so i have no idea where to begin to research this but here it is.
95 z28 When I am slowly accelerating or mainaining a constant speed, in between 1500 and 2000 rpm, there is a momentary loss of power then it comes back and is loss again. It gets real "jerky" i guess is the best way to describe it. But there is no changes on the tach, the exhuast is constant, and there no codes. Any ideas? I might put a scan tool on it this weekend and watch iac, egr, and maf while it does this.

 

i know it's a pain but check plugs and for bunt/lose plug wires..

defintately check it with a scan tool..i have killed a meter before for spraying too much filter oil on my filter

just throwing some stuff out there since it appears its a k.i.s.s problem and over looked something???

 

Yes, those plugs are a pain. And i have checked the wires and they were fine. A misfire should throw a code. Also the it still has power and can still fishtale in when shifting in to third.

 

Chased a similar problem last year on my 93. Turned out to be the connector on the ICM. Found it by accident just wiggling connectors.

 

There is no misfire detection on your OBD-I 95. Have you checked the fuel pressure?

 

Too much OBD-II work at the dealership then lol. And no i have not checked fuel pressure, but i did consider it yesterday and started looking for fuel pressure checker kits. Advanced wants $36 for one, which is better than harbor freight who wants like $110.

 

This is what I have. I've had it for 10 years. http://www.actron.com/product_detail.php?pid=16174

 

Thank you, and which ever one i get its going to have to wait atleast a week cause i have have rent coming up and a few other bills. lol

 

ok so i got the fuel pressure tester, and heres the results: with ignition on and fuel pump not running its at 42 ish. With engine running and vacuum on regulator it runs at 35 and when i pulled the vacuum off it jumped up to 45. So the fuel pump is good, but what should it run at with vacuum on the regulor? Also i revved it and held at a low rpm just enough to take some vacuum off the regulator and the pressure went up then came down alittle bit and held steady. Normal?

 

Without the vacuum line connected, you want 43.5psi, and GM accepts anything in the range of 41-47psi as OK. Reattach the vacuum line and the pressure should drop proportional to intake manfild vacuum. A 6 to 8psi drop is normal with a stock cam. You are measuring rail pressure, and the rail pressure should vary as you move the throttle and change the vacuum. Open the throttle and vacuum drops, rail pressure increases. Snap the throttle closed, vacuum increases and rail pressure drops. So far everything you posted sounds normal.

The one test you don't appear to have performed is on the road, with the engine under load/WOT/max RPM. The pressure should not drop below 40psi. To check, you tape the fuel pressure gauge to the outside of the windshield.

 

Tonight i will try this. And yes i understand how vacuum works, my question was why does it the pressure drop when holding at a set throttle position? The vacuum should hold steady, right?

 

As you open the throttle to increase air flow, the manifold pressure increases (vacuum decreases). As soon as the engine reaches stable state - i.e. throttle position is not changing - the manifold pressure will drop slightly (vacuum increases).

 

Is that caused by adjustment from the idle air control valve or is that just the way it is? (I was thinking about it at work, and thought if the iacv adjusts after i open the throttle then it would increase vacuum, because of less air flow)

 

Think about what you do with the throttle as you bring it up to a specific speed. You open the throttle a lot to accelerate, then you feather it as the engine reaches the target RPM. The fuel pressure is going to spike, and then drop as the engine operates at stable RPM, rather than accelerating to reach that RPM.

The IAC valve only increases air flow slightly as the engine RPM increases, to prevent the engine from stalling when the throttle snaps fully closed. If the valve is supplying a bit of air, you can back down a bit on the throttle, but the intake manifold doesn't care whether the air is coming through the throttle blades or the IAC passage. The vacuum will stabilize based on the air demand from the cylinders, and the required pressure drop from the throttle blades to provide only the required amount of air.

 

Did the road test. At wot and redline or any time im really getting into it, it has around 44 psi. At normal acceleration it had 38 psi, slower acceleration it had 37 psi, and when maintaining a constant speed it would drop down to about 36 psi. When it started jerking from loss of power the pressure would spike up but then drop down just as quick. My guess is it would spike from the idle air control opening up which would decrease vacuum. Am I on the right track in thinking this? And are those fuel pressures in the norm?