When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
check engine light on, something is wrong with fuel trim
Hi,
After replacing the intake manifold I started getting check engine light. The codes point to incorrect fuel trim level in both banks. I reset the check engine light 2 weeks ago. Today it is on again. I looked at the trim levels and they look ok. Long term trims were 3.1% and 2.3%. Short term trims fluctuate. If I push the gas, they can go as high as 20% for a moment, then go down to 10% and below. I reset the light again. The long-term trims reset to 0.
Do you know how to diagnose this further?
My thoughts: Both banks seem to respond together. So, it is not a problem with one of the two banks. If there is an air leak, I would expect for the problem to persist. Also, I would expect the long-term trim to reach extreme values, and not to stay at 3%. I feel that the check engine light comes on when some "short term condition" occurs. The problem is probably with short-term trim.
What sensor data is used to calculate short-term trim? Are there any other sensors except oxygen sensors that are used? Is MAF data used? Is there a sensor for throttle opening? Is there any fuel volume sensor?
Maybe if I look at the sensor's data, I could see which one is not responding properly.
Looks like you have an air leak, but a bit strange that your long term fuel trims are ok. I expected them to be higher if I look at the short term fuel trims.
The O2 sensor(s) is responsible for fuel trim, the long term fuel trim is a derivative from the short term fuel trims. If the fuel trims started misbehaving after maintenance, I would at least double check that it has been done properly (gaskets etc). If you have an airleak, you can try to spray brake cleaner or so around the intake manifold and see if your idling reacts to it.
If that is ok, how old is/are the O2 sensor(s)? They usually have a maintenance interval, which almost no one respects...
I have 3 new O2 sensor and 1 old O2 sensor. I don't understand how one old O2 sensor could always cause problems in both banks at the same time.
Do you know how to interpret the last 2 errors? What are the 4.5% limits? Before replacing manifold the car had long term trims around 9%, and there were no errors, so these should not be limits on long-term trim. The long term trim values in banks 1 and 2 always differ some, they are never exactly the same. These last 2 errors have identical values.
If I have an air leak, I don't understand why the errors might not show up for a few days.
To spray the brake cleaner, I need to remove the air filter and expose MAF. Would that affect idling?
2025 Maserati Grecale Folgore and 2024 Jaguar F-PACE SVR
I once accidentally fouled two brand new O2 sensors with antiseize. Once that stuff gets into those little holes it aint ever coming out! I mean, the stuff's designed not to burn off.. so expensive lesson. I also had a similar problem once after cleaning a Bosch ETM which used a metal gasket that was actually more like a crush washer. I never heard any vacuum leak but car definitely idled a little weird.
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Slideshow: A one-of-one U.S.-spec Mercedes-Benz SLR McLaren Roadster became even rarer after a factory-backed transformation at McLaren's headquarters.
