Boost = Hesitation

[dhoeschen]

Thanks tblake, I have had a lot of trouble with those vacuum fittings, every piece of rubber on this car is dry and rotten. I think I should replace them all just for piece of mind...

It will run without the supercharger belt!?! I'll give it a try...

[tblake]

I ran mine a week without the supercharger belt as a test to see if it gets better milage with the belt on or off. Conclusion was it gets better with the belt. Plus the HP just wasn't there.

[dhoeschen]

It sure drives better without the belt! It did have a few hiccups here and there when it needed to downshift or with a change in acceleration. Pretty minor compared to what it was doing before.

[tblake]

Double check the vacuum lines on the top of the supercharger. There is a vacuum cap up there with 2 or 3 lines coming off it. One of those lines goes to the Boost Bypass Valve. Make sure that one is ok.

Bob/Rich/Lynn, what do you think? Does this sound like a bad Boost Bypass Valve?

Before you order one, lets see what the other guys have to say as I have not had much experience with them.

http://www.zzperformance.com/grand_p...=840&catid=106

[old_master]

I didn't mean to confuse you on the resistor deal. It's a fairly common issue, but you're right, if the resistor craps out you'll have no low speed in the pump. The engine will start and then stall after a about one second.

If you don't notice the hesitation with the belt removed, that could be an indication that the fuel pump high speed circuit has a problem. Fuel demand is greater when the supercharger kicks in and if the fuel pressure doesn't increase, it's gonna hesitate and power will be reduced. Check fuel pressure and listen to the pump: you should be able to hear the difference, as well as see it on the tester, when the pump changes speed from high to low and then off.

[dhoeschen]

Did a quick test of the fuel pressure: 50psi when the key is on and about 47psi with the engine running. I pulled the vacuum line from the fuel pressure regulator and it is working as it should.

Replaced all the vacuum lines and it drives nicely and I get a little bit more vacuum on the gauge while driving...

The boost bypass valve is pulled up when the engine is running I'm going to test it with my vacuum pump to see if it has a leak... The other thingy right next to it (has a vacuum line from the fuel regulator and an electrical connection) has an open end on it - looks like a chunk of foam used to be glued on there at one point. Must be a reference to atmospheric pressure?

Need to test the high-speed circuit of the fuel pump...?

[old_master]

Key on, engine off, fuel pump running: fuel pressure should be 48psi to 55psi. Engine running at idle pressure should drop approximately 3 to 10 psi. So it sounds like yours is ok, provided it remains above about 45psi for 10 minutes after the pump shuts off, (indicating leakdown is not excessive).

Do you have access to a digital volt ohm meter and are you comfortable using it? If so, there is a way to force the fuel pump to run at high speed and see if the hesitation goes away.

[dhoeschen]

I re-did the fuel pressure test.

Key on, engine off, fuel pump running = 52 psi
Engine running = 45 psi
Engine off = 38 psi
Engine off after 7 minutes = 35 psi
Engine off after 10 minutes = 34 psi

[tblake]

Fuel pressure looks good to me. But to be on the safe side, you could tape the guage to the windshield and take it for a spin. Make it do its thing and keep an eye on fuel pressure.

[BNaylor]

Fuel pressure looks good to me too and not the problem. To the OP what is your boost indicator at the DIC reading with the supercharger hooked back up. Any specific rpm reading when the issue develops? What Autolite spark plugs did you install including heat range? Do you have a scan tool capable of reading out knock retard (KR)?

[dhoeschen]

I'll be re-installing the supercharger belt sometime today and hopefully the roads will be free of ice so I can take a safe test drive... The fuel pressure gauge has a short hose on it so I may not be able to see anything.

I tested the boost bypass valve actuator; does this seem normal? I can pull and hold vacuum on the "upper" side which hooks into the main vacuum distribution on top but the "bottom" side does nothing...

Bnaylor, the spark plugs are OEM style Autolite #606 I don't know the heat range. The e3 plugs I pulled out looked normal in color. The DIC boost gauge will peg instantly. The problem happens at any speed or RPM when I push the throttle a little more which would require some boost. My scanner does not show KR or live data.

[tblake]

Can you move the lever/boost bypass valve rod back and forth by hand? Will it move, and will it return back to where it was when you let go?

Sounding more and more like a BBP valve problem to me, but I have no experience with these. Mine is still original at 155k.

[dhoeschen]

With the engine running the lever is all the way up, I can push on it and feel a little pressure. If I snap the throttle it will move downwards some and return to where it was.

[tblake]

That sounds about right to me on the BBP valve.

Do you have any way to test exhaust system backpressure?

I've even in some cases removed the O2 sensor out of the manifold before the cat converter and drove the car like that to see if the problems were still present. Of course your car will be loud with the sensor out of the exhaust manifold.

[dhoeschen]

Exhaust backpressure was one of the first things I tested. Less than 1 psi at idle and at 2500 RPM.