Why is my car misfiring?

JIVARITO157

MEMBER
2003 SUBARU FORESTER
45,000 MILES

Getting misfires (only noticable at idle, which is wobbly, as though missing, but doesn't always throw a code) in all four cylinders. It has thrown codes a few times, 301, 302, 303, 304. Not all at the same time, but as many as three. No accompanying codes Recently had head gaskets done. Car was running fine before then. Timing belt and all marks are fine. Car has new front o2 sensor, crank sensor, cam sensor, intake air temp sensor, fuel pressure regulator, ignition coil, wires and plugs. Idle air control does actuate and pinching off the line to the throttle body/intake manifold from the purge control valve changes nothing. The car has runs on a map sensor (which I have checked with a hand pump, it starts at 4.35v at no pressure and drops about 1v per 5inHg applied, I don't know if that starting point is a little low?) And responds quickly to any artificial leaks (engine surges and immediately drops back down to it's rough idle). Smoke test showed no leaks. Alternator and battery working fine. Planning on changing the coolant temp sensor this weekend. It has deposits on it. I have not checked compression yet, but this seems an unlikely culprit as the misfires are happening in all cylinders. Throttle position sensor shows 0% with foot off gas and 100% pedal to the metal on the scan tool.

Any ideas? Do map sensor readings make sense? Could coolant sensor be to blame? Anything else I should be looking at?

Thanks.

5 Replies

RASMATAZ

MECHANIC
75,992 POSTS

Why Is Misfiring?

Refer to this link for more information:https://www.2carpros.com/articles/engine-misfires-or-runs-rough

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Tuesday, February 28th, 2012 AT 7:15 PM

JIVARITO157

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3 POSTS

Thanks, I've read that. Do you know if the map sensor readings make sense? 4.3 volts seemed like a low starting point. Isn't 5 volts normal?

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Tuesday, February 28th, 2012 AT 7:54 PM

RASMATAZ

MECHANIC
75,992 POSTS

Go to this link: http://www.youtube.com/watch?v=B8EVhFc5Yqw

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Tuesday, February 28th, 2012 AT 8:09 PM

JIVARITO157

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Yes, that's how I tested it, the question is do my numbers make sense for a Subaru or not?

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Tuesday, February 28th, 2012 AT 8:50 PM

RASMATAZ

MECHANIC
75,992 POSTS

DTC P0068: INTAKE MANIFOLD/BAROMETRIC PRESSURE SENSOR CIRCUIT, RANGE PERFORMANCE PROBLEM
DIAGNOSTIC PROCEDURE
Perform basic diagnostic procedure. See BASIC DIAGNOSTIC PROCEDURE. If DTC P0068 is only DTC set, go to next step. If any other DTCs exist, diagnose and repair those DTCs. Do not diagnose DTC P0068.
Inspect air intake system hoses for holes, cracks or loose bolts and connections. If any problems are found with air intake system, repair as necessary. If no problems are found with air intake system, go to next step.
Start engine and bring to operating temperature. Turn ignition off. Turn ignition on. Ensure A/C and all accessories are turned off. Using scan tool, monitor intake manifold pressure sensor signal. Reading should be 21.65-31.50 in. HG. Start engine and let idle. Using scan tool, monitor intake manifold pressure sensor signal. Reading should be 5.91-13.78 in. HG. If readings are as specified, go to next step. If readings are not as specified, replace intake manifold pressure sensor.
Turn ignition off. Turn ignition on. Using scan tool, monitor the throttle position (TP) sensor while throttle is fully closed. If reading is less than 5 percent, go to next step. If reading is greater than 5 percent, adjust TP sensor. See ON-VEHICLE ADJUSTMENTS.
Using scan tool, monitor the TP sensor reading while throttle is fully open. If reading is greater than 85 percent, replace then intake manifold pressure sensor. If reading is less than 85 percent, replace the TP sensor.