I've got a 1999 Mazda 626 that shakes really badly when idling. It gets a little better when put into neutral, but not much.

I've had two people suggest possibilities:

  1. Fuel filter
  2. Vacuum hose

This started happening in the last year or so. It started out being a mild shake when stopping at a stop light, but in the past 6 months or so it's any time the car is not moving.

The car hasn't needed much maintenance recently, but in the last year I've done:

  • Oil changes ever 3-4,000 miles

  • Replaced all spark plugs

  • Replaced the radiator

  • Replaced the front brakes (I know this probably has nothing to with anything, but it is something that's been done)

I think that's it. Also - it seems to get marginally better when I add a quart of oil to it (it leaks a little oil, that's another issue for another day).

I have not done much troubleshooting on this or taken it to a mechanic. I wanted to get some insight on things I could start with or try. I don't mind spending a little bit of money on easy fixes that may or may not resolve the issue, but that wouldn't be bad for the car either.

This is the RPM at which it idles: enter image description here

Any other suggestions?

  • 1
    Definitely not the fuel filter. If a fuel filter was bad enough to affect idle it would stall instead of accelerate. I'm not going turn that into an answer because there isn't enough information and because the asker shows no inclination for doing any mechanical work himself.
    – kahbou
    Mar 15, 2012 at 19:43
  • Perhaps you could provide some feedback as to what information you would suggest adding? I do some mechanical work myself, but not enough to know what specifically to describe in a question. If you have an answer for things you think this could be related to, please share it.
    – Matt Love
    Mar 15, 2012 at 19:45
  • How/when did you first notice the shaking? Describe the onset. Is it getting worse? Have you noticed anything else since then? Loss of power? Perhaps a summary of recent preventative maintenance?
    – kahbou
    Mar 15, 2012 at 19:57
  • A big one: What is the idle speed both in neutral and drive?
    – kahbou
    Mar 15, 2012 at 20:03
  • I've edited my answer with some more information. Not sure on that one, I'll get that information when I get home from work (my wife drives this car).
    – Matt Love
    Mar 15, 2012 at 20:04

12 Answers 12


Unfortunately, there are many, many things that can cause a rough idle. I'll try to cover a few things that might help with the diagnosis. I'm sure that there are many others that could be added.

  • Like I mentioned in my comment, it's definitely not the fuel filter. If a fuel filter was bad enough to affect idle it would stall instead of accelerate.

  • A vacuum leak is definitely a possibility since your car has a MAF sensor. In this case I would expect it to be worse in neutral, as a bigger percentage of the air goes unmetered, but I could be wrong about that. Vacuum leaks are quite painful to track down, but many have found that a mechanics stethoscope with an open hose at the end can be helpful in tracking them down. If there are a lot of vacuum hoses that you can't really reach you could also just replace them, they're not very expensive.

  • Injectors get clogged over time, but not always at the same rate. This could cause a misfire in the cylinder with the worst (or possibly best) injector. Try some fuel injector cleaner. This is added to the fuel tank and I would use it more concentrated than suggested on the product, or at the maximum concentration if they specify one. You can also have your injectors removed and professionaly cleaned, in which case they also measure how well each injector is flowing.

  • EGR (Exhaust Gas Recycling) is not meant to be happening during idle. Sometime the EGR valve can stick open causing a rough idle. I'm not sure if theres a way to determine if this is happening without disassembly, but it is something to investigate.

  • Raising the idle will hide the problem. Obviously this is not related to diagnosis but it is an option if you are unwilling to spend the time and money to find and fix the problem.

  • To clear fuel injectors, add hydraulic oil like Marvel Mystery Oil or automatic transmission fluid to the gas tank at a rate of 1 oz. per gallon. Drive the car normally for at least 1/2 tank for a few days. Then, add acetone to the tank at a rate of 0.6 oz. per gallon (or Berryman's B12 cleaner @ 2 oz. per gallon). Drive the car immediately at highway speeds for 30 minutes, then "floor" the car at maximum pedal from 50-80 MPH; repeat full throttle blasts at least six times or up to 30 times for stubborn deposits. Continue to add ATF to tank at 0.4 ounces per gallon.
    – Carguy
    Feb 10, 2017 at 8:57

I have a 1997 626 Mazda 2L (manual) and it shook at idle and got progressively worse. It turned out to be the intake manifold seal. Its cheap to fix about $12 for a new gasket. To diagnose it is simple too, spray where the intake manifold joins the engine with a can of "jump start" or similar and if the idle picks up you have an air leak. Mazda's are prone to fault here. Hope this helps.


Is this an automatic or a manual? The idle speed should be higher for an automatic. Do you have an engine light on? A bad misfire will sometimes turn one on, because your emissions go up with a misfire. (Unburnt fuel is being sent into the exhaust, which can destroy your catalytic converter.) Either way, this car is new enough to have a scan tool plugged into it. A technician should be able to see from that if you are getting misfires, or if your MAP reading is wonky, or if your EGR is stuck. Or you could just start replacing parts until you accidentally fix it. :)


I had the same problems and a forum somewhere told me that there was a Mazda 626 directive telling a Mass Airflow sensor going bad after a certain mileage. This islocated in the airflow tube coming from the airfilter. I pulled the 3 prong connector and the rough idling stops but it now runs on a higher idle . When I took the sensor out I broke the wafer thin sensor while cleaning with alcohol and Qtip The thing is very delicate. So don't do like me. I put a drop of crazy glue on the bottom and see is this will hold it and solved it. If not need to replace it. The darn thing is more expensive than a Kindle Color which has a more complicated electronics than this sensor. Parts store is selling for $260. Online is $100 cheaper.


Try idle speed sensor or spraying throttle body cleaner into throttle body or removing it entirely and cleaning it,especially where the idle air port entry.


2000 Mazda 626 with a rough uneven idle issue.. I figured out what it is, and below could could help. Mine never threw a CEL, but when you disconnected the EGR tube from the exhaust header and plugged it. i.e. so there would be no flow from the exhaust manifold to the intake manifold via the EGR valve. My idle significantly improved. I know its not the most preferred method to check the erg, but it seems to have help pin point my issue. When trying to diagnosis this: According to my scan tool, the MAP, ECT, TPS, Spark Adv, Injector pulse with were good. I was showing a 90% calculated load at idle, but when I bumped the throttle up to 1200 rpm in neutral or in park, the load dropped down to 12% and never missed a beat, same result when driving. Drove the car around and it preformed ok. You never notice that it had a horrible idle till you stopped at a traffic light or stop sign. Pulling away and accelerating there was no notice of a miss or hesitation. The car acted like it had a significant vacuum leak in park or at a stop light especially when warm. I did spray down the intake and all the vacuum hoses with no luck. I did replace the intake manifold gasket before hand just as a shot in the dark and I didn't have any change in quality of idle. Before all this I did replace the normal stuff, spark plugs, wires, air filter, fuel filter, cleaned the MAF. ( I was doing an engine swap ). Oh and a nod to Kahbou.. Thanks for the hints :)

  • EGR test: With car in neutral, rev engine to about 2,200 RPM and hold. Remove vacuum hose from top of EGR valve and plug the end of the tube with your finger to stop vacuum lose. The engine RPM should drop noticeably when the tube is removed and plugged; increase RPM when hose is reattached to the EGR valve. Also, with engine off, attach spare piece of rubber vac. tubing to the EGR valve and suck on it with your lips -- if suction is not tight, the EGR diaphram is bad and should be replaced.
    – Carguy
    Feb 10, 2017 at 8:46

This could be due to a dirty IAC (idle air control) valve. It is a very common problem spot. Clean or replace.

  • 2
    Welcome to the site! Good answer, but could you provide more details as to why the IAC could be the problem, where OP could find it in the car, and how they could go about cleaning it? Thanks for contributing! Jul 25, 2016 at 15:04
  • While it's not an amazing answer, I'm marking it up from -1 to 0 since there's not technically anything wrong with it. Aug 27, 2016 at 20:31

Look at the manufacture date. if it is before Nov. 1, 1999, there was a recall which "EXPERIENCING VERY LIGHT STEERING VIBRATION / SHIMMY / NIBBLE AT ENGINE IDLE OR LOW SPEEDS (PARKING LOT TYPE MANEUVERS)."



I know exactly what is wrong with the car. It will cost about 400 dollars in parts if you do it yourself. If you have an air compressor and impact and various metric sockets, the fix should be relativley easy if you have any mechanical experience.

The engine and transmision mounts are wore out casuing the engine and transmission to shake when the car is in drive stopped. When the car is in park or neutral the shake calms considerably but still not all of the way. There are 5 differnt mounts that will be needed to fix this problem. One is directly in front between the radiator and engine towards the bottom of the car. The other is under the air filter. The next is on the passenger side fender well. The other on the bottom of the firewall dead center of the back of the engine, and the last one is under the car mmounted to the frame.

To identify them the mounts are round with a bolt through them with a large rubber bushing. The engine and transmission need just a slight movement this is the reason for the rubber bushing. Anyway, I hope this helps. I just had to replace these on a car and it runs like a dream.


Have you checked the foundation bolts of the engine? Many times these work loose after hard driving and cause vibrations.

You can also consult online repair manuals which will give you complete information on troubleshooting and repairs.


This is a known issues with the motor mounts. It's not a mechanical problem, the motor mounts just wear out and the engine will shake a bit. It's potentially expensive to fix, and I wouldn't worry about it if the car checks out with a basic check. You should, of course, make sure nothing is going wrong with the computer, vaccuum, alternator, etc - but if all that stuff checks out, you can probably ignore this issue. If it really annoys you, get those motor mounts replaced.


For the Mazda 626 1999, it is your fuel pump going out. I had the same problem with mine. It shakes when it was in park, but when I put it in neutral it stops. I replaced the fuel pump and it stopped.

  • This may have fixed your vehicle's issues but it doesn't mean that all vehicles of similar make, model and year are suffering from the same problem
    – Zaid
    Mar 16, 2017 at 18:27

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