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heavy rain, misfiring & strange knock

Last night driving home (Boston area) I got caught in a torrential downpour that my 911 really didn't like. After about 5-10 minutes of very heavy rain (at 30mph on the highway) it started missing and died. Restarted, and ran OK above 3k, didn't want to idle. So I pulled off into a parking lot and tried to wait the rain out a while. When It started (started fine) it still didn't want to idle but was fine at 1200rpm with some extra gas. At this point I heard the alt belt slip when I revved it. Must be a bit loose and soaking wet, so I'll tighten that next.

Here is the strange part. As I left to limp the rest of the way home on the side streets I started hearing a knock once in a while, ~ 6 times in the remaining 5 miles home. Sounded like someone hitting the back window with their knuckles really hard at the right rear. It seemed to happen either on clutch engagement (going into load) and on deceleration (in gear). It did not sound like pinging, more like a rap on the glass.

So I have 3 issues as I see it. First, loose alternator belt. Second I should check my distributer cap and plug wires to see if they caused the miss (The wires date to 2001 with the PO). Finally and most worrisome is the knock.

I haven't driven in with it today (more rain forecast) but it started again fine in the garage last night and idled for 5 min with no problems. No new oil leaks, but haven't crawled under to look in detail.

The car is an 86 3.2 with an autothority chip, MAF, M&K exhaust, cone air filter, 129k miles, never rebuilt.

One last thing, it has an aftermarket rear spoiler that does not have any screen or cover below the vents so you can see, and rain, right onto the engine through the vents. How much rain normally gets in in a stock setup?
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Old 08-15-2008, 05:42 AM
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That's why I modified my ignition system to make it waterproof. So this is how using a Jeep Cherokee distributor cap and Chrysler ignition wires work in extreme wet conditions.
Old 08-15-2008, 08:06 AM
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The knock sound could be CV joint, loose shock, worn motor mounts, or possibly broken sway bar mount, among the possible suspects since the noise happens when the rear suspension/drive components are being stressed. You may want to put the car on a lift and carefully inspect all components.
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Last edited by ossiblue; 08-15-2008 at 08:21 AM..
Old 08-15-2008, 08:16 AM
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I'll crawl under it tonight to look at the suspension...
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Old 08-15-2008, 08:19 AM
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maybe a valve slap from unspent fuel / water mixture.
you probably ingested a bunch of water through the cone filter and modified MAF. The water could have puddled in the runners at the slow speeds and dumped into the combustion chamber at odd intervals.

or not,

the rapping usually is indicative of something in the suspension, as has been suggested.

Bill K
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Old 08-15-2008, 08:40 AM
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tedder, do you konw about our group? SEMBC
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Old 08-15-2008, 09:27 AM
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It is possible the knock is unburned fuel exploding in the exhaust, unburned from the plug wires being wet.
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Old 08-15-2008, 09:44 AM
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I had the exact same symptoms in a heavy downpour on the highway. Rather than guess at the problem I just went ahead and replaced the plug wires, plugs, cap/rotor all at once. Problem gone but obviously I don't know which specific part fixed it. No clue on the noise though so I'm interested to see what that turns out to be. Was the noise ONLY in the rain?
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Old 08-15-2008, 09:55 AM
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In my Daily Driver (77 911S Targa) I also had a strange knocking noise that would come and go and progressivly got worse. Turned out that the dust cover on one of the rear shocks had broken loose and was freely moving on the shaft. It would make a resonant thud periodically. I spent alot of time under the car looking for the source of the problem and hadn't bothered to look in my engine compartment. Duhhh Drove me nuts!
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Old 08-15-2008, 11:44 AM
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My take...I don't think the rain caused your CV joints or suspension to cause the knock "all of a sudden". If you haven't recently replace the items that 88911coupe mentioned, that's where I'd be looking also. Hey, anyone that goes by 88911coupe is "spot on" in my book
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Old 08-15-2008, 01:05 PM
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well at this point it looks like my 911 is grounded. It took me till today to get under it (wife had lots for me to do this weekend) and as I crawled around, nothing seemed loose or broken that I could see (I am not an expert fwiw). I started the car in the garage and it seemed to idle fine for about 30 seconds. Fast forward a few hours and I go out to start it to run an errand and see if the noise reoccurs. Car starts, but barely idles. 200-300 rpm, dies after 3-5 seconds and I cannot keep it running if I play with the throttle. Sounds like only ~3 cylinders running at all.

I pulled the plugs in # 2,4,5 (easiest to access) and 2,4 look OK but #5 looked like this;


seemed like black flakes of something on it and maybe a bit of oil. Popped off the distributor cap, all seemed dry and OK underneath and it has had 3 days to dry out in the garage. At this point I am starting to worry and wonder if I should tow it to the local P-car shop.

Will try to start it again tomorrow, but not sure what else to try at this point, and the condition of the plug makes me nervous.

Tedder
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Old 08-18-2008, 04:30 PM
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!@%^$^%!!

After my last post I went back out and pulled all the plugs thinking if they were fouled with gas I should let them dry out before trying again. #6 (no pic till tomorrow, camera battery dead) had somethng fuzzy stuffed between the electrodes. On close inspection, it looks like steel wool.

I didn't drop ant steel wool into the cylinder so basically it looks like something really bad has happened to #6, probably due to water ingestion. Gonna talk to the real mechanic tomorrow to see what I am in for...

F@$&!!!!!

I am a bit upset right now...
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Old 08-18-2008, 05:12 PM
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Man, I am so sorry to hear of it - I have been there, and I know you are feeling it in the pit of your stomach right now. I have to say though, that you don't know that things are that bad yet, and perhaps there is an explanation that will not involve the complete engine rebuild that I know, from experience, is dancing in your head. My situation turned out okay, and remember - there have been lots of 911s that have survived downpours like the one you went through. Patience and faith is called for. If it turns out to be the worst, I will the be first to buy you a cheeseburger and a beer even if I have to do so long-distance.

Hang in there, bud!
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Old 08-18-2008, 05:23 PM
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Water in a cylinder can do damage. Air is compressible, water is not. Trying to compress water in the cylinder can damage con rods, bearings, etc.
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Old 08-18-2008, 06:13 PM
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Quote:
Originally Posted by Doug Steinel View Post
Water in a cylinder can do damage. Air is compressible, water is not. Trying to compress water in the cylinder can damage con rods, bearings, etc.
All true, and a possibility in this case, but I like to think that Dr. Porsche assumed that at least some 911s might get rained on . I'm thinking positive here, man!
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Old 08-18-2008, 06:18 PM
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I've SPRAYED water, in pretty decent amounts, into my 911 while it was running (big thread on this BTW) and it bogged down a bit but I think it's going to take a LOT of water to hydrolock a running 911 engine.
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Old 08-18-2008, 06:25 PM
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I'm with Matty and Buck, relax a bit because most likely you do not have a catastrophic failure. Nearly all of what you describe sounds like ignition problems, especially the condition of the plugs. #6, though, is a mystery. The PO replaced the plug wires. What kind are they and how many miles on them? Moisture can wreak havoc on certain brands of wires (search the forum) and especially old ones.

I really believe you will not entail an overwhelming expense on this unless the catastrophy was coincidental to the downpour--very remote possibility. Keep your hopes high as well as your confidence, and let us know what you find out
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Old 08-18-2008, 06:54 PM
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so here is the pic of what I found in cylinder #6



The tufts between the electrodes look like steel wool and must be shavings from somewhere.
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Old 08-19-2008, 04:48 AM
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Did you pull the plug wires off the distributer cap? that is where my problem was. My cap was totally dry. The water was under the boots of the wires.
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Old 08-19-2008, 05:35 AM
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so I took the plug to my local mechanic. What was in it was not steel, but grey paper, probably from a disintegrating air filter (has a cone filter on it, when I cleaned it I got a bunch more of it out). I replaced the distributor cap & rotor, and the plugs and have been able to get it to run again, not well, but something. Now I think I have messed up the mixture (aftermarket MAF) and I can get it to idle fine but it is not able to take a load yet. Until I can get it on the road, I can't replicate the knock.
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Old 08-21-2008, 10:34 AM
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Old 08-21-2008, 10:34 AM
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