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Hi guys, bit of a long one but I thought I would share my story of a set of injectors that I purchased from eBay. They were supplied with an inaccurate data point which caused some tuning issues that left me scratching my head.
I have a turbocharged VW Beetle fitted with a Haltech Elite 1500. The car has been running for a few years with this setup so the tune is well dialed in. The car is flex fuel tuned and spends most of this time on E85. I tune the VE table in 4D with the last dimension being ethanol percentage. This way if the AFR is slightly out on petrol, correcting it won't affect the E85 map and vice-versa. I had an issue around 6 months ago when switching from E85 back to petrol. After filling up with petrol, I noticed the car ran about 10% lean of target on boost. This was very unusual as the AFR was usually very consistent regardless of ethanol percentage.
Over the next week I slowly blended E85 back in and the car was still running lean on boost. I was now close to E85 again and I had added over 10% to the VE table to get the AFR back on target. I left the car for a few days, drove it again and this time the AFR was 10% rich of target. I progressively pulled out fuel again and basically ended up at my initial VE values for the AFR to be correct.
I could not think of what caused this, and put it down to a combination of varying air temps and cylinder head temps (air cooled engine). The problem was, the magnitude of temperature difference that I was seeing between driving sessions should not cause an AFR swing of 10%. I basically forgot about the issue until 3 months later, the same occurred again.
I then looked into the issue further by comparing logs. I would overlay 2 logs that had CHT within 5 degrees, IAT within 5 degrees, identical boost, identical RPM and even identical VE table values as I chose to log this for diagnosis. One log would be on target while the other was 10% lean of target. The injector pulse width of the lean log was significantly lower. The only significant difference I could find between logs was the fuel pressure. When the AFR was on target, the logged fuel pressure was 2psi below expected, compared to the lean log where the fuel pressure was 2psi ABOVE expected.
I then went to my injector flow map and found the following values:
40psi - 1150cc/min
43.5psi - 1300cc/min
45psi - 1340cc/min
50psi - 1385cc/min etc.
There was an unusual drop in flow from 43.5 to 40psi. Initially I thought this was a data entry error, however I have confirmed that this was what the injector data specified. My target base pressure was 43.5psi. For most of its life, the car was running 2psi below base in the faulty data point. The ECU assumed the injectors flowed around 1200cc/min and the car was tuned this way. When changing to petrol, the actual fuel pressure slightly increased for whatever reason. At this point the ECU assumed the injector flow increased by 150cc or so and pulled fuel to compensate.
I have since changed the 40psi data point from 1150 to 1250cc/min, which was suggested by online flow calculators, and the tune has been perfect ever since.
Good reminder to sanity check supplied data.
For those playing at home, orifice flow is proportional to square root of differential pressure, if you are checking flow curves.
I would expect no less from eBay injectors. I have found many good quality parts on eBay, but never injectors. Most of the time, the Bosch style injectors that are found on there are completely unable to provide consistent low PW flow. Idle quality is impossible to tune because of this. Also, most of the time the coils used are wound poorly and lead to a low energy opening that varies from pulse to pulse.