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While looking at some old links, I stumbled on our 2017 conversation about tank sender feedthrough failure analysis.
My trip to town is about three miles (5 km) and descends roughly 500 feet (150 m). When I dare drop the AW70 into neutral going down the hill, foot off of the accelerator, the large tach looks to be showing normal idle rpm as best as the resolution between 0 and 1K is on the gauge. The difference I think I notice since replacing the -561 with the -951 happens when it is in gear (as much as an auto is in gear decelerating) with the engine rpm dropping further than I recall it doing with the -561 which was native to our two '89 240s. That's my only evidence for believing the later ECU cuts the injectors based on VSS and TPS inputs at above-idle rpm. I hadn't considered load derived from the AMM.
That Porsche hybrid replacement is the second version of aftermarket I've seen. The first, I believe, was built with discrete surface mount parts by one of the ECU repair outfits here in the US. It impressed me that there could be a market to repay the R&D needed to reverse engineer and design a substitute for Bosch's hybrid on ceramic. The photo of bum hybrids collected is explanation enough!
One of our Brickboard users made a few dollars many years ago cruising the yards for -951s and white label -561s sending them to Europe, I believe. Mike was interested then in distinguishing the two versions of that hybrid microcircuit to verify the fix was in the white label (yet identical part number) ECUs.
The hybrid is the obvious difference between the pink label 5xx series and those with a white label. Of the 7 symptoms given for hybrid failure, the most common, and the one I've experienced, is #4, loss of fuel pump relay ground. Interesting it is, to note #6 in the list: RPM regulation at idle. Some of the same info was posted here about 17 years ago.
As Dave Stevens notes, the 5xx series enjoys a poor reputation, although my reading has not encountered any reliable anecdotes from the community citing specific problems with the white label units having the improved hybrid.
The idle symptom might be a nuisance, but more than that, its a brain teaser, much like the tank sender epoxy failures. Might take you some time, but you'll figure it out.
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Art Benstein near Baltimore
If there is a book you've wanted to read but nobody has written it yet, you are the one to write it. -Tony Morrison
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