Fuel Pressure Low When Cold and P0300

[ian209]

Hey All,

Having some issues with my 08 MS3. After leaving my car overnight and turning the key to the on position in the morning, my cobb AP reads around 10-12 PSI HPFP actual pressure. I just changed the fuel pressure relief valve last night and it's still having the same issue. I'm also getting a P0300 code that I'm guessing is related to this as well, because the CEL comes on for P0300 mostly after a cold start. My fuel mileage on the car is also very poor (I get around 350-400kms or around 250miles a tank, which is way worse than it use to be.

Any help would be appreciated.

Thanks,

Ian.

[mituc]

Originally Posted by ian209 After leaving my car overnight and turning the key to the on position in the morning, my cobb AP reads around 10-12 PSI HPFP actual pressure.

It's not uncommon for the in-tank pumps to fail or have the filter clogged. You will have to drop the tank and check the in-tank pump, and eventually change the filter if not the whole pump assembly.

Originally Posted by ian209 My fuel mileage on the car is also very poor (I get around 350-400kms or around 250miles a tank, which is way worse than it use to be.

At what average speed?

[ian209]

At around 50-100km/hr, city driving. Forgot to mention that my fuel pressure is fine once the car starts up and I'm driving. I get around 400-450 PSI at idle and up to 1800ish at WOT. I'm also running autotech internals.

[mituc]

Originally Posted by ian209 At around 50-100km/hr, city driving.

There's absolutely no phoking way you get average speed of 50-100Kmph in city even with a police helicopter! No way! Not on this planet, in whatever city at your choosing on any continent.

Ok, so city driving... your mileage seems fine to me. In city driving these cars average about 13-15l/100km (15-19mpg) so a fuel tank of 55 litres (14.5 gallons) mean 300-350ish kms.
So if you touch 400kms with a tank (meaning an average of 12.5l/100) in 100% city driving it means that the traffic in your city is very well organized. Congrats to your city major!

[Fstrnyou]

What fuel are you running?
What sparkplugs?
What are they gapped to?
Done a compression/leakdown recently?

[ian209]

So I think the fuel pressure relief valve fixed the hard starting issue, I don't seem to get it anymore. But the weird thing is when the car warms up a bit and reaches it's idle RPM (950ish), it starts misfiring. But during normal driving and WOT, there are no misfires. I've attached an idle log, looks like my fuel trims are off. Would this cause the misfires? Waiting for a new map from Justin @freektune to see if this could be a problem.

Originally Posted by Fstrnyou What fuel are you running? What sparkplugs? What are they gapped to? Done a compression/leakdown recently?

NGK Stock Heat Range plugs, gapped to .027ish.
I haven't done a compression/leakdown but I think it would be safe to assume that it should be fine because I just got the Corksport Dankai 1 motor installed a few weeks ago.

Originally Posted by mituc There's absolutely no phoking way you get average speed of 50-100Kmph in city even with a police helicopter! No way! Not on this planet, in whatever city at your choosing on any continent. Ok, so city driving... your mileage seems fine to me. In city driving these cars average about 13-15l/100km (15-19mpg) so a fuel tank of 55 litres (14.5 gallons) mean 300-350ish kms. So if you touch 400kms with a tank (meaning an average of 12.5l/100) in 100% city driving it means that the traffic in your city is very well organized. Congrats to your city major!

It's Kilometers/hr, not KiloMiles/Hr lol.

[mituc]

Originally Posted by ian209 It's Kilometers/hr, not KiloMiles/Hr lol.

Kmph is kilometers per hour, mph is miles per hour. I know what I'm saying man...

About your idle log, the fuel pressure looks good, you're around 400 PSI which is where the fuel pressure is supposed to be at idle.
Your MAF cal is a bit off in that area but it's not very bad.

[Fstrnyou]

Fuel trims being off won't cause misfires.
How cold is it where you're at?
Does it misfire during idle when the engine is warmed up or only at cold start?
With a new engine, you might still be in the engine break-in phase. Rings might need more time to seat.
How many miles do you have on it?

[Thor Hammer]

Originally Posted by ian209 Hey All, Having some issues with my 08 MS3. After leaving my car overnight and turning the key to the on position in the morning, my cobb AP reads around 10-12 PSI HPFP actual pressure. I just changed the fuel pressure relief valve last night and it's still having the same issue. I'm also getting a P0300 code that I'm guessing is related to this as well, because the CEL comes on for P0300 mostly after a cold start. My fuel mileage on the car is also very poor (I get around 350-400kms or around 250miles a tank, which is way worse than it use to be. Any help would be appreciated. Thanks, Ian.

We know that the in tank pump is capable of 40 psi. What happens if it sits for a couple hours after running and you then turn on the key and watch the pressure? I've never tried this so don't know what the characteristics of just in tank pressure is after sitting overnight or warm.

Yet you say once you start the car you have a normal fuel pressure reading. So what's the problem? If the ITFP was shot/clogged then I doubt all would be ok at idle and while driving. Have you looked up code P0300? What is it?

Does the misfire sound make the engine stumble at a consistent rate or does it just sound rough? Does the idle fluctuate? Check for vacuum and boost leaks. Vacuum leaks will cause a rough idle at times.

Any other codes related to the assumed misfire?

[ian209]

Originally Posted by Fstrnyou Fuel trims being off won't cause misfires. How cold is it where you're at? Does it misfire during idle when the engine is warmed up or only at cold start? With a new engine, you might still be in the engine break-in phase. Rings might need more time to seat. How many miles do you have on it?

It's generally a few degrees Celsius below zero here. It only happens after the engine is warmed up. I've already put over 10,000kms on the new engine with 4 oil changes. It initially wasn't misfiring for the first several thousand kms, but after the 4th oil change (using AMSoil boosted XL 10w40) it started happening. The first 3 oil changes were using Driven RB40 break-in oil.

Originally Posted by Thor Hammer We know that the in tank pump is capable of 40 psi. What happens if it sits for a couple hours after running and you then turn on the key and watch the pressure? I've never tried this so don't know what the characteristics of just in tank pressure is after sitting overnight or warm. Yet you say once you start the car you have a normal fuel pressure reading. So what's the problem? If the ITFP was shot/clogged then I doubt all would be ok at idle and while driving. Have you looked up code P0300? What is it? Does the misfire sound make the engine stumble at a consistent rate or does it just sound rough? Does the idle fluctuate? Check for vacuum and boost leaks. Vacuum leaks will cause a rough idle at times. Any other codes related to the assumed misfire?

It stumbles randomly at idle (makes some "pop pop" noises out of the exhaust). Idle barely fluctuates. I will try to bring it to the shop to get a vacuum leak test done. No other codes, other than the random misfire p0300.

[Jeff23spl]

have you checked for injector leaks ? when you stop the car from iddling, hpfp pressure should raise and stay high for a while. IF it drop back down quickly, it could be a leaking injector...
That injector flood 1 cylinder when you start it back cold and it takes some times to dry out and recover from being flooded.

Check your plugs too, I found the stock fomoco was working better on very cold whether than an ebay set of itv22. I never confirmed if it was a bad set of plugs but it was strange. They worked well once hot but missfiring at cold...I finally rebuild and switch them for another set of new itv22 and the new engine and plug wasn't doing it. I got my injector cleaned and everything so i can't confirm what it was really.

[ian209]

Originally Posted by Jeff23spl have you checked for injector leaks ? when you stop the car from iddling, hpfp pressure should raise and stay high for a while. IF it drop back down quickly, it could be a leaking injector... That injector flood 1 cylinder when you start it back cold and it takes some times to dry out and recover from being flooded. Check your plugs too, I found the stock fomoco was working better on very cold whether than an ebay set of itv22. I never confirmed if it was a bad set of plugs but it was strange. They worked well once hot but missfiring at cold...I finally rebuild and switch them for another set of new itv22 and the new engine and plug wasn't doing it. I got my injector cleaned and everything so i can't confirm what it was really.

I think that's what's happening. My fuel pressure drops considerably fast when I turn the car off.

[Fstrnyou]

Ya, you have a leak somewhere. Not necessarily the injectors as there are many places that can leak.

[Jeff23spl]

here we use km/h ...

[mituc]

Originally Posted by Jeff23spl here we use km/h ...

Yes, but even 50Kmph average city speed is a bit too high

[Jeff23spl]

Originally Posted by mituc Yes, but even 50Kmph average city speed is a bit too high

i agree, is it coming from ecm computed speed or from what he think he do...?

On non performance forum, i often read young peoples complaining about poor fuel economy vs printed specs. When i ask about their average speed, they think they do well but when they look at the logged numbers, they understand it may not be mechanic....
My personal rating is:
Above 70, you do pretty well with highway driving, you may get close to specs.
50-70 it could either be very nice city driving or poor mixed/higway driving but it will give something toward city fuel economy.
and below 50, take the worst fuel economy spec and add 1l/100km or more accordingly.