Is this serious?

[agfours]

Hi All, (trying a re-post)

Part 1:

I'm getting a ever-so-slightly uneven gravel-like 'dieseling' sound from the power train when under power in mid rpms. Sounds a little like a truck diesel motor, but not as much bass frequency as in a truck motor sound. It is more mid to high timbre / frequency sound and has a thick presence, and can be heard over the normal cruising engine sound, but is not loud.

I can't hear the sound when the convertible top is up, but it is very prevalent when the top is down.

The sound is most prevalent between 3100 - 3500 RPM's and is present only under load, and a little more 'tinny' under light throttle than under heavy throttle. It goes away when you take your foot off the throttle.

The time I hear the sound most is when I'm under light throttle at around 3k RPMs cruising. The sound is not there when sitting still in neutral, either at idle or revving the motor. I would NOT describe it as a rattle, but rather as if the alternator bearings were made of gravel.

Part 2:

Now, not sure if this is related or not, but I hear a separate and different noise when I'm idling that is coming from the area around the heat shield under the cat -

Here's the weird part on part 2: when that sound eminates, i've noticed that the Oil pressure guage needle lifts bearly (1/8") and freezes (instead of bouncing about as usual) while the sound is eminating. This sound is intermittent, lasts about 3 seconds each time, and re-occurs about every 20 - 30 seconds on average when idleing. I breifly pushed and pulled on the heat sheild and it did not feel loose at all.

These sounds sound quite different from one another and they are in different situtations, but there are old threads that have suggested that these sounds may be related to one another (cat baffles deteriorating, or heat sheild coming loose.) I'm just not so sure, as the Part 1 sound gives a more substantial presence (more than just a loose piece of tin like in Part 2)....

This is a stock 85 3.2 with 68k miles, well serviced, etc. I recently adjusted the valves, new cap, rotor, plugs (all looked good), recent oil change with no big metal (just some fine powdery sediment). Also of note, i have a wevo coupler and new bushings in the linkage (i know that it has been mentioned that the tranny noise might be more amplified, but this sounds like its coming from the top side of the engine bay. (not that I can stick my head under the car at 55mph to hear where this is eminating from...)

I want to make sure it is nothing critical like the chain tensioner, or something else major but don't know what I'm listening for.

I know it is hard to diagnose a sound through words, but I'm looking to see if anyone can shed any light/experience that would help distill this further to help determine if this is a chain tensioner, cat going bad, heat sheild issue, or normal tranny noise.

Thanks!!!

[Targa Time]

That the "diesel truck motor" sound may be a detonation issue. Check your ignition timing and advance. And make sure you are running on premium fuel.

[lupin..the..3rd]

Originally Posted by Targa Time

And make sure you are running on premium fuel.

^^ Very important. 3.2 Carrera does NOT have knock sensors and it will not retard timing when detonation occurs!! Premium fuel ONLY.

[agfours]

Originally Posted by Targa Time

That the "diesel truck motor" sound may be a detonation issue. Check your ignition timing and advance. And make sure you are running on premium fuel.

I always use premium fuel (93 octane), but am forced to 10% ethanol fuel here in Austin.


I thought the timing and advance was all taken care of in Motronic, with only mechanical advance in the dizzy... (this is an 85). Am I missing something?

[agfours]

To clarify, this 'dieseling' sound doesn't sound to me like actual pre-ignition sounds like I've heard in other cars I've owned. It's more like the kind of sound you get when you are cranking the handle around on a manually cranked electric generator, like on antique telephones, with that gravely hissing sound but this with a little more of a metallic ringing in there.

Anyone have any additional thoughts on either Part 1 or Part 2? Sorry for such a long post above, just trying to be as descriptive as possible, knowing how hard it is to diagnose sounds through words. I tried to get an audio recording, but my phone didn't have enough fidelity to hear the noise.

[E Sully]

Have you looked inside the distributor to make sure the weights and springs are clean, and moving.

[ant7]

Originally Posted by E Sully

Have you looked inside the distributor to make sure the weights and springs are clean, and moving.

Hi ED,
Something i have wondered lately, if the distributor dosent have any electrical components ie; timing pick up etc, the only reason i can think of for the mechanical advance weights and springs to be left in place is so porsche didnt have to design a complete new distributor with a fixed shaft and hence could use the old SC model initialy for the later 3.2.
Anthony.

[agfours]

Originally Posted by E Sully

Have you looked inside the distributor to make sure the weights and springs are clean, and moving.

I'll check it tonight. Just did the tune up and looked pretty clean when I had the cap off.

[E Sully]

911 owners, Who has looked at the Mechanical Advance in their DISTRIBUTOR?

[Flieger]

He doesn't have mechanical advance on the Carrera, IIRC.

Have you checked for any exhaust leaks or loose head studs?

[E Sully]

From the other thread.

Originally Posted by jpnovak

The timing is fixed due to the crank (flywheel trigger and reference). The mechanical advance is retained for rotor phasing to maintain alignment between the rotor tip and the dist. cap during electronic timing advance.

The mechanical advance mechanisms are one of the most overlooked systems in our older 911s. Timing is critical to good performance and a lot of cars suffer from poor performance due to this easily cleaned part.

[j911brick]

Originally Posted by ant7

Hi ED,
Something i have wondered lately, if the distributor dosent have any electrical components ie; timing pick up etc, the only reason i can think of for the mechanical advance weights and springs to be left in place is so porsche didnt have to design a complete new distributor with a fixed shaft and hence could use the old SC model initialy for the later 3.2.
Anthony.

All they need to do is have a shaft with the rotor on top. The 944 doesn't use any mechanical advance.

[agfours]

Just to follow up on this thread for the benefit of future searches - Just dropped it by my trusted local Porsche mechanic... good news is that it is not a broken head stud or anything serious. He believes it is pre-ignition, likely due to the chip advancing timing too far coupled with the probability that it it running lean. Will be replacing with stock chip to troubleshoot further.

The part 2 diagnosis was a loose cat shield or baffle as when I put the M&K cat bypass on, this sound went away.

[Chuck.H]

As I understand it, the 3.2 has NO mechanical spark advance -- the weights simply keep the rotor pointed closer to the correct terminal on the cap when at higher RPM's, since the DME is advancing the spark at this time... While the two actions are not sychronized together, they help keep the spark from jumping to the wrong terminal.

Chuck.H
'89 TurboLookTarga, 332k miles