New here. Need help! P0300

energie

New Member
Mar 31, 2010
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Hi,
I'm new to the forum but not new to stangs. I recently did a cam install as well as a tune. My mods are as follows (engine wise anyway) custom grind cams (about a stage 2 comp) trickflow 75mm tb/plenum, jlt ram air intake, mac LT headers, o/r h pipe, slp lm2 exhaust.

now i just recently had a problem where my car would hunt alot at idle, thought it was the tune but the problem persisted after the tune, turns out i had a bad o2 sensor, so i replaced both, car ran fine, then i took it out a week ago, and i all of a sudden got this p0300 (random misfire) error and a p0100. my car RAN LIKE CRAP. it wanted to die at idle so i had to keep revving it to keep it from dieing. the SES light stays on even after clearing the code it comes back. now the weird thing is this, it seems that now the problem only persists when the car is at operating temperature. cold it runs fine, aside from the normal stumbling and uneven engine shaking that it always did, but when ive been driving it for a while the problem seems to pop up. i checked the cop's and plugs were just replaced not too long ago, i did a few data logs but i dont know what to tell whats good and bad, so for beginners luck, please someone share their knowledge on this topic.
thanks in advance.
 
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For the P0300 you might want to check the crankshaft pulley bolt. You say you recently installed cams which generally means removing the crank damper and front cover. If you don't re-install the crank damper bolt properly, it can allow the crank trigger wheel to move a bit which looks electrically-noisy to the PCM making it think there's misfiring going on.

Did you use a new crankshaft bolt and did you tighten it to spec?
 
i got steeda UDP's a while back so i used the bolt that came with that, but i re-siliconed the washer, but that was about it. im pretty sure i tightened it to specs. i dont think i could torque it that high because even in gear the engine would still crank over.
 
The silicone is there only to prevent oil seeping down the keyway and leaking out.

The tightening sequence is crucial:

- new bolt (OE part is TTY and must be replaced each time it's loosened)
- tighten to 66 ft-lbs
- loosen one full turn
- tighten to 37 ft-lbs
- tighten an additional 90-degrees

If this isn't done correctly most likely you'll get misfire codes.
 
wouldnt this have come up as a problem sooner than now? i did the cam install back in sept.
also, i did a datalog on misfire_cyl_1 to 8 and it seemed as though all cylinders were misfiring, then i reved to 2k for 15 seconds and the misfires went to 0. here is how someone worded it.

"Anyone else have any ideas? When rev'd to 2k it misses a total of maybe 5 times in 20 seconds. At idle he misses a **** ton. Noticeably tripping that parameter in each cylinder every 2 or so seconds. Fuel pressure is golden at 40, and battery voltage is high 13's. "
 
hey guys i have a 99 gt with cms stage 1 cams and some other bolt ons and i get p0300 when i first had my cams done i couldnt find anyone to tune it so i got a mail order tune from brookespeed and they were able to get the code to go away ... on sunday i finally took my car to have it dyno tuned and the car ran great until today its trying to dye at idle and th engine light came on and through codes p0300 and p0602 .... so im hoping its the new tune and my tuner can fix it
 
trinity_gt has given you the best answer.

A stable crank shaft possition sensor signal is critical for timing. Bad timing = misfires.

Try the suggested fix. If it doesn't work, post back.
 
I was getting the p300 and a couple random cylinder codes after the cam install. WHen i got the tune the tuner said .54 is too much gap for these cars with a few mods. He suggested i regap them .40 and i did, car LOVED it. No more codes. Also check the big tubing coming off the front of the Plenum to the air intake, i had a small crack on the underside and didnt know it, causing an idling nightmare. I also re set my idle with the cams to 850.
 
Just how lumpy is the idle with those cams?

The PCM detects misfires by monitoring variations in the crankshaft speed. Each time a cylinder fires the crank should speed up just a little bit due to the impulse that cylinder just gave the PCM. The crank trigger wheel is used by the PCM to measure these variations in crankshaft speed. A cam that is excessively lumpy can fool the PCM into thinking that the engine is misfiring because the crank speed is all over the map at idle.

I'd expect this to be over by 2000RPM and it doesn't really explain why this is a recent development. You say the cams are "custom" but about a Stg2 grind. What's your idle speed set at right now?

BTW, did you re-torque the crank bolt per the specs I gave?
 
I think my tuner set it to about 1100. Then he adjusted my old throttle stop thing on the tb itself because it would hunt, he didn't think to check my o2 sensors which was the problem. It's sitting at about 1000-1200 right now, I'll check specifics when I get home, with the new tb I installed it was at about 900. I didn't retorque it yet, will do that today if I can, but I don't get why it's dieing at idle during operating temps but fine at cold start. If the cam just threw the code I wouldt care but something is wrong
 
I think my tuner set it to about 1100. Then he adjusted my old throttle stop thing on the tb itself because it would hunt, he didn't think to check my o2 sensors which was the problem. It's sitting at about 1000-1200 right now, I'll check specifics when I get home, with the new tb I installed it was at about 900. I didn't retorque it yet, will do that today if I can, but I don't get why it's dieing at idle during operating temps but fine at cold start. If the cam just threw the code I wouldt care but something is wrong

This is most likely the problem, from what i have heard (correct me if wrong) you Never try to fix an idle issue by adjusting the TB screw. You NEED a new tuner ! When cold, the engine idles high and does not mind the extra air, but when its warm and tries to drop the idle down the air/fuel mix is way off and the car doesn't want to run.

I would attempted to set the TB screw back to were it should be and then fix the idle with a good tune.
 
well i can rule that out because since the tune i had bought a new tb/plenum and it was set to stock settings, and the car still did it. i just threw the old one back on because it didnt seem to have this problem before i put the new one on, but it still shows up with the old tb/plenum