I bought a 96 Bronco 5 speed manual last year in pretty good shape.
Everything works well, but the idle speed at coasting is hovering around 1500rpm until the vehicle slow down to 30mph at which point the rpm drop to about 700 (where it should be).
It only happens while coasting.
When truck is parked, the idle is fine and steady; I revved it up to 3000rpm for a while to see if I could replicate the behavior, but no, as soon as I release the accelerator, the rpm’s drop nicely to 700.
While driving though, if I switch to neutral, the rpm will go up for a second, and than sloooowly drop to around 1500 and stay there until I slow down to 30mph.
Spark plugs, distributor cap and rotor, wires, TPS, ICM, and MAF are new. Could it be ECM?
I would be more inclined to suspect a sluggish Idle Air Control valve. http://www.rockauto.com/catalog/moreinfo.php?pk=951203&cc=1106110
It’s not uncommon for the IAC valve to cause this problem. One way to check for a faulty IAC valve is, with the engine idling and hot, take the handle of a screwdriver and tap on the IAC valve. If the idle speed changes when doing this replace the IAC valve.
The ECU may hold the idle at a relatively high level in anticipation of the clutch being released. You might get a scan tool that can read live data and investigate the ECU’s desired idle while coasting. Assuming that your foot remains on the clutch when this occurs.
My 89 Ford Ranger ( 4-cyl, 5-speed, Dual Plug ) also did this. Was told that this was because the IAC valve controlling the “throttling down” to reduce NOX emissions. ? somewhat like the dashpot on carb engines?
Sounds like it could be unrelated to the engine but more so a problem with the transmission.
Possibly, there are some valves in the transmission that is allowing the engine to drive faster. It’s not allowing it to coast. Best suggestion is to have a transmission shop test drive your car.
There are no valves in a manual transmission that can get stuck and not allow the engine to slow down.
forgot to mention that the IAC is new as well, sorry, it was the first thing I’ve replaced to no avail