hey all ive tried before to get my truck running perfect and have still had no luck. the truck is a 90 f 150 with a 5 speed and the 4.9. the truck misfires very bad when it is cold and when it warms up it misfires under load. Im thinking it is some kind of fuel problem. the truck has a new fuel filter, spark plugs, wires and cap and rotor. also the computer throws a code about the map sensor but it has been replaced and it is still there. i was thinking about changing the o2 sensor to hopefully fix the terrible running. any ideas? i need this truck fixed asap as i am now plowing with it and the misfiring is getting very annoying. thanks

 

First of all, just replacing a bad sensor isn't going to clear the codes. The codes can be reset by either unhooking the battery for at least 15 minutes, using a code scanner if it's equipped to do so, or after the key has been cycled on and off [I believe] 30 times.

Second, is that the only code you're getting? Give us all the codes even if one seems unimportant to the issue and tell us when you're getting it [i.e. KOEO, CM, or KOER].

Third, how many miles are on the o2 sensor? They tend to need replacement after 50,000 miles but that doesn't necessarily mean they all go bad at that time.

And finally, why do you think its a fuel problem? Have you checked pressure? Do you think you may have bought some bad gas?

In short- the more you can tell us, the better we can help you.

 

Have you tried adding fuel injector cleaner to the gas tank?

 

ive cleared the codes many times< i will try scanning it again right now but i think my connector is frozen to my inner fender right now. im also getting a code about my purge solinoid but i dont think it really matters. when i got the truck 3 thousand miles ago the o2 looked new but the truck ran this way since the first time i started it. im not really sure why i suspect the fuel issue but no its not bad gas. what im trying to say is that my air fuel ratio is probably what is causing the misfiring.

 

yes ive added injector cleaner probably 10 times

 

Okay- have you checked the base timing to make sure it's correct or have you changed the base timing from the factory setting? Remember that if you're going to change the timing, you have to do so with the SPOUT connector removed otherwise it won't be set correctly.

You could also have a bad PIP sensor inside the distributor. I don't personally have experience with them but from the way other guys on here talk, it's easier to get a whole new distributor than it is to replace the sensor.

As for the air/fuel ratio, that can be affected by the o2 sensor. Go to this page to read how to test your o2 sensor- Ford Fuel Injection » Oxygen Sensor (HEGO) - and test it if you can. I would think if its bad you'd be getting a lean or rich code but it's hard to say.

 

ive left the timing all alone because sometimes it runs almost close to perfect (not very often) but like if i rev it over 3 grand it doesnt really miss

 

Is it a all cylinders type miss fire or just one or maybe two of the cylinders missing?

Not uncommon for a motor even when runs poorly at an idle or in the lower RPM range to smooth out, seam to quit missing at high RPM's. Depending on the problem/cause, likely still doing it, just not noticeable at 3grand.

computer throws a code about the map sensor but it has been replaced and it is still there.

You need to find the cause and correct this first.

If its code 72 KOER however its likely user error, you failed to floor it and let off when directed during the test?

If not post the code when you get a chance.

 

when its cold the misfire is probably 3 cyls and warmed up maybe 1 and sometimes 2. the code im getting is number 22 map sensor out of range

 

the code im getting is number 22 map sensor out of range

Ok what type code was it though, KOEO, CM or KOER?

When you did a tune up, you use good quality cap, rotor, wires and plugs? Especially the wire set. And where they model specific fit? or custom type wires you have to cut each to length and crimp an end on to fit them?

 

its koeo and yes the wires are model specific 8 mil with a brass cap and rotor. motorcraft copper plugs

 

You likely need a meter with tach function to test the MAP sensor if its frequency varying type, not sure exactly when the change was made.

You can test it with a simple volt meter first.

Test for correct reference voltage.

Unplug the harness connector and with the key on engine not running, probe VREF to ground in the harness connector, should be 4 to 6volts.

Turn key off, plug harness connector back in and turn the key back on, engine not running.

Back probe the harness connector at "Signal return" and "map b/p signal", should be 5volts no vacuum (if its a voltage varying type).

If you get nothing at all its likely a frequency varying type and you'll need a meter with tach function to test it beyond the reference voltage.

However being as you've replace it already in attempts to correct that issue, that suggests a wiring/reference voltage problem more so then a bad MAP sensor.

 

yea so other than the actual map sensor what else could it be? i thought maybe the computer was shot so i swapped it out and it made no difference at all

 

First you gotta check for reference voltage.

EVP and TPS and MAP share the same reference voltage circuit, MAP appears to be last or rather at a point out beyond the T off to EVP and TPS.

Test reference voltage at the Brn-wht wire at each of those three sensors. If you find its within range, 4 to 6 volts, at the EVP and TPS but low or non existent at the MAP senor then follow that circuit back looking for a problem. Problem would depend on what you find while testing. Lower value would suggest a slightly corroded connection or wire, zero value would suggest the wire is badly corroded connection or the wire has been cut completely through.

If its low or non existent at all three (Doubt it based on KOEO test), check the connection at pin 26 at the EEC harness connector. Verify its making connection there and then follow that circuit, the same Brn-Wht wire, out from there looking for damage of any kind between it and those three sensors.

 

reference voltage is right on at 5 volts