Friday, 14 March 2014

Diagnosing a lean condition using VCDS-Lite

I recently went on a rolling road day to get an idea of my current cars performance (Seat Leon Cupra R, 1.8T), and was happy to find it wasn't too down on power for a 9 year old car.

Unfortunately I was (ill-) advised that the engine was running lean (the air to fuel ratio was too high). There are several areas to look at when diagnosing this, but the common one tends to be an air leak in the crankcase ventilation system.

I sprayed all the pipes in the system with (flammable) electrical contact cleaner. If there is a leak, then the engine will ingest some of the fluid into the engine via the leak and it will combust, creating an audible RPM change. Unfortunately this didn't help in diagnosing, but did lead me on to purchasing a VCDS-Lite Registration ($99 / £60) that enabled me to use my KKL VAG-COM cable I got from Amazon for around £6. The free version does some basic stuff such as reading error codes, but the registered version provides extended logging capabilities.

By measuring blocks 001 (RPM) and 031 (Lambda Control Actual & Specification) it provided enough data to calculate the air/fuel ratio requested by the ECU and the actual fuelling measured by the lambda control sensor. Just to note, a faulty lambda sensor is usually easy to spot, as the figures provided by it are either flat-lining or erratic. The figure must be multiplied by 14.7 (stoichiometric) to calculate the AFR.

Anyway, after consulting with one of the tuners on, the 1.8T BAM engines typically run more lean on the standard ECU map than other cars do, which is the only reason I can think of why I was ill-advised to the running of the engine. To confirm this, I used VCDS to to log the above measuring blocks (in 3rd gear, from 2500rpm to 6800rpm) to log the data required. As you can see in the graph below, the fuelling is almost exactly as the ECU requests (as the manufacturer intended) - and where it differs is where the engine is running richer (lower air fuel ratio, therefore more fuel is being combusted). Running rich is much less deteritmental to the engine, and whilst it is an increase in fuel consumption, it also means that the fuel keeps the cylinder walls cooler. Running lean can mean detonation, which is very bad.

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