This engine is in a 1981 E-350 motorhome. It ran great out on a trip, then after being parked for a couple of days was hard to start and ran like crap. Initially I thought it might just be damp wires, but it did not go away and was blowing a lot of blue smoke. After checking in to it, it seems the front two cylinders are dead. I understand that this can mean that there is a blown head gasket, or warped heads. However, there is no sign of coolant in the oil or vice versa, and it is not loosing any coolant - just using a lot of oil!! I have not yet done a compression test, just wondering what else to look for. Could this be a bad cam as I have read these engines can have cam problems, or could this be valves??
How difficult is a re & re of one of these engines, and how difficult are they to rebuild?? I have done a bunch of newer (2 stroke) sled engines, but have never tackled anything like this....

 

I can't think of a reason you would lose two cylinders just sitting for a couple of days. I would, if you haven't done this already, check for fire at both cylinders in question. Make sure distributor cap is clean and dry. If your getting good spark to these cylinders, I would check compression.

 

I replaced the cap and rotor and then pulled the wires to each cylinder - made absolutley no difference in how it was running when the wires for those 2 cylinders were pulled.. I am wondering if maybe it overheated ...

 

just did a compression test - 2 cylinders are at 140, 2 at 110, 2 jump to 75 first crank, 90 on the second, then nothing and 2 have next to no compression... I guess it is coming out for sure haha

 

Are you going to pull the heads just to check for valve problems first or pull the engine entirely?

 

I'm thinking with the 2 dead cylinders, that I am going to pull the whole engine... What are the chances that I need to rebore given that I have next to no compression in 2?

 

I would think it's a high probability on a 28 yr. old engine that the block will have to be bored and honed. It's pretty much standard practice for any machinist.

 

I have found a 351m in a 1977 merc cougar that I am thinking about buying for the motor and swapping it, then maybe have a go at rebuilding that 400.
Does anyone know if there is a signifigant difference in the performance of these motors?? This motor is going in an old motorhome, so I am not too worried about passing everyone on the road - just don't want it too be too underpowered.

 

I've got a 351M that I just had rebuilt and turned into a 408c.i. and yes there is a big difference if you use aftermarket intake, cam and lifters, and timing set, and a 600-650 cfm carb. My engine builder was surprised at the amount of torque these engines produce with just mild upgrades. My engine with a 400 crank, CompCam 255 DEH and lifters, Cloyes timing set, Tim Meyer 9.4:1 pistons, Edelbrock intake and carb produced 339HP and 478 ft/lbs torque @ 2900rpm on the engine dyno.