Engine Knock

[Walt Fricke]

Driving along at about 30 mph on rutted icy streets, when what to my wondering ears did appear but a knocking, knocking, knocking at my, well, rear of my SC. It shows up after maybe half a mile of driving or a bit less, and only when I am on the gas. The higher the gear/lower RPM (at same speed), the worse it is. Idling, or reving in neutral, did not produce this symptom.

This happened maybe 5 miles after filling the gas tank. I retarded the ignition by about 1/4". No help. Replaced spark plugs (evidence of rich running, but old and otherwise unremarkable). No help. Checked distributor - all parts that can move could do so freely, things looked OK, though I haven't gotten around to sucking on the diaphragms. The distributor shaft appeared to have about 1/8-1/4" of axial play. Checked leakdown - close to zero for each cylinder.

Stethoscope revealed moderately increased noise in right chain housing. Pulled cover. Tensioner seemed a bit soft. Removed it and found that it had lost its upper wire clip - the one that holds the cylinder into the body. From having rebuilt the engine I know it had one. Replaced tensioner. Disassembly of old one revealed nothing unusual, though (no seals to wear on these things). And that clip is not necessary to the proper functioning of the tensioner. Couldn't fish out the clip from the nearby engine case (unlike the lucky Oh, Crap's, this is the compartment at the very far end of the case, with the big aluminum IS gear and its compartment walls between it and the sump plate). But it is thin.

Shucks, result same: fire up car and slowly drive out to well plowed highway. All is well. Open it up. Maybe that was it. Half mile down the road it does it again. Limp home.

I am wondering if I put regular instead of premium (91 ron+mon in Colorado) in the tank (about 60% full when I filled it, or if it was bad gas. And my wife had previously filled from what she then learned was the last of what was in that tank at the station). The storms we have had here interrupted the resupply of service stations as they did much else. And I am not looking forward to the logistics of draining 20 gallons or so of gas and wondering what to do with it.

Evil thought - the errant clip, despite being thin, falls to the bottom when engine is off. After oil warms up a bit (I run Mobil 1 15/50 year round) it gets moved to some bad spot. But this does not correlate with the fact that it is only under load that it knocks.

I don't know how to check gas for octane qualities (other than by replacing it). Bad gas usually ends up with an engine that is hard to start, or idles poorly, or misses when running.

Maybe I can retard the distributor to the point where the engine will hardly run at idle, and take it on the road for another test?

I don't think either a bad tranny (rebuilt just this summer and I think they are whiners, not knockers, when going really sour in the gears) or bad CVs or axle bearings would produce these symptoms. When I get down to the 5-10 mph range in 1st gear I don't hear anything untoward. Plus my CVs/trannys/axle bearings have always initially spoken with a soft voice, getting more and more insistant only as I ignored them. This came on full blown, like Venus.

I've had my share of things loosening in the valve train. When they do, I have been able to hear them at idle, or by buzzing the engine in neutral. And none have been at all temperature or whatever dependant. And even a single good inner or single good outer valve spring is adequate to 6,000 rpm or so, and we are talking 2,000 to 4,000 max here.

Color me perplexed.

Walt

[aigel]

I can tell you that a rod would not knock in this manner either. They generally do at low rpm when warm / idle and go away at higher rmps.

The fact that it seems to run on the rich side and the fact that it is winter should both help prevent pinging. Pinging is a tincan sound, not a knock at all.

George

[beepbeep]

WHat do you mean by "knocking"? Is it pinging or is it regular "knock, knock, knock"??

Pinging beacuse of bad fuel is irregullar and occurs usually at torque peak (where VE is best).

If this sound is regular there is something else to it.


I believe you might have exhaust leak somewhere. Maybe gaskets between heads and headers or a crack in the header.

It will make charasteristic regular sound when car is loaded due to increased backpressure. Check for soot around head exhaust gaskets.

[emac]

Walt

I disagree with George (sorry) but a rod knock does not go away. It is there from the moment you start the engine until you shut her down. If this is a knocking sound I would guess it's not fuel. Goran has a really good idea with the exhaust leak. Never in my P car but I have had other cars with a very distinct "knocking" sound from the exhaust, albit it is different and really not like metal against metal.

[MBAtarga]

Have you checked for a broken head stud?

[Grady Clay]

Walt,

Since it occurred just after fueling, I would empty the tank and start with known good high octane gas. Just pump the old fuel into your tow truck.

(For anyone who don’t know Walt, he is one of Colorado’s most talented Porsche people.)

I’m sure Walt can distinguish between a knock and a head leak but I’ll bring this up anyway. With all the snow, the heat exchangers will drag and could have disturbed a still sealed cylinder with broken stud(s).

Walt, does your case have a sump plate? If so, inspection is in order. If not, do you have the “can-opener” tool to get in the oil filter?

Best,
Grady

[quattrorunner]

Didn't you say that before you got into the tensioner cover, that the sound seemed to come from there? Does it still? And won't a bad bearing go away with no load and come back with load?

[Walt Fricke]

Thanks, guys

I think I'll pull the valve covers and look around. That will let me check the studs while I am at it. I hadn't thought of that (perhaps because I have so far escaped that problem).

Grady, I don't want to put the gas in my tow vehicle, since it is a GMC 5500 with a Duramax diesel. It does have a 90 gallon aux fuel tank, and some day I may use that to store race fuel, but I'm not ready to use that.

I've only had pinging or knocking once before. In an old GMC van flat towing a VW down to race in the Carrera Panamericana. At night heading south in Texas it started making noise running up hills. My buddy and mechanic knew just what it was, pulled the interior engine cover while we were rolling along, and reset the distributor. The van had been timed to suit Colorado's altitude by its former owner.

This is louder. More like when you forget to torque lug nuts on one wheel, though it goes away (unlike the lug nuts). But it is not quite the same.

This '82 engine has newly installed SSIs. I've had exhaust leaks before, from air injector plugs that came out. Those sounds were rather subtle. This is anything but subtle. But I will add that to the list.

And I guess I should bite the bullet and pull the sump cover to see what there is to be seen. Time to change oil anyway. That will make pulling the exhaust covers easier as well. I don't have a cutter, but if something is really amiss I may be able to see it in the sump drainings.

Thanks, guys

Walt