Bronco runs rough

[djjatczak]

I have a 1990 ford bronco 5.8(351) and when i start it runs fine but when the truck heats up it starts to run rought the rpm tack starts jumping and the truck starts shaking i dont know what the problem is i have ne w spark plugs the roter looks good im not shure could some one help me wiht ideas thanks.

[pez692001]

I had kind of the same problem with my 95 and i had to replace the coolant temp sensor. Maybe yours is different.

[97chevyman]

ok, there could be several causes. Now when its cold it runs in open loop, now it is possible for the coolant temp sensor to be bad and having the motor run in open loop all the time. Now if the sensor is not bad, could be the O2 sensors, giving the PCM the wrong oxygen level in the exhaust and making the engine richen up a little bit. Get a chiltons manual, test the temp sensor, and go from there. Usually you only get 80K miles out of o2 sensors, i would replace them if they have more on them, upstream and downstream. I used to have a 97 chevy, and i replaced all 4 o2 sensors ( i think your bronco only has 3 ) and my MPG went from 6 to 16, big difference. Also check your vacume to make sure you do not have a leak.

[indyram]

buy a ford repair kit consisting of 1 grenade.

[djjatczak]

Thank you for replying but it seems to me when i step on the gas it is missing and im not getting all of the sparks its miss firing i think because it kinda studders when i step on the gas and while im driving but this only happens when it is at normal operating temperature i have the distributor is good and the rotor its new same as factory.

[indyram]

When you replaced the distibutor and rotor, did you make sure to mark all your wires and put it back in the same position as when you took it off? If it is off a little bit when you put the new one in it can cause problems. Also did you replace plugs and wires also, that would be a good idea.

[djjatczak]

ok they are all in the same place i marked them there axactly the same but it was doing this before i reaplced them i just thought it could help some since there all were 14 yars old i think im going to try and replace the coolent temp sensor its not that expnsive and the oxgyen sensor is less than 40,000 miles old so i dont think that its that if this doesnt fix it i will take it to get computer codes and see what it says thank you

[bronco351]

Hey did you ever get the problem fixed? I have the same exact issue with my 93 Bronco...I replaced the fuel filter and the o2 sensor underneath in the exhause. I didnt know that there was more that one o2 sensor.

[gbic1]

sensors wont cause miss. Double check your firing order. Coolant sensor on Fords rarely go bad. Cap rotor and wires go bad often. Plugs are easy to crack when Installing. Any codes in it? Vacuum hoses off?

[bronco351]

There are no vacuum hoses damaged or off that I can tell. With me it dosent happen all the time. I mean I could start the vehicle and it may run fine for the whole day then next time I start it, it may run bad for like 3 days straight. It does it when its warm and when it cold .

[bronco351]

Ok I was told that it is most likely a vacuum leak that is the major cause of the problem, any way I can check this myself or any ideas on where to take it where they can check the vacuum?

[gbic1]

Go check your grounds. Make sure they are clean and tight. Usually there is a ground at the back of the intake for the oxygen sensors. If the computer does not see this ground it will give it a false rich reading and that will through things off.