Phantom Knock???

[Bestwestranger]

I have a 91 gst, and for the past 2 months have noticed a knock under boost. It only happens when I hit 8 or more psi, at no particular rpm range. It also is easiest to detect in 1st and 2nd gear under w.o.t., but in 3rd it seems to disappear, though there may be enough road noise that it is covered up. I found someone with a pocket logger and had it run. There was a few counts that showed up, but not near the amount that you can hear. I have had no check engine lights. It seems to run fine, but I have noticed a lack of power when the knock comes up. I have no major mods, other than a mbc set at 15 psi. I can turn it down to 10 and the problem remains. The timing is set right at stock specs. The readings from the O2 sensor seem fine, though they were non-existant untill the engine was warm (I think that is correct for it, but don't remember off hand). I have also dumped in as much octane boost as I felt comfortable, with no change. I have run several cans of injector cleaner, changed my fuel filter, and gave it new plugs and wires. The old plugs looked fine. I would like to blame it on a combination of heat and boost, but it didn't do this last year when it was hot, and it doesn't change at night when the air is cool. The guy who logged my car seems to think its some sort of phantom knock, possibly my lifters. I agree with the theory, but would like more input. Any help is appreciated. Thanks guys...

 

[philipkrotch]

try doing an oil change with a mitsu oil filter...maybe a knock sensor also. if it knocks too bad it will pull timing, resulting in the power loss you seem to notice. and u said u car hear the knock?? if thats the case, it could be bad lifter tick or u have a spun rod bearing.

 

[Bestwestranger]

I plan to do an oil change this week sometime, and flush the system while I am at it. The noise doesn't quite sound like a rod bearing, more like valve train chatter. To compare it with something, it would sound like a car that has way too much timing in it. A couple of my lifters have really been hammering on startup lately, so that is why I am leaning towards them as the problem.

 

[philipkrotch]

ya, if i were u, id pick up a set of revised lifters, theyre like 130 shipped on ebay. i got em and damn am i glad i did..no tick at all for the last 6 months. thats most likely your problem.

 

[Bestwestranger]

I was thinking about that. Thanks for the info!