Newbie to 7.3 idi's with a 1987 RV. The original 6.9 w/banks turbo addition was replaced in 94 with a 1993 factory rebuilt 7.3. It was installed with the Banks turbo by Nutmeg in Htfd (for big bucks!).. That original owner seems to have been a great owner and really took care of the unit with a lot of additions ( the banks turbo, rebuilt tranny, some kind of after mkt overdrive/ step down unit, and lots of other great stuff). The rv was then driven about 90k miles til the owner died and I'm guessing RV sat for at least ten years. It has since gone through about 3 owners and I understand no one has got it running. Has a receipt for a replacement IP and it does look newer. I picked the RV up last year. Took compression readings which were low (~100 lbs). changed oil ( no shavings, water, or bad indications). of and started adding ATF and some mystery oil to cylinders throughout the winter with block heater help and eventually got compression readings through the FI holes with readings- 5 at approx 200+, 1 at 300, and 2 at 150. The left side lifters and valve look to be normal movements. The right or pass side can't be seen as I can't get the valve cover off but do not sound off. A powerstroke mechanic friend has been helped me clean the injectors and replace the return lines.No start and one problem seems to be that when cranking the turbo spins but exhaust doesn't seem to be coming out through the exhaust pipe but does came out big as a lot of blow by from the lower end through the filler pipe and valve area.The oil press gauge does not move while cranking, A new sending unit was tried but no success and don't see oil coming up to the the valves but the valve train does look good. Hope I described al this right and don't know what to ask so any and all comments, suggestion, etc are welcome and appreciated. Thanks

 

it will never fire with compression that low.
my bet is the rings are stuck tight to the pistons causing the low compression, and letting everything blow by them into the crankcase.
you want to see AT LEAST 300 PSI in each cylinder to get it to start..... and 350 to 400 is optimal.

 

You could always do a cylinder leak down test to see if it's the rings or valves loosing the compression (hopefully not due to cavitation). If a cylinder leak down test indicates bad valves (if you hear the compressed air in the intake, or out the tail pipe) you might be able to get by with new valve guides/seats and a head gasket replacement