F-body-fan

Just curious what the roles are of the speed sensor is on the TPI engines. I honestly have never paid attention to it & just assumed it was only for the cruise control.
I guess never really cared much to be honest I had noticed several times this summer while driving the 1989 5 speed sometimes the idle would be over 1000rpm's when I would push in the clutch coming up to
a stoplight. Come to a stop & it would hang there for about 5 seconds & then drop to normal idle speed. seemed to be at mid engine temperatures. maybe 150 degrees give or take?

So today I drove the 87 automatic car & noticed it was almost on cruise control in a 25mph seed zone. I threw it in neutral & same thing. Idle speed closer to 1200 +/- & stayed high until I came to a stop.

I think its safe to assume this car has been doing this forever & I just never paid attention to it. I have owned it for 30 years, just never paid attention I suppose.
In the manual shift car it was just so much more obvious because its disengaged so often coming to a stop sign. So now I am paying attention to it.

Anybody have any insight as to what that is all about? On a warm engine, I don't really see the point unless its an emissions thing?

MikeKy

My understanding is the VSS tells the ECM that the car is slowing and adjusts the rpm. I have the 700R4 in my 55 behind a L98 with TPI and when I first put it together when I came to a stop, the rpm stayed higher for a few seconds before it leveled out. The ECM apparently doesn't know the car is decelerating without the VSS signal.

F-body-fan

I have been trying to figure out a bug with my 89 5 speed TPI. it runs good, but at cold start ups the RPM's go all over the place. Not in the way a mad MAF or other issues make a rough stumbling idle, but rather the engine is running smooth, its just keeps changing speeds. Starts at 1500, slowly drops down to 1200, then 1000, then jumps back up to 1200, then back down. sometimes will do that 4 times before settling down during warm up. But that is another topic....

All of this has made me very sensitive to what the car is doing at all times & it often has an idle speed of 1000-1200 when I push the clutch in at 25mph coming to a stop sign, & as you mention it will hang there for about 5 seconds & then drop to 750 like flipping a switch. I was sure this was Not normal & part of the other cold start idle speed issues.

Then driving my 87 automatic car & realized it does that same thing. Its almost like a cruise control at 20 mph when slowing down. Never noticed it before & I have owned that car for decades.

Vader

VSS input affects the following features and functions:

Speedometer / odometer;
TCC lockup (automatic trans);
SHIFT lamp control (manual trans);
Decel Fuel Cut-Off (DFCO);
Cruise control;
Cooling fan control;
ESC knock control enable;
Speed limiter fuel cut-off;
Highway Fuel & Spark Mode enable (HFSM);
IAC control (PID loop);

In a Y-Body with "valet" option, the secondary fuel cut-off as well.

The effects on IAC and stall-saver might be at play, but the first items to be checked are the condition of the throttle body to assure it operates smoothly and reliably (no hanging/binding) and the operation of the IAC stepper and pintle. Then there is the EGR which can affect the RPM as a result of the massive/controlled vacuum leak it creates at highway speeds.

Tom 400 CFI

I think it's for emissions, but I've wondered for years too. I find it annoying. I've warmed up cars before, then unplugged and left the IAC unplugged for "better results".

F-body-fan

I wonder if any of this is chip tune-able. I also never realized how different cold to warm idle speeds can be between proms, engine to engine, year to year. seems like there are so many different tunes, for engines
that are so similar with mostly gear ratios separating them. Its interesting for sure.

F-body-fan

The ECU is making it like driving two footed coming to stop lights. Tiny bit of extra wear on the brakes I would think too?
Funny I never paid attention to it before until driving a manual shift TPI.
hard not to notice when you push in the clutch & the idle is at 1000-1200 RPMS.

Tom 400 CFI

I agree. Annoying. Hanging throttle is annoying, and not helpful to high performance driving, or any kind of driving involving a clutch and the desire to have a seamless engagement. And with an auto; engine fighting the brakes? Dumb.

I'd love to hear anyone's comments about about the ECM tuning of the IAC as it relates to driving. I wish it would simply handle target idle speed and that's IT. From a driver's perspective, the following throttle and following veh speeds is an interference to driving.

Like I said above; I've gotten engines to op temp, settled down idle speed and then unplugged 'em and left it. After that, the driveability once warmed up, has been the best there on forward. But the cold driveability has varied greatly, veh to veh. On one, it needed a touch of gas pedal to start COLD (below 20*F) and hold the throttle cracked for ~30 seconds or so, then it was fine. Another, the engine acted super lazy when cold; idled ~400 RPM, stalled easy, AC cycling would kill it...thing sucked. (Vortec 305). Another, (Vortec 350), was totally unfazed by it. Started cold fine, ran cold fine, AC, no issues, it didn't care. Weird.

Tom 400 CFI

Thinking about this more, on the '89 Kart, I have no VSS b/c I lost the instrument cluster (speedometer). It's symptom is that it idles really low after it's been driven under a load. It'll idle nice, as it should all day long, until I drive it a few hundred feet or more (put a load on it)...then it idles super low. Weird. I should try disconnecting the IAC in that. IDK why I haven't, yet.