HELP!!!!! Running rough, poor starting

My 1996 tacoma 2.4l was get into a problem. Let start it from beginning, my truck have experience hard start when weather really hot, when I stop for food or gas usually i have to wait 15' to be able to get it start, i can't fix it so run it as is until last month when it first get a rough running so I disconnect battery hope it will reset the ECU and get it back to order, next morning it get worse, can't run any above 5mph, if I rev it a little more it stop. So I went ahead clean the MAF with no help so I replace the new one but I bought the aftermarket one off ebay for $60 and it running just fine for arround a week. Last weekend, the weather a bit cold so anytime I drive back from work at night it running really slow and hard to get it above 40mph. Next morning I switch the O2 sensor after CAT to the Front and it seem working fine, so i thought it could be the O@ sensor problem. BUt, yesterday, it seem to be back, right now check engine light with code p0115 which is insufficient coolant temp but I have just replaced a new coolant sensor last week so it couldn't be a problem. I have read an article say even OBD II say p0115 is coolant but it may have a bad o2 sensor, but to me my truck seem have a bad MAf because of the symptom, rough starting and running rich, stumple, and get a "gun shot" anytime I give it more throttle. So Please help, Should i change MAF or O2 sensor, or i have other problem because these sensor is really big money for me.

Thank you

 

If you have changed the coolant temp sensor, double check the wires going to the sensor to make sure there isnt a break. Also, replace the intake temp sensor which is located near your MAF. I was having a similar problem and changing these two sensors fixed it.

 

I replaced the fuel pump and fuel filter less than year ago. so I don't think that would be the problem. I have check the connect coolant temp sensor, it's seem fine to me, And i also check for resistance of intake air temp 2000ohm when cold and close to 100 ohm when i apply heat. So right now I'm leaning toward the MAF, eventhough it's new but who know it cheap and trash. One more thing i would like to add, When i start the engine in the morning after seating whole night, it start right up, no problem, if I stop the engine and start back up right away, it'll start no problem, but if I let it seat for mor than 5 minute it will not start, i have crank more than 5 time to start it.
Thank you

 

On the P0115 code, which sensor did you replace? There is one sensor on the front area of the head for the gauge, but the sensor for the computer is located on the back of the cylinder head. Two different sensors.

As for the rough running engine, if the ECT is reading cold (Broken connector or failed sensor usually defaults to -45 degrees F) then the computer will run VERY rich because it thinks that the engine is very cold. I would look into the sensor on the back of the head first, if that is not the one you replaced.

This is a pic of the one on the back of the head. Camera view is as if it were at the firewall)

 

I replaced the one as you show, on the back, I will double check, I'll let you know, Thank you.

 

OK, the sensor is good, I check for resistance and show no problem, the connection seem good. I forget to mention, my truck sometime run rough sometime run good, not bad all the time. I make a test drive after connect everything, seem to be good. But when I stop and start up, I see smoke coming out with the smell, rich condition, must try at least 2 crank before it begin to start, it shake a bit with smoke but after that seem stable and run fine.
Thank you.

 

Leaky injector?

 

Good possibility there. Or a ruptured fuel pressure regulator. Too many things to guess at really.

 

After doing some research, I think it may have a backfire, because I hear a pop sound, anytime I accelerate it, coming out of the intake air. So it could be a bent valve problem. Do you thick bent valve may cause a problem? But it is not like this all the time, some time running really smooth. without any problem, idle good, run great. The poping sound come after I start and drive for 3,4 minutes, after 3,4 minutes if it's stop, run great no problem, but if it's not stop pop sound it will like that until i stop and start it again. Keep in mind if I can start it up it will idle without any sound, smooth like it should be. So do you think my truck have a bent valve or not?
Thank you

 

Do you think bad EGR could cause backfire, stall, rough running?

 

Start with the cheap stuff first, plugs and wires and filters. Are you getting spark to all cylinders? Bad coil? If it idles good sometimes, it shouldn't be a valve. Sounds like a sensor or air/fuel delivery issue. Do Tacos have a cam position sensor? Could be affecting timing? O2 and MAF are possibilities too. EGR valves are cheap. Take it off and shake it. If it rattles it should be ok.

 

Thank you for helping me out. So yesterday, I reset the check engine light, and it came back on this time no more code p0125 but p0171, I think it could be a bad o2 sensor vacum leak or leak exhaust pipe because i can smell the smoke if i open the hood. Ok i think i should describe my truck symptom again because i don't think i got it right the last time. It will start up in the morning after 2 crank or if i allow it to cool down after an hour even outside temp is 50F, if i start it back up right away after stop it will start up with 2 crank, it will not start if i let seat for 30 minute, just crank. If i accelerate it after stop for light, it will stall and have a pop pop sound i think come under hood but if i past 20mph it will run ok. So please let me know what you think. I have change the spark slug and wire, the distributor got a bit oil in the cap.
Thank you for the advance.

 

I know this is ancient, but you described the exact problem I had last weekend. Particularly the low coolant error code.

Symptoms:
Engine was starting to act like it was starving for fuel, losing power slowly until I could barely crawl up hill at 10 mph. Finally, I had to nearly floor it to keep it running. Limped into a auto parts store lot and shut her down. Opened the hood and she was smoking so badly I thought she had a electrical fire. Looked underneath and the muffler was smoldering. I had heard some rattles earlier for a few weeks and thought I had a bad muffler with disintegrating baffles. So my first thought was that a baffle had broken loose and was blocking the exhaust. So I figured, "time for an exhaust replacement.

Earlier that day, I had experienced what seamed to be a draining battery. Each start resulted in a slower and slower crank. Battery dying, for some unknown reason.

While sitting in the auto parts lot I cracked open the exhaust coupler part way in front of the exhaust to see if it would run. I used the auto parts store code reader and came up with a 'coolant level' and 'downpipe O2 Sensor code'. I called for a jump, and waited. They came and she jumped with no problem. AND she was running great! I cleared the codes, checked the tail pipe, and exhaust was free flowing. Jumped in and continued on with errands.

Got a mile, and she began losing power again. Crawled into a gas station lot and stopped the engine. By the way, the coolant gauge was reading COOLER than normal all this time. So at the gas station, I decided to check the coolant level. It was topped off. So my guess was bad coolant sensor.

I let the motor cool, and push started her. Popped off and drove a quarter mile till she wouldn't move at all under torque. So I pulled over and kept her running with the hazard lights flashing. If I touched the accelerator, she acted like flooding or starving for fuel. Didn't know which. But she would run IF I just let her idle. While sitting there, thinking, I noticed that with each flash of the hazards, she would nearly die. Turn off the flasher and she ran smooth. "Interesting", I thought. And a lightbulb went on in my brain and I turned on the headlamps to see what would happen. She really lost power at that! ALTERNATOR???? Had to be!! So I called for a lift, and ran to pick up a fresh alternator. Installed it on the street, fired her up and voila!! Ran perfectly!

A low voltage read from the alternator must have caused a super rich fuel mixture because the coolant sensor was reporting an engine not achieving operating temperature.

The fuel dump caused a massive increase in CAT burn, which sent 1200 degrees of exhaust gas to the muffler, which nearly melted down. All because the alternator wasn't charging the system.

Man, do I miss the days of an bad alternator just leading to a no start. All these sensors are interconnected so closely that you simply chase your tale for hours instead of simply being stranded.

If this sounds familiar, check your alternator voltage.

Cheers!

 

Wow, that's crazy but totally plausible. Appreciate you taking the time to type that up and with good details. That will help someone if the future and sounds like help save someone tons of time. Great work in figuring that one out. If you're ever in Norfolk, VA a cold one is on me.