The problem

My 2001 Toyota Camry CE (4-cylinder gasoline engine) has run rough and shaky the last several weeks and several hundred miles. The car has about 226,000 miles. I bought the car new and have generally done regular maintenance. In general, the car has served me well for these 17 years and given me few problems.

Check engine light appeared and resolved

In June 2018, the car was running fine, but the Check Engine light appeared. I used my OBDII scanner and got code P1135, which apparently is a bad upstream air fuel ratio sensor. That's a very easy repair on this car, so I replaced the sensor with a new one and cleared the code. The Check Engine light went off and stayed off, and for a week or two, the engine continued running fine. Please note that the only symptom of problems at this point was the Check Engine light itself.

Evidence of engine trouble

A week or two after the air fuel ratio sensor replacement, I started hearing and feeling occasionally that the engine was not running 100% smoothly. It was as if the engine would suddenly slow down and then speed back up once in a while. The problem has become more consistent in the ensuing weeks. At this point (August 2018), most accelerations produce this rough, shaky behavior and sound, where the engine goes fast then slow, fast then slow, fast then slow, probably three times per second. This happens both while accelerating to highway speeds and while accelerating to 25 MPH on residential streets, most of the time but not every time.

I made this video to illustrate the problem. The audio quality isn't great. You can hear the problem best at 4:22 when I'm accelerating to 25 MPH; you can also see the video shaking up and down slightly as the engine goes fast then slow, fast then slow, fast then slow.

What I have tried

  • Oil (full synthetic) and filter are recent (four months ago, 2,800 miles ago)
  • Fluids are topped off (oil, coolant, power steering, brake)
  • For what it's worth, the car passed its biennial emissions inspection four months ago
  • Replaced upstream air fuel ratio sensor (as mentioned above) (one month ago, 600 miles ago)
  • Replaced air filter (it was due) (two weeks ago, 300 miles ago)
  • Applied Gumout Regane Complete Fuel System Cleaner (10 days ago, 200 miles ago; it's about half gone by now)
  • Replaced spark plugs with platinum spark plugs that claim to fit my vehicle (today, just before taking the video)

None of these actions seem to have made any difference.

Update: code P0171

A day or two after replacing the spark plugs, the Check Engine light returned but with a different OBD-II error code: P0171, System Too Lean. The interwebz say this could indicate a variety of problems, including a mass air flow (MAF) sensor problem.

Advice appreciated

What should I try next? I am a DIY kind of guy who works with computers for a living, so I won't attempt any major auto repairs. If my best move is to go to a mechanic, please say so. But if there are other simple and relatively inexpensive things I should try, I would love to hear them.

  • Welcome to the site @GaryS. What codes do you have now?
    – GdD
    Commented Aug 3, 2018 at 12:28
  • 1
    Check out the TPS "Throttle Position Sensor" and see if that's providing a clean, relevant, consistant signal. If that checks out, move on to the MAF "Mass Airflow Sensor" and see if that is also behaving in an appropriate manner. These are shots in the dark, but it's where I would start - given the information you've provided so far.
    – SteveRacer
    Commented Aug 4, 2018 at 10:01
  • 1
    Oh - and also check all the intake plumbing after the MAF for leaks or cracks, especially in those "accordian" sections and bends. This might be as simple as an unmetered air leak, due to engine torquing over and opening the crack. That's exactly the kind of behavior I'd expect if you have an intermittent unmetered air leak due to a crack in the flexible intake hoses.
    – SteveRacer
    Commented Aug 4, 2018 at 10:04
  • Update: today I got a new Check Engine light with code P0171, which can correlate with your MAF sensor comments. Nicely done. 👍 I have updated the question accordingly. Commented Aug 5, 2018 at 23:37

3 Answers 3


I agree with the above stated suggestions. "Lean mixture" can mean any number of things, especially vacuum leaks. Hard to diagnose. In our 2012 Kia it was a bad valve cover gasket. Six months for that diagnosis. In my 06 Sentra I had a very dirty throttle plate that caused power surges. Seafoam should clear that right up. Good youtube videos on that. Can't spray air filter or MAF. My sentra also had bad ignition coil problems later on. Easy to replace and test. Plenty of youtube videos on that. Rockauto is a good inexpensive place to get them (can be pricy; mine were $120 ea. from a mechanic. $50 online). Denso brand is high quality OEM stuff. Looks like you would need two, as they each control two spark plugs. You can test the coils by unplugging the spark plug wires one at a time from the coils while the car is running. If the coil is good, you should hear a clear change in idle speed/smoothness. If nothing changes, most likely bad. Coils are located on the side of you engine, just follow the spark plug wires. However, my bad coils were causing huge clouds of smoke and scary noises, for what its worth.

In my experience, bad sensors will probably send a very specific code. A bad MAF sensor in an 01 camry would be P0101. I had a crankshaft position sensor problem in the Sentra that gave a specific code for that sensor. Not all sensors have codes, but a lot of the important ones do. Your lean code is a very general code meaning "something is not right with the air/fuel mixture, and I have no idea what it is." It is most likely not sensor failure, but possibly dirty sensors (you can buy MAF aerosol cleaner for a few $$) or a vacuum leak (air sneaking in where it shouldn't without letting the computer know).

Surging could also be fuel filter. Kind of a pain to replace, but doable. Again youtube (my personal favorite, can you tell?). You could clean the idle control valve or the pcv valve. After all this you'd be grasping at straws and would need the advice of a mechanic. Good luck.

Jared A.

  • "Can't spray air filter or MAF." Are you saying don't spray Sea Foam on the air filter or the MAF sensor? Commented Aug 8, 2018 at 10:59
  • "You can test the coils by unplugging the spark plug wires one at a time from the coils while the car is running." Are you sure? Is it harmful to the car? Is it dangerous? My wife would kill me if I electrocuted myself. Commented Aug 8, 2018 at 21:42

Intermittent power loss can have a variety of causes, you could have a vacuum leak, fuel or spark problems. Things you can do yourself that are relatively easy (keep in mind this is generic, I don't now Camry engines specifically but it should be applicable):

  • Fuel pressure: replace your fuel filter, a clogged one can give just this symptom and they're cheap. If the problem persists then measure your fuel pressure if you can (you need a gauge for that). Low fuel pressure could be a bad fuel pump or you have a clogged fuel line. You could also have vacuum forming in the fuel tank if the gas cap's busted. A bit more complex but still doable for a home mechanic is checking your injectors, a clogged injector can cause this sort of symptom
  • Spark: worn spark plug cables, ignition coils or distributors can cause this. I'd start with an inspection of the cables and distributor, if the cables are worn or oily then they need replacing, if the distributor contacts are dirty then clean them. If the contacts are worn replace the cap. Take pictures to make sure you get any cables back in the right place! Your camry may have a fully electronic distributor in which case you probably don't want to mess with it, but you could replace your coils instead
  • Vacuum leaks: you need a vacuum gauge to test this. Usually what you'd do is pull the vacuum servo line and read off of it, you'll want to refer to a manual on that to see whether you want to do that at a particular RPM and what reading you should expect. If the vacuum is low then you've got a leak somewhere, and finding that can be a lot more involved. A diy vacuum leak check is to use a length of vacuum hose as a stethoscope, listening for sucking noises around your vacuum system

If you don't have the right tools for this work then don't pre-emptively replace fuel pumps or other more expensive parts, you could spend a bomb and still have a rough running engine.


Clean your MAF sensor. See if you have any old hoses that might have broken open.

If you shop around, buy several cans of Seafoam, (round without the red hose). Big box stores should sell it for eight dollars or so.

Read instructions, Add to gas, look for the YouTube video for your model to run thru your intake. Expect a lot of white smoke so only do it very early in the day or very far from prying eyes that would call the Fire department on you.

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