Knock knock knocking on heaven's door.

January 10, 2007 2:41 AM

My Horizontal Land Transport Vehicle (a Toyota four-door sedan) is in the throes of an engine knock-fest.

Clackety clack, clackety clack. It's engine knock time with my car.
Symptoms:
1. A faint knocking sound from the engine area. Unobservable to anybody but me, but I want to nip it in the bud before it gets worse and I have huge bills mounting up.
2. Only occurs when gassing the car from a dead stop. Diminishes at the 25 MPH point.
3. I've played nice and juiced the engine with the higher octane fuel it recommends. No dice.
4. I've pretty much ruled out the transmission, tires and steering, and exhaust system. Though in my case, "pretty much" means "I'm not entirely sure."

What could cause a knocking that persists even after refueling with premium? My mechanic thinks it's a fouled or dead spark plug; I'm in the dark.

But here's the catch twenty-two: If it *is* a plug, I'll have to drop the damn engine to get at it! A six-hour job, apparently. Seems that nowadays, plugs in many cars are inaccessible. Scotty, beam me back to the 1970s, si'l vous plait.

Questions: any ideas on what might be involved, other than a plug? Will the engine get damaged if it persists? If the answer is "a spark plug," can I handle this without dropping the engine? My car's at the 70K point, and I don't wanna replace plugs till 100K!

posted by Gordion Knott to travel & transportation (11 comments total)

can I handle this without dropping the engine?

It would help a lot if you actually told us what the car is. You know - year, model, engine size - that sort of thing. "Four-door Toyota sedan" is not going to get you any useful information.
posted by Kirth Gerson at 4:03 AM on January 10, 2007

I didn't want to harsh anybody's morning mellow with a lot of data, but the car's a 2001 Lexus GS300, with a 3.0-liter inline six-cylinder engine and power transmission.
posted by Gordion Knott at 4:29 AM on January 10, 2007

How long have you been driving the car with lower-than-required octane gas? If it's on the order of months, it will take more than a single fill-up for the system to re-adjust.

Have you changed the oil regularly? With the correct grade?
posted by Thorzdad at 4:35 AM on January 10, 2007

Thorzdad, yes to the change of oil (last change three weeks ago), with the correct grade. But I went over 20K on an old air filter (my bad). The engine is peppier with the new one, but no change in the knocking.

As far as octane, I read in an article that most modern cars adjust for octane differences via fuel injection, and have been using low octane fuel ever since. My bad again, but I switched back. But it's only been three weeks with the correct octane.
posted by Gordion Knott at 5:05 AM on January 10, 2007

Three weeks is more than enough time for it to adjust for any octane changes, as long as you have gone through at least one tank of gas. Engines knock more when they are hot - is your cooling system OK ? Check the coolant level, radiator and cooling fan(s).
posted by rfs at 6:18 AM on January 10, 2007

While it's true that modern engine management will allow an engine to run efficiently on a wide range of octanes, most engines seem "happier" with whatever octane the manufacturer recommends. I have a Maxima as well as a VW GTI and they both recommend a minimum of 91 octane. I've run them both with lower grades (to save a couple of dollars per tank) and, while they run acceptably, they both really do perform better with the 91 octane.
YMMV...literally.

As long as you aren't getting a Check (or Service) Engine Light (or any other warnings), I'd say run a few more tanks of premium through your car and see what happens.

If the knocking is still there, call around to some of the local shops (Like PepBoys or Advanced Auto or whomever) and see if they will hook your car up and read any diagnostic codes that happen to be stored in the computer...for FREE. Many places will do this for free. If anything is truly amiss (misfiring, O2 sensor, etc.) it will show up in the codes. Proceed from there.

And, while highly difficult to access, the plugs on most cars aren't totally inaccessible. If you are seriously thinking of performing your own maintenance, I highly recommend you arm yourself with the Haynes manual for your car. (the recommended Haynes manual for your car is actually their Toyota Camry manual #92007, per this page on the Haynes website...same basic car, different sheet metal, I guess)
posted by Thorzdad at 6:24 AM on January 10, 2007

drop the engine to get to the plugs?

wikipedia says the lexus GS has, since 1998, the toyota 2JZ-GE engine, which is the exact same engine in my 97 supra. i've changed the plugs before; they're right on top, no dropping anything involved.

(there's some plastic intake thing on top that you have to remove, which makes it a minor hassle, but hardly a six-hr job!)

not sure where you've got this idea from, but it's nutty.
posted by sergeant sandwich at 6:25 AM on January 10, 2007

It sounds like it's in the timing or indeed a bad spark plug.....

You mention the 6-hour job changing the spark plug, which leads to me to believe you have a V6.....

If your mechanic needs six hours to change a spark plug, I would find a new mechanic.... There are specialty tools to roll the engine forward to access the rear bank of plugs, taking a competent mechanic about one hour total to change all six plugs......

I would get a tune up and go from there....

More info (year, model, engine size) would help the hive advise you.....
posted by peewinkle at 6:26 AM on January 10, 2007

According to this, Gordion is correct that his engine is an inline six (and I'd be surprised if he weren't).

This page tells (sketchily) how to change the plugs on a '98 GS300. No mention of moving the engine in any direction.
posted by Kirth Gerson at 6:43 AM on January 10, 2007

This could be a carbon chunk that was knocked loose in one of your cylinders. Or something much, much worse.

Have your mechanic:

#1 Verify the condition of the plugs to determine if one or more cylinders are running lean and causing detonation. (This can be caused by a clogged injector). If this doesn;t work:
#2 Do a compression test of all cylinders, trivial to perform with an inexpensive gauge. This will also let you know if it's valve related. Sticky lifters/ect. If this shows nothing:
#3 Determine the exact location of the knock using a stethoscope. It it's inside the oil pan, it's probably time for a new engine/rebuild. (Or a new car. whichever)

If he has access to a bore scope, he should use it.

This is all very general, but time tested.

Who's your mechanic?
posted by IronLizard at 8:01 AM on January 10, 2007

I would strongly recommend taking this to experts.... at a Toyota/Lexus automotive forum. Just Google "Lexus forum" and go with the most popular ones.
posted by Doohickie at 9:36 AM on January 10, 2007