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Cold Engine Knock, Check Engine Light

I have a 2006 Corolla LE with about 93,600 miles.

It was about 16 degrees Fahrenheit this morning and I drove on a cold start, and noticed the engine knock a bit when idling. Later noticed check engine light come on. After the car fully warmed up, 15 minutes later, the knocking seemed to subside.

I don’t think I’ve replaced the spark plugs in the longest time, so could it be this issue?


When the check-engine light comes on, codes are stored that can help diagnose the problem. It’s not very sensible to guess at the problem with knowing those codes. In most US states, auto parts stores will read the codes for free.

As for the spark plugs, most of us don’t have your owner’s manual, so you should tell us if they’re due for replacement or not.

The plugs are rated for 100k miles; and if bad wouldn’t have this symptom.
If the trouble code is P0171 it’s probably a leaky intake manifold gasket; a common problem for this engine in the cold.

what should I be looking for when the spark plugs go bad, symptom wise?

Plugs are working hardest under heavy throttle, so a miss or shudder when accelerating is the typical symptom of worn out plugs.
However, even before there are symptoms worn plugs put more stress on other ignition parts because they need higher voltage to fire.

I have 43k miles on my 2006 Matrix (same engine as your Corolla), but I plan to change the plugs at 60k miles because they’ll have been in the engine for such a long time and I don’t want them to get stuck.

Get the code read and report back.

Autozone pulled up P0171, left side

The guy told me to put in fuel injector cleaner to clean it up? Is this correct? I’m leaning more towards the leaky intake manifold gasket…


Plugged injectors will not clear up after 15 minutes of driving

A marginal intake gasket may swell up and do a better job of sealing after 15 minutes of driving

Do you hear any hissing with the hood open and the engine is idling cold?

I haven’t heard any hissing with the hood open. The temperatures have been better this week aside from Monday, so when I start the engine revs to 2000 RPM before tapering off, as the engine warms up I can idle at 800 RPM with no issues.

The leak might not be bad enough to hear hissing.
Another way to detect a leak is to spray a little propane, starting ether or carb cleaner around the manifold.
If the idle speed or smoothness changes there’s a leak.
Instead of sucking in air with oxygen it sucks in the spray and the fuel/air ratio changes.

P0171 is simply “fuel trims system lean”. Db suggested a possible gasket leak because that would allow excess air to be drawn into the mix and could swell and seal as he suggested.

A temp sensor can also allow lean operation at cold without tripping a specific fault code other than “lean operation”. Normally, the ECU bypasses the oxygen sensor when the engine is cold to enable the engine to run rich, but if it doesn’t know the engine is cold it’ll run lean based on the other inputs. 800 is low for that engine when cold (assuming cold weather), and that would be consistent with a temp sensor problem.

The good news is that the temp sensor is easy for a shop to test. But you will need to bring it to a shop and pay for the diagnosis (whatever it turns out to be). Most shop waive the diagnostic charge if you also have them do the repair. But it’ll take some “hands-on” to fix it properly.

So less than 1000 rpm when the car is in Drive, warmed up 15 mins in, and waiting at a stop light in 35 degrees weather is not normal? And in this conditions its not rough idlling.

800 rpm after the engine has warmed up is normal.
There are lots of folks out there with the same symptoms and changed the manifold gasket, just do a search.

One add’l thing worthwhile to check. Probably already been done, but if not. Make sure the oil level is ok, the correct viscosity per the owner’s manual for these temperatures, and both the oil and filter are in good condition.

Warmed up, 800 is normal. Cold, it would not be.
I agree with db and circuitsmith regarding the possibility of a leaky intake manifold gasket. And circuitsmith’s suggestion to use ether to find out is a valid test. My thought of testing the temp sensor was intended as an additional idea, not a challenge to others’ suggestions, based on the information available at the time.

I’m just saying an intake manifold gasket that leaks in extreme cold seems to be a rite of passage for this particular engine.

Hopefully the OP will post back when he has a chance to check it out.

hey everyone, thanks for your help!

i unplugged the battery to clear the ECM and popped open the intake gasket over the thanksgiving break, what I saw was the original black OEM rubber gasket pretty worn out. in fact there was some nasty black rubber deposits building around the gasket onto the engine itself. after cleaning it all out, i bought the new orange gasket from a toyota dealer and put it on

not sure if this was the root cause, since i unplugged the battery earlier and the error code vanished for a while, but I haven’t seen it yet! also mechanics were charging 2.5 hours as repair time to do the intake gasket, so I saved like $200!!!

thanks a bunch guys!!!



It sounds like you fixed the problem and saved money by doing it yourself