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Old 11-15-2021, 10:28 PM   #1
Wilford Brimley
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Default Random missing

So my vo started doing something a little odd, when I'm cruising at 60 or whatever it will randomly miss and go to 10.5 afr then bounce all the way to like 16(this all happens fairly quickly) then back to normal. Could this be a cap/ rotor maybe? It has new plugs and the wires mayyy be questionable.
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Old 11-16-2021, 08:31 AM   #2
Otto Mattik
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Quote:
Originally Posted by Wilford Brimley View Post
So my vo started doing something a little odd, when I'm cruising at 60 or whatever it will randomly miss and go to 10.5 afr then bounce all the way to like 16(this all happens fairly quickly) then back to normal. Could this be a cap/ rotor maybe? It has new plugs and the wires mayyy be questionable.
Haynes article that touches on some of the basics of AFR mysteries:

https://haynes.com/en-us/tips-tutori...ratio-your-car


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Old 11-16-2021, 10:11 AM   #3
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Lol yeah I got all that. I dont get the random misfires though which is what I assume it is since it goes so rich and then Tries to correct itself
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Old 11-16-2021, 01:35 PM   #4
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- if there is an exhaust leak between head and O2-sensor then that can cause weird AFR readings which cause the ECU to do weird things. If AFR gets too far off target it could cause misfires.
- new plugs: did you check gap distance before installing? (No, they are not always gapped correctly by the factory)
- a leaking oil seal on the dizzy shaft can cause oil to drip into the dizzy cap causing misfires
- another potential problem on 740, 940 and 4cil 780 is the paravan seal. If that is leaking, rain water can drip directly on top of the dizzy cap causing misfires.
This water can linger between the black cover and red cap.
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Old 11-18-2021, 01:22 PM   #5
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Gap is good I did that, ill check the other stuff thanks!!
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Old 11-18-2021, 03:39 PM   #6
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On my 1987 745 T, I developed a very infrequent misfire than was only apparent when cruising on the highway. Problem was eventually traced to a failed terminal on the ignition coil high voltage lead. The OEM replacement came as part of a kit so I ended up replacing all the wires. Problem solved in my case. I did find it odd that my problem was so infrequent. Drive a couple of hundred kms with no issue, one or two misfires and then good for another couple hundred kms. It did seem to occur more frequently in colder / damp weather.

When you get a misfire, the AFR will momentarily go lean because the misfire introduces a slug of O2 into the exhaust system (in addition to unburnt fuel). Depending on the signal filtering on the O2 signal, the ECU might not respond to a single misfire. If it does, I expect that after going lean it would then swing rich before correcting to target (assuming the misfires do not persist).
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