CEL flashing on downhill only Express 1500 4.3
 

Hi Guys I have a 2007 chevy express 1500 with 4.3L engine with 238,000 miles it runs great, i have recently been having this issue where when driving on freeways or highways and have a long downhill run the CEL will blink until the van is level again, it does not sputter or run rough just the light flashing and goes off soon as van levels out, does not do it on uphill just downhill, any ideas? wanted to check here before i take it to shop, thanks in advance for any help.

May I suggest you take it to Autozone or similar parts house and get the actual trouble codes read, then report back with them.

Thanks for the speedy reply, i had my neighbor pull the codes and it reports p0300, p0171 and p0174

Could be several things hopefully, others can help, but here are the code descriptions

p0300-https://www.obd-codes.com/p0300

p0171-https://www.obd-codes.com/p0171

p0174-https://www.obd-codes.com/p0174

My take is the 171, 174 is causing the 300 misfire. May be a dumb question, but is your gas tank full, part full, does it matter on the fuel level when you get this?

How long has the fuel pump been in service?

Does your van have a serviceable fuel filter? I cannot remember when the returnless system started.
Also have you checked your gas cap?

It doesnt matter if the tamk is full or what level CEL light will still blink, probably the original fuel pump, dont know about fuel filter. didnt mention that sometimes the light will stay on then go off after about 50 miles of driving

One of the fixes/causes on those links was a faulty or dirty MAF sensor. When coasting down a hill you force more air than the engine demands....perhaps that is causing temporary fault. Try cleaning the MAf sensor.

check for things that lean out the fuel system on both banks.

view the fuel trim data at idle and 2500 rpm. if it is more than +10% all the time, the issue can be low fuel pressure, bad maf.
it the ft is adding at idle but normal(0 to 10%) at 2500, you have a vacuum leak anywhere from the air filter snorkel to the intake manifold.
the hard decell will accentuate a vacuum leak