93 F150 misfiring


Hi all. My father in law has a 93 F150 with a 5.0 V8. Just over 100k miles. The engine has been misfiring for most of the summer and he has had no luck in getting it to run right. I know more about GM cars than I do Ford trucks. So I am hoping someone here has some experience to share with these vehicles.

I took it for a drive. The truck starts just fine, warm or cold. It misfires any time it is running. The misfiring is sort of random, but constant, at least once a second. It is not a super hard bucking like I have experienced in my cars, but a softer misfire. Sometimes it is worse than others, and it does not seem to depend on load or speed or gear or whether or not the TCC is locked. It does it right from the start, so it does it in both warmup mode and warmed up mode. He said it goes through considerably more gas since it has been doing this. The check engine light never comes on while driving. It also does not matter if it is running off the front or rear fuel tank, it still acts up the same.

He had the plugs and wires, and cap and rotor changed, and he said it made no difference one way or the other. Since it is drinking more gas, it leads me to believe it is either getting too much gas, or not burning the gas thats there.

I pulled the vacuum line off the fuel pressure regulator and there is good strong vacuum from the engine. It also made the engine run a little faster with the regulator unhooked and the vacuum hose end plugged. So at least from what I know, it means it is working.

Another thing it did sometimes was want to intermittently idle high at cold startup. So I took off the IAC valve and sprayed a little carb cleaner in the bore, and sprayed a little WD40 in the valve tip to make sure it was sliding properly. I didnt get to check the next cold start, but with a warm start it still misfired the same.

I also cleaned out the little foam filter that was under a little snap-on plastic cap. No difference, but I figured might as well.

I looked around and checked all the connections and all vacuum lines which seemed fine.

I am thinking one of the next things to check is if the oxygen sensor is coated and giving false readings and making it run too rich. I looked and I am not sure where the oxygen sensor is on this engine. I am used to seeing them in one exhaust manifold. Is it down by the cat or something?

The other thing Im thinking to check is if the ignition coil may be weak. I have a multitester and have tested my GM coil packs but this is different. Which connections should I test between, and what kind of readings should I be getting where? Also, does this engine have a separate ignition module and/or pick-up coil that could be tested?

Or if there are any other things to check for, any common issues with these trucks, Im open for suggestions.

Hopefully I can get somewhere on this truck. He does not drive it that much (but has gone through several tanks lately so it is fresh), but really hopes to have it running good by deer season.


This truck should have the TFI-IV ignition module and these are very prone to failure due to heat.
Most are mounted on the side of the distributor with a pair of screws that are located down in a hole. You would need a 4 MM or 7/32 deep socket or nutdriver to access them.

Some parts houses will test them with a machine but that does not always give a true indication as to whether they are good or not. It all depends on what part of the circuitry is failing and when it fails. Often they act up when hot and then are fine a few minutes after the engine is cut off.

If you change the module, you MUST use the special electrolytic grease that is packaged with the new module. Failure to do so will ruin the new module.
Hope that helps. (Do a net search for TFI-IV failures and you’ll see that it was a common problem.)


If the truck has over 100,000 miles on it, check for a worn timing chain. A worn timing chain can cause the problems you’re having.



Thanks guys, Ill check it out again pretty soon. I found some pages on the Autozone site on how to test the ignition parts.

As for the timing chain, what is the best way to go about checking that? What Ive done before is remove something from a valve cover so I could see the valvetrain, then turned the crank with a breaker bar one way to take up the slack, then took up the slack in the breaker bar, then used a little trig to figure how many degrees the crank needed to turn before the valvetrain started moving again. But how many degrees of slack is acceptable? My 6000 wagon had 7 or 8 degrees of slack and 85k later still runs good.


Testing the ignition module is a waste of time. They may test fine and still be bad. Heat is what does them in.

You should also check the fuel pressure regulator to make sure it does not have a ruptured diaphragm. Pull a vacuum on it and see if it holds.

I don’t use degrees of movement as a gauge of whether a chain is worn or not. My preference is a timing light. If the chain is worn the timing mark will be dancing around a bit. A 100k miles seems a bit young for a chain failure.

The O2s are going to be underneath in the header pipes, but I would be surprised if this is the problem.
I would add that I’m assuming here no one has been dinking around with the distributor. Ignition timing is critical and if someone has toyed around with the timing without disconnecting the SPOUT connector the timing will be way, way off.
This will lead to loss of power, lousy mileage, missing, etc.


Yes, ok, come to think of it, improper timing might cause all manner of problems there. And it occurs to me that the Mass Air FLow sensor can be involved, also.


You mentioned you changed the plugs; were you able to tell from the old spark plugs which cylinder was misfiring? If not, I would pull the new ones one-by-one and try to find the offending cylinder. If there is just one cylinder that is missing, and you have checked for spark, I would swap the fuel injector out with another cylinder, run for a while, and see if the misfire follows the fuel injector. Unfortunately, your ECU doesn?t monitor misfires, so you have to figure it out the old fashion way:)


Thanks for the replies everyone. Hopefully I will get this truck figured out. I hadnt thought about bad timing originally, but it does make sense that it is a possibility.

He had the plugs, wires, cap, and rotor changed by the Ford garage before he told me about the problem. I can pull the plugs and see if one or more looks different from the rest. Since they are fairly new it should really show.

As far as the timing and distributor, what is the SPOUT connector? I can look and see if the distributor has been pulled. There should be good evidence if it has, since everything visible under the hood besides the wires and cap is covered with cooked dust.

I do have an inductive timing light and could do a quick timing test. Which cylinder (by physical location) do I clamp the wire to?

It misses even when it is started stone cold, every time. It is not a heat issue. If the module is bad, it should be bad when tested. How about a weak ignition coil, maybe some of the internal windings burned and the coil is now weak. Could a weak spark cause it to do this?

About the pressure regulator. When I pulled the hose originally, there was no smell or residue of gas in the hose. Does this mean the diaphragm is good? On my cars, if the diaphragm is torn, gas will draw up the vacuum hose.


If the engine has a distributor, remove the cap.

Now have someone turn the crankshaft by hand in one direction. Observe the rotor in the distributor as the crankshaft is turned in the other direction. If the crankshaft can be rotated about ten degrees before the rotor begins to move, the timing chain is stretched/worn. Or the plastic teeth on the cam sprocket is starting to come apart.

If there is no distributor on the engine, then use the same method while observing the valve action.



Well I finally got a chance to look it last week and it appears to be fixed now. I tried my best to check the ignition system and TFI module (mounted to the drivers side fender), but all the connectors didnt exist on the TFI as my instructions said. I checked the coil and it was good. I checked ohms between the module and distributor and it was good. I checked the plug wires and they were all good for ohms, but seemed a bit loose. I checked the distributor connector, and cleaned it out. The same style pin connectors went bad in the washer pump connector awhile back, so I wonder if these may have loosened up too. I cleaned the connector and tightened them a bit with a baby screwdriver. I didnt get to check the spark plugs themselves. The distributor housing looks like its never been messed with. I didnt get to check the timing.

After doing all that, I started it and he could tell right away that it wasnt missing anymore. So maybe the loose plug wires, or distributor connector, was it.

Surprisingly he sold the truck to his friend who really liked it, then bought a 2005 F150, but said he likes the 93 better. Who knows what will happen, the friend hasnt paid him for the truck yet either. The friend just gave me his old Escort wagon, which Im not sure what to do with yet. Another thread for that.

Thanks everybody for the replies. Ive learned a lot about Fords so far.