Evidence based diagnosis - logging to troubleshoot cruise surge

langsbr

Lieutenant
Apr 5, 2017
868
I've been experiencing a surge at cruise speeds, in the 2800 - 3000 rpm range. I just completed a long trip last week, and while I didn't log then, I am fairly confident in being able to reproduce the issue. I've found that it doesn't matter the gear, but the rpm that I'm in, when cruising. It occurs in 4th, 5th, and 6th in the 2800 - 3000 rpm range. The problem is that 85mph is where it occurs in 6th, so I have to drop my speed to 80 or less to avoid it.

I am suspecting VANOS solenoids, as previous logging revealed no issue in AFRs, timing, or throttle. I've not done much VANOS logging, or troubleshooting and was wondering what I should be looking for to confirm VANOS solenoids are the culprit. I am adamantly opposed to swapping parts because it's 'likely'. If it's an electronic part, it should be able to be diagnosed, and visible malfunction in a log.

My thought is log VANOS requested vs. actual and if they don't match I would suspect an issue. The issue with that thought is that currently I don't have a known good baseline to say for sure that is expected behavior. Very few logs I see have VANOS logged, so it's a bit of a mystery to me.

Anyone have suggestions?
I
 
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tisdrew

Corporal
Jun 27, 2017
109
@langsbr I have this symptom also but logging my throttle vs pedal, I see my throttle has some variable behavior with a static pedal input. It's so minor that I haven't bothered to attempt cleaning my TB. I'll go log some vanos stuff like @Jeffman and post back in a day or two.
 
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