Frank3

This past weekend I needed to travel to Dayton, OH from MA with my 2002 Chevrolet Tahoe. After I was on the road about 3-4 hours the check engine light came on. I kept going going but did notice that if I slowed my speed down to 65 it would begin to flash then eventually go out. If I increased my speed above 72-75 the light would come on after a while. I was able to repeat the slowing down and have the light go out. When I reached my destination, I had it scanned and just a P0300 was found. I had them clear it and when it was time to head off to my next stop, the light stayed out for a long time until I was pulling up a long hill doing about 72. When a tractor-trailer in the right-hand land pulled out a couple car lengths in front of me and the light came on at the exact same time. Are we looking at a heat soak condition, partially plugged fuel injectors... where to start , I don't want to just throw parts at it. The Tahoe only has 51443 miles on it.

cleveland63b

when did you change the fuel filter last? do you have access to a scantool?

Frank3

I changed the fuel filter about a month and a half ago. I do have a scan tool that I used after I got back home to find the P0300 back again. It's not a really sophisticated scanner, it is an actron CP9135 that is old, but is still newer than the Tahoe. When in Dayton, I put a couple cans of injector cleaner in and filled it with name-brand gas.

cleveland63b

if your scanner can access misfire data you need to narrow down which cylinder(s) are missing and go from there. Spark plugs been replaced at all?

Frank3

Sorry it's been a while before I could get back to this problem. I was able to borrow a new scanner from my friend. I hooked it up and sure enough, it showed ONLY a pending P0300. So while watching the data stream as the engine was running and waiting for it to get fully warmed up and go into closed loop, I raised the engine speed up. I found out that when the RPMs were under about 1800 everything was fine. When I increased it to 2000 rpm, the check engine light started flashing on the dash after about 30-60 seconds of running at that speed. As I lowered the RPMs to about 1500-1800 the flashing check engine light would go out after about 30 - 60 seconds. I could repeat this over and over. So now that I could get this to repeat on demand, I raised the speed to 2000 rpms again to get the flashing check engine light.
I had it at the dealer and was told the Tech-Tool told then the Tahoe failed the mis-fire test 4 times and passed 197 times during the trip I took to Dayton, OH. they suggested top tier fuel and fuel injector cleaner.
I then took the scanner and attempted to get it to display mis-fire data by cylinder. there was not option to display this data on my 2002 Tahoe. Being puzzled, I then took the scanner and connected it to my 2014 Impala and started it up. Sure enough, there was the option and I could see the cylinder data.
Now, I really don't know what is going on. Is it possible the 2002 Tahoe is so old it will not support displaying the misfire data like the 2014 Impala does? What can I do now? The condition repeats with the engine running at 2000 rpm consistently. It has a new fuel filter, top tier fuel, fuel injector cleaner, followed by new fuel injectors and new plugs and wires. I really don't want to just keep throwing parts at it. Ideas of what to do next?

kevinkpk

Not sure about your Tahoe, or any one for that matter. Does it have two crankshaft position probes? If so, it might be one that the ecm looks at over 2000 rpm

Frank3

I think there is only one crankshaft position sensor. I was contemplating getting that replaced, but then you have to do a re-learn, and I am just throwing more parts at it.

Frank3

Well, I thought I would post an update as I have this error nailed down. I did a lot of searching on the internet and ran across some youtube videos that were essential in getting this persistent error fixed. I want to give credit to "dogbyte555", "The Car Doctor" and "The Disgruntled Mechanic" for their input, ideas and suggestions. As stated, the flashing check engine light kept reoccurring and was really getting annoying. It wasn't until I connected up my scanner/tester and started the Tahoe to view live data that I started to run down the right path. The engine was still in open loop, so I sped up the engine to warm it up a little faster when I discovered that the light would come on and start flashing at 2000 RPM. When I would drop it below 2000 at about 1500, the light would stop flashing but I could see a pending P0300.
Looking at live misfire data, I would see misfires MAINLY occurring on cylinders 1 and 6 with a small quantity of misfires on cylinders adjacent to 1 and 6. According to the videos, a crankshaft relearn will correct the problem. So I performed a crankshaft position sensor relearn, then once completed, went to watch the live misfire data. When I revved the engine up to 2500 RPM, the misfire data show persistent 0 counts of misfire data. So the problem is now gone.