My 17 year old sons 88 F-250 5 speed manual 4X4 w/5.8 (no air conditioner) is having some issues idling. When he starts it up in the morning to warm it up it oscillates from a normal idle (800-900) to what sounds like 2500 to 3000 rpm's (truck does not have a tach) It runs at a normal idle for 5 seconds then instantly up to the higher numbers and stays there for about 15 seconds then back down, then back up ... you get the idea. While driving, when we come to a stop the motor idles really fast and only sometimes will drop down to a normal idle, most of the time we have to turn the motor off and restart to get it back to a normal idle, then after we take off again ... same thing at the next stop. My neighbor has a shop, good mechanic (26 years as a Ford dealer mechanic, 18 years as a lead diesel tech) took the throttle body off cleaned it and replaced it, we replaced the IAC with no change. My neighbor said it could be the ECT sensor, going to try that next but thought I'd run this by you guys first. Appreciate any ideas guys, thanks in advance

I know the idle numbers I included are most likely NOT factory correct, I'm just trying to describe what the motor is doing so as to get info to solve the kids dilemma

 

Have you or the mechanic pulled the codes?

I'd say it's a good first step to look at the IAC. Have you checked for any vacuum leaks?

 

RIKIL, Thank you for replying, yes on both questions, he and a machinist friend both have checked and found no faulty codes or vacuum leaks.

 

HUM ... nobodys experience the idling issues I described?

 

I'll first say I am not 100% sure what your truck has or doesn't have in terms of sensors. My truck is 4 years newer than yours and what I discuss below is on the basis that our trucks are similar, but not exact.

I think the problem is between your first post and my questions we covered a lot of what people would suspect. I do understand the behavior you are talking about. Aside from what you had mentioned that you checked/fixed, my thought would be the O2 sensor(s). Not sure if you have them at all but if you have one that is not working well or if you have an exhaust leak, it MAY cause a rise in RPM. For example, if you have an exhaust leak and one of your O2 sensors (my truck has 2, one on each side) is detecting a lean condition due to the exhaust leak, the computer should be trying to add more fuel to the motor which would cause an increase in RPM. That theory is sort of a stretch.

The other thing is that if you aren't getting any codes then you need to start looking in places the computer doesn't monitor. Have you tested or checked the fuel pressure regulator? Try pulling codes again (KOEO & KOER)

I just re-read your OP...I am not sure if the ECT would cause something like this but I might suspect the ACT sensor first. Also, might be good to test your TPS to make sure it is responding properly. Lastly, could be your MAP sensor (or is it a BAP on SD trucks)? Same sensor, check it.

If you have more questions about these things, post back, I'll help where I can.

 

Thanks, Don't think there is an exhaust leak. I've had both exhaust manifolds surfaced because both were leaking. The exhaust system is new 3" from the catalytic converter back ... the catalytic converter had corroded and pieces were restricting the flow so the exhaust guy "knocked" the pieces out (no emissions inspections in this part of Alaska) One of the air supply manifolds was good the other was bad so it got replaced. When my neighbor mentioned the ECT sensor, I remembered, a year and a half ago when I had a new radiator installed, the shop owner told me his tech said the ECT sensor had a very small leak and the plug in connection seemed loose and in bad condition, I got a new sensor today and will try that. Will let you know the outcome, thanks again.

 

I had the same issu on a 96 ford ranger i know its completely different truck but the TPS was my issu slapped a new one in and all the problems got fixed

 

Well, here's where I'm at ... Installed a new ECT sensor, no change. I bought a new TPS today and will try that next ... will let ya know the outcome. Thanks again

 

BINGO .... Installed the TPS and trucks runs great, thanks for the advice guys ... Much appreciated

 

Glad you found the root cause. Thanks for the feedback!