I own a 1987 F-150 with the 4.9L straight six, which has 108,000 miles. The problem I'm having is that at idle the engine misfires badly. I have had it on an analyzer, and all sparkplugs are firing. It has a new cap, rotor, coil, plugs, wires, air filter, fuel filter, and manifold pressure sensor. I have had the injectors cleaned, and also took them out and cleaned them myself. Fuel pressure and volume is well within normal. The first 3 cylinders have 160 pounds of compression, the 4th has 145, the 5th has 140, and the 6th has 130. I had the EGR valve off, and it is functioning normally. Removing it and blocking the hole in the manifold produced no new results. After having it put on an exhaust gas analyzer, it said that the hydrocarbons were 5 times over the normal, and the engine is running lean. The misfire is completely random; disconnnecting one plug wire at a time yields the same result on each cylinder - a drop in RPM when taken off, an increase when reconnected. This problem has been plaguing the truck since I bought it in August 2002, and really makes it hard to get the vehicle moving without over-reving the engine as it has a manual transmission. Since it is a manual, I can feeel every time it misfires because the stick will shake back and forth as the engine jerks in its mounts, which is very annoying to a person who is good with engines and should be able to make one run properly. Almost forgot, disconnecting the oxygen sensor makes no difference, however disconnecting the wiring to the manifold pressure sensor makes the idle smooth out, however it raises it to 1500 RPM. I have to set the timing at TDC, otherwise the engine will knock due to the lean mixture. Can anybody help me with this? It would be greatly appreciated.

 

Sounds like possibly a vacuum leak. Pull off the throttle body and I'll bet you find the gasket sucked in. It's an easy thing to change and the gasket is cheap.

 

*****I forgot to mention in my original message that I have been through the entire intake system, and there are no vacuum leaks, as I have checked and replaced all the gaskets. Sorry I forgot to mention this, but good call, Racerguy, that was one of my first guesses too.
Scott

 

*****My bad again, I lacked to mention that I have also ben through every vacuum line, from the brake booster to the fuel pressure regulator line - nothing is wrong.

 

MAF sensor?

 

Hey there,
I have got the same problem you got.
My dad has a 1988 F150 4.9L and it sounds like its doing the same
thing you just described.
this is what I came up with.
When he bought this truck the guy told him he had this same problem
and had the EEC (computer)replaced. but after replacing it ran fine
and now acting up again.
I have tested just about everything.
ECT (Engine coolent temp),ACT (Air charge Temp) Sensors OK
MAP and EGR OK
Timing ok and Throttle position sensor ok.
We did change the idle air control valve because it was sticking a
little.
I have 5 volts coming from the EEC to everything EXCEPT to the Knock
sensor. I only have 2.6 volts! and from what I understand, everything
is fed around 5 V from the EEC.
So I pulled the EEC connector off at the firewall to see if there
was any bent or damaged pins (none),then I checked the wires out to
the Knock sensor connector (OK).
But I did find That Pin #40 and #60 are Power grounds and one of them
i think # 60 had resistance of 5.1 ohms. My next step is to repair
this ground wire and see if this will bring voltage back up for
Knock sensor. if not computer will be replaced. could of went bad
twice because of bad ground.
If i find out anymore i will let you know.
anybody, any clues???

 

If I was just guessing I'd say my next try would be a MAP sensor. If you know someone with a similar truck you can swap that in a matter of minutes and try it.

 

*****Where are the MAF and Knock sensors? I know what the terms mean, but do not know where they are located. Also, to all of you who have posted replies here, thank you very much for your help and ideas so far.

 

Ford Trucks dont use a mass air flow sensor.
They use the Manifold Absolute pressure (MAP)sensor to tell the EEC
how much vacuum the engine is pulling, in return the EEC can advance
Timing and control the injectors (rich/lean).
We checked and replaced the one on this truck but did not help it.
The Knock sensor on the 4.9L is on the block between the Distributor
and coil. It will have 2 wires to it, a light green with a black stripe and a black with a white stripe, Pull the plug off the sensor
and check the light green wire, it should have 5 volts to it.
the black/white is the signal return to the EEC.
the knock sensor contols timing. it sences spark knock, if this
happens then it retards timing. check the voltage to this sensor
and let me know what you came up with. i am fighting the same
thing you got.
thanks