Hey everyone,
I just bought a '91 Pontiac Firebird last weekend. Well, driving home from work last night, it started to make a loud knocking sound from the engine compartment (I still don't think it's the engine, though) It was knocking from the back area of the engine compartment, and the knocking was strong enough to vibrate my accelerator pedal while I was driving. It was still able to accelerate and operate normally, except for that loud knocking noise. At idle, it would almost not make that noise, but when I would drive it, it would vibrate in accordance to the speed which I was going. I've been told everything from a knocked rod to a messed up flywheel to loose bolts on the torque converter. I need to check all those out, I guess, but I need to know where to start. If anyone has any ideas or suggestions, please let me know. Thanks.

1991 Firebird Formula
5.7L V8
Automatic Trans.
137,000 miles

 

Start with the cheap, easy, and obvious. That would definitely include the converter bolts. A cracked flex plate will make a noise that sounds unpleasantly similar to a rod knocking.

What was the oil pressure while all this was going on? That's the surest indicator of whether there's a problem inside the engine, or outside.

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"So many Mustangs, so little time..."
ICON Motorsports

 

Well, the oil pressure stayed at about 30, it didn't change from the time when it wasn't making the noise to when the noise was present. I checked the converter and the flywheel(flexplate) and didn't feel it to be loose or have anything missing. The cover that goes over the converter wasn't there, so it was pretty easy to get into it(after I jacked it up for what seemed like forever). I noticed tonight that when I shift it (park, rev. neutral, drive, etc), the noise will usually cease for a second or two, then start back up. What gets me is that it still seems to have power like before, only with this annoying sound. The guy that usually helps me out when I have an auto-related problem won't even mess with this car. Too low to the ground, he said. If you have any other suggestions, I'd be glad to try them. I sure don't want to have to pull the engine if I don't have to.

 

Take a look at the exhaust system too.

 

 

definatly sounds like a cracked flexplate to me too.. it may be hard to see if the crack is near the top, also that cover not being in place could point to that being the problem too.. with the motor idling, take a length of heater hose and put it to your ear using it as a stethoscope to listen around the engine compartment to try to pinpoint the noise...