Intermittent start, then stumble on acceleration?

Tiny
GROJGUY
  • MEMBER
  • 2005 KIA SPECTRA
  • 2.0L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 100,000 MILES
The vehicle listed above is an EX-model.

The original motor ran low on coolant and blew the head gasket. So, I installed a used (under warranty) replacement motor (from 2009 car, 93k miles). While I cannot say 100% I made zero oversights, I will say that I am *extremely meticulous* (hardware/software engineer by profession). I did everything by the book, reconnected all hoses/wires/clips/zip-ties exactly as original, and had no parts left over (yay!).

Fuel in tank was 3+ years old, so I pumped most of the old fuel out of the tank (gauge was on E), and filled half-full with new fuel. Tried starting, and got good crank but ZERO ignition. I suspected residual old fuel in lines, so I disconnected fuel line disconnect in engine bay, and flushed new fuel through the line (except from quick disconnect up to the fuel rail/injectors). Eventually it sputtered a few times, but was weak and then cranked extensively without firing again. I thought maybe it was flooded, so I tried many attempts to flood-clear. Finally on one try, as I double-tapped the throttle while cranking, it fired and I was able to rev it up and nurse the throttle to keep it running for several minutes.

I then shut it off, and immediately it refused to fire again. I happened to check diagnostic codes and found a P0340 Camshaft Position Sensor (circuit A) code. So I removed the Camshaft Position Sensor, cleaned it, and re-installed.

Immediately it started and idled nicely. However, if the throttle was quickly pressed, it bogged and almost died, then came back up and searched RPMs at 100-200 above idle for about 10-20 seconds, then settled back to idle. If throttle was *slowly* depressed, it revved up normally with little or no stumble.

After idling nicely for 3-4 minutes, I shut it off. Then immediately it failed to start (fire) again. So, I removed the Camshaft Position Sensor from the old motor, cleaned it, and installed it. Immediately it started. But it still exhibited the same bog issue with rapid throttle application. And on several subsequent start attempts, it did not fire. Definitely somehow relates/correlates to CMPS. Still intermittent and not reliable but seems improved compared to the other Camshaft Position Sensor.

I have read that there is a lobe on the cam (used by the Camshaft Position Sensor) that can come loose. I am wondering if that explains the strange intermittent starting that correlates with changes to the CMPS. Apparently, to check it requires removing the valve cover.

And then, what could explain the bog on rapid throttle? Perhaps a related issue? Or another different sensor?

After some additional research, I see that I should check the IAC valve, and TPS. I have spares of those from the old motor, so it could be a good experiment to swap them and see what is the result.

It appears that diagnostic codes do not consistently assert quickly on these cars, which is disappointing and somewhat defeats the purpose.

The main concern is the intermittent start/no-start. That tells me there is an intermittent component that could completely stop working anytime and leave the wife stranded. Not good!

Thank you!
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Tuesday, August 29th, 2023 AT 10:21 AM

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Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 42,235 POSTS
It sounds like you have a fuel pump that is weak but to be sure let's do a fuel pressure test. This guide can help, and you can rent a fuel pressure gauge from the auto parts store:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Standard Value: 343 kpa (3.5 kgf/sq.cm, 49.8 psi)

Here is how to change the fuel pump out and drain the remainder of the old fuel from it. I would also use a flashlight to inspect inside the fuel tank for debris. The camshaft position sensor I would replace. Check out the images (below). Please let us know how it goes.
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Tuesday, August 29th, 2023 AT 5:32 PM

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