Help 7.3 missing, no codes, did buzz and contribution tests

[Probuilder]

injection pressure is @ 730 psi, when cold, then drops to 602 when warm.
Now injector buzz test when engine cold:
1,2,3,5,6 all buzz good.
4,7,8 buzz slow.
When engine warmed up:
All buzz perfect, with new injectors in 7 and 8.

Could this be an oil problem in the rails?
Compression on all 8 cylinders is good: Cold - between 375 psi and 389psi
Warm: 378 psi and 387 psi.

When cold engine misses, when warm miss goes away. With or with out the FU Chip, now.

I think the motor is possessed

[907DAVE]

Do you have a good way to check fuel pressure?

I would be curious to see what the pressure actually is in the head.

You can pull the 1/8" plug from the rear of the driver side head, and screw in a gauge to monitor this.

Its possible that the check valve/ screen/ vibra-lok fitting at the end of the fuel line that feeds the head is restricted.

[Probuilder]

Originally Posted by 907dave

Do you have a good way to check fuel pressure?

I would be curious to see what the pressure actually is in the head.

You can pull the 1/8" plug from the rear of the driver side head, and screw in a gauge to monitor this.

Its possible that the check valve/ screen/ vibra-lok fitting at the end of the fuel line that feeds the head is restricted.

I am running AE Scanner - Ford ProLine Pack. Can't I check it with that system? At least at the Fuel Filter I know I have 79 to 81 psi, that is what the return spring is set to.
How high does the gauge need to read, if I put it on the back of the head?

[907DAVE]

If you are running 80 psi fuel pressure (which seems high), then the fuel pressure at the rear of the head should be very close to that.

Also be sure to check the pressure under a load (driving) at both locations.

[Freedomisntfree]

wow... im dying to know what ur problem is.. seems like youve done everything but swap out PCM with a different one. if you can get your hands on one, throw it on, if no changes throw it back on ebay and get your money back .. i just sold mine for $200 after my auto to manual swap. if you have allready ruled this out and i missed it on this thread plz disregard ... good luck

[Probuilder]

I am running between 79 - 80 psi at the fuel filter return.
At the back of the heads I am running 76psi no load and 69 psi under full load.

[907DAVE]

Why is the pressure so high?

Might try dropping it back down to 60.

[Probuilder]

We installed a different fuel pressure spring in the return. This was done over 2 years ago, to smooth out the idle, and cold weather starts. It also brought the fuel economy up just a touch. It is still with in Specs from International and ford.

[Probuilder]

We have taken all mods off, returned the truck back to stock. The truck will run great until we install the FU Chip, then everything goes back to the bad running motor.

Just like this post:

Here is what I have done since last post.
Did the buzz test, everything sounds good on all 8 cylinders. Contribution test is good on all 8 also.
I am lost as to what is going on here.
Now the truck doesn't have the miss, but also has very low power even with the FU chip on 100 hp setting. It feels like the chip isn't even in the truck. my boost used to go to 26-28 psi now it only goes to 15 psi max, blows black smoke really bad. Even smokes on the idle, it is also black, sitting in garage for 3 to 5 minutes, when warm and idling, will burn your eyes really bad.
Oh MPG is down to average of 8.

I took the new injectors out of 7 and 8, put old ones back in, truck still has no miss, but the rest is still the same- no power, smokes on idle when warm, smokes driving it, with the FU chip in.

Take chip out and everything runs perfectly. No smoke, no miss, nothing, and has power, contribution test good, buzz test good, boost up to 26psi, 16 mpg average. I am LOST

Any ideas now? Ready to trade truck off, but can't with it running like this.

Could there be thick oil trapped in a injector boss that causes the solenoid to buzz correctly but injector not work right??????

[907DAVE]

Guess I missed that post......

Would have to be a calibration issue then.