Fuel Trim Question

So my car engine was replaced under warranty, it has the large IMS bearing. Before I verified this, I was researching the IMS Solution and as part of this I read the pre qualification instructions for the install, as well as watching the youtube video. One of the items to check for is the "fuel trim" the comment on the video from Raby states this should be 1.04 or lower, my car is showing 1.06 on Bank1, Bank 2 shows 1.04 but this was 1.06 for a bit before going down. Can some one provide some feedback on this, what am I looking at?, What can cause this anomaly? This was capture with Durametrix.

The engine is running lean, so the DME is adding fuel to compensate. (More air is entering the system than is being reported by the MAF) What are your RKAT, FRAU & FRAO numbers on both banks? Here is where I would start: If you have access to a smoke machine, that's the easiest way to find intake system/ vacuum leaks. (If you're in the Houston area, hit me up!) Clean (or replace) your MAF.

I am out at the office now, will post the other reading later today. Unfortunately I am very far from you, South Florida. LOL

South Florida has lot's of smoke machines

My smoke machine's use has been: 50% turbo boost leak detection 25% EVAP leak detection (using nitrogen, not shop air) 23% Lean condition troubleshooting 2% smoke detector testing Find a shop that specializes in turbos. I've smoked a TON of BMW N54/N55, and Subaru engines. There always seems to be boost (BMW 30FF) codes once people start hot-roding their engines. Also any place that has strict emission testing will have one to find EVAP leaks.

Here is the RKAT, FRAU & FRAO readings.

Since your RKAT readings (fuel trim when engine is at idle) are very low 1~2%, I don't expect any intake air leak. All the other numbers look good to me too and I would say nothing to worry about. 1.06 FRA is not bad. Perhaps change your fuel filter if it's a MkI and also run some fuel injector cleaner.

What are your MAF numbers at idle on a warm engine?

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Ahsai is correct. Most vehicles don't throw a DTC until 10% (1.10) so it's not a horrible lean condition, just a small one. Fuel filter or fuel injector cleaner might clear it up.

Here are the MAF numbers with the car hot. Will change the fuel filter and run an injector cleaner. Is there a prefer cleaner outhere?

to clear things up, my understanding is that your car will not return a real frao number unless it is a turbo. frau is the closed-loop long term fuel trim number (just called fra for us non-boosted folks); ecu will throw a code if outside of the .7 to 1.32 range. again as per ahsai, rkat is the idle long term fuel trim, and ecu will code if outside +/- 4%. fyi, your car's computer works as follows: there is an airflow sensor in the intake that determines how much air is coming in. the computer takes that value and looks it up on a fuelling map which determines how much fuel should be added by the injectors. the o2 sensors on your car monitor the products of combustion in the exhaust; if there is too much unburnt air or too much unburnt gas then the air/fuel mixture is incorrect and the computer adds/removes fuel (trims) it to correct. initially short term fuel trims, if the condition persists then the computer converts them to long term fuel trims. also, your car uses narrow band o2 sensors that only work in a … narrow band … around the ratio of 14.7 parts air to 1 part fuel; anything significantly outside of this and the sensor is no good. your car's computer tries to maintain 14.7 to 1 during idle and cruising, but when you are hard on it (engine hot, full throttle, max rpm, etc.) then the computer adds a bit more gas to keep things cool and prevent damage - it richens the air/fuel ratio to around 12.5:1. this is outside the operating range of the narrow band o2 sensors so the computer no longer has any feedback as to whether combustion is happening as per intended; this is open loop operation (computer only knows inputs - fuel, airflow, throttle position - and not outputs). not a big deal as computer is throwing enough fuel at it to prevent damage (damage occurs when there is not enough fuel to quench, things get hot, and holes get burnt in piston tops). in turbo cars maintaining the proper air/fuel ratios is much more important so they got wide band o2 sensors that operate in a … wide band … and can return frao numbers to properly richen the fueling at wide open throttle, etc. a fuel trim over 1 means the engine is adding more fuel than the computer fuel maps say is required; this is due to (a) fuel system not performing to spec (not enough gas) or (b) more air than expected (air leak, wonky air sensor). realise that this whole trim system is designed to address components going out of spec as the engine ages. as stated by ahsai, if there was an air leak it would have more of an impact at idle when the engine is moving less air (a 3.4 litre engine is moving 1400 litres per minute at idle but 6800 at 4000 rpm; a 100 litre per minute air leak would have a 7% impact at idle but only a 1% impact at 4000 rpm). your trims seem fine.

Thank you, this is all very informative and educational. My car has no symptom or issues, I was just checking thing (I should know better, if it aint broke dont fix it, LOL) and notice the FRA numbers at 1.6, I remember seeing a video from Jake Raby in reference to the IMS Solution, on this video they reference to an FRA of 1.06 as been a problem. Jake has some other posts around here commenting on how he has seen multiple engines blown up do to lean conditions caused by weak fuel pumps, so the FRA number kind of made me curious. This car has 101,000 miles, I have all the service records but do not recall seeing a fuel filter listed, so it may be original. I am wondering if the fuel pump is something to be replaced as a preventive item also.

Your FRA is 1.06, not 1.6, which will definitely be a problem.

Yeah no kidding, my mistake it is 1.06..Just edited my post. Thank you good catch.

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now there is the question. the ecu is not magic; it monitors a number of things (misfires, airflow, products of combustion) but is relatively poor at monitoring fueling. the only indicator of fuel system performance is the o2 sensors and their measurement of products of combustion in the exhaust. now, the ecu has say 5 modes of operation: start up - a special fuel map designed for engine start, with tests of the catalytics, air injection system, etc. idle - a fuel map designed to keep engine running when no throttle applied. closed loop - this is when the car is cruising and computer is trying to maintain air/fuel mix of 14.7:1 with feedback from o2 sensors. open loop - car is working hard and computer adds a bit more gas to keep things cool; no feedback from o2 sensors. limp - computer has decided something is broken (airflow sensor, o2 senors) and relies only on throttle position to determine how much gas. thinking only about closed and open loop, the computer can 'trim' the fueling in closed loop to address aging components using feedback from the o2 sensors. the computer will trim up to 1.32 before it throws a code. note that this is not 32%, and i really don't know what that number means other than it is relative to a setpoint of 1. in open loop the computer richens the mixture to 12.5 to 1. this is an increase in fueling of (44.7-12.5)/14.7 = 15% one question i do not have the answer to is whether the computer uses the fuel trims from closed loop operation to determine fueling when operating in open loop (ie, it says i had to add this much extra gas in closed loop, so i should add this much extra gas in open loop as well) or whether it says that i am throwing so much extra gas at it in open loop that i can ignore my closed loop fuel trims. if the latter is true then there may be a situation where the computer is almost at max fuel trim in closed loop (ie, 1.3 or so) because of a failing fuel pump or whatever, and can't add the 15% extra fuel at open loop, gets no feedback that this is in fact the case, and cooks the engine. what makes this more of a concern is that problems with the fuel system (injectors, pump, old filter; fortunately the fuel pressure regulator tends to fail to too much gas) become magnified the more you ask them to put out; ie, a failing fuel pump may work fine when cruising around, but when asked to put out lots of gas when you step on it, it underperforms. and you getr no feedback that this is the case. so, either the fuel pump becomes a replacement item (much like the water pump and ims has become) or you get a pair of wideband o2 sensors and monitor your fuel system performance yourself.

Thing is the ECU can ask for a particular amount of fuel to be delivered, but if the fuel pressure is not accurate for what ever reason, pump going bad, clogged filter, the ECU has no way of telling and thing will just work until something blows up because of over lean condition. Same I guess if the injectors are not working correctly. Am I wrong on this?

Are you trying to look for problems?

that's my understanding. you would get a code it if was happening in closed loop, but not in open loop. thing is, the fuel pump *should* be delivering the same amount of fuel regardless of demand (ie, whether in open or closed loop) but who knows, esp with the 'lifetime' fuel filters in the second gen 996.

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LOL, yeah I guess this is what happens when work is slow.

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So I tried to buy a fuel filter and the local shop tells me my car does not have one. I understand is "life time" but can it be changed regardless? Is the filter and integral part of the pump?

Only the early 996 had a replaceable fuel filter. Many/most/all? newer vehicles have non-replaceable "lifetime" fuel filters these days. The "lifetime" filter is replaced when you replace the fuel filter. I've seen the filter material being sold separately for other cars. It's kind of like a sock that goes on the end of the pump. The ones I've dealt with (BMW) were very easy to replace. I don't know if that's what the later 996 has or not.

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The primary O2 sensors will identify a lean condition and continue to up the duty cycle of the injectors until the maximum compensation is reached (ecu limit). Somewhere during this time, the ecu will trigger a CEL. If you choose to ignore the CEL and run the car despite the "smart" ecu warning you, then you could damage your engine. Likely, the ecu would go into limp mode, as self preservation as well.