I have spent the past three weeks searching threads for ideas of what might be causing my problem. I have found lots of ideas that kind of fit but to this point nothing has eliminated the issue.

It is a 1993 F250 351 4WD. The truck starts and runs fine if the engine is cold but once it warms up to operating temperature it develops a miss. The truck bucked and stuttered severely under light load (such as maintaining speed going up a hill) but was also apparent under hard accel. I thought tranny or transfer case but even with the truck running in park I could feel the engine shake and the miss through the exhaust if at operating temperature. The truck also sounds like it has an exhaust leak near the driver side manifold. I have searched the manifold, donut gasket and crossover tube but was unable to find the cause of the sound. The odd part is that the noise goes away depending on RPM, so I am wondering if the sound is actually related to the miss.

I started by giving the truck a tune up. Plugs, wires, cap, rotor, pcv, fuel filter. I had only recently purchased the truck so I have no idea how old any of these parts were. The plugs were generally normal with just a little indication that the truck may be running rich. The fuel filter looks like complete crap and could very well have been from the factory. It had rusted to the bracket and made for a lot of fun removing. When complete I checked the timing and it was dead on. I also made sure to route the wires so that the there is no inductive effect.

I noticed maybe some improvement but nothing dramatic. The truck never threw a CEL but I decided to code scan it anyway. I used an Actron EEC-IV scanner but found no codes (KOEO & KOER). Next I replaced the Coil, Condenser, and TFI Module (fender mounted). I considered replacing the Pickup (aka: Stator or Hall Effect Sensor) but in reading a few threads, this didn't seem to fit with the symptoms. The threads did mention that the bearings go out on the distributor so I checked it for movement but it was tight.

Again if there was any improvement, it was hard to identify. In climbing around checking the truck I noticed a rattle from the catalytic converter. I removed the cat and found it broken internally, along with one of the EGR tubes. The truck is deemed historic so I will not need to go for emission testing, so that isn't my concern right now. I plugged the EGR ports to the cat and installed the empty cat back onto the truck. I also tied up the EGR tubes to eliminate rattle. If I ever get the truck running properly, I can worry about fixing the emission components then.

This actually made a dramatic improvement to how the truck ran. The truck does not buck and stutter severely now. The miss is still there but now it is most noticable when under light load. If the truck is pushed into a passing gear it is hard to tell if it is still missing.

The next thing that I had listed as a possibility was the Fuel Pressure Regulator. While attempting to get to the FPR I ended up breaking a couple of the plastic vacuum line ports to the air injection system, I have not replaced this yet. I have a feeling that I would have to remove that massive bracket and tube assembly a few more times anyway, so I have held off. At the moment, I have just routed the air injection tubes to circulate and plugged the vacuum lines.

The FPR didn't seem to make any difference (neither did the air injection system). I then tried plugging the vacuum line to the EGR, didn't seem to make any difference either but did confirm that my CEL does work.

At this point I was really scratching my head and wondered if maybe it was base engine, maybe it had a valve problem and was losing compression. I grabbed a compression tester and checked, but all cylinders were close in compression (ballpark of 125 and no variation over 10 pounds). Then I went back to looking at vacuum lines to see if there was a leak somewhere. I hooked up the gauge to the brake booster line and the needle held steady with not fluctuation. I chased a line to the MAP sensor. I noticed that if I disconnected the MAP and capped the line with my finger, the RPM would increase but it would also smooth out (and that exhaust leak noise would largely go away). I replaced the MAP but it once I plugged it in, it acted the same way as the old MAP. While doing this I also noticed that the PCV valve is buzzing like crazy. I took it out, checked the line and reinstalled it... still buzzing.

I then thought that maybe I had a bad fuel injector. I was able to verify that the injectors are getting a signal, I also used a screw driver to listen and verify that they are opening. I have no idea if they are providing proper fuel but since the truck's exhaust is smelling of unburnt fuel, I am thinking this might not be the case.

I talked to a buddy of mine who worked for a Lincoln Mercury dealer for the past 20 years and while he told me he is not an expert on this truck (had no L/M stablemate) he recalls the coolant temperature sensors becoming carbon fouled. I figure I might as well give it a shot... and will also install the Intake Air Temperature Sensor (Air Charge) at the same time.

I installed the Engine Coolant Temperature Sensor, Temperature Switch and Air Charge Sensor. The Air Charge Sensor was terribly fouled. As soon as I had all three parts instead, the truck ran like a dream. No miss and it felt more powerful... until I tried driving it the next morning. I stopped to get fuel and noticed it acting up again after leaving the station (the station is right down the road). I tried adding Seafoam after getting home that evening and it seems to have only intensified the issue.

I am planning on ohm testing the spark plug wires and verifying that everything is connected securely. The issue when hot and the raw fuel smell makes me think insufficient spark/high resistance connection.

Beyond this I am really just guessing... I have no idea. Other than the Temp Sensors, the ideas that I have will be getting more expensive than the parts that I already tried. The out on a limb ideas are: replace the fuel injectors, replace the fuel pumps, replace the evap purge solenoid.

Sorry for the ridiculous right up, I just wanted to provide all the information that I could. Any insight that you can provide will be greatly appreciated!

 

Have you tried replacing the tps they can do lots of weird stuff.

 

It is funny you mention that. I had replaced that about two weeks ago so I figured that it was fine. However... this morning while test driving it, the truck began to bogg down during light acceleration. I would press the pedal to try to compensate but it wouldn't help, the the CEL light finally came on (first time yet). I got it home and ran the code scan and found that it had two TPS related codes. 121 & 122. I checked the connection of the wiring harness but it is secure. I am curious if I just received a bad TPS from Autozone.

 

If it's a Duralast or Valucrap I wouldn't be surprised.

A real Motorcraft is under 50 bucks at rockauto.com.

 

I have a 95 with a 351W. Lately ive had problems with erratic idle and hard shifting. I've replaced the tps and its still doing it. I was told to adjust the sensor position and that it might be reading too high. I've also been told to check for vacuum leaks. But the problem is that at idle it idles to high and runs at higher rpms. This causes it to shift hard and when coasting the rpms will go up and down in a steady pattern. Wondering if anyone has a tip to help me get my truck happy again.

 

Since this made a remarkable change to the way it ran, it should be your area of scrutiny.

It's very possible that you have alot of carbon build up in the intake area and after replacing everything and getting it running good, you might have knocked some more carbon loose and plugged up things again.

Also check your EGR valve.

I would also consider taking the entire intake system off and cleaning it out.

I've worked on vehicles for years, and Ford trucks have been notorious for carbon build up in the intake.

If it were mine, I'd take the intake system off and have it vatted to clean it out.

 

1. Check TPS voltage, it should (sweep) move up and down when open and close throttle blades. I have my '94 set at .921 v, and it likes this voltage setting.
2. Have you performed a visual check of the HEGO (02 sensor)?
3. You mentioned FPR was good, you checked pressures? Any bleed down?
4. Disconnect battery, and check all underhood and frame grounds.
5. While battery is disconnected, pull the ECM under the drivers kick panel, and check for an aftermarket chip, then check for burn marks under the ECM lid.

 

Check your ECM under the driver fender liner in the firewall. This 1994 computer that controls all the sensors in your vehicle is likely shot! Most computers from 1994 (on your desk or your car/truck) are likely non-functional. In addition, check your ICM as well. Both are easy to check and replace... New ECM about $150 at rock auto with core exchange. If these two components are not working, you'll never be able to get things going. If the brain is dead, the body won't work.

 

Thread is 8 years old.