Misfires when cold (no repair codes)

My engine randomly misfires mainly when it has not been heated up all the way and only while I'm revving up.

I've had mechanics replace my Mass Air Flow sensor, Battery, Clutch, spark plugs, spark wires & boots, distributor rotor & cover, valve cover gasket and the spark plugs and wires again. I also tried fuel system cleaner a few times.

I'm hoping someone reading this can provide some input that can help me get my Tacoma back on the road without worry. Thank you!!

 

Maybe try another ecu

 

What does "ecu" stand for? I'm new here

 

The electronic control unit? (Mass airflow sensor) as I put it. I spent nearly $400 to have that sucker replaced. The guy told me that's half the cost of the dealers. Do you think I need to buy the OEM part?

 

If you are misfiring on cold start and it gets better with warm up, that is an indication that the engine is not getting enough fuel. You have replaced a bunch of parts already, but you also need to check the fuel pressure (make sure within spec) and the ECU coolant temp sensor (have it tested). If those are good, I would then have the injectors pulled and cleaned.

 

Many many things it could be... as a test, when you turn her off for the night wait 1/2 hour and with a rag loosen the radiator cap and let her sleep. In the AM put the cap on but not fully seated so it doesn't build pressure. See if she misses then... do this a few times to confirm the issue. Just drive her enough to see if the miss is still present... I dont want you blowing coolant all over the engine compartment hot footing it to work. If she doesnt miss in the AM with no coolant pressure over night means that there is coolant seepage. If she still misses we will move on to the next. My next assumption is if you have an EGR system, there might be an issue with it... there should be no EGR flow while she is warming up. Doing some tests will set off the CEL so you should have a way to reset the light.

 

I used a combustion leak test kit on my radiator and the result was negative. No combustion fumes in my coolant.

 

I had a similar problem on my 2.7L a few months back. I tried a lot of the common fixes (MAF, spark plugs/wires, cleaned throttle body, tried new coils, etc.) but it ended up being an issue with worn exhaust valves on cylinders 1 and 3. I ended up having the head rebuilt (to the tune of $2000, including labor ). The major difference between my problem and yours is that I did have a check engine light on (P0300, P0301, and P0303) that would come on at cold starts when the truck would misfire.

 

Hopefully a code will come through soon. Can you describe how your truck was misfiring when you had worn valves? My truck has nearly 330,000 miles on it.

 

It would only misfire on cold starts (i.e. if it had been sitting overnight or longer). It misfire at idle, so it would feel very bouncy and rough at idle, but then once the RPMs increased it would run smoother. There wasn't too much of a loss of power or anything. Once I ran it for a 5-10 minutes and the engine warmed up, it would run just fine for the rest of the day.
As I said before, I was getting check engine codes-- those would always come on during the first few minutes the truck was running. After they came on, I usually read them immediately (I had a code reader in the truck) and would reset them, and then they wouldn't appear again until the next day when I started it up for the first time. Unfortunately, my truck had only 175,000 when this happened.

If this is your issue, it's likely that it will have low compression when cold. You can try running a compression test and see what happens. That's how the mechanic finally diagnosed my truck (after two months of me just about pulling my hair out over it).

It could also be a head gasket issue-- that's what I thought mine was. You might have a small leak into one of the cylinders, so it misfires when cold because it has to burn off the coolant. I think there's a test you can do to see if there's exhaust gas leaking into the coolant, so maybe try that? I imagine that there would also be some noticeable white smoke coming out of the tailpipe on startup if that's the case.

Hope this helps.

(Edit: I just saw that you already did the exhaust gas test)

 

Thank you all for the advice!