Beretta.net

I'm having a difficult to diagnose problem on my 3.1MPFI.
I recently did the following:
-plenum cleaning and new upper intake gaskets
-new front valve cover gasket to fix oil leak
-that paper gasket at the water pump outlet
-EGR and EGR intake passage cleaning and new gaskets
-IAC cleaning
-TB cleaning and new gasket
-the oil pump drive shaft O-ring to fix oil leak (with new brown o-ring and ultra copper around the top of the flange, seems to be working so far)
-TB coolant bypass
-thermostat and new seal
-new Rapidfire plugs (all 6)
-new O2 sensor
-injector resistance tests (all 6 ohmed out at 12.5)
-coil resistance test (all 3 ohmed out at 5700-5800)

I did not do the plug wires, the autozone duralasts i got wouldn't snap onto the coil terminals, they kept slipping off, so i put the old wireset back on for now.

I noticed that at the front and rear of the plenum, there is like a 1/2" gap between the bottom of the plenum and the top of the lower intake. I think it was there before, like it looks like its not sealed at that point but it really is? Am I right about that? Do you know what I mean? I can see the gasket on each side. Is this normal? A few of the rubber boots on the plenum bolts ripped some, i managed to seal them all, but don't know if this could be a vacuum leak source.

Anyway, my problem is this. When in top gear (3rd with my 3t40 tranny) and when the lockup is engaged, when i am barely gassing it at all and am at or below 2000rpms, i get a bucking. Its not a severe buck, but definitely noticable. The bucking ONLY occurs under the above described conditions. The idle is smooth, acceleration is good, gas mileage is very good (26.4mpg average even before I did all this work). Like when I'm cruising along at around 50mph, at 1800-1900rpm barely even touching the gas in top gear, is when the bucking occurs. Its not even really a hesitation or a noticable fluctuation in rpms, just a
bucking. The bucking was worse before i did all this work, and is better now, but its still there.

A few things, the old PCV grommet seemed worn in that the valve wasn't held securely in it, i could lift it up a bit with my fingers, not all the way out, but some of the way before the grommet gave resistance, and the hose was ripped a bit around the valve. Probably should have done the grommet and PCV hose right then, but I didn't (the valve itself is new). The IAC gasket was old and torn in a spot, but I reused it, should've replaced that as well. The cruise control doesn't work and the cruise control servo has a plastic inlet to accept a small vacuum line, that plastic is broken and the line running to it is disconnected. I don't know how old the plug wires are so maybe its them, but it feels like either a vacuum issue(it only occurs at low rpms/near closed throttle when engine vacuum is highest and leaks are most noticable) or a tranny issue( TB cable misadjusted? TCC?). I'm gonna get a vacuum gauge to test it, as well as a backpressure gauge to see how the exhaust is doing.

Has anyone on these forums ever had a similar type problem with a 3.1 Beretta? Any clues or leads would be great and appreciated.

~Thanks~

Check for vacuum leaks with a propane source, carb cleaner, starting fluid, etc...  Spray around at different places that you think might be leaking.  If it changes then you have a leak.

Check the TPS for a nice smooth ohm reading change.

I'd replace the wires.  It sounds like an ignition problem to me.  One of the hardest things to light is high load at low RPM.  VE is very high so the spark has to jump at near the highest cylinder pressure possible.

Thanks Rex, what does "VE" mean?

-I am going to try the TPS, get a new wireset, recheck the new plugs for tightness, redo the PCV grommet/hose situation, put on a cone filter airbox and a breather filter on the front valve cover, and spray solvent around all vacuum points as best I can and see if anything is noticable. I want to get a vacuum gauge to see where the vacuum is at at idle and under load.
-I also want to test the exhaust backpressure. Since I installed a new exhaust a year ago the intermediate pipe hanger (that hook shaped thing welded to the pipe) has been loose the whole time causing the exhaust to rattle against the frame when I first start the car and when I put it in reverse. Its possible this vibration has damaged the cat internally causing a plugged exhaust condition.
-I had the white, deposit free look on all the old plugs and the O2 sensor, indicating a possible lean condition. Fuel filter is new 2 years ago, should check fuel pressure but that only tells me part of the story, right?

This type of diagnosis is really hard, hope that all these different avenues get me somewhere quick!

Oh yeah, does anyone know about the weird looking seal on the plenum I described earlier? Basically I can see the front plenum gasket and even touch it with my pinkie. It really doesn't look sealed, but the bolts are in as far as they will go and I'm pretty sure I aligned the gaskets up good before I tightened the plenum down.

VE is Volumetric Efficiency.  Basically how much air actually fills the cylinder vs the total volume of the cylinder.  

100% VE would fill each cylinder of a 2.8L v-6 with 0.466L of air.  
50% VE would fill each cylinder with 0.233L of air.

This effects dynamic compression ratio in the cylinder, which effects BMEP (brake mean effective pressure) which partially determines total TQ output of the engine.