My 87 740 Turbo has been running rich for the past 8 months. New spark plugs would turn black within 10miles, then begin to misfire so that the engine stumbled, stalled, but would always restart, at least briefly. (Of course this car is too old to have an OBD system). All of the usual elements which can affect the fuel system/mixture were checked/renewed including fuel pump relay, fuel pressure regulator, radio suppression relay, coolant temp sensor, oxygen sensor. Voltages from the air mass meter to the fuel ECU were right on spec. Timing and cylinder compression were all normal. There was no evidence of an ignition problem except misfiring caused by the plug fouling. (When spark plug insulators turn black, they become conductors, so the electric charge runs down the insulator instead of jumping the gap and the cylinder doesn't fire.) I tried a reman fuel ECU ($60 from Programa) with no change. Finally, I read carefully in my Green books, which I bought with the car when new, that the ICU (ignition control unit) not only retards timing in response to knocking but also can richen the mixture, which can also reduce spark knock. Until then, it did not occur to me that the knock sensor could be involved but I then tried running the engine with the knock sensor disconnected, and the rich running entirely corrected! The knock sensor was abnormally flogging the ICU which then commands the ECU to lengthen injector on-time, producing all of the symptoms. Of course a new Bosch knock sensor would be the next step. Anyway, I think all of this shows why, as cars really became computer controlled since the 80s, an OBD diagnostic system also became essential.

 

The ability to communicate and see the data stream went a long way toward diagnosing vehicles. Back in the 80s, only GM had live data streams and the rest had a flowchart to test/replace parts until the problem was corrected.

Good news is that those knock sensors are very inexpensive.

 

Thank you, I didn't know GM had that capability in the 80s. I guess the Roger Smith era wasn't totally bad!

 

This is a update to my previous posts about my 87 740 Turbo running rich. It turns out the problem was not a defective knock sensor. The rich running had always been somewhat intermittent and soon recurred with the new knock sensor. I finally tried replacing the aftermarket air mass meter which I had installed after the engine rebuild. The car had run well with that for almost 10000 miles. I reinstalled the original Bosch amm which was on the car when new and was working fine before the rebuild. That finally cured all symptoms. I was surprised that the aftermarket unit (made in Taiwan) failed because it had worked well for 10000 miles and has essentially no moving parts. (The mixture adjustment screw on the air mass meter had not helped the symptoms.) I had also measured the voltage from the aftermarket air mass meter to the fuel ECU; it was slightly high but close to spec (within 1 volt). I far as I can tell, the Bosch units are not available new anymore. In any case, the car runs perfectly now.

 

I am confused...you are surprised that a Taiwan electonic part failed?

 

 

I'd never use Asian parts in important areas like sensors, any electronics for that matter! If not new, then better used, speaking from experience here, couldn't care less where the i phone is made--totally irrelevant!

 

Most of the vehicles people drive daily have lots of Asian electronics in them, usually with excellent reliability.