Car runs fine under load. No hesitation, good power for a B23F, normal fuel economy (20 in town, 25 highway). This is a B23F/M46 with just under 400k miles.

At idle, the engine almost dies and the ECU periodically "blips" the throttle to about 1500 rpm to keep the car running. New Bougicord wires, NGK plugs, Bosch cap and rotor. O2 sensor and AMM only 2 years old. Throttle body has been cleaned. Any ideas?

Thanks for the suggestions, guys.

About 5 years ago I had a surging problem that ended up being the hall sensor in the distributor. But in the process of chasing that down, I replaced the IAC, FPR and coolant temp sensor. The accordion hose was also replaced at some point.

Wondering if this could be caused by uneven compression due to a HG leak? I seem to have to top off the expansion tank every 3-4 days and there is no obvious leak. I've also noticed the temp gauge (there is no compensation board on an '83) sometimes goes up and then comes back down at idle. The motor is original and 34 years/400,000 miles old so I wouldn't be shocked. Then again, it pulls hard enough to "bark" the tires in 2nd which would be rather odd if compression is low.

Hi,

The best answer to this is to do a compression check and read the plugs looking for one of them to being super cleaner than the others. A Coolant using cylinder will get itself steamed cleaned and yet the compression can be fine. If it gets worse it's possible to warp valves, on less hearty engines but I wouldn't keep pushing using Coolant too long. A pint or so in this time frame is talking to you as not normal!

Rough idle could be the water mixture effecting the burn.
Do you see wetness in the tailpipe or moisture boiling out the back even on these hot days of summer?

If you have antifreeze in the Coolant, you should smell that sweet fragrance somewhere outside or in the car.
Check the top side of the transmission for Coolant streaks. The console box has a drip pan and a duck bill drain bib over the transmission.
If the pan overflows the padded areas under the carpets foot wells catch it during turns.
If this was winter time, you would have a foggy or smeared oil look on the inside of the windshield.

Pressurize the Coolant system "cold" with a reversing Mity-Vac pump. Looks for leaks or a slow drop in pressure in a "wait and see" short time period.
That water is going someplace!

Phil

Hi

Idle speed and roughness issues usually relate to an air flow disturbance up setting the mixture balance to all the cylinders.
There can always be one cylinder that is a little bit more sensitive to conditions. Not all the cylinders are going to exactly the same due compression wear or valve deposits either. It's life with its twists.

Since it appears to be sporadic. I would look at things that can be changed from air leakages or signal drop out.
The Throttle Position Switch is the first one to come to mind. It fits in between what the throttle plate does and it's how the Idle Control Valve gets told to respond.
You want the IAC to control all of the idle air input. It has a limited range of flow.
Make sure the throttle plate is absolutely closed in the bore and not being held open by that silly stop screw. It gets adjusted to high in most cases.
It is a stop screw only! I barely give it a quarter turn or less!
Make sure the switch is set properly by using an ohmmeter. Look for a good repetitive continuities. Responses every time the throttle opens and closes.
This can be checked at the ECU on certain pins it's voltages to check out wiring harnesses on these mid eighties cars but you will need more information from the gurus on the board.

The Idle program is different from going OFF idle or onto power on/running. The Electronics have to get in time with transitioning mechanical mixtures. The new or good 02 sensor helps and you changed that.
The FPR works in here too. Tied in with vacuum signals and is independent of the ECU. You say the engine is fine there and the Mileage is good too! So, I would pass on it.
If it starts good from an eight hour stop over it's not leaking down either.

The IAC is working but might be hanging or lagging. The vane could be dirty or it has worn some brush grooves into the armature from many years of use. They get tapered and narrow in width and lose contact from excessive carbon dust. If it's over 100-150 K it's possible depending on how much idling did over its time under the hood. Another of life's variables.

Air leakages have to include the accordion hose for cracks along with vacuum hoses with splits on the very ends. I find them more on the under side where you can not see them! Another one of life's twist. The flame trap hoses, becoming swollen from exposure to oil vapor will loosen, of which, can mess with air input.
Do you have any idea how old the intake manifold gasket is or injector rubber seals?
Both are very reliable but can age.

Darn life and gray hair, if you got it!
(:-)
Phil

Fuel pressure regulator?