Here's the story. About four days ago I got about 800lb of stone dust that I needed to do some work last night. I left the dust sitting in the truck all weekend and Monday, and then yesterday evening me and a buddy went to go do the work.

We got out of my driveway and I turned and started to accelerate, then there was a noticeable jolt and suddenly I heard the RPMs dropping and felt the truck slowing down. It was like I had thrown it in neutral and let off of the accelerator, but I had done neither. The last time I felt something like that was when the timing chain snapped in a Dodge Sprinter that I had been driving (although at the time I didn't connect those dots). I pulled off right away and let a guy pass. I sat there idling for a second, and I went to pull out, and... nothing.

Nothing in the way of problems, that is. The engine purred happily and as I accelerated up the hill growled in a healthy way. We drove the 12 or so miles needed to get where we were going with no issue, did the work, and drove back home just fine. Sitting here thinking about it, I think once a year or so ago my truck did this to me, but it ended the same way: I let off the accelerator for a few seconds, got back on, and it was like nothing had happened.

Usually I kind of write stuff off like this as "eh it won't happen again" or "it's an old truck," but this one has the potential to be more serious so I definitely want to know if you guys have any input here.


Truck specs (in case you miss my sig )
1983 F150, 4.9L with C6 automatic

 

Things that have happened to me;

The parking brake cables stick and cause a drag and make the brakes hot in the rear. If you don't use your parking brake, then it's not the problem though.

The rear spring hanger broke. That would cause a jolt, but not necessarily slow you down. The hanger will break and the spring will fly up and hit the bottom of the bed.

The tranny may have a leak, and it will be low on oil. When you start out, the oil is cold and there is barely enough in there, and when you make a turn it sloshes to one side and you lose drive for a couple seconds till the oil comes back. Once it runs for a few minutes the oil gets hot and expands, and the problem will go away the rest of the day. Of course if you do have a leak this problem will get worse till it won't go at all.

 

Hmm interesting. I almost never use the parking brake so #1 is out. Just from seeing my truck sitting I don't think a spring hanger broke but I'll double check, but like you said, that wouldn't have caused the quick loss of power. The transmission thing though has me thinking. When I bought the truck back in '07, I know that it had fresh fluid in the transmission, but it has not been touched at all since then. Granted, it's only probably got another 14k on it or so, but I suppose just the age alone could wear down the oil and it could be very slowly losing some over time as well.

It's still got a couple hundred pounds of stone in the bed and I'm going to run it up the road here after dinner to unload it, it requires me to take the same turn out of the driveway up the same hill so let's see what happens here.

 

Check the tranny oil color on the dipstick. If it's old, it should be maroon. Pink is good. Very dark maroon or brown is bad and the fluid should be changed. If you haul heavy a lot with it, you should get a oil cooler. I burnt the oil in the c6 in my old 1980 twice pulling a trailer. I then got a oil cooler for it and the problem never came back.

 

It's more than a one-time problem now, I was hauling maybe 600lb this time and was cruising at about 45mph down a nice flat straightaway when it happened again. This time it would not "re-catch" and when I pulled over the engine shut itself off in drive. This worried me because it was exactly how it felt to blow that Sprinter timing chain, so I threw it in park and quickly turned the key to start it to be sure. It fired up without issue, and once again, we made it to our destination (and back home later) without any more problems.

I did check the transmission fluid. I am partially colorblind so asked the friend who I was working with to double check what color it was, and we agreed it was in fact brown. It was not murky, which I guess is a decent sign for what it's worth, but I'm going to schedule them to do a trans flush at work. I happen to have an OEM filter laying around still in the box so I'll take it with me and hopefully this issue will be resolved.

It does seem to happen when I'm hauling heavy-ish, which never was too frequent but I've been doing work with stones and bricks lately so the cargo weight has been more than usual recently.

 

Sorry for the double post but it also just occurred to me that maybe it's a spark plug/distributor problem? Could that cause these symptoms? Spark plugs are at a minimum 8 years old, the distributor is original for as much as I know anyway.

 

Is the engine still running fine when this happens? just acts like it looses it's gear? sounds like transmission issues to me. Never heard of an engine causing an issue like that.

 

Right, it acts like it falls out of gear but it doesn't slip into neutral, either, so it doesn't suddenly rev high. The engine doesn't knock or shudder at all, the RPM just drops slowly as the truck gradually coasts slower and slower. The accelerator pedal has no effect, and when this happened Saturday, when the truck coasted to a stop, the engine shut itself off as soon as I hit 0 mph.

 

Each time this happened you were carrying a load, so vapor lock is a possibilty. Another is failing ignition components, such as the module and pickup assembly, and at the age these trucks are, a wiring harness turning to dust, with lots of bad connections.

Why on Earth have you not changed your plugs in so long?

 

Reading between the lines I think he is saying the engine is running fine, but the truck is not moving. Correct?

 

Right, when this occurs, there seems to be a slight jolt while I am driving, and then basically the accelerator is unresponsive. There is no sputtering or missing or anything, the engine just loses RPM as the truck coasts to a stop. The first time it just idled once I got stopped, so I accelerated like normal and it ended up fine. This second time, as it coasted to a stop, the engine shut itself off, but I put it in park and turned the key and it fired up again without issue.

I have it scheduled at work on Wednesday, so they'll be able to do it while I'm doing my job (I work at a Ford dealership in the parts department). I have them changing the transmission filter and oil, and also I figured to have them do a radiator flush while it's in there (I know it has nothing to do with my issue, but it's time for new coolant). We'll see if it works, I have some more heavy hauling ahead of me.

 

How old is the fuel filter? My buddy had the same symptoms on an 73 Chrysler New Yorker foot in the gas and slight jerk (from the engine quitting) and coasting, if it was going fast enough to coast a ways he could let off the gas long enough for it to pump the carb full again and keep going but had to roll to the side of the road several times to restart the car when going slow because of a restricted fuel filter.

 

There's a new transmission filter and fluid in, and the truck definitely seems to shift smoother than it used to, but this problem still is persisting, and it's leading me to think it's not transmission related.

I talked to a coworker today who turned wrenches for decades on Dodges, but also owned a Bullnose from brand new back in the day, and he said his actually experienced the same problem. He claims it was related to some kind of module mounted on the outside of the distributor, and that Ford had a recall out for it in the late 80s but they never seemed to be able to fix it right. I checked my truck after I got home from work, and I can't seem to find any kind of module mounted on my distributor. My coworker was referring to his '85, so maybe his was EFI?

Either way, could this be a distributor issue? Because of how intermittent this is, and the fact that it instantaneously repairs itself, it leads me to believe it's some kind of electrical malfunction as opposed to a busted part. Just saw another thread posted about this same thing, interested to see where it goes as well.

 

He is talking about the TFI module. Ford did have a lot of problems with it, mainly heat problems being mounted on the distributor. They later moved it off the dist to its own heat sink in a remote location.

His 85 six cylinder would have had the TFI even though he had a carb, since his engine had a computer also. Yours is a earlier model, it may have it or may not. I don't know when they started using it, I bet California models used it first. It looks like a rectangle shaped plastic piece mounted on the side of the dist with wires coming out of it.

If you don't have it, then your dist has a vacuum advance with a vacuum line going to it, and you have a larger silver square ignition box mounted on the inner fender. This is the duraspark II system Ford used.

Think of the TFI system as being "dumb". It will fire the coil by itself if it has to in a fail safe mode, but normally is just sits there and only fires the coil when the engine computer tells it to. There are no timing functions at all, it's all decided by the computer. They used this system on all the EFI engines and these early sixes with a carb, as well as a few of the v8's with a carb.

The duraspark II system is a stand alone system that is "smarter". The "smarts" are in the dist and are mechanical in nature, centrifugal advance weights and vacuum advance mechanisms control the timing, and the module fires the coil when the pickup inside the dist tells it to. This is the older system that has been out since about 1974 or 75.

 

I am rethinking this because it seems that 83's possibly had EFI and not a carburetor.... I was under the impression that EFI started in 86/87? The ignition/EFI control module on the side of the distributor can cause some very funky problems much like what you described and the supply fuel pump in the tank failing could also have the same results.

I've worked on 79 and older and 87 and newer but haven't really ever seen an 83... They just don't exist around here for some reason?