very expensive), then thinking about the how the ECU is probably checking the flow via the MAF sensor, (and external test of exhaust gases are ok) then it could be related to the MAF sensor input (airflow/temp sensor).
It could still be unrelated, and it could be a multiple combination of factors, so all I can suggest is to tackle the easiest/cheapest first and try to eliminate as much as possible.

 

Thanks again Avos,
I did take your advice, starting with the simple approach. I cleaned the air filter, which seems to have corrected the issue. You mentioned discarding the K&N that I have been using, but my easiest step was to clean and re-oil it. The fuel trims have been near zero on the 3 times I have monitored them since my last post. In fact the one that was in negative territory (Bank 2 - Long Term) seems to be reading between zero and +3.2 The others are all fluctuating slightly, near zero. No CIL's so far. I am hoping the issue is behind me, but a bit more time will tell. I am also hoping the EGR P0400 code I originally got was related. I have not ventured into cleaning it yet. I hope I do not need to. If not, and cleaning would rectify the P0400, that would be great, as a new one appears to be pretty expensive.

When cleaning the K&N, I re-oiled after cleaning and took care to apply the oil evenly, but sparingly, to the inlet (bottom) side only. When I opened up the air box, there was oil residue in the bottom, so perhaps I had been over zealous with my previous application. If that is the case, it may serve as a cautionary note to others who use K&N. Will have to read up on the K&N issue you noted, as I had not been aware of any controversy around them. I will see what transpires, as the prior CIL's were not frequent, however the fuel trims seem to be better.

If my hunch is correct, negative fuel trims could be one indicator that it is time to change an air filter.

 

You probably meant Stevetech ;-)

Nevertheless the issue you mention is the only issue I have with the K&N filter, as you need to follow the instructions carefully to avoid over oiling.