3.2 making knocking noise under load

[kjb]

Hello,

This really sucks: I'm afraid my 86 carrera has snapped a head stud!

Driving back from the coast yesterday, I suddenly heard a new noise: some type of knocking under load in 4th gear going uphill with the A/C on.

I let the car cool down for a few hours while eating dinner with some friends. Later at night when we drove home, the noise still seemed to be there when I got on the gas above 3500 rpm. The car also seemed to heat up quicker than usual and run a little hotter.

If it were the engine pinging, I think it would have showed up last weekend, as I drove the car really hard with a group of other Pelicanites and I haven't filled the car up since.

After browsing some threads here on the forum, my suspicion is that at least one head stud has snapped. As I don't have access to a garage, I will take my car to a mechanic this week to have it checked. Is there anything else I should have them check?

The valves were adjusted three months ago and I assume all head-studs were OK at that point in time. 121k on the odometer w/ a top-end rebuild at 89k.

Aaaah,

J

[cowtown]

You're right to check the studs, but given the overheating/under load nature of it, it could also be running lean. You could check out the fuel system as well - clogged filter, low pressure, bad injector, bad tank of gas, that type of thing.

[Jack Olsen]

Are you sure it's not your CV joints?

[kjb]

Originally posted by JackOlsen
Are you sure it's not your CV joints?

I'm not sure about anything at this point.

The knocking noise was proportional to engine speed and only appeared above 3500 rpm or so. C/V joint noise would be proportional to vehicle speed, wouldn't it?

Cowtown: thanks for the tips. I might as well change the fuel filter (which is due) before heading over to the mechanic.

/ J

[Jack Olsen]

Engine speed and vehicle speed are going to be locked together, so long as you're in fourth gear. I had a CV knock that I could only generate under moderate load in fourth gear, much like you're describing, except starting at a lower RPM.

Compression and leakdown time?

[kjb]

Originally posted by JackOlsen
Engine speed and vehicle speed are going to be locked together, so long as you're in fourth gear. I had a CV knock that I could only generate under moderate load in fourth gear, much like you're describing, except starting at a lower RPM.

OK, I'll add that to the list of things to check. Thanks for elaborating!

Compression and leakdown time?

My car has burned less than a cup of oil in the last 2000 miles, so my guess is that the valve guides and rings are fine. Compression was 175 all around 5000 miles ago, when I bought the car. I guess it never hurts to check again, though.

Thanks for the input, guys. My mind is more at peace than last night.

/ Johan

[nostatic]

didn't they do the headstuds when they rebuilt the top end?

[kjb]

Originally posted by nostatic
didn't they do the headstuds when they rebuilt the top end?

You would have thought so, but the previous, previous owner just did the valves & the clutch. Otherwise I wouldn't have been as worried.

I just replaced the fuel filter. I poured the fuel out of the old filter into a container, and it looked really goopy. There's a bunch of sediment in the bottle. This give me a slight degree of hope.

I'm going to take the car out for a short drive to see if anything changed.

/ J

[Ho Hum 74]

How's your oil pressure? I'd be REALLY worried about a rod bearing failure only because that's exactly what happened to me.

You should get between 15 and 20 psi per 1k rpm under warmed up conditions.

Tristan

[kjb]

Originally posted by Ho Hum 74
How's your oil pressure? I'd be REALLY worried about a rod bearing failure only because that's exactly what happened to me.

That's something that struck me too. The oil pressure is above what it needs to be according to the gauge (which was replaced in March '02 by PO). If a bearing fails there'd be some shavings in the oil, correct?

I just came back from a quick ride, and the noise is still there. It's not very loud, but there's definitely something changing when I get the engine under load. My problem is that this is my first 911, so it's the only refererence I have.

I'll just take the car to the mechanic to get it diagnosed.

Thanks for all the input,

Johan

[kjb]

Here's a picture of the gasoline I dumped out of my old fuel filter. There's grit swimming around in the brown stuff at the bottom. Could I have gotten a bad tank of gas from Shell, causing all this?



/ Johan

[Por_sha911]

"Grit swimming in brown stuff..."
That doesn't sound very good. I suspect that you may have a lean condition caused by a clogged fuel filter. Have you replaced the filter? Do you have the ability to check your fuel pressure?

[Kurt B]

3.2's do not break headstuds like SC's. Ask around.
I wouldn't worry about a busted stud. My car pinged like the dickens. I put the stock chip back in and it went away. There's the possibility of bad gas and a host of other things.
But a 121k miles -> busted headstud -> a CA car? no freakin' way. I have 200,100 miles on my car as we speak. In fact, my car's real beatings didn't begin until I bought it at 123k miles in 1999.

[kjb]

Kurt,

I think you are right. My mechanic ruled it out after I told him my observations.

I'm still having it looked at today since I'd been driving over 100 miles on that tank of gas before I noticed any knocking. On the other hand, I only put 15 gallons of gas in the car the last time I filled up. If the gas doesn't mix well in the the tank while driving, symptoms wouldn't show up right away.

I'll be a wiser man when I find out the answer.

/ Johan

[Ho Hum 74]

Here's what I'd do to rule out bearing failure:

Dump the oil from the sump plug (underneath on the sump plate). Look carefully at the magnetic drain plug. There will be some metal around this. However the real metal you'll be looking for is non-ferrous and will not be on the plug. What you want to do is look at the screen in the engine. Hopefully you don't find any gold/copper looking flakes in the screen. If you do, I'm afraid this is pretty bad news as it means that the bearing has worn down to the copper and will need to be replaced ASAP otherwise it will start spinning and destroy your crank and rod surfaces.

Tristan

[kjb]

Tristan,

I understand the severity of a bearing failure, and it kept me awake the other night.

At this point, I've left it to my mechanic. His initial assessment was that I don't have a bearing problem, as that would give a slightly different type of sound and would not go away when I let off the gas.

BTW, the sump plate went away sometime in the 83-85 timeframe (possibly with the introduction of the Carrera). My car only has a drain plug on the side of the case.

I'll post when i get a verdict from my mechanic. Incidentally, when I drove my car in this morning, I couldn't hear any noise. Time will tell.

/ Johan

[Ho Hum 74]

I'm hoping it turns out good for you Johan.

Best of luck
Tristan

[Tim L]

I had an 84 3.2 119,000 mi and a bearing went bad. It took me until the thing was completely gone before I figured it out. The sound could only be heard under hard acceleration at higher revs. It sounds like a soft knock or almost like an exhaust leak and only under heavy load. You can't hear it by just revving the motor. I think you might be able to tell by opening up your oil filter and looking for bearing parts it’s worth the $5 for a new filter.

Tim

[vash]

good luck johan! keep us posted.

[kjb]

Hello again,

I picked my car up yesterday afternoon after a very thorough run-through. Several people drove my car and couldn't provoke any abnormal noises under load.

I either have an extremely vivid imagination or the problem went away when I changed fuel filters. It could be bad gas as well, I suppose.

So what do I do if I find pieces of bearing in my oil filter? Start crying?

/ Johan