Would a disconnected vaccuum line to a fuel pressure regulator cause a vehicle to run lean at moderate rpms and could this generate knock? I was out in the 91 Z today and I knew something wasnt right. The idle was higher than normal and car felt like cruise control was engaged. Searched for leaks and couldnt really find much. On the drive I was at about 3200-3500rpm and heard what literally sounded like someone knocking on the door 3 quick times. Slow down and come to a stop. Pop hood and didnt see much in the way of any obvious issues. Left the car out of gear and a couple gentle revs...nothing...gently increased the rev spurts till about 4k rpm and nothing. No funny rattles or noises. Good oil pressure and fuel pressure. All the basic stuff seamed to be in order. About 5 minutes later it happend again so I babied the car home. Lots more searching so I poped off the VC and all rocker arms are still attached and adjusted. So on reassembly I noticed the fuel pressure regulator vacuum line has a 1" split. Guess this was the vacuum leak as it immedietly idled at its usual spot upon start up. Repeated the testing of rpm revs gradually getting higher and a couple gentle runs on the street and a few decently aggresive accelerations and no noises out of the norm. Anyone have an opinion? Thanks for any replies guys.

Kris

 

While the leak would admit a little more air, the MAP should detect the slight drop in manifold pressure and compensate a bit. The lack of vacuum at the FPR and resultant higher fuel pressure would likely have a much greater effect and actually tend to enrich mixture more than the small vacuum leak woul lean it out. This would be especially true at a larger throttle opening/higher load. There may have been a slightly leaner mixture localized at the #5 or #7 cylinder due to the vacuum leak right near those two intake runners. However, for that to cause a detonation the leak would have to be more significant, IMO. I'm guessing the higher rail pressure and richer mixture across all cylinders was more likely the cause, but I could be mistaken.

 

Wow! Not as simple as I thought lol. Thank you very much for the detailed info. Does what I describe sound like knock cause believe it or not I have never experienced this noise before. Heres another quick question. My engine is a 355 with 9.8:1 compression, Edelbrock 64CC heads, HSR, Accell 24lb injectors,Accell 58MM TB, Crane Gold 1.6 RR, Full MSD and Edelbrock TES headers, Comp Cams 08-305-8 cam....as a general idea what should my FP be set at? I know this question is beat to death but would appreciate any suggestions. Again...thanks for the replt.

Kris

 

Just a quick update. Was checking a few things out and found what may have be a major problem. Looked at my MSD distributor and cap and noticed the cap was looking strange. Decide to pull it and then saw that the pin on the cap for the coil wire was completely seperate fromthe cap.Upon further inspection it appears that the center pin on the cap for the coil wire was melted off! Yes melted. The pin was still fully intact with the coil wire and there was a melted hole in the cap where I could see the rotor. This cap was less that a year old with less than 2k kms of drive time. Im guessing it must have be cracked either before or during install and that the spark of my MSD coil and 6AL was just to much. Will take pics and report back after a few test runs.

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