After 10min highway driving I used the engine brake to slow my 1988 302 down. Unlike usually, the engine just stayed at the revs the speed and gear forced it to be. If I hit the clutch, it was slowly ramoing up the RPMs. If I brake with the gear in to pull the RPMs back down, it would stay wherever I pulled them for a while, before it would slowly ramp up again. Every once in a while (I think with A/C clutch engaging) it would drop to about 500rpm, stumble and then go to about 1500rpm.
The vacuum hoses are intact and at least a few weeks ago, the truck was dying when I disconnected the IAC (still have to repeat that test). Plenum/intake gasget is fairly new, so I'm rather confident that there isn't a huge amount of unmetered air coming in from somewhere. TPS? MAP sensor? No check engine light has come on, but I'll pull the codes soon anyway. No problems on the highway at all.
Anybody else ever had those symptoms?

 

Sounds like a mechanical problem to me. Somwthing is binding. Is your throttle cable gunked up or bound up or does it return freely? Are both throttle return springs working?

 

The throttle feels just as normal, no problem. If it would stick, wouldn't the rpms then go back up as soon as I hit the clutch (after I pulled them down with the engine)? They stay down and only ramp up slowly, though.
Even more weird is that the problem turned out to be intermittent. It happened the day before yesterday after the highway drive there and on the way back (3 hours later). Yesterday, there was no trace and unplugging the IAC stalled the engine. I've been having starting problems (turning over but not firing - starts with one cylinder at a time it seems) lately and it's been running rich (EGR error codes). Right after starting, there's the smell of unburnt fuel in the air (was it flooded due to old, leaky injectors?). Could a faulty fuel pressure regulator give me these symptoms (high idle, rich, bad starting)? I'll check fuel pressure as soon as I can!

 

You need to check for codes.

Take care,
-Chris

 

Common problem on Mustangs. Clean the two large connectors that go to the engine. Scrap all the terminals clean.

Good luck,

Shannon

 

Sounds like a vacuum leak. Or a idle air issue.

 

It's probably the throttle position sensor. Mine liked to hang around 2200 RPM and it is an intermittent problem. I got a new one for $32 at AutoZone and it fixed that irritating high idle.

 

I'm starting to think it's the TPS too. I checked the codes and all I got were the EGR codes that I got already before the idle problem (I'll replace the EGR system together with my little project.
The only disturbing error code I got was a memory code: 96 (fuel pump relay). Is that relay sitting under the driver's side of the hood where one big connector from the computer is going? It might explain the starting troubles I also inconsistently get since a few days back. I couldn't check fuel pressure, because the cheapo gauge in my set doesn't fit on the shrader valve