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1974 Oldsmobile Cutlass Engine making Loud Knock!

Tiny
79TRANSAM455
  • MEMBER
  • 1974 OLDSMOBILE CUTLASS
  • V8
  • 2WD
  • AUTOMATIC
  • 10,000 MILES
I own a 1979 Pontiac Trans Am with a 1974 Oldsmobile 7.4L 455 in it (and a TH-350 auto tranny). I have spent the last year rebuilding bits of the engine in hopes to drive it for my wedding (in 5 days!).

Before it ran fine. I changed the spark plugs, spark plug wires, distributor cap, converted over to an electric radiator fan and a holley blue series fuel pump, installed an edelbrock torker intake and a 800cfm edelbrock performer carb. I also changed the lifters, pushrods, and rockers because I noticed 2 of the lifters were busted (the retaining clips werent even on them). Everything sounded great while driving it around. It would accelerate at WOT with more power than ever and no unusual sounds or hesitation at all. While driving down the highway I experienced a loud "Pop". The positive battery cable made contact with my headers and melted threw giving a direct contact between my battery cable and header. My car died instantly and once I fixed the wire and got my car started again it had a frightening Knock that is RPM driven. The more RPM the louder it gets.

It gets more violent with the more RPM you give it and it is a pretty good sounding knock too (like hitting the engine block with a hammar). I thought it was my lifters so I let it warm up for 20 minutes and then revved it up to 2500 rpm for a while and then brought it back to idle. This didnt solve the problem but when it was at 2500rpm the knock would dissapear for a second and reappear again. (Like the knock was skipping every other knock). The knock The knock occurs at idle about 4 times a second from what I can hear. I changed my lifters just now and changed the oil and filter and retorques my non adjustable hydraulic lifters to 25lbs (factory specs) but the sound is still there.

My Wedding is in 5 days and this has been my dream car that Ive always wanted to drive away in so please any help is greatly appreciated. Im to nervous of driving it to a shop since the closet one is 10 miles away and I dont want to be stranded on a highway.
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Tuesday, December 8th, 2009 AT 9:25 PM

6 Replies

Tiny
BUDDYCRAIGG
  • MECHANIC
  • 2,262 POSTS
I'm sorry to hear about your trouble.

But let's just get right to it.

You didn't explain if the knock speeds up with the RPM, you only said that it gets louder.

If it does speed up, I would suggest a load balance test and see if the knock is less apparent when one of the cylinders are dead.
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Wednesday, December 9th, 2009 AT 12:10 AM
Tiny
PEAR69
  • MECHANIC
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I will not sugar coat my opinion just because you are getting married -- and BTW Congradulations and good luck.
You mentioned that you changed the lifters because two of them were busted and the retaining clips were not on them. Did you find the retaining clips? Or are they somewhere in the engine? Did you check the cam for any damage or inspect it before you installed new push rods?
Were the old lifters hydraulic or solid?
What is the oil pressure? Test the oil pressure with a tester - do not rely on the gauge.
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+1
Wednesday, December 9th, 2009 AT 12:20 AM
Tiny
BUDDYCRAIGG
  • MECHANIC
  • 2,262 POSTS
I'll let pear69 take over from here.
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Wednesday, December 9th, 2009 AT 12:29 AM
Tiny
79TRANSAM455
  • MEMBER
  • 3 POSTS
To answer your question, yes the knock speed increases as I increase the RPM
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Wednesday, December 9th, 2009 AT 12:44 AM
Tiny
79TRANSAM455
  • MEMBER
  • 3 POSTS
Yes I found the two retaining clips in the bottom of my oil pan. The Old lifters as well as the new ones are both hydraulic. I know the oil pressure with these oldsmobile engines run low oil pressure are idle so thats why I keep the rpm around 2000-2500 in hopes that the lifters or whatever the sound is goes away. I have taken the valve covers off and can see a pretty good amount of oil being squirted out of the pushrod around 2k RPM.

I am concerned about the fact that this noise occured right after my electrical failure (when my batter cable hit my headers). The second I fixed the wire and started my engine up again the noise was apparent (this noise has never occured up to this point).
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Wednesday, December 9th, 2009 AT 12:50 AM
Tiny
PEAR69
  • MECHANIC
  • 1,482 POSTS
The fact that you started to hear this knock when the battery cable hit the headers may or may not be a coincidence. Start the engine in the dark and look for sparks around the plug wires - spark plugs - ignition coil ETC. If no sparks, then try what buddycraig said. Start the engine with one plug wire off -- do this 8 times - once for each cylinder. If the knock goes away or is reduced, then you have a serious spark knock and the issue is with the ignition system. Which I personally doubt. If the oil pressure is low + you are running the engine at a high RPM + sometimes at WOT + the hydraulic lifters were busted (which may have hurt the cam) = all of this adds up to a rod knocking inside of the engine. Rule out all of the ignition system items, but I would say you have a rod knock caused by an oil pressure issue. Unless you can tear down the engine - do all the machine work necessary - I am afraid the car will have to wait for the first anniversary.
BTW - if you want to take the car to the shop that is only 10 miles away - ask them if they will tow it.
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Wednesday, December 9th, 2009 AT 8:57 AM

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