Comet still grunting like a big truck when accelerating

[Pete W]

Hi,

Thanks to you guys my 3 speed manual 61 Comet with a 144 is starting up on the first try and idling fine. My initial problem hasn't gone away though. When I press on the accelerator over 50 mph (maybe lower but under load) the engine sounds like a truck and grunts. It doesn't sound healthy. When I let off the gas it goes away. I rebuilt the tranny, balanced the shaft with new U joints, new clutch, rebuilt carb, new fuel pump, new ignition. All trying to find the cause of this low pitch grunt. It runs fine with only a slight vibration when in grunting mode. I could leave it this way but it is annoying and it bothers me. It sounds like the engine and not the tranny because it stops the minute I let off the gas regardless of speed. It seems like it gets worse the longer I'm on the road. Driving over 50 mph has become unpleasant.

Please tell me all I need to do is switch to high octane fuel...please. (<:

Thanks,

Pete

 

[jamyers]

Hard to describe noises online, I know...

If it's a low whine or continuous grunting only under load, I'd suspect either the tranny gears or the rear axle. Unless you replaced the tranny gears and/or the shafts, the tranny is still suspect.

The way to tell them apart is whether or not it's dependent on vehicle speed or a particluar gear (engine speed). If the noise changes with vehicle speed no matter what gear it's in (I'll bet that 50mph in 2nd would have your engine would up pretty tight...), then it's rear end. If it changes the same way in each gear (follows engine rpm), then it's the tranny.
Often it's not easy to tell them apart, and sometimes you'll swear the noise is coming from one end of the car when actually the problem is at the other end (like my old 2.77 tranny that had me convinced my rear end was completely gone).

Searching for the silver lining in your cloud, this might be a good time to think about swapping out that super-light-duty 2.77 gearbox for a later bulletproof 3.03, and maybe trading the also-super-light-duty 7.25" rear end for something stronger (anything else would be).

 

[Bort62]

exhaust leak?

 

[jahearne]

Check wheel bearings in the rear end.

 

[pedal2themetal45]

I'm thinking its timmig...
unhook the advance's at the dizzy and plug them. see it that stops it... it might not run good but if it stops the growell then reset your timming at a lower point by just 2* and see if that fixex it. if its better but still there drop it a couple of more... (IE: 12* to 10* and so fourth.).
tim

 

[Pete W]

Tim,

I can't get the vacuum off at the dizzy because the nut is frozen. Is it OK if I remove the line from the carb and plug it up there? Do you mean drive it around like that to see if the growling stops?

All of the noise stops when I release the accelerator. Wouldn't a drive train problem make itself known at all times except for when I depress the clutch pedal? It is constantly spinning.

Pete

 

[Bort62]

The problem you are describing really sounds like an exhaust leak to me, dude.

 

[Pete W]

Ian,

Are you are saying there is a leak in the exhaust system as in the exhaust minfold or the pipes/muffler? If I rev up the engine in neutral it runs great.

Pete

 

[Bort62]

I am saying that under load you may be hearing the leak in your exhaust more.

It is really hard to quantify a sound without hearing it, however. Do you know what an exhaust leak sounds like ?

 

[66 Fastback]

Can you give a better description of "sounds like a truck"?
Do you mean it sounds like a diesel?

I like the exhaust theory as a truck sound, but if it sounds like a diesel could it be a timing issue causing some pinging/rattlling?
Doug

 

[Pete W]

When I press the accelerator going over 50 mph it rumbles like a bigger engine which it is not suppposed to. It happens immediatly when I press the accelerator and stops when I let off. I had a crack about half way down my exhaust pipe one time and and it was a more high pitched, sputtering sound. This sounds to me like a big, throaty engine.

I would like to invite everyone to Pechtree City, Ga. to come hear the noise. I'll supply the adult beverages. (<:

 

[Anonymous]

Just a comparison for you. I had a worn out pinion bearing in my rear end, and I experienced a resonant hum around 30 MPH. I know it is rear end related as it started when I swapped in a temorary 7" rear to hold me over after my old one died, until I could rebuild an 8" to swap in. Also, the noise only occured while my foot was on the gas. (i.e. when I was using the engine to apply torque to the bearing)

Does the sound change pitch with vehicle speed or engine RPM?

 

[Pete W]

Is it OK to take the car out in the driveway and but the back end on stands and run it up to speed in gear and see what I can see?

 

[Anonymous]

Just a comparison for you. I had a worn out pinion bearing in my rear end, and I experienced a resonant hum around 30 MPH. I know it is rear end related as it started when I swapped in a temorary 7" rear to hold me over after my old one died, until I could rebuild an 8" to swap in. Also, the noise only occured while my foot was on the gas. (i.e. when I was using the engine to apply torque to the bearing)

Does the sound change pitch with vehicle speed or engine RPM?

hey, I had that with my old stang. wow. I didnt know

 

[jamyers]

Ugh, diagnosing funny noises can be a real pain, especially when you're trying to describe them, even moreso when you're typing the description...

Now I'm thinking you've got an exhaust leak. Small cracks will sound high-pitched, but a larger leak can get really deep and loud under load. At idle, or in neutral, there's just not enough flow for the leak to be heard over the rest of the noise, but under load there's a lot more exhaust gas trying to get out.

Take a length of heater hose, hold one end to your ear, and poke the other end around the exhaust manifold and pipe with the engine idling - I'll bet you'll find a leak spit/hissing at you. If that doesn't work, get the rpm's up (turn in the idle speed screw), and have a helper hold a piece of cardboard or wood over the end of the tailpipe. If you still can't hear any leaks, then your exhaust is good. Don't forget to take a look at the muffler.

(To answer your question about noises changing from load, yes - driveline gear noises (tranny or rear end) will happen under load, coasting, or decellerating, usually just one of those times. If it's happening all the time, then suspect a bearing or something constant. Most often gear noises are a whine, bearings like to growl or click. Like I said, describing car noises is a fun all its own.)

 

[Pete W]

I couldn't find any exhaust leaks but I did notice one thing when I took it for a drive. When I let off the gas over 55/60mph the growl increases for a spit second. There is also a vibration going on also. It makes the rear view mirror shake. I rotated the driveshaft 180 and it didn't help. I checked the rear end for wobble and everything is tight. I recently took off the cover plate ond refilled with oil and all of the gears look good. I'm still baffled.

 

[jamyers]

Does it make the noise when you've got it nearly floored in a lower gear?

Have you checked the motor / transmission mounts? The engine could be rocking over and transmitting noise/vibration to the chassis.

Hmm...me baffled too...

I tried to describe my funny noise awhile back, and just realized that my digital camera will record video (and sound). If you could get some way to record what you're hearing, that might be a help.

 

[Anonymous]

My 2c worth...
If it's happening at around 50 mph, I would assume the car is in high gear at that p[oint.
That being the case, it is not as efficient, and therefore the engine is under more load, than it would be under low or second gear.
If it sounds like a "truck" in high gear under acceleration, then that would lead me to believe it is detonation under load...

What grade gas are you using?
How hot is the engine running?

May have some carbon build-up in a couple of culinders, how do the plugs look?

Mike.

 

[Pete W]

Hi

I just jacked the rear of the car up and I notice something interesting. Looking at the back to front, the left side tire comes off the ground a good two inches before the right one does, and it stays this way as I jack it up. The left jack stand slides under the left axel well before the right stand will fit. When I lift the car with the floor jack the jack is centered perfectly in the middle of the pumpkin so everything should be balanced. I don't think the car did this when I bought it last year. I replaced the rear shocks about last winter because it still had the originals on. The new shocks raised the car rear of the car up significantly. I can't remember if it was lopsided like this before or after the new shocks. Maybe it has nothing to do with new shocks at all. Just maybe, it has something to do with my vibration problem.

Pete

 

[Bort62]

You don't have a rear tire rubbing on something do you ?
'
Get out the measuring tape and figure out what is broken.