Consistent though slightly changine engine misfire problem

Dear Click and Clack,

I listen to the show whenever I get a chance, and I have never had a reason to ask a question until now.

The Problem: I drive a 2002 Toyota Camry 4-cylinder that I bought used in November. It had low mileage (56,000) and had seemed like the previous owner had taken good care of it. I will provide the extensive detail below of the problems I have been having, but at the moment, the car misfires often and hard when it is absolutely cold then proceeds to get better (almost to the point of perfect) as the engine warms up. I have been to two different dealerships at least five times for help with this, and no one can figure it out.

History: After owning it for a few months, I started having trouble with the engine misfiring (and during that time I also moved from Houston to Nashville). I took it to the closest dealership (Dealership #1) which couldn’t figure out what was wrong with it because they said all of the read outs were where they should be. Eventually they blamed the problem on bad gas and dropped the tank to clean out the water and debris.

The problem improved but it continued to occur, so after a month of running fuel system cleaners through every tank, I took it back in to Dealership #1. This time they now said I needed to replace all four fuel injectors and the mass air flow sensor for $1500. That seemed ridiculous to me as I know that fuel injectors don’t go out very often especially not all four on a car with only 64,000 miles or so.

After looking around online and in the Mechanics Files, I called around for prices and chose to take it to another dealership across town (Dealership #2) that gave me the second cheapest price for that work and was rated very highly by its customers. Their first recommendation was to change the coil pack in the third cylinder because the error code followed that coil as they moved it. I warned them that despite what the computer might be saying right now, I have had multiple misfires constantly. They said this would fix it, and they recommended a throttle body and top engine clean to remove the carbon deposits from misfiring so often.

Dealership #2 did the cleaning on a Friday while they ordered the part, and replaced the coil on Tuesday. The day I picked it up and started it, I could tell that the problem was not solved. I drove it for a day with no improvement and dropped it back off again. While the cleaning seemed to help it run smoother, the coil pack made no noticeable difference.

That Thursday I was trying to leave town to drive to Texas for my best friend’s wedding, so Dealership #2 looked at it all day often with three mechanics including their master technician giving it a lot of time. After spending all day with it and looking at the timing among other things they had no great epiphany. They recommended running some of their stronger fuel injector cleaner through it as I drove to Texas. I did that and it seemed like the car was running much better, but again, because the car was warm on the entire drive it was hard to tell for sure.

Once I got back to Nashville, I started it up the next morning to head to work. The engine started and revved up to about 1500 RPMs and then down to about 800 RPMs which is its typical range at idle. I put the car into reverse and held the brake for a few seconds, and the RPMs lowered to 500-600 which caused the car to misfire often and hard. I let the engine sit there like that hoping that maybe something was getting pushed through the gas line and it would even out. As best I can tell, the only thing that will even it out is the engine warming up. On the third day of this happening and experimenting with it, the engine actually misfired so much that it died.

I took the car back to Dealership #2 yesterday (Friday) though unfortunately I could not drop it off and let it sit over night. They let the car sit until after lunch and looked at it as cold as they could get it. This time they said new codes had popped up for the mass air flow sensor, so they changed that out and sent me on my way.

This morning I woke up, crossed my fingers and started my car…and it is still having the same problem. I don’t know a lot about cars, but I do know a little bit about science. At this point my guess would be that when cold the fuel pressure is lowered for some reason and is not getting enough gasoline to the engine, or perhaps there is a seal or connection somewhere that is wearing out which is shrinking as it cools and then expanding to fill whatever hole it is in as it warms up.

At this point, I have spent just under $1000 on the problem (which is close to an entire two-week paycheck for me), and have replaced an ignition coil, the mass air flow sensor, cleaned water and debris out of the gas tank, pushed two heavy-duty fuel injector cleaners through, and cleaned the throttle body and top of the engine with no great improvement. No one seems to be able to figure out why the car is now running better when it is warm but even worse when it is cold.

Any and all help would be greatly appreciated.


Clueless (Daniel) in Nashville

Has anyone checked the coolant temp sensor for the computer? If this sensor has failed where it’s telling the computer the coolant temp is at operating temperature when the engine is started cold, the engine will run too lean for that condition and run rough.


I too suspect that when it’s cold the engine is not getting enough gas. But not due to coefficients of thermal expansion. I’m wondering of anyone has checked the temp sensor.

When the engine is cold, the oxygen sensor loop is bypassed in the ECU to allow the engine to rrun rich enough to operate without the fuel mix being leaned out based upon the oxygen sensor signal. If the temp sensor is not providing the proper signal, the ECU does not know the engine is cold, does not ignore the oxygen sensor, and the mix gets leaned out and the engine starved. This will not always throw other than a misfire code, and perhaps miscellaneous codes, as the ECU may not know the temp sensor signal is bogus.

This is just one wild guess for the list. Others may have more ideas.

I also wonder of anyone has checked the vacuum for stability. A vacuum leak is always a possibility with erratic operation situations when the ignition system checks out good.

PS: Tester, you beat me! I have GOT to learn to be more concise!

First, I can’t believe I just read through all of that.

Second, you won’t get an answer from Tom & Ray here. This is just a discussion board where assorted random riff raff talk about cars and stuff.

Third, other that what parts have been changed you need to be able to say what else these dealerships have done (has anyone even checked the fuel pressure, for example?), and a report of the actual error codes that they are getting from the computer would be helpful.

We also have to just assume that things like fuel and air filters are up to date, etc.

See, there are lots of possible issues. You’ve had people actually spend time looking at the car and able to put their hands on it. So you might think that you provided a lot of information - its just that most of it wasn’t really what someone would need to know.

Anyway - has anyone tested the coolant temperature sensor?

You both beat me - least concise?

No. To the best of my knowledge that has not been checked, but since my car has spent dozens of hours in the shop at this point, I am honestly probably not aware of every part or system that has been tested.

As I told Tester, I do not believe that the temp sensor has been checked, but I am not sure. I have also not been told that they have tested the vacuum for stability, but with as many hours as this car has been in the shop, I am probably not privy to everything that has been checked.

I personally had the car taken in for the 60,000 mile service which replaced the air filter, spark plugs and a thorough inspection. I know that both dealerships have also checked the fuel filter.

I can’t give you the specific numerical codes that I have been getting, but when I have personally checked the codes at an auto parts store, I have received codes for Cylinders 1-4 misfiring and random/multiple cylinder misfiring (as you would probably guess). I am unaware what code they received yesterday that led them to the mass air flow sensor or what the second new code that showed up was.

PS: Oh, and no to the best of my knowledge the coolant temperature sensor has not been checked. I will try to call today to ask.

What is the status of the spark plugs? Have they been changed recently? If the plugs have 20K miles on them, or you just don’t know their history; I’d change them for a new set. Old plugs can misfire when cold due to a too wide gap or bad gaskets where the porcelin meets the metal at the base of the plug. This era Toyota specs new plugs every 30K miles so a new set is relatively cheap and they are likely due for replacement.

When one coil pack goes bad, others can be marginal or going bad as well. The coil packs on this car are easy to access and change.

When cold the motor has a “cold” start feature which uses sensors to tell the computer to feed a richer fuel mixture to the cylinders. Richer fuel needs a better spark to ignite, which goes to my comments above re the plugs. Sometimes the cold start isn’t working properly and not feeding the richer mixture. This would cause the car to stumble and stall out when cold, which sounds similar to what you are call misfires. Others have commented on checking the coolant temp sensor as this is the key sensor to the cold start system.

When the car is cold and running badly have someone observe the tail pipe. If the exhaust is black and smells “gassy” the motor is getting too rich a mixture (too much gas). If the exhaust is clean, or just puffs of white (water condensation) then the mixture could be too lean (not enough gas). This observation could give a clue to help in the diagnosis.

I observed the tailpipe this morning, and the exhaust seemed to be coming out clean with what I would consider your average exhaust smell. So this could support the not enough gas theory.