Merc 350 MAG MPI fuel rail/injector removal and inspection

Fuel rail cleaned, new injectors installed, ready for reinstallation:

uc


uc


I’ve cleaned the original set of injectors and will likely keep them as a backup for use in the port engine should I get to repeat this experience on it. On the water testing in the next few days to hopefully determine if this was my root cause...
 
On the water testing over the weekend unfortunately revealed the same intake popping condition remains. :(

I have performed a compression test on the engine this evening (cold, no oil added to cylinders, 322 hours) and generated these readings:

  • Cylinder 1: 152 PSI
  • Cylinder 2: 150 PSI
  • Cylinder 3: 133 PSI
  • Cylinder 4: 152 PSI
  • Cylinder 5: 150 PSI
  • Cylinder 6: 145 PSI
  • Cylinder 7: 150 PSI
  • Cylinder 8: 154 PSI
Would those with experience in this area be concerned over cylinder 3? Is it too much of an outlier?

Any insights are appreciated...
 
Had similar issue on our previous 320 with the cf3 module. Turned out to be a bad fuel pump. Don’t recall if it was high or low pressure pump. You can cancel this out by swapping cf3 module between motors to see if the problem follows the pump.
 
The fuel pumps have not been replaced on this engine yet. Earlier this week I purchased a fuel pressure gauge and verified key-on pressure at 43 PSI but know that pressure needs to be verified under throttle and load conditions to be certain they’re working properly. I appreciate the feedback indicating bad pumps caused a similar issue for you...a ray of hope.

The pumps are becoming quickly becoming the last items that I can imagine being the culprit. I’m running out of ideas.

I really thought the injectors and fuel rail cleaning were going to be the answer.
 
stephenm27 said:
On the water testing over the weekend unfortunately revealed the same intake popping condition remains. :(

I have performed a compression test on the engine this evening (cold, no oil added to cylinders, 322 hours) and generated these readings:

  • Cylinder 1: 152 PSI
  • Cylinder 2: 150 PSI
  • Cylinder 3: 133 PSI
  • Cylinder 4: 152 PSI
  • Cylinder 5: 150 PSI
  • Cylinder 6: 145 PSI
  • Cylinder 7: 150 PSI
  • Cylinder 8: 154 PSI
Would those with experience in this area be concerned over cylinder 3? Is it too much of an outlier?

Any insights are appreciated...
Click to expand...
Probably leaky exhaust valve on cyl 3. It is less than 10% off from some others so something to watch, but I doubt it is the culprit.
 
Had a similar problem with a 5.0. Had 170 lbs. on all 4 corners and 150 on the middle cylinders on both heads. Pulled the heads and they were both cracked between the to exhaust valves in the middle. Common failure point.
 
scoflaw said:
Had a similar problem with a 5.0. Had 170 lbs. on all 4 corners and 150 on the middle cylinders on both heads. Pulled the heads and they were both cracked between the to exhaust valves in the middle. Common failure point.
Click to expand...

But would a failure of this magnitude manifest itself as "intake popping"? Wouldn't other symptoms be expected?
 
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