Engine knock with no boost

[USA_muscle]

Hello all,

I have an 86GN that has began to knock. Now here is the weird part, I am not under any boost when it does it. Just subtle changes in RPM. Running down the highway at about 2K-2100RPM, asI gently press the accel pedal and the RPMs go up about 200-400, the knock gauge will start changing lights. I have a SM and although I have not had time to pull a complete set of numbers to give you guys, I can tell you that during that drive, the retard would range from 0-about 6 (at which point I would take my foot off the pedal and decel which would bring it back to 0). I am well below the point at which my turbo starts building pressure (about 2900-3K) when all this happens. When I got home I, noticed 3 things when I popped the hood: 1) the engine made a noticeable putting noise ("putt, putt, putt") which sounded like exhaust to me (also louder under the car, but have not had time to jack up and climb under; possible culprit???), 2) the fuel pressure had dropped to ~37 (no vac) from previously set 42 (set in Jan when I did a tune up), and 3) a slight fuel odor (not strong but noticeable) but could find any leaks around rail, injectors or regulator and nothing I could see under the car as far as leaks. Turned the pressure back up to 42 but still no change in the aforementioned noise. Looked for loose or cracked vacuum hose(s) but did not find any. I do not see any smoke from tailpipes, temp is normal, have not pulled plugs yet but they looked good when changed in Jan. I can pull some numbers when I get home this afternoon. In Jan I changed oil (w/filter), plugs, fuel filter, cleaned air filter. Any thoughts??? Sorry so long just trying to provide as much info as possible.

 

[sscamino]

Check the stupid stuff first,filter arrow forward,wires (loose or to wronge plug),pvc tube vibrating against the other vac tubes,anything else vibrating and giving false knock(just tap stuff to see if its hitting anything),injector harness check driver side above egr valve(as it gets hot it softens wires and they ground out and make inj. go wide open gas out the tail pipe),one of the screen could have come loose on the Maf sencer in the air intake ,cracked header manifolds. Sounds like cracked manifold between 3 and 5 or pass side in between wrinkled section or the cross over look for soot (black carbon).

 

[USA_muscle]

I figure the putt, putt is an exhaust leak because I had a similar issue in a 5.0 stang I used to have. Actually lost a small chunk from the passenger side header gasket on cylinder 2. Made a similar putt, putt sound. I will tap away when I get home later.

Could the exhaust leak have anything to do with the knock issue? This is my first forced induction car and although I have had it for going on 4 years now, the only real issue I have had is an ignition miss and a coolant leak a couple of years ago.

 

[sscamino]

Possibly,if there cracked get back to me and Ill tell you how to fix them. Fuel preasure issue could be loss of vac,wiring connection or ground and or pump going bad. The black steel lines get bent and can crack under them,rubber pieces comming off top of motor crack from age. These also supply vac to the regulator.

 

[USA_muscle]

found a crack in the downpipe right after it exits the turbo (3" downpipe) but did not see anything else on the headers or the crosspipe. Tried all of the tapping around and nothing registering on the knock meter. Pulled the MAF and it looks good. Wires hooked up correctly. Pulled plugs (except for 6; I hate that one and didn't have time to putz with it) and other than a reddish tint on the insulator they look good and have a .0.32 to 0.33 gap. Could the downpipe crack cause the knock issue?

 

[QUIP'6']

Downpipe will not give a knock signal. You can also check accessory noise by removing the serpentine belt and drive for a short run to check for noise from a/c. Alternator or water pump. Also another thing to check is number 3 exhaust lobe they are known to wear

 

[QUIP'6']

Need to take off rockers and do a leak down test

Sent from my DROID RAZR HD