I have a 1991 F250 4x4 ZF 5 speed w/4.9liter 300ci 6cyl. The last 2 days I have noticed that the engine won't idle down between shifts, and when put in nuetral the engine races. I checked and lubed throttle cable, and all linkages and springs on the throttle body, and replaced the TPS sensor. No help. It is still doing it. Has anyone else had this problem, and can you give me a clue as to what I should do? Maybe idle control valve, MAP sensor, I don't know. Thanks.

 

Check for vacuum leaks also.

Jason

 

I posted this before.
I do not know if this will help you, but my truck idle's at around 2000 rpms. When I got it the seller said that was normal, bull----. I replaced ever sensor, upper intake gasket, vaccuum hoses, check codes and it stills does it. I have been all over the internet and find people with the same problem but no answers. What I discovered about a month after I got it, if I hit the gas (rev the motor for a brief second) right when the motor starts running, the idle goes down to normal about 98% of the time. If it still idle's high I just repeat the above step. I been doing this for 3 years and 15,000 miles. I think mine is the computer, but I am to cheap to replace it. I figure I just keep driven it until it breaks down. It could also be one of the new sensors I replaced was bad when I got them.(autozone brand)

 

Pull the codes FIRST before randomly changing parts, this could be a mechanical or electrical problem and if there are no codes then you have eliminated all those possibilities and can focus on the mechanical possibilities.

 

I've been doing the same procedure too. It's a hit-and-miss procedure though since mine will react differently at times. I just shut her down and restart the engine; works most of the time. The problem with this is that now there isn't enough dash pot for shifting gears (manual trans); as in, not enough off-throttle idle for smooth shifting.

Conan, maybe you can address why this procedure works at dropping the idle down, although not all the time. Are we fooling the computer somehow? Possibly something to do with the MAP sensor? Just an uneducated WAG on my part.

 

"What you say? It idling at 2k rpms? Yeh brah completely normal. Its legit bro"

Did the PO honestly expect you to believe him when he said that is normal?

 

He also said ever thing works beside the ac. Right after that I check the wiper's and they did not work. I got him done to $2000 before the wipers, then I got it for $1900. After I got it home I hooked up some ac charge for about 15 seconds and the ac started working. Then the list goes on from there, fuel gauges stop working,(still do not work) gas tanks crossfilling,(only has 1 tank know), auto 4x4 hubs did not work,(went to manual) Radiator support rusted out, (put new one in)

 

My I6 also does this upon start up and if you tap the gas it will idle somewhat normal. If I let it idle to long after I first start it up the RPMs will start fluctuating so bad that by truck dies. The only code I have is high voltage on EGR and I blocked it to see if it helped but it did not. I do have a vacuum leak though. My break peddle hisses for a minute when I let off of it but that has been going on longer than the racing RPMs.

There is another guy on here that posted about the same thing. One guy posted on there that his was doing the same thing so he replaced the MAP and it fixed it for a while but it came back.

 

Sounds like a sticking idle air control valve.

Sounds crazy, but try hitting the idle control valve a few times with a wrench or a screw driver.