90Iroc

Okay guys, I need your help again. My car, 1990 Iroc 305 TPI 5 speed, has been doing this "jerking" thing at 45 mph (1500-2000 rpm-->roughly) for over a year now. It doesn't do it all the time. It really pissed me off two days ago though. I went and adjusted my pressure regulator to 39 psi at idle. At the WOT setting it reads about 47-48 psi. Okay, a long time ago someone tried to help me with this but I rarely have time to mess with it. But this morning I made time. From idle in neutral (5 speed) has a pressure of 39 psi. When I raise the rpm by hand the pressure starts falling off! It went down to 35 and 34 psi all the times I tried it. What does this mean?? Is my pump no good? Thanks for taking time to read this, and as always your input is greatly appreciated!

Vader

IROC,

What it may mean is that your FPR diaphragm is possibly getting slightly more vacuum as the engine RPM is increased. You should "tee" in a vacuum gauge to the FPR vacuum line to verify the vacuum reading at the FPR while you do this.

Just my guess.

90Iroc

Hey Vader, what should the vacuum reading be at idle and then at the "WOT" setting? What does it mean then, if that's the case? What would be the "fix"? Do you not think it's a fuel pump or injector problem?

90Iroc

Anyone, Please help me. Your thoughts count!! This has been looked at 42 times, 42 heads are way better than one!

Arawn

I have no idea how to help you but my car will do the same thing. When in 4th gear doing around 40-50 (around 1500-2000 rpms) sometimes it'll start bucking. Drives me nuts cause there's a lot of 45 mph roads around here.

90Iroc

I just went and drove it with the gauge in my lap. My pressure never gets anywhere near my WOT setting! It might go to 44 psi or 45, but then it starts going down immediately after it hits that. And then it's at 45 psi for a very very very short period of time when I'm not even hammering it, like just taking off from first gear, then it shoots way down to 35, 36, or 37 psi. When I floor it, it doesn't go near the 47 psi mark either, it's just pretty much stable at 41 then. Any ideas?

Desert86Roc

If you push on the throttle, and the Fuel Pressure goes DOWN, then it is time for a new fuel pump pronto....

90Iroc

That's what I'm fearing man! I guess now would be a good time to go ahead and order those lowering springs, huh? I could go ahead and pop them in the back since I'll have to drop the axle. Thanks for listening and reading all this stuff! If anyone else has any advice, or whatever, please go ahead and say it.

JAY87GTA

Is the pressure steady or does the needle bounce? Mine was doing something similar but the needle started bouncing after I changed the pump. I guess that Walbro pump is too much for the regulator and is blowing the diaphram to far open, the spring can't compensate. I've got a Holley one coming because I can't get a damn regulator diaphram anywhere around here. :ar15 parts stores. Later :burnout