legacycentral bbs

So, on my '92 SS autotragic I just installed a boost gauge from the local parts store - on general principle. At some point I'll be modifying this one for fun, too, and for now it will be diagnostic. Besides, I like knowing what's going on in my car as I drive.

Is a rapidly fluttering / oscillating boost reading an indication of something like burnt exhaust valves? On stock boost control, which feels smooth and normal BTW, the boost gauge's needle is just a blur highlighting the range of 8 to 10 PSI.

I've never seen this before on my '94 SS's (hehe, "ess esses") boost gauge, which is a much older simple Autometer gauge (range -30 inHg to +20 PSIg).

The newer boost gauge was from the local Autozone off the shelf (range -30 inHg to +30 PSIg).

Actually, I'm wondering if perhaps our old stock boost control solenoid maybe just regulated this sloppy and now this newer boost gauge responds quickly enough to show it? Whereas my older gauge responded slower maybe...?

It shouldn't oscilate....

Where did you tap into for the boost gauge and what size vacuum line did you use?

Have you tried testing the old gauge on this motor/car to see how it reacts or vice-versa, move the new gauge to the old car?

I have not swapped gauges between cars; I'd like to not have to re-mount them again. However, I may purchase another boost gauge as a troubleshooting step here at some point, since I will need one eventually for my '91 SS (once it runs again, anyway)

I tapped on the passenger side of the intake manifold, on the front runner - same place as my '94's boost gauge tap, and as far as I can tell, same size boost gauge line. (hard plastic tiny tube going to boost gauge, adapted to the tubing shown below:


Here is a short video of the boost gauge in action, taken this morning on the way to work.


(also just noticed my trip counter's digits are all screwy and half-set, perhaps I should reset that)

I suppose you can't really see much here, but it does exemplify the gauge fluttering under boost. It felt like a normal stock boost pull to me, didn't feel lurchy or hestitant or anything...

May be a clue: under vaccuum, the gauge is relatively stable. As it approaches atmospheric, the oscillation begins. At full boost, the oscillation appears to be fluttering between 2-4 lbs of boost reading. So confused.

I also seem to have heard a little mechanical noise that seems to be associated / timed with the flutter, which appears to be coming directly from the boost gauge. I wonder if I have a faulty gauge....?

lose the plastic line do a full rubber vacuum like and where you tapped ya line into put a piece of cotton in the line that connects to your vacuum nipple. it fixed my flutter on my boost gauge

My suggestion would be to keep the plastic line. Smaller the line the more responsive the boost gauge....which is important, but may contribute to the flutter.

If the gauge is not fluttering under vacuum then it'd be more diffcult for me to say the gauge is the problem. How does the engine/car feel? Does it feel like boost is not consistent?

Mine does the same thing. Has ever since I got it. For the record, it has done it with 3 different gauges, and with plastic line and rubber line. Also with and without restrictor pill in the gauge line. First spot, I had it teed into the brake booster line, now I have it teed in right off the manifold before the map sensor.

Still have yet to figure it out.

Update: ive seen flutter under vacuum too just not as pronounced. I still get -22inHg slowing down in gear, and -18 at idle. Those readings are about the same as I get on my 94. The deeper into vaccuum I am the more stable the gauge is. Slowing down in gear is very stable readings.

I do not experience any stutter or hesitation or anything...feels like good smooth stock boost.

This leads me to believe it may be perhaps indicative of relative valve health...?

Hmm....



Sent from my SAMSUNG-SGH-I337 using Tapatalk 4

Honestly....if you don't have any issues.....I wouldn't get too worked up about it.

No worries, I'm not worked up over it or anything, but I am definitely curious to know what this is a symptom of.

I'm glad to hear at least I'm not the only one to have experienced this. I would definitely like to eventually have a stable boost reading on this car but for the time being, it doesn't matter - was just a nice "to have" since I had the car in the garage anyway working on a couple other things, so I tossed the gauge in there in anticipation of later adding an intercooler and turning up the boost.

Restrictor pill, huh? I wonder if my gauge came with one of those...

oh, also, mine did it on the stock turbo and a new 16g. Also before and after a full engine rebuild. When i first noticed it, i though it might be valve or turbo related, but my work since then has disproved that. Might have to do with being tapped into a line off of one of the intake runners instead of centrally, but that is kinda grasping at straws.

That said, i got a digital gauge so I didn't have to listen to the shit rattle anymore (2 different mechanical gauges, one Autometer and one Prosport and they both made a slight rattling while the needle was moving back and forth).

i put some cotton in the the tube that connects to my vacuum tee and no flutter and holds at the 8 lbs of boost. yes mine set at stock boost for now lol

Interesting. I would have immediately thought it was where I tapped into the intake too, except my 94 is set up the same way and never saw flutter.

Im gonna have to try cotton in the line to the gauge.... I suppose its not going anywhere if its not past that tee.

Sent from my SAMSUNG-SGH-I337 using Tapatalk 4

just a small piece. i stuck mine on the nipple then put the vacuum line from my boost gauge over it. or google boost gauge flutter fix and the show pixs. like i said just a small piece like off a q tip

Gotcha, I should have thought to google that myself lol...thanks!

Sent from my SAMSUNG-SGH-I337 using Tapatalk 4

Aha! Finally, a reasonable explanation!

Cant seem to paste here on my phone at the moment but ill summarize:

VDO gauges respond so fast, the pulsing of the boost control solenoid is represented by the flutter.

Inline restriction, such as cotton or a fuel filter, is a common fix. So is going to larger diameter hose to the gauge (from 1/8" to 1/4" usually).

Makes sense now

Also, I can see our bcs is indeed active all throughout the throttle range...not just closed until near 9psi. This is the difference I knew I felt between my mbc and stock bcs on the 94.... mbc really works your turbo hard at part throttle vs stock boost control. Perhaps why my turbo has decided to take up chimney smoking after running an mbc for 7+ years.

Sent from my SAMSUNG-SGH-I337 using Tapatalk 4

I put a bit of cotton in mine last night, and it helped. Apparently my restrictor pill was little more than a coupler, but the cotton greatly reduced what I was seeing. Instead of +/- .5 psi, it's down to +/- .1.

Yep, finally got around to pulling the boost gauge line off and stuffing the cotton from the head of a q-tip in there; completely fixed my flutter. Looks like this car may be not so happy with the premium gas quality at Fred Meyer, possibly -- now that the needle isn't jumping all over the damn place I can see I have 6psi wastegate boost now. Anyway, flutter resolved!