I bought a '90 E-350, 4.9L to replace my rusted but dependable '70 E-300 240c.u. six. They said it needed a head gasket, which it did. I replaced it but the engine ran rough and rich. I found it had a bad MAP sensor but with the new one it would hardly run at all, still rich. Compression readings from 1 to 6 (after the new head gasket)were: 90,90,60,60,60,60. Naturally I checked the head when it was off and it looked good. It was clean but sooty, no oil, no warpage and no burnt valves. I took it apart again and pulled the pistons. Numbers 3 through 6 (the ones with 60#) had broken rings and lands. Numbers 1 and 2 were fine. I cut the ridges (which were minimal), honed it and put in new pistons, rings and rod bearings. When I started it, it ran the same as before, rough and rich, regardless of which map sensor. With the new map sensor it would only start cold. The new compression test results are: 90,90,90,90,90,90. (the exact same compression as numbers 1 and 2 before the ring job). A wet test raises the compression to about 95. There is no blowby and no spindown. The rings are tight! A leakdown takes about 20 seconds at 100# with the piston at BDC. The exaust is not clogged. Vacuum is low and steady at 12# at idle (when you can get it to do so). It goes to 20# steady at about 2500rpm. The air and coolant sensors are within spec.. The fuel pressure is 60#. The timing is on the mark. Yes, I checked the cam timing (I know your thinking that now!). The van was comercially owned before and I received the complete service history with it. They obviously had the same problem before the head gasket blew. At least four shops looked at it and they replaced everything but the map sensor (?) including: O2sensor, computer, EGR, EGR position sensor, injectors and the ignition module. One mechanic (the last one) checked the cam timing also. I think I found the reason for the blown head gasket by reviewing the service records. Someone put stop leak in the expansion tank (the heater core was shortly there after replaced) and it clogged the little hole in the bottom of the tank. They were probably checking the level by looking at the tank instead of the radiator. I'm now at a total loss. Low compression, low vacuum yet good ring seal. The odd thing is that occasionally the thing will smooth out and run normally for about 30 secound. I will not mention my suspicions so as not to throw any of you experts off track but I need help!

 

If it was comercially used, How many miles are on the odometer? One thing it could be is that the intake and throttle body are all carboned up. Another thing (yes I know I've said it before) it could be is the exhaust system. Alot of times the front catalytic convertor will come apart and plug the back convertor up. And that will cause a bad idle, due to the tremendous amount of backpressure building up in the front of the exhaust system.


1988 F-150 Custom to XLT Lariat conversion, 5.0 EFI (formerly 300 I6), 5-speed M5OD, 3.08 rear gears (will be upgraded to 3.55's), Summit headers, Flowmaster muffler W/ dual 3.5" tips 24" long.

Pioneer CD, AC, Tilt, Cruise, Mobile Radio, Lightbar (Force 4 LP6000), Federal Signal PA300 Siren W/100W Siren Speaker. Caution Customizing in progress, Watch for flying tools & Parts!

 

Check the vacuum tree that the MAP Sensor connects to on the intake . It may have sufficient carbon buildup that the MAP doesnt always get the vacuum needed , this would explain the intermittant smooth running . Its easy to check , the vacuum tree unscrews from the intake easily .

 

Thats a good possibility. I'll check it. Thanks