58 captured repliesDiscussion pagination complete · Source accounts, not a confirmed diagnosis
THIS DISCUSSION / SOURCE-DERIVED OVERVIEW
Discussion summary
YOUR RESEARCHEngine Knock / Lugging
Leading reported evidence
Replace plugs about 10000 miles
Performed · outcome unconfirmed
No successful repair reported for this finding.
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Reported fixes
0
Unsuccessful attempts
1
Discussions
Read evidence and context
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Performed · outcome unconfirmed
I had changed my plugs about 10000 miles before I heard the tapping.
Leading evidence: 'Replace plugs about 10000 miles' (no explicit attributable resolution; this is a reported action or suggestion). Across 1 matching discussions, the most frequently mentioned possible cause is 'good idea' (1 discussions; 0 report identifying it and 1 suggest it). The most frequently discussed action is 'Adjust speed of the engine' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.
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Performed · outcome unconfirmed · Original poster
This has happened before but went away when I changed the spark plugs (which were due).
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Suggestion
The sensor 'senses' vibrations or knocks and the PCM alters the timing to adjust the speed of the engine.
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Suspected cause
Not knowing why the engine is knocking, which it should not, replacing the knock sensors is probably a good idea.
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Suggestion
It's automatic and goes without saying...One should always check hoses and belt, engine in or out.
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58 eligible posts · 0 unverified posts · 1 discussions match the subject in their title or opening post.
Rennlist: 1 discussions
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Outcome without a verified action link
New plugs and the problem went away for about 4K miles and the better part of year.
I'm hearing some engine knock with medium to hard acceleration below 3K RPM. I know we're not supposed to lug these engines, but sometimes you don't have a choice when you're down to 2nd gear (e.g. slowing down for a light that turns green). This has happened before but went away when I changed the spark plugs (which were due). Current set only has 4K miles on them. No CELs or faults. Bad coils? Bad tube?
Thoughts? Thanks in advance.
Car has 34K miles. Plugs are not OEM, but are approved for use on my car
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Oil consumption is minimal, I go through a quart every 5K miles.
I can hear the knock if at slower speeds. Other times it might be because I'm driving next to a wall and I can hear the engine sound.
I'm confused. This leads me to think that the new plugs solved the knocking issue. Yes? No?
I'm curious because my 996 mk2 engine knocks if I aggressively accelerate below 3k rpm.
I'd replace the pugs since you said it went away (did it really go away) with the last plug change. Maybe pull your plugs and check and gap them.
Bad gas?
At 30K miles, there was knocking. New plugs and the problem went away for about 4K miles and the better part of year. The knocking is back now.
Bad injectors did cross my mind. Maybe its because of the cab roller thread and the fact that my motorcycle is having injector problems. I'll put int some cleaner and see if it clears up since it's easy to do.
If not, I'm going to swap out the plugs, tubes, and coils. Would rather piecemeal, but getting to the plugs on these cars is a pain.
I would say it did go away. If it didn't, then it's more audible now.
Put a bottle of octane booster in your tank to boost it up, see if this sounds goes away or not. If so it's pinging and will help you narrow down the cause.
Your coils should be OK at 34000 miles. Now if the car had done 100000 miles would be a different matter. I had a tapping from the left side of my engine recently and found it to be a loose plug. I had changed my plugs about 10000 miles before I heard the tapping. Check to see all your plugs are tight if the knock sound has gone louder since the plug change. Do you hear the knocking from a particular side at the rear of the car? I could hear my tapping even as it ticked over and standing still. It went worse as I accelerated and went away about 3000 revs. You will find it easier to remove the exhaust box either side when changing plugs. Its still a royal pain in the **** but easier to get to the plugs, sleeves (pre2001) and coils. Good luck.
Thanks Charles, I'm a novice, so please excuse the green questions. If the knock sensor is faulty, then a new one would tell the PCM to retarding timing. If the knock stops, is that it or do I still have to address other underlying issues?
Thanks Chris. Please see note on being a novice above. Can you please elaborate?
Agree, I was not a fan on changing the plugs on this car. Hence the thought of changing coils and tubes while I'm down there. But the knock sensor seems to be easier to access, so I may do that first.
The engine has relatively high compression, without correct ignition conditions it might cause pinging or knocking. Fill it up with premium and put an octane booster something like this:
https://www.autozone.com/fuel-and-en...ster/53483_0_0
With high octane gas in it, drive it and see if the knocking goes away. If it does it will help you narrow down the cause. Coils/Plugs/Knock sensor etc
i had the same issue, i would notice it when accelerating from a U-turn in 2nd gear it would slightly knock
I also use Chevron 91 (as high as they have here), so i knew it wasn't bad gas
I added a bottle of Lucas fuel system cleaner, and the knocking has gone away
Would a bad knock sensor throw any codes/CEL? How can you tell if one or both are bad? Have read it's rare that they fail.
Pretty sure mine pings sometimes at half throttle in 4th-6th from 3-5k rpm (disappears at WOT). Car runs great, no codes. Will try the Lucas cleaner and see if that does anything.
A bad knock sensor would not throw any codes. The engine will run advanced timing and when it knocks, it won't detect it.
We've also seen engines that were burning oil detonate and take out rod bearings because there is so much carbon on the pistons, it actually raised the compression ratio!
Read up on how to remove the generator when getting to the knock sensors.
Those small air tubes have a tendency to break easily. Then you have a lot of trouble on your hands. BTW the knock sensor is there to detect a knock and nothing else. Replacing it will NOT cure your problem.
That's what I was wondering, but Charles says otherwise above.
The prices of knock sensors are all over the place ($20 - $200). What am I missing? Also, there seems to be a dearth of online resources re changing out the 996 knock sensors - anyone have a resource?
Absolutely if a knock sensor is bad, it will not pull timing (that's what the knock sensor is for, to tell the ecu to pull timing) when needed, and, let the engine ping. Under many conditions, high load low rpm will cause the ecu to pull timing based on voltage readings from knock sensors.
Bill If there is a bad knock sensor on board, the PCM with throw a code up. His PCM has not done that, so I take it the sensors are not his problem. The sensor 'senses' vibrations or knocks and the PCM alters the timing to adjust the speed of the engine. In other words the sensor is there to inform the PCM which in turn protects the engine by it's action. His sensors have done their job. If they had not then, he would have seen a code which he has not.
OK Bashat I am not real sure so will accept your comments. Ok I would change the sensors or at least test if they are working correctly. He could have a bad one and the engine via the PCM, could have gone into LIMP MODE. My initial post was for him to be careful removing the generator and to read up on how to accomplish this correctly to 'get to' to the knock sensor. I do know that one of them is right behind the generator and that evap lines can be damaged doing this job. .
I think Hurdi's comments are on target to the OP. Not knowing why the engine is knocking, which it should not, replacing the knock sensors is probably a good idea. There are 2 knock sensors, one for each bank. And given that they are quite a ways in there on top of the engine under the intake plenums, a lot of stuff needs to come out of the way. The vacuum lines and hoses are definitely going to be in the way of accessing the sensors, so his comment on damage to the oft brittle plastic hoses is on point. Be prepared for fixing/replacing that rats nest of vacuum tubing when going for the knock sensors. And bonus, you will probably bust the plastic coolant hose connector next to the alternator so be prepared for that. Things I would replace "while in there" - the coolant hose connector, the vacuum lines, the vacuum tank and the oil filler pipe. Since the TB has to come off, a new O-ring for that. Might also be a good time to replace the SAI valve and check valves since they will be accessible.
Hurdi was absolutely confident about the advice he offered, that is until he wasn't....
no offense Hurdi...good on you for putting your thoughts out there
Just makes me chuckle when people realize that their beliefs, opinions or reality could be incorrect....
Lol, not yet. I haven't done anything but worry and post. I'm going to get around to it today. Then I will probably check the plugs, coils, and tubes. Then will be the knock sensors and potentially some tubes and hoses as necessary - I'm not sure I want to tackle this piece yet. Curious as to why no one recommends hose inspection / replacement as preventative maintenance when the engine is out of IMS / RMS.
And someone else re-confirmed it, ha!
My question is this: does a knock sensor provide feedback to the ECU when detonation occurs in the proper way? If no, they can go bad not because they aren�t sensing properly, but because they aren�t sensing at all.
Also, to the OP: how do you know it�s a predetination knock and not a rod knock?
Detonation will sound like a can of marbles rattling around inside the engine. Rod knock would be more consistent, the tapping relative to rpm. Which one does it sound like?
So dumped in some fuel treatment and the knocking seems to have stopped after about a quarter tank of treated fuel. Does that mean I just had some bad gas in there? Or maybe the fuel just sat too long?
Maybe the treatment cleaned off some carbon buildup, lowering compression enough to stop the pinging. Or it raised the octane level just enough.
I put out a bottle of Lucas in mine yesterday. Heading out for a 50 mile drive tomorrow. We�ll see if it makes a difference.
Fill up with the best premium fuel you can find (no ethanol if possible!!!) toss in more fuel system cleaner and take it out for a good old "Italian tune up" 50-100 miles. Report back.
I think that I may have had some slight carbon buildup, just enough to cause some predetonation
i don’t think the crappy California gas helps (even chevron)
glad the fuel cleaner worked for you also
I've gone through a tank per month for the last two months. I'm trying to drive her 2x per week, but life seems to get in the way. I'm thinking of putting in half tank increments to keep it fresh.
I usually put in Shell. I think I just going to make treating the gas a normal part of the routine - that and just driving the damn thing.
I don�t think gas will go bad in a month. You could try a different gas station (on the off chance that it�s the gas).
Something else is wrong.
These engines have over 11:1 compression, which means that unless everything is perfect - ignition timing, low carbon buildup, low oil burn in the combustion chamber, proper cylinder head and piston temp, proper fuel atomization, etc. - having any predetination will ultimately lead to engine failure. If you put octane boost in the tank, you are only going to lower predetonation, but you might not eliminate it completely. Some predetonation is inaudible and cannot be felt, but instead, only reduces power a small amount and is otherwise undetectable. However, damage is being done and will ultimately lead to bearing failure, a cracked piston or ring, etc.
So what would your course of action be? I don't hear the ping, the indy might not either. What should we be looking for to identify an issue? Call me lazy and cheap, but I'm not really in the mood to replace the knock sensors, plugs, tubes, and coils without knowing what's wrong. Even then, if detonation is inaudible, how do I know I've fixed it.
I'm not arguing, I just want to get to the bottom of it. Thanks for the input.
I thought about a coupe, but the first time I drove to Stinson Beach with the top down felt like I made the right decision.
Hope you get your issue sorted out.
This is true, you only regret the cab when the top is up. I rarely regretted the cab in LA.. When the weather warms up here, I'm sure I'll be glad I got cab.
I would do the knock sensors. They are cheap (not sure why some sensors are $200, because even videos on YouTube for the Boxster knock sensors are claimed to be closer to $25). Since you did the fuel treatment, you are probably burning/cleaning some carbon buildup or the injectors are spraying better. But whatever is causing the issue will likely return unless you keep cleaning the system out.
Out of curiosity, how often do you get the engine up to temp and drive hard at higher RPM or redline?
Like I said, I�d try the knock sensors or borrow a Durametric from someone, since sometimes the knocking sensor will show up there (not always).
I've gone through a couple of tanks of Shell V-power, one of them was treated injector cleaner. The knock has not returned. No errors no CELs. I took it to a well respected indy and they said there was nothing wrong with the car. They also added that they have never replaced knock sensor on a 996 - another indy has told me he's only replaced one knock sensor in the 996.
Immediately before I heard the knocking, my butt dyno was telling me the car felt a bit sluggish. I don't know if you can tell, this is 2nd / 3rd gear roll on wide open - 10 minutes before the knocking sound.
https://www.youtube.com/watch?v=pOJfnk-6wuw&feature=youtu.be
Car seems to have picked up that lost step during the last tank of gas. I'm just going to drive it. If I hear the knock again, knock sensors will be replaced.
Yes, the one treatment of injector cleaner and no more knocks. I think it may have been the fill up a gas station in the mountains combined with not driving the car enough. Then I had a sticking lifter - resolved itself as soon as the indy fired it up, again not driving enough. Moral of the story, drive the car.
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