i have a1993 f150 4x4 5.8 auto. it has a rough idle when it warms up ,i have had new plugs,cap, rotor. the garage has checked for codes and scope checked . they say all is well but cant tell why its idling rough .ive run injector cleaner thru other than rough idle it runs great.old plugs look perfect and no black exhaust pipes.cant find any vacuum leaks, truck has 65000 miles on it. thanks for your help u

 

How does it drive when it is warm? Any problems accelerating or any hesitation or missing?

 

runs perfect, no miss excellent acceleration decent fuel mileage,starts great hot or cold.

 

Hmmm.... original ignition coil?

 

appears to be original but shop that did scope check said its ok..???

 

I don't know. I have a big problem with my 302 when it is warm. It does not accelerate properly. It will hesitate, stumble, and sometimes knock especially on an upgrade. I've tried everything under the sun at considerable expense.... Wish I still had my old F100...it was much easier to diagnose. The Dealer has come up empty also. Very frustrating.

 

I find it strange that you don't get any error codes. Have you run the diagnostices yourself (as opposed to the dealer)? At least your o2 sensor should be out of range.

In frustration, I piggy-backed a wire onto the signal lead from my o2 sensors so I could read the actual voltage during idle. This is tricky, and you can ruin a sensor if you apply voltage to the signal wire, so make sure you don't check its resistance with an ohmmeter.

There are two or three wires to the o2 sensor on most Fords. If there are three wires, one is ground, one is 12V constant for heating it up, and the other is the signal. If there are 2 wires, the case serves as ground. I found that on the three wire units the case is NOT grounded. Unfortunately, I can't remember which wire is the signal. I believe the white wire is the heater. I can check it and get back later.

Anyway, with an analog (digital won't work) voltmeter with at least 20000 ohms/volt sensitivity, the voltage should swing constantly between 0.3 and 0.7 volts or so. It swings about once per second, that's why the digital meter is not too good for this job.

If the voltage is low, the mixture is lean. If high, then it's rich.
However this is confusing, because the computer is also adjusting the mixture while you are reading it.

Incidentally, if you have an old sensor you want to check, you can put it in a vise, connect your voltmeter, and heat up the tip with a propane torch. As the gas from the flame removes the oxygen from the tip area the voltage will rise. (This is getting too long).

If there is any vacuum leak, it can fool the sensors. I had this happen - caused by the vacuum operated valve from the smog pump. Pumped fresh air at idle, causing the computer to think the mixture was lean, therefore enriching it, making the problem worse.

Hope this helps.


Winford

 

i will try anything, so thank you for any ideas. i may just put a new o2 sensor in out of frustration and also make sure the egr valve is clean as it appears to be woking ok.thank you

 

OK, but a new sensor is about $60.00

Winford

 

ouch, well ill be checking the one thats in there first,so ill be rounding up an analog meter

 

Check out another thread in this forum on misfiring at idle and low speed pulls only when engine is warm.

 

>i have a1993 f150 4x4 5.8 auto. it has a rough idle when it
>warms up ,i have had new plugs,cap, rotor. the garage has
>checked for codes and scope checked . they say all is well
>but cant tell why its idling rough .ive run injector cleaner
>thru other than rough idle it runs great.old plugs look
>perfect and no black exhaust pipes.cant find any vacuum
>leaks, truck has 65000 miles on it. thanks for your help
>u

This may be a long shot, but does your truck have A/C? My daily routine is to start my 93 F-150 in the morning and let it warm up for 5 to 10 minutes on cold mornings. I would put the climate control on defrost to clear the windshield and let the engine warm up. Once the engine warmed up and the idle dropped to it's normal level, it always seemed to idle rough. I couldn't figure out why. Come to find out the engineers at Ford decided to wire the A/C compressor to come on when the climate control was set to DEF and MIX and well as NORM and MAX A/C settings. Owner’s manual doesn’t say anything about this, but the shop manual does. So while the defroster was on, in the background the A/C was running and loading down the engine causing it to idle rough.

Good luck
Dan

 

you sound like your driving my truck im still in the process of trying and testing so far plugs look perfect,pvc new,o2 sensor new ,coolant sensor ok,cap,rotor new,no vacuum leaks,ran injector cleaner thru, replaced fuel regulator,fuel filter ok.... ill try diffrent settings on mya/c heater talk to you later..scott