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Hello,
The engine starts fine and seems to idle good. When I drop it into drive it sometimes will stall right then or once I try to move the car it dies. If I wait a little longer I can get it going and drive down to the end of the street and when I reach the stop sign it dies as I come to a stop. I restart it and drive about another mile to the traffic light. Then if it does not stall when I slow down then when the light turns green and I try to accelerate it can stall. after about five to ten minutes of driving it is fine. No issues. It also seems to take a long time to warm up.
I am not getting any codes on a regular basis. I have seen some misfire codes but that makes sense because it is stalling.
Here are some of the readings I get:
When the engine is cold
LTFT = -14.8
STFT = -18.8 to as much as -20% and even -25%
O2 sensor voltage sits up at at max around 1.1V (I have my scan tool set to graph it so I am not certain of the exact number.
If I rev up the engine I do see the O2 sensor voltage go low to around 0.2V. So it seems to be giving readings.So last fall I had a mechanic do a parts change and he changed all the coil packs since he saw misfire codes. He also changed plugs and I think the wires as well.
The car has over 200K miles on it now.So in looking at the fuel trims, I leads me to the idea that the engine thinks it is running rich and is backing off of the fuel trims. So it makes me think that it is not getting enough air or thinks it is getting too much air due to bad sensor readings.
I checked some of the obvious. I checked that the air filter is clean and that the air passage from the outside to the air filter is clean, (no mice nests).
I suspected the intake air bypass control thermal valve might be stuck closed. It would fit the symptoms. If I remove the hose from the value to the intake manifold the idle increases a lot but will still want to stall if I try to drive it like that. It seems a little better. Then I replaced that hose and removed the hose from the air intake that feeds the valve and I get a vacuum that I can feel by blocking it with my thumb. the idle does react. (I do not have vacuum gauge) Then once the engine is warm I do not get any vacuum so it seems like the valve is doing its job.The throttle body was replaced a couple of years ago but I do not know if they replaced the idle control valve or if they cleaned it.
So I really could use some advice as to where to go from here? I really would like to take care of this myself rather than go to a shop and have them charge me hundreds and throw parts at it.
Once the car is warm the STFT gets better and the LRFT gets better but they both all seem to still be negative. LTFT around -7%
Hi, It sounds like your on the right track there:
“So in looking at the fuel trims, I leads me to the idea that the engine thinks it is running rich and is backing off of the fuel trims”
The engine (computer) “sees” the fuel mixture as being very rich via the O2 sensor high voltage signal. Normally the voltage should oscillate between 200 and 800 millivolts. You need to determine if it’s really running rich, or is the O2 sensor sending bad data. I’m guessing it’s not running too rich because of the way it stalls. Try comparing the O2 senor readings when the engine is cold opposed to when when it’s fully warmed up.
Thanks Importech,
This is my daughter’s car and she is returning from her first year of college tonight so I drive the car to work today and captured some screenshots to give some good data rather than from my memory. 🙂
This first one is where I turned on the key, waited a few seconds then started the car. You can see the O2 sensor graph. The gauge to the right of the graph has number display in the middle of it. That is set to intake air temperature.
This next screenshot is right after the cold start where you see the O2 sensor come up. This is right after the first screenshot.
I took some more when it was still idling and cold but there is not much that changed. The O2 readings stayed close to the same. The idle starts at about 1500 RPM and as it warms up it goes down below 1,000, I am guessing around 800-900 RPM. It is fairly steady and smooth. No hunting.
So after driving for about 40 min I took this while on the highway driving at about 45 MPH (rush hour traffic) and trying to keep the speed steady. The second half of this graph was pretty steady.
Then when I arrived at work I parked and let the car idle and took this last screenshot.
http://tinypic.com/r/125omee/9I also took one with the RPM at a steady 2500 RPM when warm sitting in park. I can post that one if it would be helpful.
What strikes me about these is that the O2 voltage is sitting a little high? Is that true? If I remember right it should be around 450mV when warm? I guess this would explain why the fuel trims are low.
Thoughts?
Thanks Dave T
Well, the images did not exactly work well. MY first time trying to post pictures on the forum. I will have to figure this out later this evening.
Hello,
I will try again to upload the first two images. The last two seem to work.
From a cold start:Shortly after a cold start:
So in looking at these. I see that the O2 voltage starts really high before I start the engine, but I would think that is normal. Then the engine starts and it goes down to zero and rather quickly it comes up to around 820 mV and pretty much stays that way. It comes down slightly.
But then after the roughly 45 minute trip it is still up around 750mV.
So my question is if this is normal?
I guess I need to watch some of the other videos regarding the O2 sensor readings.Thanks,
Dave T
Hi Dave, Is the lower right gauge air flow and intake air temp? If it is airflow at 12 to 14 that would be really high for idle. Should be around 8 or 9. Maybe you’ve got a dirty mass air flow sensor.
Sorry, I was mistaken. It doesn’t have a mass airflow sensor. It’s a calculated value based on throttle position and load. But back to the o2 sensor, they normally start out low voltage then come up as the sensor warms up, then it oscillates between about 250 to 850 millivolts at idle. Looking at the graph it does show some oscillation, but then returns to the high side of the graph. See if the sensor looks ok physically (not damaged or covered in oil), and inspect the wire harness near the sensor to see if there is any chafing.
Thanks everyone for the suggestions. It seems like it may indeed be running rich since I can smell some unburned gas in the exhaust. I have questioned the O2 sensor since the exhaust system was replaced fully about 4 years ago the the tail pipe and muffler only lasted 3 years so I question the quality of the parts that were put in at the time.
I did replace the fuel infectors which improved how well it runs but did not fix the stalling problem. I need to get the probes so I can hook up my oscilloscope to view the infector timing to be certain they are not being told to be on longer then they should. Most everything else seems fine.
I really don’t want to take it to a shop where they are going to make quick assumptions and throw parts at it then charge me $1000 and it probably still will not be fixed.
Dave THave you checked your fuel pressure regulator? Also your oil level. I’ve just had this problem and oil being sucked up the PCV hose was causing rich trims. On some cars it’s connected with a vacuum line. It should be dry to the touch, sometimes the diaphragms rupture and fuel ends up being sucked into the vacuum line.
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