Ill try to be as specific as possible but I am no motorhead. 1991 F-150 4.9, EEC-IV PCM, TFI-IV(DI) ignition, E4ODTransmission 90,000 miles. Bought the truck in 1995, and it started missing about 2 months later. Took it to a dealer and they replaced the oxygen sensor,which didnt fix it. Gave up on them fixing it,(ran out of money to throw away at the swap till you drop dealers) and the condition has gotten steadily worse. Seems to happen more frequently on cold wet days or hot humid days. Does not matter if the engine is hot or cold. Ran code checks, System passes, no fault codes, or stored codes. There seems to be a certain RPM that the truck dosent like, when I back off the accelerator at about 35 mph,and the transmission shifts, the truck starts missing or surging or bucking or something screwy. If I hold the accleretor at this point and keep the same speed,rpm,and gear combination, it will continue to miss until I step down, or let up, But anyway it sure is annoying. I checked the timing with the spout jumper out and it is 10 degrees BTDC, which is the spec.and timing is steady at idle. After plugging in the spout jumper the timing advances with throttle. Changed the Ignition module, changed the Inductive pickup (PIP) in the distributor. Changed the plugs, wires distributor cap and rotor button,changed the fuel filter, Still there. Coil resistance checks good on primary and secondary. Coil voltage on the primary is at 13.7 volts. Havent done a compression check yet,although that is probably my next step. Anybody out there good enough to fix this dog that I bought? I just dont know what to do next.

 

Have you tried cleaning your MAF (Mass Air Flow) sensor? Has the volume and pressure of your fuel pump been checked? Has the ECM been checked for shorts, etc.? Take it somewhere that specializes in computer diagnostics if you're not doing it yourself.

 

Humidity related problems always seem to be electrical in nature. Since it happens at a certain speed and/or throttle position, I would suspect a bad throttle position sensor or TPS. The computer should flag this, but maybe it's not quite bad enough to register a code. It is also possible that it is the computer itself that is faulty. Do the spark plugs reveal anything, is one or more of them fouled? What about vacuum, is the engine basically healthy?

 

I am not real sure bit i dont think i have a mass air flow sensor. I am going to check the other stuff.I am going to do compression so ill check all the plugs when i do that. Thanks alot,Ill keep you posted.


[This message has been edited by bigwhitetruck (edited 08-05-2000).]

 

The same thing happened to me past 55 about it would keep jerking like it was mis firing. well then the engine light came on i took it to the dealer they told me the electrodes on the sparks were completely gone. a tuneup and bam its all good now!

 

The same thing happened to me past 55 about it would keep jerking like it was mis firing. well then the engine light came on i took it to the dealer they told me the electrodes on the sparks were completely gone. a tuneup and bam its all good now!

 

Well I have some news, not really sure of my results though. I did a compression check, all the cylinders were pretty even, although it did take two strokes to reach full compression. The compression on the first stroke was around 100, and the second was around 170. It was pretty much the same for all the cylinders. I put 3 squirts of oil in each cylinder and retested. It still took 2 strokes to reach full compression and the readings were the same, so I am assuming that the rings are ok. Could I have a valve not seating? Is this enough to cause the problems I am having? The plugs all looked really good, no fouling, looked like the normal picture in the book. If a exhaust valve isnt seating on the intake stroke could it cause a mixture problem? Could I have Back pressure? When I checked the exhaust pipe i held a piece of paper in front of it and once in a while it pulls air back up into the tail pipe. Is this normal?Or is this just a symptom of the same problem i see at 35? Do you guys think its mechanical or electrical, or vaccuum? I am going to do fuel pressures next, and then vaccuum. Still dont know what the hell I am doing but It is challenging and I am learning a litle. These damn trucks are a little tuffer than the stuff I do at work. But they say an electrician can do anything, I am wondering who said that. Thanks in advance for any help you can give.

 

Sounds like the engine is mechanically healthy. If you have even compression on all cylinders, and it doesn't change with some oil, than the rings and valves are good. I'll say it again, this sounds like an electrical problem, though without a computer code it will be hard to diagnose. If the computer or a device driven by the couputer such as fuel injectors, or electrical solenoids were to malfunction, th e computer would not know it. It does not monitor devices that it controls, only sensors that feed it data. An example of this would be a fuel injector. The injector is driven by a transistor in the computer, and connected to the computer by various wires and connectors. If the transistor was intermittant, or one of the wires or connectors were corroded, the injector operation would be faulty, but the computer would never know since it doesn't monitor its outputs. The only way the computer would sense the faulty injector is by other sensors such as exhaust oxygen or manifold pressure. Their readings would have to go out of range long enough to trigger the error message, and record the code.
Unfortunately sometimes the only way to troubleshoot these type of problems is to replace parts, or find a mechanic that knows this particular motor well, and might have seen this problem before.

 

I had almost identical problem with 300ci in a 1967 f100. Nobody could figure it out. Finally, after 2 years of agony, & several tune-up & parts changed--I put another complete distributor assy. in it. The problem was gone forever. What was particular about my problem, it would only miss-out @ certain rpm range in 2nd. gear (man. trans.) Good luck!

 

Its me again. Ive got a little info to add. I did a fuel pressure check as per the Haynes manual. My fuel pressure at the schrader valve with the key off(no vaccuum) is 55psi on tank #1 and 57psi on tank 2. This is the mid to high end of the acceptable range for this test.(50-60 psi) When I did the test with the engine running the readings were 45 psi on tank #1 and 47 psi on tank #2 at idle. This is the extreme low end of the acceptable range.(45-60 psi) I could see the needle on the gauge bouncing as the injectors fired. When I pull the vaccuum hose from the fuel pressure regulator, the pressure increased 10 psi(to 55 psi), and the idle quickened, and the motor sounded much less bogged down.(I didnt know it could sound that good) I thought I may have a clogged fuel filter, so I changed it even though I changed it last year when I tried to fix this problem. I saw no noticeable increase in fuel pressure so I am guessing that the filter was not restricted. When I clamped off the rubber return line I saw a significant increase in the fuel pressure (Actually an overpressure condition which has to be hard on the fuel pump), and again the idle was smoother and faster. So I unclamped the return line and the engine bogged down again. When I did the static pressure check (where you see if the pressure stays for one minute when you cut it off) the pressure stayed steady at 45 psi. I am wondering if I could have a weak spring in the fuel pressure regulator to where the restriction is opening too wide at high vaccuum, allowing too much flow and lowering fuel pressure at high vaccuum conditions. I ran out of daylight but I went to advance and picked up a pressure regulator for 25 bucks. The way I see it its worth a try. Next step change the regulator,then Im gonna check vaccuum. Any thoughts, could we have solved this mystery? Only time will tell. I wish I had a garage. If I were to disconnect the vaccuum line and plug it off, the fuel pressure will still be within the acceptable running range(extreme high end of the range) at idle. Could I run it this way because the pressure will drop as demand for fuel increases?I am sure the fuel pump is not an accurate metering device. Thanks again.

[This message has been edited by bigwhitetruck (edited 08-05-2000).]

[This message has been edited by bigwhitetruck (edited 08-05-2000).]

 

It is not uncommon for these regulators to go belly-up. Sounds as if you are on the right track.

 

One other thing that I forgot to mention. When I shut off the truck i hear a buzzing noise for about ten seconds comming from the throttle body area but i am not sure what it is. Is this normal? What am I hearing? Also there is something called a bypass valve for the thermactor that has a little filter on it.It comes on a couple of minuted after I start up. It makes a fluttering noise and it cycles on and off about every 30-40 seconds. My motor idle rpm drops dramatically when this cycling occurs. Whats up with that?

 

Installed new fuel regulator today, saw no increase in fuel pressure so I am assuming it was ok. I still may try running with the vaccuum disconnected and plugged if I cant find anything else. I started doing vaccuum checks, EGR valve was fine as far as mechanically.I didnt test the feedback resistance yet. It held vaccuum like it was supposed to. When I tested the EVR (you know the one that regulates vaccuum to the EGR) I found out what the buzzing noise was that I Heard was. The EVR resistance was very low and when ever I applied vaccuum with a pump to the Input port it would make that buzzing sound,and relieve immediately. Come to find out that it is supposed to hold vaccuum until the solonoid is energized. Went to Advance and asked for an EVR valve and they kept telling me its EGR. After much arguement and I had to show the big dummy in the book, He said "oh you mean the EGR vaccuum solonoid"? Well anyway they dont have them at advance or any other parts stores (I called around),so I guess I am at the mercy of the dealership parts department,which were closed because It is Sunday. I sure hope this fixes it....I got my fingers crossed. Any thoughts guys? Thanks.

[This message has been edited by bigwhitetruck (edited 08-06-2000).]

 

The Haynes Manual is wrong. The evr valve is not supposed to hold vaccuum on the input port. I got the brand new one and tested it, although the new one does keep vaccuum longer and makes a whole lot less noise. Ill keep the new one so I can boast about all the parts I have changed and still not fixed it. The only reason I am keeping this site updated is because I know there is somebody who has the same problem,maybe this will help. I checked my throttle position sensor and it increased voltage and resistance very slowly with throttle, so I am assuming it is ok.(.5 volts to 5 volts) I checked my EGR positioner, and again it was very smooth with no jumps in resistance,the numbers didnt match up with what the Haynes manual said it should, but i went down to auto zone and tested a new one and the numbers were exactly the same,so again I say its ok. Im running out of stuff to check. Intake air temperature sensor and coolant temperature sensor are my next checks. The guy at advance told me to take the truck to a transmission place and have them check it out. He said that it sounds like because its a gear,speed combination , and that it happens when i lift the acclerator, it may not be a miss that I feel but that my transmission may be confused about whether to coast or something. He said that they can also check the catalatic converter, which is the origianal one, and that it may be plugged causing backpressure. All I know is that I am very tired of fighting this thing, it may be dealer time. I noticed you guys have gotten very quiet. Anybody out there?

 

I have a '95 with the same problem. I wanted to bring this thread back up and see if you ever fixed your problem. I don't think it is a transmission problem because mine has a manual tranny.