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Spark Plug Reading Anyone?
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Tom
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Changing my spark plugs in preparation for a smog test. Any spark plug whisperers here?
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dr bob
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3 looks a little lighter, so might get a harder look. Maybe a borescope shot of the top of the pistons for some correlation. If you fail on NOx or are even a little unexpectedly high, initial focus might go to that injector.
There might be a case for a DIY test fixture with a fuel pump or cannister, a simple pulse controller with timer, and a graduate cylinder you can use to measure the flow mass. I might be tempted to use a good scale that reads to grams, and just weigh the output flow. I've used paint thinner for the test fluid to reduce the chance of fire-or-worse, and do it outside. Do each injector individually for flow in the same amount of time. Compare.
Do you have EGT on each pipe?
There might be a case for a DIY test fixture with a fuel pump or cannister, a simple pulse controller with timer, and a graduate cylinder you can use to measure the flow mass. I might be tempted to use a good scale that reads to grams, and just weigh the output flow. I've used paint thinner for the test fluid to reduce the chance of fire-or-worse, and do it outside. Do each injector individually for flow in the same amount of time. Compare.
Do you have EGT on each pipe?
dr bob
1989 928 S4, black with cashmere/black inside
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Free Advice and Commentary. Use At Your Own Risk!
1989 928 S4, black with cashmere/black inside
SoCal 928 Group Cofounder
928 Owner's Club Charter Member
Former Ex Bend Yacht Club Commodore Emeritus
Free Advice and Commentary. Use At Your Own Risk!
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Tom
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Funny you mention that. I've had my eye on these cheap testers for a while now -- they all seem to come from the same place and get pretty good reviews.dr bob wrote: Sun Jun 21, 2026 8:38 pm 3 looks a little lighter, so might get a harder look. Maybe a borescope shot of the top of the pistons for some correlation. If you fail on NOx or are even a little unexpectedly high, initial focus might go to that injector.
There might be a case for a DIY test fixture with a fuel pump or cannister, a simple pulse controller with timer, and a graduate cylinder you can use to measure the flow mass. I might be tempted to use a good scale that reads to grams, and just weigh the output flow. I've used paint thinner for the test fluid to reduce the chance of fire-or-worse, and do it outside. Do each injector individually for flow in the same amount of time. Compare.
Do you have EGT on each pipe?
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Problem is I'm eleventy-two percent sure these plugs split time between the stock injectors and the bigger Five-O injectors I've been running recently. And the stock injectors are a mixed set -- 3 of them were professionally cleaned a few years ago, but one is an older used one I cleaned myself with carb cleaner and home-hacked controller to open and pulse it for cleaning. Smog results have been quite good this way (knock on wood). Once I'm done with the smog test, maybe I'll install new plugs with the new injectors and see if the same pattern develops. I have a single EGT sensor, but could try an IR gun on the header pipes.