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Old 11-29-2020, 12:34 PM   #1
Jhebbard
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Question 740 Throttle Pump

OK, I know my 1992 740 turbo LH2.4 only >simulates< throttle pump action. My question is, how does it do this?

I've been trying for several years to rescue my (former) daily driver from the throes of death caused by old age and sitting outside i Florida's heat and humidity. Little problems like the relaxing spring tension of silicon-bronze spring clips and the like are causing intermittents that suggest most of the problems are electrical connections.
I did replace the AMM which seemed to help for a while.
At present the engine (a fully rebuilt 1993) starts nicely, and the air-fuel meter (F-A M) shows a gradual progression from lean (red) to nice yellow ("normal") as the coolant temperature sensor warms up. But here (and anywhere during warm-up) stabbing the throttle stalls the engine as the F-A meter plummets to full lean. Releasing the throttle restores a nice idle at "normal."

My question: what function drives the mixture rich on quick throttle action, the AMM, the throttle switch, obviously not the on-board O2 sensor-it's too slow. Does anyone have a print-put of the Bosch software this thing runs on?

Any help will be appreciated. I'd love to re-chip this beast (it has a Garret turbo) but it needs to run before that investment.
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Old 11-29-2020, 03:01 PM   #2
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At present the engine (a fully rebuilt 1993) starts nicely, and the air-fuel meter (F-A M) shows a gradual progression from lean (red) to nice yellow ("normal") as the coolant temperature sensor warms up. But here (and anywhere during warm-up) stabbing the throttle stalls the engine as the F-A meter plummets to full lean. Releasing the throttle restores a nice idle at "normal."
Most engines start at rich when they're cold, coming down to "normal" 14.7:1 as the warming parts are better able to keep the fuel vaporized. Worse on older carbureted cars, but some injected cars need help too. I wonder why yours (apparently) works backwards?

Quote:
But here (and anywhere during warm-up) stabbing the throttle stalls the engine as the F-A meter plummets to full lean. Releasing the throttle restores a nice idle at "normal."
Checked for the obvious? Fuel pressure, vacuum leaks, split intake hose, something not plugged in?
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1990 740GL, now in extra-small economy size.
506,000 km so far...
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Old 11-29-2020, 04:31 PM   #3
Jhebbard
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(being backwards...) I assume it has something to do with the O2 sensor. The air-fuel meter reading I refer to has a separate O2 sensor. Both sensor are three-wire (heated) but, you know, take time to heat up.
The last time the car ran OK and accelerated on demand, I has just replaced the AMM. The new AMM has less than 1,000 miles on t, but I haven't checked the signals. That's partly why I would like a information on the FI computer programming.
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Old 11-29-2020, 10:37 PM   #4
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Both sensor are three-wire (heated) but, you know, take time to heat up.
20 to 60 seconds
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Old 11-29-2020, 10:42 PM   #5
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but, you know, take time to heat up.
.

1993 NA 240 with a pinch larger injectors...quick cold start, but a rich mixture...so I increase RPMs to prevent stalling...within 60 seconds, LH 2.4 reduces injectors' fuel amounts...engine runs fine.
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Old 11-30-2020, 01:39 PM   #6
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Whoa!
What drives enrichment on throttle actuation? I assume Air-fuel ratio shooting lean on stabbing throttle could be fuel pressure, but the car runs fine, intermittently and pumps replaced recently.. Need more information on exactly what controls the injector timing on acceleration....
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Old 11-30-2020, 01:59 PM   #7
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What drives enrichment on throttle actuation?
FPR - Vacuum drops in suction, and FPR increases the fuel rail pressure.
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Old 11-30-2020, 02:05 PM   #8
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Need more information on exactly what controls the injector timing on acceleration....
Should be default or learned fuel 'database' in ECU.

Learned Database - Based upon AMM/O2 mostly...if AMM/O2 out of specs, then default profile might be used...all depending.
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Old 11-30-2020, 09:15 PM   #9
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RE: Florida's heat and humidity.

Terminal connections may loose their electrical potential with time...I'd use electrical spray-contact cleaner.

Running lean would not be expected with a CTS (coolant temp sensor)...usually, a rich mixture would be expected when defective. I always replace the CTS with every vehicle bought.

I would do this Throttle Body Adjustment first, then report back what idle RPM is...your lean mixture appears to be related to atmospheric air not going thru AMM.
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