October diagnostic challenge (SOLVED)!

[Jeremy airjer W]

Gas tank always full or half at least? Had a ford CV that when the fuel level got low, the heat from the return fuel would swell the pump. It would start off great and run good but then it would stumble and if you shut it down for a little while it would cool enough to start again.

Return less fuel system and fuel pressure held steady the whole time.

[Jeremy airjer W]

OK, let me throw in a couple questions, you said you have spark, how about injector noid? Have you tested the CKP, and cmp sensors?

[gquist]

The scan tool numbers are generated by what the pcm wants, most of the time, not what is actually happening, hard to go further without scope test readings, I am guessing that if the throttle body was sticking it would throw codes, that is the only component on the car that that actually have the sensors backed up to check against themselves.

[gquist]

Did you scope the correlation between the crank and cam sensors? Pattern to the pcm from the module?

[Jeremy airjer W]

Did you scope the correlation between the crank and cam sensors? Pattern to the pcm from the module?

Didn't have to. I overlooked a valuable piece of data!

Here's a hint and how the diagnostic process unfolded

The first night I verified the complaint with the customer. I then went on a test drive with the scan tool and watched MAF, APP's, TP, and CKP. When the vehicle stalled all of those looked normal. Just like the customer said it started after sitting for a couple of minutes and drove fine. I brought it back to the shop and decided to check back pressure. It was normal. I decided at that point it would be easier to figure out what was right and start narrowing down the possibilities. The next day I taped the fuel pressure gauge to the window and grabbed the spark tester. It stalled on the test drive and I eliminated the fuel pump, the crank sensor, and the module because I had fuel pressure and I had spark. Since the no start was a steady crank with no sign of a start and stall I also eliminated the TAC module (if it was stuck closed the vehicle would still start and then immediate stall). I then new I had a fuel control issue but I had to figure out if it was a power supply problem to the injectors, the PCM not supplying ground, or a sensor giving bad data to the PCM.

After sitting for a couple of minutes the vehicle again started but this time it did not drive normally. It felt surgy and the transmission didn't seem like it was shifting correctly. I made it back to the shop and once again scrolled through scan tool data and then it hit me. I overlooked the info from this sensor. A couple quick tests and the problem was solved.

[gquist]

Not much left other then the cts that would make it stall as far as sensors go, you said the cam sensor was good correct?

[Jeremy airjer W]

Cam and crank are good. Coolant temp never dropped spiked or flaked out.

[Jeremy airjer W]

We are so close I can almost taste it!

[Scott K]

Well if it has to do with fuel metering, something with the MAF reading incorrect? Does it pass the tap test? Map sensor?

[overdalimit]

02 sensor.

[Jeremy airjer W]

The sensor I overlooked is the MAP sensor. With the engine running the MAP was reading 2.4 to 2.6 volts which is about a volt higher than it should be. As I increased RPM's the voltage would drop below 1.3 volts and then shoot back up to a "normal" reading. Once I saw this I decided to check to make sure my input voltage was correct, the sensor ground was good, and that engine vacuum was correct (I removed the MAP sensor and checked it at that port). Everthing was correct so the MAP sensor was replaced. I test drove the normal "extended stalling loop" twice with no more symptoms.

I wish I would have watched the MAP sensor during the stall to see what it was reading.

My theory is it read high voltage which would make the computer think the car was at a very high elevation. The higher the elevation the less air, the less air the less fuel. If the fuel mixture is too lean the engine will not start.

What do you think?