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3) Failed fuel pressure regulator can also cause over-rich mixture. If Susan is still listening we can help her find it and sniff the correct thin hose.
In God-knows-how-many miles over the past 30 years of driving and maintaining cars with Bosch fuel injection systems, I have yet to see a queefed out fuel regulator, but, I have seen them blamed for all manner of car problems. Years ago, I cured myself of reflexively pointing a finger at the fuel pressure regulator (FPR) after heeding the advice of an FI pundit who said that a Bosch FPR *must* be *spot* on or else it is junk. I was having some drivibility problems, so I check the FPR and found it to be a couple of PSI off spec. I knew the readings were accurate because I used a gauge which was as good as it gets - a high quality industrial item which had been calibrated in the instrumentation lab of a nuclear power plant. So I sprung for a new FPR. Because this was long before Al Gore created Internet shopping, the FPR was not cheap. Thinking I was about to enter FI Nirvana, I plugged in the new FPR only to find the pressure readings to be exactly as before.
I recently had the opportunity to check an FPR again whenever my daughter's 240 suddenly started to return 17 MPG. I tee'd my trusty gauge into the fuel rail and took some readings. I wasn't concerned that the gauge's calibration is about 20 years out of date because I have learned to only look for good or way-bad. Also, it allowed me to check the residual pressure of the system which can really expose a fuel consumption problem.
All was good pressure wise, so I turned my attention to the most spiteful task on a 240 - checking and, God forbid!, changing the FI coolant temperatue sensor. I want to meet up with the maniac who located that thing where he did - must have been a discontented engineer who got his goodies by thinking up perverse ways to bring misery to mechanics; he probably was Sven The Heater Box Designer. Anyway, I used to go to the trouble of pulling the FI temp sensor and checking its response in a pot of water as it heated, but I got bored with that approach after wasting time pulling one too many good sensors. Now I leave the sensor in place and read its resistance cold and then with a fully warmed engine. During this most recent foray, when I found the temp sensor to be reading 550 ohms fully warmed, I knew I had solved the problem.
I learned something: Previously I had thought that a thermistor such as the FI temp sensor either worked or did not work at all. I didn't realize that it could creep out of tolerance.
Check the FI coolant temperature sensor.
Rich
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