Idle issues

junkyardjeff · May 25, 2015, 11:06:10 PM

I picked up a 95 F150 with a 5.8 that needed a trans so after installing a trans and fixing a few other things from sitting a year or so I am now driving it but it has a idle issue,when I take it out of park and let it roll before hitting the gas it will die and it will want to die when I am stopping and its just barely rolling and will stay running if it stops quickly. Its not setting any codes and ran a couple tests but nothing shows up so where do i start looking.

GPster · May 26, 2015, 09:56:51 AM

Just jumped into my mind, because of the transmission change, does the transmission have a lock-up torque converter that is not unlocking? Vehicles with smaller engines have so many switches that adjust the idle speed to keep them running for any low speed load. Or maybe there is some other change do to the transmission change. Was it an exact exchange or maybe it requires some re-programing. GPster

junkyardjeff · May 26, 2015, 05:33:09 PM

The used trans was a exact match to the original including the number and works like it should,I am beginning to think it has something to do with the speed sensor system since the speedo jumps around and a new speed sensor did not fix the problem. The speed sensor is in the rear end so I am going to remove the cover to check the thing the sensor reads off of,the idle only acts up just after the truck starts moving.

rooster · May 26, 2015, 08:59:20 PM

maybe brake booster has a vac leak, pinch the line see if it does it.

junkyardjeff · May 26, 2015, 09:18:58 PM

The plug for the speed sensor had been replaced so I checked the connection and found that the two wires were just twisted togather and taped with the wires touching each other,since the connector wont stay on the sensor I will grab one from the junkyard since I have no idea if the wires are connected right since the aftermarket plug has the same color on each wire.

rooster · May 27, 2015, 11:15:07 AM

You might like this:

junkyardjeff · May 27, 2015, 05:59:53 PM

I was wondering if that part was bad but after watching that video I think its working like it should,it will idle fine without having to keep a foot on the pedal but if the truck starts to roll then it wants to die.

rooster · May 27, 2015, 07:48:35 PM

Quote from: "junkyardjeff"the truck starts to roll then it wants to die.

To get the truck to roll are you putting it in gear, or just not applying brakes to roll?

Is computer acting like it should? self -dignistics with key in on position, RPM's should flare up on start-up for a few seconds.

You place a volt meter on TPS sensor and see if that changes when you begin to roll. 1 bad sensor can take them all off line, or goofy output. Just a thought.

junkyardjeff · May 27, 2015, 08:21:16 PM

Letting it roll in neutral or slightly moving in gear with foot off the gas it will want to die,the IAC is working as it has high idle when cold and the RPMs flare up when starting so I do not think its that,the speedo is not steady so i changed the speed sensor and still the same but I think the problem has something to do to the jumpy speedo.

rooster · May 27, 2015, 08:42:37 PM

Quote from: "junkyardjeff"but I think the problem has something to do to the jumpy speedo.

I do remember reading something about what you are saying and if I remeber right it had something to do with AC voltage leaking from the alt diado's and making things not read right (speedo jumping around). Do your test again with alt disconnected. I'll see if I can find info
found it:

junkyardjeff · May 27, 2015, 09:55:12 PM

My speedo does not move when sitting still but I will disconnect it and go around the block and see what it does.

junkyardjeff · May 28, 2015, 05:37:05 PM

Its definately not the alternator.

rooster · May 28, 2015, 06:18:50 PM

Quote from: "junkyardjeff"Its definately not the alternator.

ok!

Maybe see if you can create the idle problem while the car is sitting still, by moving the wire harness abit.

The speed sensor thats in the rear end, is that for the anti-lock brakes? The ABS light in my 88 f150 has been on for 20 years.

junkyardjeff · May 28, 2015, 06:43:58 PM

That speed sensor is for the anti lock brakes,cruise control and the speedo,the trans can also accept one too but there was never one on the trans but has the plug in the wiring harness for it.

rooster · May 29, 2015, 06:49:48 AM

Quote from: "junkyardjeff"That speed sensor is for the anti lock brakes,cruise control and the speedo,

Is it posable is disconnect a fuse to those 3 things, then do the test? Just a thought.