Alright, first, a little info about my truck. It's a '94 F150 4.9L 5 speed manual and has 263,000 miles on the original engine. It has been running great and recently I have done the following maintenance items;

new valve cover gasket
new upper intake manifold gasket
new inspection cover gasket
new oil pan (over size drain plug was stripped out)
new oil pan gasket
new 192 degree thermostat (old one was always slightly open, thus never getting to operating temp)
new coolant temp sensor


The front fuel tank is not working and apparently has not been working for quit some time. Unfourtunately, it has a full tank of fuel. So, I got a siphon pump and started draining the fuel so that in a couple of months I can fix it. I drained 2 gallons of fuel and put it into the back fuel tank.

This is when things started going south. I drove the truck for a few miles after I put the fuel in the back tank and it drove fine. A few days later I started the truck up and the entire truck was shaking at idle. VERY rough but consistent idle. So I drove the truck around the block and found that at low rpm, it feels like the truck is only firing on 3 cylinders. It is VERY rough! The shifter bounces around at low rpm while under a load. If there is minimal load then it still misses, but it is not as noticeable. At higher rpm, it runs smoother but is down on power. For example, there is a hill here that I can normally tackle in 5th gear at 40 mph with the gas pedal floored and can barely maintain speed up the hill. Now, I have to hit the hill in 3rd gear at 40 mph in order to maintain speed, and the gas pedal is close to being floored.


So here is what I have done to try to diagnose what is going on.

1. Tried unplugging the green hose from the EGR. Got an increase in power but no decrease in the engine miss.

2. Tried disconnecting the coolant temp sensor, no change.

3. Unplugged the fuel pressure regulator, no change.

4. Filled up the fuel tank with 7 gallons of good Chevron fuel and drove the truck 55 miles, no change.



I am starting to think that the bad fuel that I put into my back tank gummed up a couple of fuel injectors. What do y'all think?

 

Maybe the injectors, maybe the fuel pump. How much gas was in the tank when you poured in the old gas?

Pull the computer codes first and see if it tells you anything.

Fill it up all the way with premium gas and a large bottle of Techron or Berryman's B12. Or Marvel's Mystery Oil. Whatever. See if that helps.

You can use old gas but only a gallon to an otherwise full tank will keep it from, well, gumming things up.

I did the same thing in a Honda and they just ran injector cleaner through it. No worries after that. But I also had it gum up and ruin a Holley electric fuel pump I was using to drain the old gas out of the tank. Hopefully you fall into category A.

 

The rear tank had 5/8ths of a tank of fuel in it when I poured the 2 gallons in it. I have since filled up the tank with good Chevron fuel. I haven't noticed a difference yet, however I haven't put in a bottle of Techron in it yet either and I am planning on doing that (I typically use it anyway and swear by it).

I would assume that if the fuel pump were starting to go bad then the truck would idle fine but when under a load it would buck and miss since the fuel demand at idle is so small. I guess I need to dig out my fuel pressure tester and check it just to make sure.


As far as the codes, this truck has always had a persistant CEL so I've never bothered to look and see. I may do this tomorrow just for grins.

 

Alright, here are the codes that I found;


81 - MAP sensor problem


72 - looks like another MAP problem


33 - EGR valve opening not detected, I caused this when I unplugged the green hose.

52 - Power Steering pressure switch always open or closed - This is my ongoing code.



So it looks like it may actually be a MAP sensor problem. I have already checked the MAP sensor hose and checked for vacuum leaks and have found everything to be ok.

 

Cool, at least the MAP sensor isn't too pricey or labor intensive to replace.

 

Pull vacuum hose from MAP sensor and cap vac line. Does engine RPM change? Pull electrical connector from MAP sensor. Does engine RPM change? If no to both, I'd suspect bad MAP sensor.

81 is pointing out a problem with your TAB/TAD solenoids, thermactor system for emissions.

BVA is right about the MAP sensor. I would replace and clear codes, drive and check for codes again.

 

Pulled vacuum hose from MAP sensor while running at idle. The engine RPM lowered slightly till I stopped the vacuum leak, then idle returned to normal with a miss still present.

I disconnected the electrical connector from the MAP while the truck was at idle. There was a VERY slight change in engine RPM (barely audible) and the RPM then went right back to where it was.

What are the TAB/TAD solenoids and thermactor system? Is that something to do with the air injection?

 

Well, I just ordered a new cap, rotor, plug wires, plugs, and MAP sensor. Ordered everything through Rock Auto and the grand total, after shipping was $70!!


After the parts get in I'll replace everything, including the fuel filter that I have laying around, and I'll post back up as to if it helps.

 

Yup, air injection. The valve are somewhere on the hose exiting the AIR pump, follow the vacuum hoses back to the solenoids.

 

Don't forget the fuel filter, if it's plugged up that can cause similar issues with driveability.

Sam

 

The parts from Rock Auto just came in today. So, tomorrow I'll be replacing the cap, rotor, plugs, plug wires, MAP sensor, and the fuel filter. We will see how she runs after all of that. I'll post up after it's all done and go from there.

 

Alright. I have now replaced the cap, rotor, plugs, wires, MAP sensor, and the fuel filter. Here is what I found.


1. The plugs were worn down so the gap was at about 0.060 on average. All of the plugs had a nice brown color to them except for the back cylinder. It was solid black, but not wet. Considering this truck has 263,000 miles on it, I see this as pretty good.

2. The cap was in bad shape. The little metal tabs on the inside of it were worn with a half moon shape from the rotor going past them.

3. The fuel filter was almost stopped up completely. I could not noticeably blow through it, but when I tried to a small stream of fuel came out of it. After I removed it, I tilted the fuel filter so the fuel came out the inlet and a lot of black fluid came out once the fresh fuel had finished coming out.



So now, the truck runs better but the miss is still there and it is still down on power. At least now I could drive it around town without being afraid of snapping a U joint from the miss being so bad. On the highway in 5th gear, it still jerks around and misses a lot. More than what I am comfortable with.

 

Hopefully driving it a while with clean gas and a double dose of fuel system cleaner will clear it up.

You'd assume the EEC codes would tell you if the TFI module was going north, but that would be my next item to check. So many engine issues get blamed on fuel when ignition is actually the culprit.

The gas may have been the straw that broke the camel's back on some things that were already close to the end of life.

 

A while back I had printed off how to trouble shoot a TFI module going bad but I can't seem to recall where I put it. I do know that on the truck I was working on back then, the TFI module was going out and it acted a LOT different than what my truck is doing. I just can't see how a bad tank of fuel could cause a TFI module to start acting up. This really does feel like the engine is just not firing on all cylinders, and it's a VERY rythmic pattern. This leads me to believe, with all the things I have replaced so far, that it is a fuel injector problem.

I'll drive the truck around town tomorrow and throw a good dose of Techron into it and we'll see what happens.

 

Now I am 100% convinced the problem is fuel related, and not ignition related.


I drove the truck to a local autoparts store to pick up some Techron, and when I parked the truck, smoke was pouring off of the rear catalytic converter! See, the truck has a small transmission fluid leak (typical of Mazda 5 speeds) and so it gets some on the rear cat. But it has NEVER smoked like this before. Alright, so what would make it smoke more now than before if the fluid leak was the same (which is it). An increase in exhaust temps will do this. So what causes an increase in exhaust temps in the catalytic converter? Either an overly rich condition or an overly lean condition. If an overly rich condition were present(faulty ignition) then I would see black smoke coming from the exhaust when I've got the gas pedal floored, which I don't. So, an overly lean condition it is.

To give y'all an idea on how much power has been lost, I had the truck floored in 5th gear doing 60 mph with no headwind and level road. After 2 miles the truck was doing 63 mph. So just to maintain highway speed in 5th gear, the truck is almost floored.