PATRIOTIC1911

iTrader: (2)

So I have a 91 Firebird with the 305 TBI. The engine was rebuilt to factory specs about 6,000 miles ago. Last night while at my brothers, the car was idleing in the driveway for maybe 20 minutes engine temp was running 220. I something started knocking. Put a wood handle on my valve covers and everything internal there sounded fine. Once I drove home and the car cooled a bit, I started it and no knocking. Nothing throughout the day today either. I'm assuming, it was just running hot and something started knocking. I feel a lot better if I had ideas as to what it may have been. Engine sounded great all day today, though it never came near 220 temp either. Any ideas as to what it may have been?

cdjones85

It could have just been timing off for some reason. Running a low octane will cause a knock to develop. Ive even heard that a water pump going out can sound almost identical to a rod knock. I recently did a lot of knock investigation because the 91 Firebird 305 tbi i just got started knocking 3 days after i got it. I was hoping for something simple. The oil pressure was gone so i knew i was in trouble. I threw a rod out the bottom yesterday so now i am preparing for a 350 swap. I hope yours is not the same case as mine. If your oil pressure is still ok you are probably alright. good luck.

Schurkey

How does timing get "off"; and then heal itself later?
At idle?
Do you mean "fuel pump"? Never heard a water pump knock; have heard plenty of engine-driven fuel pumps knock/tap.




I have seen/heard bearings knock/not knock/knock again. Take oil pressure readings with a trustworthy gauge.

PATRIOTIC1911

iTrader: (2)

My oil pressure was fine and I run 91 octane only. I didn't hear anything all day today. It was weird. My brother and I were chatting next to the car. It had been idleing for a bit with the fan kicking on as it should. Then all of a sudden, we both start hearing this knock. The second it started, it stopped both of us in mid conversation. It was that noticeable. We just couldn't pin point it. It did seam to be more on the drivers side than the passenger side.
I live literally two minutes from him and in that time after driving, the temp of the car went down and i didn't hear it any more. Could it maybe have been exhaust manifold related or something?

PATRIOTIC1911

iTrader: (2)

Pulley bearings or something??

Schurkey

I was thinking spun main or rod bearings. Having spun, sometimes they overlap enough to take up clearance and the knock goes away--along with the oil pressure.

PATRIOTIC1911

iTrader: (2)

Wouldn't main or rod bearings continue to knock once they start? As far as my oil pressure gauge in the car, it's fine.

Schurkey

Usually. Not always. Sometimes it comes 'n' goes for awhile. It always gets worse over time--or the engine explodes.
Unlikely to be crank bearings, then.

PATRIOTIC1911

iTrader: (2)

I guess for now, I will just have to wait and listen to see if it does it again. This isn't a daily driver. Even in the non-winter months, I only get the car out a few times a month. I'll just pay attention to it and keep an eye and ear tuned. The car doesn't drip anything and doesn't burn oil one little bit. Thanks for what insight you all were able to give. Maybe it was just a fluke thing.......

mmadden55

Hot spotting from idling causing some detonation knock. antiknock sensor will generally pull some timing to stop that but at idle there isn't any to pull.

PATRIOTIC1911

iTrader: (2)

I've never heard the term "hot spotting" before. You up for explaining?