5.3 Runs good for 1 driveway pass
06-19-2016, 09:40 AM
#1
Teching In
Thread Starter
Join Date: Jun 2016
Posts: 20
Likes: 0
Received 0 Likes
on
0 Posts
5.3 Runs good for 1 driveway pass I just got done building my 5.3 with a Texas Speed 224R cam, pac springs, and texas speed push rods. I also did flat top pistons and 8.1L injectors. I got the motor back in and hooked up and went through a quick break in then drove it to the end of the driveway and back and it ran great. I went to pull out of the driveway and it started to run like crap, missing like crazy and no throttle response. Checked the codes and nothing was there. Shut it off and let it sit and then it runs great for another pass to the end of the driveway and then starts running like crap again.
Things I have tried:
New plugs
New wires
New MAF
Good fuel pressure
Followed the harness start to finish looking for issues
Alternator tested good
PCM is receiving adequate voltage
93 octane fuel
I know it needs a tune with the new cam but could the tune really make it run good for a little bit then like absolute crap? The only other funny thing I have noticed is my tach no longer works, and it worked just fine before I rebuilt the motor.
Things I have tried:
New plugs
New wires
New MAF
Good fuel pressure
Followed the harness start to finish looking for issues
Alternator tested good
PCM is receiving adequate voltage
93 octane fuel
I know it needs a tune with the new cam but could the tune really make it run good for a little bit then like absolute crap? The only other funny thing I have noticed is my tach no longer works, and it worked just fine before I rebuilt the motor.
06-19-2016, 03:42 PM
#3
Teching In
Thread Starter
Join Date: Jun 2016
Posts: 20
Likes: 0
Received 0 Likes
on
0 Posts
Plugged it in to a MAC computer and still pulled no codes and nothing seemed out of line. It was picking up misfires on all cylinders but 1 and 7
06-20-2016, 07:48 AM
#5
Teching In
Thread Starter
Join Date: Jun 2016
Posts: 20
Likes: 0
Received 0 Likes
on
0 Posts
Dual exhaust with glass packs. The MAC computer picked up RPM fine so there's in issue with the gage somewhere. I'm going to drop it off to a buddy with hp tuners today and he will se what he can do with tuning
Trending Topics
06-24-2016, 09:44 AM
#9
Teching In
Thread Starter
Join Date: Jun 2016
Posts: 20
Likes: 0
Received 0 Likes
on
0 Posts
Tune didn't work. Got it to idle though and my fuel pressure is barely 50. Replaced fuel filter same story. Replaced fuel pump and had good fuel pressure at idle but still ran like crappie. Now I'm back to barely 50psi at idle and it does not jump up at all when rpms go up.
Pump has 12 volts at all times
Return line has a nice steady stream
Replaced FPR
Edit: pump is a brand new AEM 50-1200
Could this be tied to a vacuum issue? Or are my used 8.1L injectors possibly at fault? This kinda seems odd as my computer picked up misfires on almost all cylinders
Pump has 12 volts at all times
Return line has a nice steady stream
Replaced FPR
Edit: pump is a brand new AEM 50-1200
Could this be tied to a vacuum issue? Or are my used 8.1L injectors possibly at fault? This kinda seems odd as my computer picked up misfires on almost all cylinders
06-24-2016, 11:12 AM
#11
Teching In
Thread Starter
Join Date: Jun 2016
Posts: 20
Likes: 0
Received 0 Likes
on
0 Posts
Not quite sure what your getting at here but I was watching live data from the MAC computer and my tuner guy did the same through hp tuners
06-24-2016, 11:58 AM
#13
Teching In
Thread Starter
Join Date: Jun 2016
Posts: 20
Likes: 0
Received 0 Likes
on
0 Posts
I'm looking for input now on why my fuel pressure would be so low.
Things I have tried:
-new fuel pump
-new filter
-new FPR
-voltage at the pump
-checked for kinked lines
-fuel injector cleaner
Things I have not tried
-swap back to old fuel injectors that I know worked
-different fuel pressure Guage
-?
06-24-2016, 12:06 PM
#14
50 psi at idle is not that low for a return fuel system with fuel rail regulator and certainly not low enough to make it run like crap at the end of the driveway. Which BTW is very descriptive. The data that needs to be analyzed is sensor data. When you post that for people to look at you will probably get some real help.
06-24-2016, 12:16 PM
#15
Teching In
Thread Starter
Join Date: Jun 2016
Posts: 20
Likes: 0
Received 0 Likes
on
0 Posts
50 psi at idle is not that low for a return fuel system with fuel rail regulator and certainly not low enough to make it run like crap at the end of the driveway. Which BTW is very descriptive. The data that needs to be analyzed is sensor data. When you post that for people to look at you will probably get some real help.
06-24-2016, 12:33 PM
#16
Fuel pressure should be approximately 58 with the engine running and the vacuum disconnected from the fuel pressure regulator.
With the engine running and fuel pressure regulator connected to vacuum the pressure will be less, probably close to the 50 you are getting.
Fuel pressure is not the issue here.
What do the fuel trims do when it starts to run poorly?
With the engine running and fuel pressure regulator connected to vacuum the pressure will be less, probably close to the 50 you are getting.
Fuel pressure is not the issue here.
What do the fuel trims do when it starts to run poorly?
06-24-2016, 12:44 PM
#17
Teching In
Thread Starter
Join Date: Jun 2016
Posts: 20
Likes: 0
Received 0 Likes
on
0 Posts
Fuel pressure should be approximately 58 with the engine running and the vacuum disconnected from the fuel pressure regulator.
With the engine running and fuel pressure regulator connected to vacuum the pressure will be less, probably close to the 50 you are getting.
Fuel pressure is not the issue here.
What do the fuel trims do when it starts to run poorly?
With the engine running and fuel pressure regulator connected to vacuum the pressure will be less, probably close to the 50 you are getting.
Fuel pressure is not the issue here.
What do the fuel trims do when it starts to run poorly?
I know he started with an 8.1L tune and it was starving for fuel so bad he had to leave the injectors open for as long as possible and it was still starving for fuel. That's when he noticed low fuel pressure. I'll see if I can get more technical data than that
06-24-2016, 12:54 PM
#18
I get maybe 54psi with the vacuum line disconnected. Should psi really remain the same as I rev the throttle up?
I know he started with an 8.1L tune and it was starving for fuel so bad he had to leave the injectors open for as long as possible and it was still starving for fuel. That's when he noticed low fuel pressure. I'll see if I can get more technical data than that
I know he started with an 8.1L tune and it was starving for fuel so bad he had to leave the injectors open for as long as possible and it was still starving for fuel. That's when he noticed low fuel pressure. I'll see if I can get more technical data than that
I wouldn't bother trying to read fuel pressure while revving it up, if it has more pressure with the vacuum unhooked than hooked it confirms the regulator is working. That's good enough.
But to answer your question, as you crack the throttle, there is less vacuum in the intake, it will increase the fuel pressure only as much as the vacuum is reduced and only up to the 54psi max. As soon as you let off the gas, vacuum returns and fuel pressure will return to 50 or even dip lower as the vacuum will be high as the rpm returns to normal.
Stop looking at fuel pressure, start looking at fuel trims.
Do you have any exhaust hooked up beyond the o2 sensors? If not, it could be diluting the exhaust enough to throw the reading off.
06-24-2016, 01:06 PM
#19
Teching In
Thread Starter
Join Date: Jun 2016
Posts: 20
Likes: 0
Received 0 Likes
on
0 Posts
54 is fine, it won't cause the problems you are having.
I wouldn't bother trying to read fuel pressure while revving it up, if it has more pressure with the vacuum unhooked than hooked it confirms the regulator is working. That's good enough.
But to answer your question, as you crack the throttle, there is less vacuum in the intake, it will increase the fuel pressure only as much as the vacuum is reduced and only up to the 54psi max. As soon as you let off the gas, vacuum returns and fuel pressure will return to 50 or even dip lower as the vacuum will be high as the rpm returns to normal.
Stop looking at fuel pressure, start looking at fuel trims.
Do you have any exhaust hooked up beyond the o2 sensors? If not, it could be diluting the exhaust enough to throw the reading off.
I wouldn't bother trying to read fuel pressure while revving it up, if it has more pressure with the vacuum unhooked than hooked it confirms the regulator is working. That's good enough.
But to answer your question, as you crack the throttle, there is less vacuum in the intake, it will increase the fuel pressure only as much as the vacuum is reduced and only up to the 54psi max. As soon as you let off the gas, vacuum returns and fuel pressure will return to 50 or even dip lower as the vacuum will be high as the rpm returns to normal.
Stop looking at fuel pressure, start looking at fuel trims.
Do you have any exhaust hooked up beyond the o2 sensors? If not, it could be diluting the exhaust enough to throw the reading off.
06-24-2016, 01:33 PM
#20
Without being there looking at scanner data while it's acting up, I cannot help beyond the following.
Check 02 sensor wires to be sure they are on the correct side. If they are mixed up, it will cause 1 bank to go dead lean and the other to go dead rich, but only after it goes into closed loop.
Unplug the MAF, you said it was new but not if it was aftermarket. Only the GM ones are proven to work correctly. By unplugging the maf, it will set a SES light but it will revert to the speed density tune. If it doesn't run any better, then it would probably rule out the MAF as a cause.
With the MAC scanner, you should be able to watch fuel trims, see if they go positive or negative and how each bank relates to the other. This info will help others to help you.
Check 02 sensor wires to be sure they are on the correct side. If they are mixed up, it will cause 1 bank to go dead lean and the other to go dead rich, but only after it goes into closed loop.
Unplug the MAF, you said it was new but not if it was aftermarket. Only the GM ones are proven to work correctly. By unplugging the maf, it will set a SES light but it will revert to the speed density tune. If it doesn't run any better, then it would probably rule out the MAF as a cause.
With the MAC scanner, you should be able to watch fuel trims, see if they go positive or negative and how each bank relates to the other. This info will help others to help you.