engine vacuum fast fluctuations

[gmbellew]

vacuum

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been fighting a rough idle since summer. full tuneup and TBI rebuild in the last couple months. compression test shows 180psi on all cylinders, so the valves aren't leaky I don't think.

I finally put the vacuum gauge on it to see what was happening, even though MAP readings are steady at the OBD1 refresh rate.

idle - fast fluctuations 18-20psi
2000rpm - fluctuations settle a little.
3000rpm - fluctuations settle a lot. but still not steady.

snapping the throttle does what is expected to average vacuum reading, but still has fast fluctuations.

Guessing either a lifter(s) has gone bad or a cam lobe(s) has worn down?

other thoughts on the next steps in the diagnosis?

 

[Snoots]

Check your IAC valve.

 

[gmbellew]

IAC counts are good and steady. I think because the computer refresh rate doesn't allow it to see the fluctuations in vacuum with the MAP. MAP readings are also steady and are the average of the fluctuation range at idle or steady throttle.

 

[fast 99]

That has all the symptoms of a valve train issue, worn cam, loose valve guides, ect. The fluctuations are fairly regular.

Also, a broken valve spring. If the throttle is snaped open and it backfires it is indicative of a possible broken spring.

Disregard data update speed. Systems back then weren't very advanced.

 

[gmbellew]

That has all the symptoms of a valve train issue, worn cam, loose valve guides, ect. The fluctuations are fairly regular.

Also, a broken valve spring. If the throttle is snaped open and it backfires it is indicative of a possible broken spring.

Disregard data update speed. Systems back then weren't very advanced.

any way for me to further diagnose without tearing it all down?

 

[fast 99]

Pull valve covers watch valve action, inspect springs.