1984 4.9L problem. Help??

Here's a thought.

What condition is your catalytic converter in? I had one on a 460 that had big chunks that would intermittently move around and almost totally block the exhaust flow. Very similar intermittent total loss of power, you know the old potato in the tail pipe deal... As a side note, that truck passes the state mandated emissions test with flying colors, even with a hollow catalytic converter.

This bad behaviour sounds familiar. 351CJ suggested you check the exhaust system, a good idea. I have a few others. Check the egr valve. Check the vacuum advance. My main suggestion is to check the wiring terminals to the coil. This bad behaviour showed up on my truck, same as yours, and it liked-to drive me even craizier than I already am. It would come/go, come/go and always at the worst times.....highway, crossing intersections etc. Anyway, I got 'real' determined to fix this and started checking the wiring. I'd already replaced the ignition module. I pulled off the wiring to the coil and noticed the terminals completely mashed. When the wiring harnass was made, the crimp machine 'missed' the fat part of the terminal and only mashed the very end of it and the wire. This broke all but 3-4 of the wire strands. The coil was getting power through only these few strands of light-guage wire! read Electrical Breakdown! Replaced both terminals....been right ever since! This was the problem in the wire harnass for years back then. I think your problem is electrical and if it's not the wiring, look at the module.....see if it has leaked 'goo' at the bottom. The module will overheat and the epoxy inside will leak out the bottom. If this is the case and you have to replace the unit, create an air-space between the mounting site and the unit. Getting some air flow under the unit...helps. Also check the stator in the distributor and all associated wiring/connections. If a carb is involved, check the float and the choke actuating mechanism(s). Hopefully you will find the cause of this bad behaviour!

Dave S(Texas)

I have similar problems with my 1984 F150 right now, same engine but 3 speed auto tranny. I am really leaning towards the Duraspark module that is located on the left front fender. These units were known to fail and these are the symptons they would show. My truck, the last time I had it on the highway, would suddenly start bogging out at highway speeds, 100 km/h usually, and slow itself down to around 60 km/h before it would come out of its fit and start driving normally again. As you described, it acted as if there were no fuel getting to the engine. It would do this quite a few times in a 60 km stretch of highway, but I was still able to make it home without a tow truck.

I don't think there is any way to test these modules to know if they are going bad, but I priced one up and they are only $43 to replace. My first car, which was a 1986 Tempo, occasionally did the same thing, even just on rural roads and the Duraspark module was replaced to fix it. It was fine after that.

Chances are we have the same problem. Good luck with your truck.

Sharky

The truck has all the service records and notes from its previous two owners. Says in the notes the converter has been gutted. I will check it out though. Thanks

Fuel pump was no help. Same thing, almost in the same place every time.Went a little longer this time. Started acting up right at the bottom of a hill before my house. The module is a Big A. I assume it has been replaced. Engine was rebuilt about 55k ago. Haven't checked the coil yet. Cold out. I did notice something a little odd that I've seen a couple of times. First, under the air cleaner cover and around the top of the carb there is a creamy, milky looking slime. Not much, but enough to notice. I don't know carbs so I don't know if this is normal. Also it's spitting out black water from the tail pipe. Pretty good stain on the snow. I don't know much about vacuum lines so that might be left for a mechanic. The previous owners had just took it on a road trip from Kansas to Montana and back so I'm guessing this problem is just starting or has something to do with cold. Guess i'm going to have to call a shop next week if I can't figure it out. Thanks again.

Put on an ignition module. Same thing. Took a little longer but it still did it. Stop and let it idle for awhile and switched tanks. Ran fine for awhile then did it again. I'm done.

Sounds like a blown or leaking head gasket... Milky slime could be oil/water mix and the water coming out the tailpipe doesn't sound good either. Does this water come out all the time?? Some water dripping from the exhaust after you 1st start is normal as the water vapor condenses but should go away after that. have you done a compression or leakdown test??? What do the plugs look like???

I'd do a little more investigating before I threw any more parts at it.

Your carb might be icing up. This is common when it is cold and damp. I'm not sure if your engine has the plate underneath the carb that has coolant flowing through it, but it if does, make sure that the coolant is actually getting to the carb plate. Also, make sure that your thermostat is working properly. If the engine isn't warming the coolant sufficiently, this could cause the icing problem too. It would also explain why your oil is milky. If the engine isn't warming up enough to remove the condensation from the oil and crankcase, you can get the milky oil condition.

If you can, take the truck out on another run and drive it until it acts up again, then pull over, shut off the engine, and remove the air cleaner right away. You may actually see a frost like substance down in the venturis. Also make sure that all the intake air tubes are hooked up, especially the one for the heater tube that goes from the exhaust manifold to the air intake. Make sure the heat riser valve is functioning on the air cleaner as well.

Here is a little reading for you about carb icing:

Good luck Chris

You might be on to something there. The heater works ok but I did notice the temp gauge doesn't heat up much. It's supposed to have a 180° thermostat and I don't think it gets near that. I was thinkng of one of those grill covers that you see on some trucks. Never heard of carb icing but I'll check out the links. Thanks

The water has usually been on the first start when I pull it out of the garage. Then again, I usually put something on it and only drive it once before I find out I justed wasted my money. Got some spare parts now though. I have a friend with a compression tester. I kind of wanted to do that anyway. The first plug was a light brown, chaulky looking on the tip. The other five were black.