When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Need help figuring out a friend's 2016 engine problem
A friend of mine has a 2016 C7 that is having serious problems that I think are fuel related. I was able to get some real time data, here's what I experienced and read:
1. Engine starts OK, then starts stumbling, then revs start oscillating, then rpms would drop down to around 500-700, and the engine would struggle to continue idling.
2. Throttle application doesn't help much if at all.
3. Long term fuel trims on both banks are bottomed out at -25% (engine running rich?)
4. Fuel pressure at engine start is around 80 psi, then drops down to 50 psi (which I think is correct).
Just got off the phone with my friend, and he said that his driver side window wouldn't go up more than about 5 inches, so they disconnected the battery to hopefully set the window index back to the default position, and they decided to try to see if the car would run enough to get it on the flatbed.
Lo and behold, the car runs fine. Ran fine when they drove it off the flatbed at his house and into the garage. So we're obviously dealing with an electrical gremlin; now, how to isolate it!
After touching the disconnected battery power cables together step on the brake peddle. This will assure that the residual voltage in any circuit is completely drained.
Also wait for ~ 1 = 2 minutes, and a little longer if you are working on air bags.
A friend of mine has a 2016 C7 that is having serious problems that I think are fuel related. I was able to get some real time data, here's what I experienced and read:
1. Engine starts OK, then starts stumbling, then revs start oscillating, then rpms would drop down to around 500-700, and the engine would struggle to continue idling.
2. Throttle application doesn't help much if at all.
3. Long term fuel trims on both banks are bottomed out at -25% (engine running rich?)
4. Fuel pressure at engine start is around 80 psi, then drops down to 50 psi (which I think is correct).
What do I need to look into?
TIA for any help, and have a good one,
Mike
You need to look at data in your scan tool to see why your fuel trims are so negative…I’d be looking at your O2 sensors, MAF in grams/second, ECT, and MAP sensor…something like a stuck open EVAP purge valve can cause this rich condition because the intake manifold is pulling in fuel vapors from the EVAP canister.
Thanks. So, quick question: which way is the fuel trim going to indicate a rich/lean condition? IOW, does -25% mean the engine is running rich and the ECM is pulling fuel, or the opposite?
And thanks to all for the suggestions, we'll be working them as weather allows (his car is parked in a carport).
Thanks. So, quick question: which way is the fuel trim going to indicate a rich/lean condition? IOW, does -25% mean the engine is running rich and the ECM is pulling fuel, or the opposite?
And thanks to all for the suggestions, we'll be working them as weather allows (his car is parked in a carport).
Have a good one,
Mike
Negative fuel trims means the ECM is “pulling fuel” and it’s rich…don’t know what kind of scan tool you are using but it can be helpful if you know what you’re looking at…what codes are set if any ??…BTW, a stuck open purge will cause the car to run rich and once all the fuel vapors are burned it will then run lean because you basically have a vacuum leak…I would see what the ECT reads after the car sits overnight and should be close to the outside temperature…read MAP sensor inches of Mercury or PSI with ignition on…you can try to disconnect purge valve and plug the intake manifold connection and the MAF reading in grams per second at hot unloaded idle should be around the liter displacement of the engine.
When I saw it (before the battery disconnect/reconnect) the poor critter had about a dozen: both O2 sensors voltage low, both O2 sensors voltage high, etc. When we get back together (we live about an hour from each other) I'll check again.
Thanks for the understanding of the fuel trims, one of these days I'm gonna remember this stuff!
Being that you saw O2 sensors “low” or a lean condition fuel trims will go positive…when O2 voltages “high” fuel trims will go negative…seeing both of these may be that stuck purge valve…a rich condition then a lean condition…disconnecting the battery will erase those DTC’s.
Being that you saw O2 sensors “low” or a lean condition fuel trims will go positive…when O2 voltages “high” fuel trims will go negative…seeing both of these may be that stuck purge valve…a rich condition then a lean condition…disconnecting the battery will erase those DTC’s.
UPDATE: I haven't been able to get with my friend for a while other than by phone, but he's still working on the problem. Got the purge valve changed out, which seemed to work for a while, but the symptoms came back. So I'm leaning towards an electrical problem, and I always lean towards dirty/bad grounds first. Problem is, I can't find a diagram showing the ground locations for a C7. C6 and C5 out my ears, but no C7. Does anyone have a diagram of the ground locations for a 2016 C7 coupe?
Thanks for all the help, and have a good one,
Mike
What do your upstream O2 sensors look like on a scan tool ??…if they are high the fuel trims will be negative and will pull fuel…doubtful BOTH O2 sensors are bad…did you look at the MAP and MAF sensor data Pids ??…if MAP sensor is skewed you will have fuel scheduling issues !!…DON’T think it’s a ground issue…grounds don’t mess with the fuel scheduling but this is from a 14 if you want them.
We live about an hour apart; so I'll get my baby scan tool out there this weekend to get the data, and let you know. In the mean time (just for prosperity), do you know of a diagram showing the ground locations for the C7? IDK that there's a problem there, but my C5 experience with ground locations and corrosion always makes me check those out.
Need help figuring out a friend's 2016 engine problem
