I have a problem with my engine idle speed. I have a 1987 stock convertible with manual (4+3) transmission.

The Problem: When I push in on the clutch while rolling, the engine idle speed tends to go up above 1000 rpm (generally to around 1300-1400 rpm) and it stays there until I have come to a complete stop. Once the car is stopped, after a couple of seconds the idle speed drops slowly to 700 rpm and stays there until I accelerate again. Being in various gears or in neutral has no effect. If I race the engine while sitting still, the idle drops immediately to 700 rpm when I let off the gas. I had adjusted the IAC and throttle position several months ago without any effect on this issue; also checked both recently and they did not require adjustment. Vacuum at idle is slightly above 20" and steady.

Anyone ever seen this problem? Could this be "normal" operation? (It's been this way ever since I bought the car 2 years ago.)

Any thoughts would be appreciated.

 

You could have a vacume leak somewhere, that can do some crazy things...also, you could have some stuck, sticky linkage.
Just a couple of thoughts, I'm sure more will chime in

 

When you are moving the idle speed is controlled by the Warm IAC Park Position vs Coolant Temp table, not the Desired Idle Speed vs Coolant temp table.

The Desired idle speed control works when you are stationary, and it sounds like this is working ok.

The Warm IAC Park Position vs Coolant Temp table sets a position for the IAC valve between 0 and 145, as a function of coolant temperature, with a generally decreasing trend, as coolant temp rises.

My guess it that your throttle stop is adjusted too far open at idle, allowing more air than was intended based on the IAC park position setting.

Try adjusting your throttle for minimum air and reset the tps to 0.54 volts.

If this doesn't help you can always go with a custom chip with the IAC Park position re-adjusted for your airflow.

 

I appreciate the responses. As soon as I can, I will recheck the TPS and IAC, although I'm like 99% sure that they are set correctly per the Helms FSM procedures (including disconnecting the timing bypass during the adjustment procedure).

 

Check throttle shaft play on drivers side of TB. Over time the bore of TB will be elongated and the shaft is able to walk back and forth. This could cause the blades to get hung up on bore. Worth a look anyway
Know anyone with a scanner or laptop with software. If you could see what the IAC is doing during these transitions it would sure help you diagnose it.

 

Agent 86 --

Is there a specific procedure for checking throttle shaft play? Or do I just disconnect throttle and cruise cables and try to wiggle the shaft? If I'm just "eye-balling" it, how much play is too much?

 

You should be able to see and feel if there is excessive play by just grabbing the linkage end and wiggling it around.

 

I have reset the idle speed and TPS setting per the FSM. TPS was off a hair (was .56v vs .54v now). Also tried to jiggle/wiggle throttle shaft... did not seem to have any play.

I watched the IAC position with scanner software (@8192 baud) and observed the following (from my memory):

- with engine warm and in closed loop, idle was between 725 & 750 rpm; IAC position was at 20 steps. Rev'd engine and released accelerator & IAC would return to about 20 (sometimes 19, sometimes 21 or 22). Engine speed would return to between 700 & 775 rpm.

- while driving, IAC counts sort of follow accelerator - increased throttle = increased IAC counts; decreased throttle = lower IAC counts. (didn't observe IAC at constant throttle)

- push in on clutch and release accel. at approx. 30 mph, IAC goes to 32 and constant; rpm = approx. 1300.

- leave clutch pushed in and vehicle slowing down, IAC remains at 32; rpm holding at 1300.

- below about 5 mph, coming to a stop, IAC decreases slowly to 20 & rpm follows back down to 750.

(I hope I remembered this all correctly!) Does this tell us anything?

 

Well it looks like your rpm is following along with the IAC steps so it seems all good there. tequilaboy's post sheds some light on the workings of the IAC which goes along with what is happening with your rpm at stationary idle and when car is moving. My rpm does what your does to some extent so maybe this is somewhat normal

 

Bama,

I replied to your pm, and now I have just seen your latest posts.

The IAC behavior that you reported is extactly what I would expect.

The table looks like this:

Temp Deg C IAC Counts
-40 145
-16 145
8 113
32 100
56 80
80 50
104 50
128 50
152 50

Looks like my spacing didn't work, but you get the idea, 2 columns.

Your value of 32 is even lower than what the table suggests, so there may be some more factors involved. The tps plays a role as well, that's why proper adjustment is important.

Oops forgot there are also gain factors involved which control how quickly the IAC reacts to a command change. My gain factors ar cranked up so I can raise and lower the idle quickly on demand.

 

Thanks again for the responses!

I won't be able to do anything with this until Saturday, but what I plan to do is close the throttle plates a little more at idle and maybe adjust the TPS back up to .54v at the new throttle position to see if that has any effect (per tequilaboy's suggestion).

It seems logical that if I can get the IAC to move to around 30 counts at zero-speed idle and give 750 rpm, then that will closely match the non-zero-speed counts that I've seen. (The object there would be to create a "bumpless" transfer between the IAC tables.) Of course, it could be that the computer will be smart and increase the non-zero-counts to compensate for what I've done!

Is it possible that I have a leak in a vacuum circuit that is only there when the car is moving? In other words, is the operation of any of the vacuum valves dependent on the vehicle speed being non-zero?

Edited: Grrrr! So much for doing anything this weekend! My "honey-do" list is tooooo long! It'll be next weekend before I get to experiment with throttle position vs IAC count while moving.