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Pre OBD vehicle diagnostics

  • sleepdoc
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01 Dec 2017 21:08 #15450 by sleepdoc
Pre OBD vehicle diagnostics was created by sleepdoc
Hi, I am deep in a rehab of 1986 3.2l Porsche Carrera engine. The fuel and ignition control are by a very early Bosche Motronic fuel injection system. There are about 6 sensors.
Flywheel speed sensor.
Crank position sensor.
Exhaust oxygen sensor.
Mass airflow sensor.
Aiflow intake temp.
Head temp.

I am able to investigate the sensor signals, the primary and secondary ignition,the current profiles from the fuel pump,starter,alternator. The injectors.

There are accessable
Computer outputs controlling ignition, injector timing and duration, idle speed air control.

However there is no OBD network or plug.

When I setup my picoscope I will likely be able to collect enough information to recognize weak or failing parts in the manner "SD" has worked so hard to teach us.

So, my question is about how to get a handle on fuel trim. I realize that the O2 sensor data is a prime avenue, but can anyone think of other data I can access that logically can be reduced to a model of what inputs/signals are the computer reacting to and in what direction.

In a broader sense, Same question. Has anyone come across a body of resources that gives insight on diagnostic work on pre OBD vehicles?

Thanks for your thoughts.
Mark Robinson

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01 Jan 2018 12:30 #16348 by mx5tc
Replied by mx5tc on topic Pre OBD vehicle diagnostics
Mark,

Having owned two 1975 Volvo 164s with early Bosch FI and a 1986 240 with a little better Bosch FI, I can sympathize with you. Certainly harder to diagnose without an OBD-II port but not impossible. I was able to keep all of my Volvo's running with out any help from the stealership armed with only a Radio Shack DVOM and Bentley manual.

My problems on the 164s were limited to defective ECTs (nasty little fault that caused engine to stop after it hit operating temp...effected temporary fix with appropriate range resistor in place of sensor while I waited for replacement), bad grounds on control module and leaking fuel injector to fuel rail hoses.

On the 240, I had a fuel pressure regulator fail ( car ran pig rich, did fuel pressure check with manual gauge to ID), a clogged cat (diagnosed by process of elimination and banging on cat to see if honeycomb was loose...did not have an exhaust back pressure gauge), a failed O2 sensor (could not positively diagnose this with cheap DVOM but previously failed fuel pressure regulator and high miles on sensor (140k) were enough to gamble on said part...I now use 100K as a replacement interval for inexpensive 02 narrow band sensors based on numerous failures I have come across in vehicles with 100K plus miles), and a failed MAF (good crank, no start condition with good spark and good fuel pressure)

Having a scope to troubleshoot the 02 sensors, MAF, and waveforms from ignition and injectors would have made all my repairs a lot faster. With respect to the 911 and fuel trims, I would suggest using a wideband 02 sensor to monitor fueling and trims from idle thru WOT and a dialback timing light to monitor ignition timing and advance at various engine speeds.

FWIW, if you are not a purist intent on keeping the 911 original, I would carefully remove the OEM control system and replace it with a tuneable modern speed density system like a Megasquirt that can support multiple O2 sensors (1 for each bank) and electronic wastegates.

Mitch
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01 Jan 2018 13:15 #16350 by borntoroll
Replied by borntoroll on topic Pre OBD vehicle diagnostics
I think you can just measure injectors and O2 with your scope. if you reset memory (engine hot) you will get baseline and correction when in closed loop.
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01 Jan 2018 14:46 #16351 by mx5tc
Replied by mx5tc on topic Pre OBD vehicle diagnostics
I suspect that is correct but the biggest problem one faces in tuning older high performance cars with primitive ECUs is fueling at WOT which is typically controlled by Open Loop fueling tables vs the closed loop narrow band O2 sensor(s).

The big advantage of a good wideband O2 sensor is the ability to monitor proper fueling from idle to high load at WOT. OEMs often have open loop tables set overly rich to "protect" high performance engines at WOT, this condition can be easily identified doing pulls with a wideband in place. Similarly, exhaust system and/or air box modifications from OEM may cause an engine to run leaner at WOT.
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01 Jan 2018 15:08 #16352 by mx5tc
Replied by mx5tc on topic Pre OBD vehicle diagnostics
Mark,

Additionally, I forgot to mention that data logging capability is another advantage of installing and using an aftermarket wideband 02 sensor. Having the ability to review a graph showing RPM, boost and air/fuel ratio on a WOT pass thru the gears can be a very useful tuning diagnostic.

AEM, Innovate and Zeitronics all have data logging software packages that are very helpful for tuning high performance engines. Logworks 2 allows you to calculate torque and power during a WOT pull. You can acquire more useful tuning information in a few WOT pulls than you'll get in a day of scoping narrow band O2 sensors and looking at closed loop fuel trims.
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07 Aug 2019 02:23 - 07 Aug 2019 02:24 #32565 by liao.chun
Replied by liao.chun on topic Pre OBD vehicle diagnostics
That's a pity, no idea how people handle it without OBD. Can it be just a lame app that doesn't show info? Try out this one https://play.google.com/store/apps/details?id=com.motordata.obd , maybe it'll work.
Last edit: 07 Aug 2019 02:24 by liao.chun.

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