You could very wellhave a bad fuel pressure regulator. Try unplugging the vacuum line to the regulator while it's running, the pressure should go up a little, andsee if the idle smooths out any when you unplug it. Let us know what the results are.

 

The TPI article said proper fuel pressure had to be confirmed under three circumstances.

I don't remember what each tells ya', but obviously each readin' is important...

Early c4's have an exhaust resonance problem at 2000 - 2200 rpm. I doubt it's tied to the motor tune problem.

 

Just unplugged the fuel regulator and pressure went from 28 to 38psi, still rough idle though. Do you think the noisy cat could trip the knock sensor. Thanks.

 

Yeah possible, but I still doubt that's what's goin' on here...

 

Do you think its my fuel pressure regulator based on the psi increase.

 

OKAY...

I just went back and looked at the TPI article. It says 43.5 is proper pressure for most years of TPI.
https://www.corvetteforums.com/upfil...DAF97A25E6.jpg
https://www.corvetteforums.com/m_37283/tm.htm

 

43.5 with NO vacuum.

...and 1 psi drop for every 2 inches of vacuum. You got a 10 psi difference, which means 20 inches of vacuum (20 INCHES OF MERCURY IN A BAROMETER).

AIN'T NO WAY. Check your reading again. If the numbers you got are right, that's the problem...

 

I've got a 15-18 inch's of vacuum on my motor. If I had 20 inch's, the change from 28 to 38 is just right( 1985 had 40psiunlike the 1986 and later.)or am I missing something here. What is the normal reading for vacuum on a 1985.

 

Mine runs fine with 32 psi fuel pressure. If yours didn't smooth out when you unplugged the vacuum line then I feel like you have something else causing your problem. Try this....loosen the bolt thats holding the distributor down just enough so you can turn it. Crank the car, then turn the distributor both ways (raise and lower) just a little and see if it runs any smoother. be sure to markthe distributor somewhere so you will know where to set it back if this doesn't help any. Thats basically how I found my problem. My timing was set too high, which BTW was set by mr goodwrench.

Hope your engine hasn't jumped timing.

 

Yeah, BUT, the article says when you plug back in the vacuum line to the regulator, you will get a 1 PSI drop for every 2 inches of vacuum.

You're gettin' a 10 PSI difference, which means a 20 inch swing in vacuum. That's way off!!! A hurricane doesn't cause thatmuch variationin barometric pressure