5.7 Misfires

[Stockdale Dave]

Hi..first post so educate me if I've posted in the wrong forum. My 1997 Tahoe with a 5.7 (190K Miles) has a misfire constantly on cylinders 5 and 7. Mechanic hooked it up to scope and saw a tremendous amount of codes...mainly left bank runing rich, O2 errors and 5 and 7 misfires. Replaced all plugs, wires, cap, rotor, fuel filter. Compression check on all cylinders is ~160-180. Fuel pump is 6 months old. Cleaned throttlebody and airflow sensor.

 

Cylinders 5 and 7 still misfire. Running with the scope in monitor mode shows them misfiring 60 plus times in 0ne minute run time. In line spark monitor shows fire getting to plugs. Other cylinders running fine.

 

Ideas?

 

Thanks in advance.

 

Dave

[cammiller]

I've seen this before. I found the pressure regulator leaking. It is located under the upper intake plemun and the vacuum regulator sits above the runners for the 5 and 7 ports.

[Stockdale Dave]

I've seen this before. I found the pressure regulator leaking. It is located under the upper intake plemun and the vacuum regulator sits above the runners for the 5 and 7 ports.

 

 

Thanks for the response Cam. Is that a fuel pressure regulator? I don't see a reference to a pressue regulator in that area in my manual. From what I am understanding in your post it's a pull the upper plenum and replace the regulator? Thanks again for your time and effort.

 

 

Dave

[cammiller]

Yes, it is a fuel pressure regulator and yes it is under to upper intake plenum. It is an easy repair if that is what the problem is. I wonder if you could use a small mirror. Remove the snorkel and stuff, open the throttle plate, and look toward the back on the passenger side while having the fuel pump running. The one I did you could see the fuel dripping out of the one end. Just so you know, the regulator looks like the ones you see on the outside of the fuel rails, only it is in the intake and is surrounded by vacuum.