One of the more persistent problems on Project Dumpster Fire is my inability to measure the air-fuel ratio. Tailpipe sniffers are thin on the ground and even used units are fetching over $200 on eBay. I could build one, but the rabbit hole is deep there. I could also buy an in-car air-fuel meter with wideband O2 sensor, but that would require a lot of faffing around.

Luckily, I had a solution sitting on my dining room table! A while back I got an Amazon price alert (thanks CamelCamelCamel) which, sidenote, fill me with even more dread than Steam “An item from your wishlist is on sale” emails, for an oscilloscope that I’ve had my eye on for a while.

I am an electrical engineer and have a lot of professional toys but not a ‘scope. No longer!

Anyway, back to the Saab, I do not own a Bosch KDJL 7453 Pulse ratio thingy. In fact I haven’t even looked for one because I figured I wouldn’t find one. That said, I do (now) own an oscilloscope.

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SCIENCE!

I ran the Saab up to temperature, hooked up the scope to the measurement port, and with a little fiddling with the triggering method had a mostly usable waveform.

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I don’t remember what causes that weird spike when the waveform falls, or how to get rid of it, but ultimately it didn’t change the reading.

Looks like despite having leaned up the mix considerably, I was still reading at about a 10% pulse ratio, where the spec is 50%. As I dialed it leaner, the idle started hunting a little, but eventually I got the ratio to where the hunting was centered at 45%.

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Good enough.

What is the net effect?

No idea. It is still covered in tools, trapped in the garage, and in need of a hot start relay.

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But progress!

(NOTE: No actual science as used in this process. Just tools that would ordinarily be present in a lab.)

Bed hog doggo for your time.