Rough Idle

I have a 2003 Toyota Matrix and either due to it or coincidentally, after washing the engine compartment, the car now idles rough when it is warmed up to operating temperature. The car runs out fine and does not misfire when driving. I took it to the Toyota Dealer and they said it might be a fuel problem. A tech had dried off all of the spark plugs and said he could not find any moisture and he had checked all of the electrical connectors & vacuum hoses, as I had, and found no problems. The car was put on a diagnostic machine at the dealer and also at 2 separate auto parts stores and it comes up that it is misfiring on all four cylinders when it is idling. How could it run if this were true? The Toyota tech said that it was possibly bad fuel, so we ran several tanks of quality gas through and this has not helped. I have also cleaned the throttle body with cleaner and a brush, have run fuel injector cleaner through it and also went back and put dielectric grease on the plug boots. This did not help either. So, I went back to old school ways to check the misfiring by disconnecting the connectors to the coils one at a time and found that one made very little difference to the running of the car at idle. Could this possibly be the problem?

Does anyone have any suggestions?

Thank you.

Yes it is very possible that you damaged something while washing your engine compartment and it is possible that all four cylinders are misfiring. Exactly how did you wash your engine, what method? I can’t believe a “Toyota tech” said maybe it was bad gas, that is a real copout. There are a number of sensors that could be damaged by spraying water under the hood, including but limited to your cam and crankshaft position sensors and your mass airflow sensor. There could also be water lingering in a wiring harness or connector causing a short, despite the “Toyota ‘bad gas’ tech” claiming that he couldn’t find any moisture.

I would guess moisture buildup in some component would be a source of the problem. Moisture in a supposedly sealed place can be hard to get rid of. A drive through death valley when it is 114 degrees would be a likely fix. Not familiar with the car but you need to find a source that is involved in all cylinders.

Thank you for your answers. I washed it off with a hose and purple degreaser, trying not to spray under, but just around on the surface. I have done this several times in the past with no problems. If it is the sensors would it show up on the diagnostics? I would not want to have to start replacing one sensor after another without knowing that it was the bad one. That could get expensive. However, after reading your answer, I checked online about the Mass airflow sensor and it does say the check engine light will come on and mine has been on since I did this. Do you think a bad coil could cause it or you think it is more likely the moisture or the sensor? I have already recleaned the spark plug compartments and regreased them. Do you have any suggestions as to how to dry the wiring harness or connector? if it is that?
Thank you.

If the check engine light is on then it very likely holds an answer. Didn’t the Toyota tech pull the codes?

Go to a big box auto parts store such as Autozone. Most will read the codes for free. Write them down (actual code numbers like P0123) and post them back here.

…and I am mystified by the “Toyota tech.” I’d avoid them in the future.

My son has the car at college and I’ll get him to take it to an auto parts store and write the numbers down and post it back here. When the Toyota store put it on the diagnostics, they did not give me a number, they just said it was misfiring and to try to gas.
Thank you.

i was thinking since it does it after the car is warmed up, it be possible that a temp. sensor is messed up. the one i’m talking about is the one for the ecu, not the temp gauge on the car unless it’s a double temp sensor. one thing i’ve learn form check engine lights, they sometimes can be something completely diff. on what they say it is.

If it were one bad coil it would only be misfiring on one cylinder as your car has 4 coils, one for each cylinder. I doubt all four have gone bad all at once. But for sure you need to find out exactly what codes are causing your engine light to be on.

I’ll look into this as well. Thank you.

Based on your statement about checking the misfiring by disconnecting a coil one at a time and finding a cylinder in which this made very little difference, I’d consider one of 2 things.
One is minor and that could be a failed spark plug, which could have been caused by water in the plug well.

Two is a bit more alarming and that’s the possibility of low compression. If the engine was idling fine before the washing I would not think that low compression would be an issue. I only mention this because of the comment about disconnecting a coil.

Thank you for your reply. It did run fine before I washed it. Do you think it could be a bad coil?
Thank you.

doubt a bad coil. it would missfire all the time. what ever the problem it is, for some reason it’s only after the car is warmed up. that is why i thought it could be the engine control temp sensor, which is like a coolant temp sensor, except for the ecu to know how hot the engine is,so it can give proper fuel/air ration and so on. i’m also thinkin it could be a bad maf, and maybe a throttle postion sensor. but until you post some codes we really are shootin in the dark. i did have spark plug work lose one, and the only time it missfired was at high speeds.

Thank you for your answers. My son should take the car tomorrow to the parts store and I’ll post the code. I have been looking at the schematics (on the Toyotanations Toyota Matrix Service Manual) and seeing where everything is for the things mentioned.
I appreciate everyone’s time and answers.
Thank you.

Here are the codes:
Any help would be appreciated.
Thank you.

Would you open up the back of your TV/DVD player/Cable box/amplifier in order to “wash it”?
If not, then you understand why it is not wise to do this in the engine compartment of a modern car. That might have been okay with your '55 Chevy, but it is not advisable with anything manufactured over the past 15 years or so.

God only knows which electronic components have been damaged by this “washing”.

I wish you sincere good luck.

Man, now that is a list!!! Just WOW!

P0300 - Random/multiple misfires detected.
P0301 to P0304 - Misfire detected, cylinders 1 to 4.
P0171 - System too lean, bank 1
P1300 - Ignitor malfunction, Cylinder #1
P1310 - Ignitor malfunction, Cylinder #3
P1315 - Ignitor malfunction, Cylinder #4

I have a feeling that the ignitor(s), not the coil, is(are) the culprit. I’m not too familiar with the ignition system on this car, since they are typically bullet-proof in reliability.

I may be wrong, but on the coil-over-plug systems, is the ignition module also part of the coil unit?

Ha, answered my own question. Your coils are bad. The ignitor circuit is a part of the coil, and your having problems directly due to ignitor malfunctions. If the circuit checks out, replacing the coils is the only other solution.

There is not a distributor nor is there a rotary button and I would assume that it is electronically done. The coils must be the ignitor, I would assume. I had no problem with the car until I changed the radiator and heater hoses and then washed over the antifreeze. I did not steam clean it or wash up under it and didn’t wash under the cover of the top of the motor where the coils COP are and where the injectors are. I have washed it like this several times before and never had this problem. I do think my sleeve caught on the clip of accelerator cable, but I did not think I jerked anything out of whack. I could have gotten something wet, but it has not happened before when I did it. I did wash over the Mass Air sensor, where the air filter is. I had heard the leaning out of the fuel mixture would cause one of these motors to misfire. What would cause the leaning out? I appreciate your answer.

When it was idling rough and I pulled the wire off of each coil one at a time and one did not make a lot of difference in the running at idle speed.

Truthfully, you may have triggered the P13XX codes when you unplugged the coils. If the CEL was not reset since your test, this is a possibility. I was assuming you had the CEL reset after the coil test.

The lean condition may be the root of the problem. The mis-fires could be lean misses. This could be from low fuel pressure, a vacuum leak, or a dirty fuel injector. I’d check the fuel pressure, manifold pressure, and listen to the injectors for one that is not clicking the same as the others. A failed injector would post a code, but a dirty injector may not. And the ECM doesn’t monitor the fuel pressure.