[killakev]

Bought my first GTO (1968) yesterday. It has the original YS 400 engine in it but was converted to a manual 4 speed munchie manual trans at some point in it's life. The car hasn't been on the road since 2003. The motor runs good when idling and sound just fine when it's revved. The person I bought it from had the car running already so it wasn't like I found this car and just started it up. But of course I have limited info on what he did to get the car running. I was told the car wouldn't start when he got it, he did some involved work on the distributor, not just a cap and rotor, added new gas and it fired right up.

To shorten this up, basically the motor starts to make a "clanging" noise after you start driving down the road...and the noise gets louder and nosier as she heats up when continuing down the road. I pull over and it sound wonderful in idle..no noise. Revved it up in idle no noise. Head down the road and she starts clanging away and it sounds really bad. When this first happened i thought it may be collapsed lifters but there is no noise in idle. Even adjusted the rocker arms a little but no change.

Talked to a few people today more mechanically inclined than me and they suggested either timing or lower end problems such as connecting rods or piston pins. I'm looking for suggestions from the wealth of knowledge here on how to troubleshoot what the actual problem is. I did check the timing, specs say 9 degrees BTDC. Seemed to slightly backfire at that setting, sounded better when moved to around the 12 BTDC region. It is not the original distributor but looks like a later model electronic version with a blue cap and the ignition coil in the top of the cap. Any suggestions to test if it is the timing or bad connecting rod or pin?

[Squidward]

I've had a few con rod bearings go bad in Pontiacs. They tend to be a "floating" knock, and severity can vary as you rev. They also seem to knock at a greater frequency than lifters. They can also kind of go away at idle. Lifter knocks are slower and more regular. Rod knocks also can be influenced by pulling a plug wire and seeing if it goes away.

Your timing sounds pretty conservative, so I can't imagine that would be it.

I've also had the under load clanging, but it was a flexplate loose bolt. You have a manual, so I can't speak to that.

Other possibilities are bad rocker stud, bent pushrod, or some other valvetrain issue. Pull valve covers to see if everything is good. Harmonic balancer issues can also make clunks. Sometimes exhaust leaks can also be tricky to diagnose.

Do you feel it as well as hear it? Bad u joints might be possible.

[killakev]

I did pull the valve covers off. Did not find any "severely" loose rocker arms. Oil was coming through the push rods. No noticeable bend push rods, I was ready to just get new hydraulic lifters to change them out but thought again about it. Doesn't sound like a exhaust leak, have had those before. This is a clanging noise under load. And I mean BAD. So bad that I baby it back to port. The lower the reve is in gear the worse, but you can hear it just as well if you get on the accelerator.

[dmac]

flywheel/clutch

[killakev]

Definitely feel it as you go down the road. Interesting thought that it could be the universal joints.....The car does move up and down as it idles and going down the road. The motor mounts look good and I did not see any separation. That would be awesome if it was just the universal joints. I think I will pull the drive shaft and see how the joints feel. Any other suggestions?

[Will]

Check the transmission mount too.

Also, your description of the noise as "clanging" is odd to me. A rod knock or bad lifter present as a knock or tick, not a clang. A rod knock literally sounds like someone is hitting the engine with a hammer - a loud, sharp, solid rapping or knocking sound. A rod knock might not be heard at idle or when free-revving with no load but it will present itself as soon as you load the motor, like letting the clutch out to get going from a stop. It's a bad noise that will get worse very quickly and make you think twice about driving the car at all.

"Clanging" sounds more like something in the exhaust or drivetrain. I once had a transmission mount go bad which allowed the tranny to shift around to the side and hit the exhaust pipe and the driveshaft tunnel with the u-joint and that made a heck of a lot of noise that was obviously coming up through the floor.

I would check the clutch/bellhousing area, the engine & trans mounts, the u-joints, the exhaust connections and routing, and all the suspension mounting points.

[huddie]

Maybe flywheel and starter.

[Tim Corcoran]

Could it be pinging from detonation? If the engine is stock as you say and your running it on todays pump gas it could be detonation

[craigs69]

I agree with Tim. Under load and particularly at lower RPM makes me think detonation. You definitely need to run non ethanol 93+ octane fuel. If you have a gas station that sells 100+ octane, try adding a few gallons to your tank and see if it gets better or goes away entirely. You could also add some octane boost to your fuel.

As for your timing issue, you might also want to check and see that the previous owner did not use the original resistor wire from the coil to power the HEI distrubutor. You can verify that you are getting 12V to the distributor by sticking the positive probe from a volt meter up into the positive plug on your distributor cap so you can check voltage while the car is running. You need at least 11V to run your HEI decently. Once the wire is attached, let the car run for a few minutes. The resistor wire will show 12V when you first start the car, but you can watch it steadily decrease down to 7 or 8 volts after a minute or two. I encountered this problem when I purchased my car. The previous owner used the resistor wire to power the HEI and it gave me fits until I figured it out.

[killakev]

Originally Posted by Will

Check the transmission mount too.

Also, your description of the noise as "clanging" is odd to me. A rod knock or bad lifter present as a knock or tick, not a clang. A rod knock literally sounds like someone is hitting the engine with a hammer - a loud, sharp, solid rapping or knocking sound. A rod knock might not be heard at idle or when free-revving with no load but it will present itself as soon as you load the motor, like letting the clutch out to get going from a stop. It's a bad noise that will get worse very quickly and make you think twice about driving the car at all.

"Clanging" sounds more like something in the exhaust or drivetrain. I once had a transmission mount go bad which allowed the tranny to shift around to the side and hit the exhaust pipe and the driveshaft tunnel with the u-joint and that made a heck of a lot of noise that was obviously coming up through the floor.

I would check the clutch/bellhousing area, the engine & trans mounts, the u-joints, the exhaust connections and routing, and all the suspension mounting points.

Well, I would definitely not describe the sound as a "tick". Maybe my description of the sound is off but just as you described the noise comes when putting load on it and definitely makes me think twice about driving the car at all. It sounds really bad....so I'm not driving it. I guess I need to get a knowledgeable person to take it for a short ride so they can hear it for themselves. Any North or Central New Jersey experienced guys out there?

[killakev]

Originally Posted by craigs69

I agree with Tim. Under load and particularly at lower RPM makes me think detonation. You definitely need to run non ethanol 93+ octane fuel. If you have a gas station that sells 100+ octane, try adding a few gallons to your tank and see if it gets better or goes away entirely. You could also add some octane boost to your fuel.

As for your timing issue, you might also want to check and see that the previous owner did not use the original resistor wire from the coil to power the HEI distrubutor. You can verify that you are getting 12V to the distributor by sticking the positive probe from a volt meter up into the positive plug on your distributor cap so you can check voltage while the car is running. You need at least 11V to run your HEI decently. Once the wire is attached, let the car run for a few minutes. The resistor wire will show 12V when you first start the car, but you can watch it steadily decrease down to 7 or 8 volts after a minute or two. I encountered this problem when I purchased my car. The previous owner used the resistor wire to power the HEI and it gave me fits until I figured it out.

I put 10 gallons of new high octane fuel in it, probably 92 Octane since it wasn't Sunoco. Didn't make a difference. As far as the distributor, the resistor wire is not being used. It currently is rigged and is getting 12 volts directly from the battery.

[ELKHORNAOG7]

If it is doing it when under a load what does it do when driving and you let up off the gas peddle does the noise also go away or do you still have the noise as long as the wheels and driveshaft is turning??????----BOB

[killakev]

Originally Posted by ELKHORNAOG7

If it is doing it when under a load what does it do when driving and you let up off the gas peddle does the noise also go away or do you still have the noise as long as the wheels and driveshaft is turning??????----BOB

The noise goes away when I let off the gas pedal and coast. I will be working on the car after work tomorrow. Going to go through the distributor and timing 100% to rule that out. I was thinking about the U joints but the noise isn't coming from under the floor, it's coming from the engine.

[killakev]

Forgot to add that I'm also going to pull the push rods to see how they look and re assemble as per Pontiac shop manual. Will also take compression readings and post tomorrow night with results.

Any test I can do to determine if there is a lower end problem?

[killakev]

And again I forgot to add that at low RPM's, such as starting off in first gear after driving and getting the motor warmed up, the sound reminds be of a diesel engine rumbling and chattering.

[killakev]

Originally Posted by Squidward

Rod knocks also can be influenced by pulling a plug wire and seeing if it goes away.

So if all else fails tomorrow, I should pull a plug and drive down the road to see if it becomes quiter and if so, it's that cylinder's connecting rod?

[killakev]

Originally Posted by dmac

flywheel/clutch

Can anyone elaborate on this idea?

[killakev]

Originally Posted by craigs69

I agree with Tim. Under load and particularly at lower RPM makes me think detonation. You definitely need to run non ethanol 93+ octane fuel. If you have a gas station that sells 100+ octane, try adding a few gallons to your tank and see if it gets better or goes away entirely. You could also add some octane boost to your fuel.

I will get some octane boost tomorrow and see if there is any change.

[WDCreech]

The octane boost that is sold in stores won't do anything to help your problem! They're just snake oil. There are chemicals that will stop the detonation, but they are expensive and some are dangerous.

[dmac]

Originally Posted by killakev

Can anyone elaborate on this idea?

I was thinking loose flywheel bolt, something hung up in clutch.

Your description as 'clanging' makes me think of metal on metal like a bell. Rod knocks I've had I describe as a knock, with the sound loud but muffled by the block.