I do love getting my head around a problem car, especially when its been to other garages for weeks!
This ST200 came in with a lumpy idle and occasional misfire, as well as stuttering on a part throttle.
The usual process of diagnosis is plug it in like anyone else, and monitor the live data.
But with the absence of any codes, its important to know what the live data means and what it should be for a particular car (thats the specialist part)
The long term fuel trims at idle seemed fine, as were the short term trims. It was following lambda, MAF values within tolerance - It all looked good, but it didnt sound it.
Mode6 is a particulary usual tool for isolating misfires on individual cylinders, but we couldn't read it, in this instance, I updated the firmware to the latest version for an ST200, and we could then see what was going on.
Cylinder No.1 was showing the most signs of a potential issue, the plugs had only been replaced last week, but the ignition coils were very grubby and showed some signs of green corrosion, fitting a spare used set helped, but cylinder 1 was still not happy.
Even though the plug looked fine, I replaced just that one plug, and this seemed to resolve the off beat tone from the exhaust and a clean misfire counter.
Before we get to the road logging side, its important to look at the history of this car.
It didn't have its original engine, at some point being replaced with a JTJA engine from 2014. It was very clean internally with no signs of heat stress or cracking, and compression was excellent, but that meant we had an engine with much older components with an unknown mileage adding to the variables.
We made the owner aware, and carried on with further testing.
Straight away the car felt flat, and a very distinct misfire/hesitation was felt on a part throttle below 2k, data was obtained with a WOT pull in 3rd gear, and another in normal driving to replicate the behaviour so it could be seen on the logs.
Something was clearly not right, with injectors maxing out at +25% on short term trims, and the long term trying to pull all that fuel out, with -21% a typical figure, and the part throttle showed a lambda registering lean when requested to richen with zero trim response.
After looking at all the usual, ruling out air leaks, dirty air filter/MAF, even a blocked catalyst or an injector issue, I decided to disconnect the pre-cat lambda sensor to watch the behaviour, it was then I saw a change.
Once the lambda sensor was removed, it was obvious it had an issue, with a very light tan, almost white appearance .
Again, a good used sensor was fitted, checked at idle and then road tested again, and it was perfect, doing bang on the 163 g/s MAF we would see from a 200, no stuttering and no misfires.
This did take me a few hours to get to the bottom of the problem, I was fair with what was charged and kept the cost down where possible, what the owner now has is a car that finally drives right, and money can be spent getting that cambelt replaced and the leaking cat back sorted, then we can talk about tuning!
I enjoyed this one, putting together patterns and behaviour where I've simply spent way too much time on these cars the past 12 years but the customer benefited - Immense job satisfaction!
