Thread: Engine starts then dies when warm, but drives fine

Does it idle well? Any codes?

almost sounds like a leaking fuel injector, but I don't want to start throwing things out until we know more. I'd get a scan tool that can read fuel trims and misfire data. It'd be nice to be able to see the maf, ect, map, iat, vss, and other sensor data before we go throwing parts at it.

Originally Posted by Mitch R

Checked the fuel pressure regulator, and it did not smell like fuel. I confirmed that the problem seems to go away when the MAF is unplugged, so I sprayed some MAF cleaner on it and stuck it back in to see if that helps. I have a spare MAF i may try as well. IAC was carboned up pretty bad so I took that out and cleaned it as well. Seems better but I'll find out tomorrow when I really drive it again. At idle it is pig rich though LTFTs of -19 to -22. Only code was the one I caused when I unplugged the MAF so i cleared them.

Best case scenario is you get a scantool that can map engine data. You can verify your maf sensor, o2 sensor, fpr, fuel pump, and map sensor without ever tearing into anything or even unplugging anything! ScannerDanner's channel goes into depth about diagnosis using these methods and is a really good resource if you have a few hours to kill and learn. There's actually a video he has on a lessabre that has this exact issue and ended up being an intermittent weak fuel pump that was just replaced a few months back with a new unit. You can really do some cool stuff when you understand fuel trims.

If you want to dig further, I would start by checking the fuel pressure key on engine off, idle, 2000rpm, and idle with fpr vacuum removed. This will verify the fuel pump is working correctly and "sort of" test the injectors. If you find the fuel pressure is too high and doesn't change with vacuum removed, it's your fpr. If it's too low, it could be fuel pump or leaking fuel injector. I bought a fuel injector pulse tester for $30 off ebay and it works miracles for this test. Also got my fuel pressure gauge from harbor freight for cheap.

If all that checks out, you'd really need an oscilloscope or really good scan tool to test the o2 sensor thoroughly.

Originally Posted by Scottydoggs

i was helping out a guy on the fb dhp page, he was stuck at -22 ltft no matter what we did to his tune. then im like its got to be a stuck injector, try pulling the plug off each injector one by one and watch the fuel trims. he got to cyl 3 and the trims went way down. found himself a stuck injector, replaced em all and trims are normal now.

Agreed. Power balance test is a great tool when coupled with fuel trim data!

Yeah. I'd use the scan tool to test everything as outlined above coupled with testing the fuel injectors. I'm not a parts swapper, so I don't recommend replacing stuff without testing it first. You are correct. Vacuum leaks cause positive (lean) condition not rich negative fuel trims.

Originally Posted by Mitch R

It ended up being the maf sensor, I had a known good sensor laying around and once I swapped that in, the problem went away completely.

Yet another reason I hate the maf sensors in our cars... glad you got it fixed though!

What is the normal fuel trim? Mine sits at -18 and ive noticed it's harder on fuel. As well as picking up a p0300 code recently

Ill have to investigate on mine. Last nighg was -18.75 now this morning iy is at -21 at an idle. 🤔 my p0300 code only comes up when im at wot in second gear it starts missing bad. Ill have to do some more testing on it later this week.