1996 Suburban misfire

[Ryan_25]

I am hoping that one of the ASE techs can help me with this. If you have a Tech2 scanner that you can scan this thing with that would be helpful too. ;)

1996 Suburban 2500, 7.4 Vortec, auto trans.

I am have a misfire issue when under load. It idles great and drives nice when no trailer is hooked up. When towing heavy loads the misfire is extremely bad. When towing lighter loads it is there but not as bad.

I have replace cap, rotor, plugs, wires, fuel filter, fuel pressure regulator, coil, upper intake gaskets.

No codes in the computer.

I am now at a loss. I have read many forums and have done what most people have recommended. It seems to be a fuel or vacuum problem. I just cant get it figured out.

Help is appreciated.

Thanks

[ExplorHer4x4]

sorry,  a ford tech here so no tech2.  Have you had a chance to watch for any ignition arcing at night? Its way easier to see at night braketorquing, w/ a friend.

what about possible cross-fire. look out for wires that are in firing order AND run next to each other.

Are you getting ant DTC\'s or check engine light? even generic codes would help identify affected  cylinders.

also do you run better fuel when towing?92octane

Im not going to be the best help being a FORD guy but, this is all important info to figuring this thing out.

[Ryan_25]

Replaced distributor and injectors last night

[fasteddie]

vortec injectors are know to cause odd things let us know if that got it or not

[smichaelR22]

we did a Multec 3 conversion with a higher flowrate injector while we were at it.  still problems.  runing with no codes.  

i signed out a techII from work this weekend so were going to play with it tomorrow and see what happens.

[fasteddie]

what did the plugs look like the last vortec i saw do this had an almost undectectable head gasket leak on the number one and three cyl. had to do a compression leak down to find it. sorry i have not been able to be more help but no comp.

[Ryan_25]

Compression

1 135
2 135
3 125
4 130
5 110 Hmmmm
6 130
7 130
8 125

[fasteddie]

number five seems low but i don\'t recall specs off my head  like i said though the last one was just like that but 1 and 3 were just out of spec and would leak quickly

[silverado432]

Ryan Sorry for not checkin the forums sooner...I will check out the specs tomorrow at work on SI...But I dont like those compression readings..I will check that out tomorrow and I will post up tomorrow either during work or after!!! Again sorry for the late reply...that was my fault

[Ryan_25]

The heads are getting a valve job. Cylinder 5 was not seating at the exhaust valve.

Should be back together tonight and running smooth again.

Remember that big blocks use very low compression. Maybe 8:1

[fasteddie]

yes they are low but number 5 being an odd ball was a tell tale