P0303 check engine light when cold.

I've been chasing a P0303 error, long cranking to start, running slightly rough since the weather got cold. The problem clears up as the engine gets to operating temperature, and if I reset the trouble code no more Check Engine light until the next cold morning.

After searching on "P0303" here and elsewhere, I pulled and inspected the spark plugs...found plug 3 showing signs of lean combustion with white coloration on the electrode, the rest were tan with light black coloring on the metal interior of the plugs.

New Toyota/Denso plugs gapped to spec, and with 196K miles on the odometer I popped for a set of Toyota rebuilt fuel injectors.

No real change in behavior after ~160miles, and it's still throwing P0303 and occasionally P0304 & P0300 during cold startup. Only improvement I see via the scanner is the Long Term Fuel Trims have dropped. Was idling at +3%, now idling at -7%. At highway driving it was maxing at +12%, now maxing at +3%. Seems that the new injectors flow more/better which is not surprising.

I checked compression and found all cylinders in the 174-180psi range.

I'm stumped at this point, having given up and now sitting in the waiting room of my local Toyota dealership. Will post their findings shortly...the next part I was going to throw at it was an EGR valve...we'll see what they come back with.

 

Relating to temp, have you changed out your water temp sensor and your intake temp sensor? Water temp sensor is on the back of your head by the fire wall and the air intake temp sensor is on your air box. Mine took a while to crank in the cold and when I changed those two sensors it cranks right up now without hesitation. Just did yesterday without issue and it was 31 degrees out

 

Following up: I picked up the truck as the dealer closed this evening, no diagnosis. They were able to reproduce the misfire briefly after letting the engine fully cool, but the freeze frame data was not revealing as to the cause.

They did a leakdown test on all four cylinders and it passed. Other than that, not much on the $89 diagnostic bill to indicate what other steps they took, and the tech had gone home for the day.

As for the two sensors mentioned above, that's a great point. I've been watching the coolant temp on my scanner and it tracks with reasonable precision to the dash gauge, peaking at 186degF...so not likely the coolant temp sensor.

However I've not paid any mind to the intake air temp so I'll add that PID to my scanner in the morning and measure against ambient air temp before startup, then watch it rise a bit as under-hood temps climb. Thanks for taking time to post this. I'll also verify the coolant temp starts at the same ambient temp reading as the intake air temp before startup.

More to come...

 

Update: both Intake and Coolant temps show 54degF this morning. Upon startup, IAT drops ~2degF immediately which makes sense due to airflow.

My thinking is to pull the EGR valve and look for indicators of problems there.

Edit: I've ordered a new EGR valve as well as a PCV valve. I don't like throwing parts at it, but Winter is approaching and I don't want surprises. At 196K miles and hopefully many more to come, it's not entirely wasted $$.

 

In the meantime you can just pull the EGR and clean the fine wire with TB cleaner. The wire gets gunked up and stops sending a signal

 

EGR has a wire filament sensor element??

 

HAHA Im sorry, I meant to write MAF. I was reading your post and wrote EGR

 

Thanks for clarifying. I used aerosol MAF cleaner to hose down my MAF innards a few weeks back when this first started, no effect.

I only use paper filters anyway, no oiled K&N type cotton gauze ones...so not surprising there wasn't a problem w/ the MAF.

 

While I'm waiting for the new EGR and PCV to arrive, temps have dropped below freezing and it's running crappy again. Spark knock climbing hills at highway speeds, rough idle, and Check Engine light is on. I picked up a can of SeaFoam and ran 1/3 of it into the vacuum fitting behind the throttle body that goes to one of the VSVs on the fenderwell. Let it soak ~20minutes, started up and let the smoke show ensue. Quick test run to blow out the rest of the gunk, back home and repeated with another 1/3 of the can. It's soaking now and I'll repeat one more time with the remainder of the can. More to come...

 

Im wondering if you may have gotten some bad gas to begin with

 

It would have been several tanks of bad gas over the past few weeks, from 2 different Shell stations and a BP station. I have occasional bad luck but that would be pushing it.

I finished the 3rd SeaFoam treatment, let it soak for 15min. Smoked quite a bit again on startup then took a spin. After the smoke cleared it is idling smoothly, no spark knock I could hear, and the check engine light hasn't come on since I reset it. Long startup time is the only symptom I haven't checked yet so we'll see how that goes, going to run errands and pay attention to startup time and overall behavior.

 

Update: it runs great! Startup cranking time is much shorter and normal, idle is smooth, however still a bit of audible spark knock on the long uphill in OD. The gas in the tank is Shell 87 octane.

I picked up a bottle of Techron cleaner and dumped it into the tank, next fillup I'll switch to BP or the other Shell station just to rule out gas problems as the spark knock cause.

Have to wait until tomorrow morning to get a good test of cold-start behavior.

 

keep us posted

 

Update: took it out for a pizza run after sunset at 31degF ambient and dropping, it set the Check Engine light within a few seconds of startup. P0300, P0303, P0304. Darn. Ran really nice after it was warmed up, though.

I pulled down the service manual and resumed reading. Disconnected the EGR vacuum line and pulled a vacuum on it, heard the valve opening and making a nice solid click when vacuum released and it closed. Very unlikely the EGR valve is my problem.

I moved on to the MAF sensor, disconnected it and probed for resistance across the two terminals specified in the test. Got 3.782 KOhms, which is slightly lower than range for 32degF which was 4-7 KOhms. Engine is completely cold and it's presently 29degF outside, so according to the chart the resistance should be on the higher end of the 4-7K range. I verified the pins were clean, re-tested again, same exact reading.

Not sure if this test is checking the Intake Air Temp sensor or the MAF itself, as both are integrated into the housing and electrical connector. IAT readings on my scanner were fine as mentioned previously. Local parts stores only carry rebuilt "Cardone" sensor, so I ordered a new Hitachi sensor from RockAuto instead. Should be here Wed-Thurs timeframe.

 

bump

 

Did you replace the wires? Seen my fair share of bad spark plug wires on these 3.4L motors causing misfires. Makes sense when cold too, since as the wires warm up, the conduct better.

Also try moving the coil packs to a different cylinder to see if the misfire moves. I bet you need new wires though. I use NGKs personally.

 

I think he's got the 4 banger... your statement still stands tho could be the plug wires

 

No wires, this year 2.7L is coil-on-plug. I did verify the boots weren't cracked, and I swapped coils 1 & 3 but the misfire didn't follow it. I'm seeing misfires on 4, and the random P0300 code as well.

 

Update: MAF looks like the culprit based on last night's test with the Ohm meter which I repeated this morning and got the same 3.7xx KOhm reading.

I pulled the EFI fuse and let the stored fuel trim data clear, sure enough it fired immediately instead of taking a long crank time. Long Term Fuel Trim had been -12% on the scanner before clearing, and it took a lot of cranking to start. Soon as I cleared the computer, start time was immediate.

So it looks like I've got a MAF sensor sending bad data to the ECM, but not bad enough to set its own code. New Hitachi OEM MAF sensor is on the way and I'll post the outcome when it's installed on Wed-Thurs.

 

Subbed to hear the outcome. Hope the new MAF will clear this up!