The permamently installed vacuum gage I have just started doing this funny twitching thing. Doesn't do it at idle at all, but does it under moderate load (from around 5-10 inches of vacuum) under lower rpm ranges, like 1300-1750. easier to see when the w/TCC locked-up becasue I can hold rpm and load. It is barely perceptable but it is there! doesn't do it at higher rpm's under any load and no sign of it at higher loads (lower vacuum levels) at any rpm, or at least it's not perceptible.

I think I have an intake gasket leak on one runner OR a valve guide/seal is on its way out.

I have TBI, 350, Performer Intake, crate motor heads ported to 240/190 flow, 9:25 CR, mellow cam for mileage testing/PROM burning testing (Eelbrock 3702 = 194/204 w/.396"/.442" right now) and didn't change anything to make this happen. Comp test was 185-200 for all cylinders (told you it was a mellow cam)

Any ideas on what this is? I've never seen an engine do this before. Maybe vacuum leak somewhere? hmmmm

 

FB,

A "twitch" in a vacuum reading generally indicates a valve problem, either in lift (opening) or sealing surfaces. It's also possible that a completely dead cylinder will create erratic vacuum. Operating at lockup with moderate load can create fairly high cylinder pressure, which may be challenging the sealing abilities of the valves. A slightly burned or bent valve might be a culprit. A spark plug blowing out under load might also look the same on the gauge. Not knowing how much deflection you're experiencing, it's difficult to determine.

It's also possible that the opening (lift) is being affected in another way. Excessive coking on the rear faces of an intake valve would impede flow, causing the symptom. Flow at idle ranges wouldn't be high enough to be affected, and higher RPM ranges would tend to hide the erratic needle.

------------------
Later,
Vader
------------------
"Make Me Bad"
Adobe Acrobat Reader 4.0

 

Whe I say twitch, I mean about 1-2inches MAX. You can hold the gage in your hand and feel the internals twitching and hear it, too. I check the gage on another engine, too. Not the gage. No completely dead cylinder, I guess, cause comp check was ok. Spark plugs ok, too.

Good point about the high cyl pressure. Maybe I have a leaking intake (and/or exhaust valve???) and when the cylinder fires, the pressure causes flow by the non-sealing intake valve and into the intake manifold, which causes the twitching on the vacuum gage. No backfire, though, and heads don't have many miles on them. Could be, I guess?

Could be bad valve guide, too, I guess but would be less of a problem w/lower vacuum, I thought, and worse w/more vacuum (higher vacuum would suck more air through the bad guide)


Oh, another weird thing, if I hook up the vacuum gage to the full vacuum port of the TBI unit (front of TBI) which pulls from only one TBI bore, the gaage MOVE ALL OVER THE PLACE!!! ie 4-5 inches of movement. I figured it did that because the port is only on one side of the 2bbl TBI and therefore the gage was reading the intake sucking "pulses" of that one side, only (ie 4 of the 8 intake sucking events). Should that be happening? Are those pulses (from one side) enough to do that? or should vacuum be steady there, too??? And it also does it from the big vacuum port on the Performer intake, just behind the TBI/carb mount area, on one of the runners??? you know, the place where a plug is installed when you get it... I figured it would do it there, too, because it is a runnre to one cylinder and is getting the pulses from one cylinder, only???

I have always hooked a vac gage up to full manifold vacuum port, not the one-side of a TBI full vacuum port.... Carbs have full vacuum ports that "pull" from the plenum, right, not one side of the carb...

[This message has been edited by FastBroker (edited October 09, 2000).]

 

Usually a minor twitch, say 1 to 2 in.Hg like you said, means some sort of ignition system problem. I wish I knew the answer to your second question, but Vader will have to cover that. Good luck!

 

Why would ignition problem cause vacuum gage twitch? Ignition/spark and resulting pressure/flame/expansion should not enter intake, right?

This is killing me, man. You gotta see that needle twitch when it's hooked up to that runner port or the TBI! That can't be good, man! something ain't right...

 

Irregular power production/cylinder imbalance due to minor ignition system problems causes a slight stumbling of the engine (which you may not feel or just feel as a slight and erratic shake in the car if it's a minor problem) that will definitely show up on the vacuum readings. A complete ignition misfire on one cylinder can even cause a 4in.Hg fluctuation in vacuum readings. Also, I just researched your second question in some of my engine books and it is essential to only connect the vacuum gauge at the intake manifold and not to ported vacuum. I think you just want to check it at a spot where you have a macroscopic effect from all cylinders rather than where one cylinder/intake runner dominates the readings. Hope this helps, but I'm pretty sure Vader can clear this up some more.

 

I did not hook it up to ported vacuum. I hooked it to a full vacuum port. sorry for the confusion. Ported vacuum would work ok to see the problem, but it just shuts off at lower throttle settings. I actually did hook it up to ported once just to check and it still did it.

I just can't remember heraing or seeing anything like this in all my past engine work. Gotta be something wrong w/my engine...

Engine does run a little rough here and there, not much though. Figured it was my PROM waaay off. I'll fully check out my ignition/distributor tonight (wires, pickup coil and module) . You just gotta see that needle move to know what I am talking about.

Vader, HELP!?!? Would/could different vacuum hookup locations cause this?

 

Check the ignition thoroughly, I've had that slight fluctuation happen to me before and that's what caused it. I suppose a bad fuel injector or two could do that as well. You have to consider the power production of the engine as a function of time (small time: at 1000RPM each cylinder fires every 0.12 seconds, and they're all out of phase with each other too), and when those cylinders don't fire evenly or fire weakly there will be some small fluctuation of power production, and therefore some small fluctuation of vacuum as a result, as the power production and airflow through an engine are very related. Whew! That's the best explanation I can give for the ignition thing, but I'd say the services of Mr. Vader are needed on the other questions! Good luck!

 

It's TBI and the injectors are ok as I had them cleaned/flow checked. Needle fluctuation is smaller, like I said, when gage is hooked-up to central vacuum port, like where MAP sensor is. If hooked up to the other ports I mentioned, needle is ALL over the place.

 

Any other ideas/experience on this???