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Engine misfiring on a hot day- unable to fix

Posted: Thu Aug 01, 2013 11:57 am
by neon_tony
I have a quaestion for you guys because I have been unable to solve this on my own or at any mechanic shop (so far)

Basically on a hot day my car starts misfiring once it reaches normal operating temperatures, it seems to be made worse with the air conditioning or with added weight in the vehicle, but I am guessing that is related to the extra heat when the car is working harder. It happens at low speeds and high speeds, and has been doing this for quite a few months consistently. The engine codes which I pulled from the computer are:

P0068 - related to throttle position sensor
P0300 - multiple cylinder misfire detected
P0352 - related to ignition coil primary circuit
P2305 - related to ignition coil secondary circuit
P0123 - related to throttle position sensor

Its relevant to mention that I am running a stock sx 2.0 block with an RT head, a stock intake manifold, rt injectors and fuel rail, and an RT computer. Also I am running a high flow cat. I do not have a heat shield on my headers so I was convinced the heat from it was causing the misfire so I got some high heat (2000 f) heat wrap and wrapped the wiring harness which runs next to the manifold and wrapped the coil and wires up. I also replaced the coil pack, the plug wires, the spark plugs and the throttle position sensor. However yesterday it was hot and it started doing it again.

I also noticed that it started misifiring mutiple time between 2500-3000 rpms in 2nd gear, and this made me think is there any way that the RT computer is causing this because I do not have the magnum intake installed? I also remember last summer I was running into a similar misfire problem at high speeds and high heat when I was running a green bastard box intake and 62 mm throttle body. I figured that was caused by the fact of not have idle control etc, so I went back to the stock intake manifold and thought nothing of it. it is weird because its not caused by having activating full throttle or beast mode but rather seems to happen almost randomly when Im just touching the throttle, or trying to keep the rpms consistent.

I am very confused and frustrated as I have to drive home to canada from Maine very soon and after that am driving to Houston to begin my masters. Do you guys have any idea what this might be? Or have you come across this before? Any and all help would be GREATLY appreciated. Let me know what other info I can provide.

Posted: Thu Aug 01, 2013 12:05 pm
by r/tguy02
after you put all those new parts on it, did any of the codes come back, and if so which ones?

Posted: Thu Aug 01, 2013 12:14 pm
by neon_tony
The same codes were present in the computer after all of the parts were replaced but I never deleted them out of the computer. Also when the misfire occurs the CEL starts to flash. The funny thing is as soon as i turn the ignition to the off position then back on the misfire stops as does the light flashing. It quickly comes back and I just keep turning the car on and off.

Posted: Thu Aug 01, 2013 1:24 pm
by occasional demons
What year? Assuming 2003 - newer.

If you are still getting P0068 with the stock TB, it may be related to the P0123 code.

P0068 is more than just a TPS issue. The 62mm TB will surely cause that code on it's own, but if it is removed, P0068 should not return, unless you have a bad TPS/wiring/connector, or a significant vacuum leak somewhere.

There is no difference between Magnum injectors/fuel rail and the regular ones.

Check the coil connector, and the continuity of the outer wires to the PCM.

If you used off brand cheap plug wires, or the plugs aren't gapped correctly, it could cause the P2305 code.

Posted: Thu Aug 01, 2013 2:21 pm
by neon_tony
ya its a 2004. Interesting about the P0068 code, I will look for vacuum leaks as i just replaced the TPS... Im just baffled as to the source of this misfire problem.

Posted: Thu Aug 01, 2013 5:57 pm
by occasional demons
Disconnect the battery, (ign off). After you have the terminals removed, reach in and turn the headlight switch on, then shut it off after a few seconds. This totally drains any capacitors that may preserve logged data. Reconnect the battery and drive it, see what codes reappear.

Since you don't have a lean code, your injectors are probably ok, but if the coil secondary circuit code returns, I would look at the plugs/check the resistance in the wires. The primary circuit, again check the coil connector for corrosion/damage, and the continuity from the connector to the PCM connector.

Those are the obvious red flags as far as the misfire issue.

Spray penetrating oil or something similar around the vacuum lines, IM flange, TB etc. to see if the idle changes any. A big enough vacuum leak should make it idle higher tho, except possibly a manifold flange leak.

Was/is there any oil accumulating in the spark plug tubes?

Posted: Thu Aug 01, 2013 9:15 pm
by neon_tony
there was oil accumulating in the spark tubes but it stopped when i replaced the valve cover gasket

Posted: Fri Aug 02, 2013 12:33 am
by kc2005ptgt
Stupid question, but could it be a plug, wire, or coil pack? Have you checked these? I noticed you mentioned it got hot, perhaps it was damaged in the heat.

Anyway, I had a P0300 and P0301 code in my PT GT and it turned out to be plugs. Car bogged bad, actually stuttered at times, and scared me poopless!

Posted: Fri Aug 02, 2013 12:46 am
by occasional demons
A VC gasket will not keep oil from entering the tubes, it seeps in from underneath. New tubes is the best answer.

If you look at the seals around the top, you will notice the tube sticks up higher than the seal. The oil would have to climb over the top of the tube, and not run down the top of the VC to get in there.

Search "spark plug tubes" to find multiple threads on replacing them.