My car idles steady when cold, but after warming up a bit the idle will drop every other second from 900 to ~600. *Sometimes* when the engine is warmed up this doesn't happen. Additionly the volt meeter also drops in unison sometimes (or at least it did today). I haven't measured voltage at the battery while this is happening yet since I only noticed the voltage drop this morning on the way in to work.

It's been doing this since I've owned it for the last 6+ years. If it was a vacuum leak I would expect the RPMs to go up, not down, right? I've done a lot of work to this car, including:

replacing all hoses (vacuum and fuel)
replaced fuel pump,
set fuel pressure,
new battery,
replaced the relay board
new voltage regulator.
new plugs, wires, set timing and replaced the points.
new injector
new injector o-rings.
plus other things I'm not remembering at the moment.

All of the above work was not done to fix the idle issue, but to solve other issues or as routine maintenance. None of them had any effect on the idle. Car runs good and strong, otherwise.

Any ideas of what I should check to remedy this?

I believe its called lean idle hunting. Paying close atention to where the knob on the ecu is, turn it clockwise one click at a time to see if it smooths out. If the ecu knob makes no difference on idle speed then your tps needs to be adjusted or repaired.

Adjusting the ECU doesn't change things. I had wondered about the TPS.
Question:
If while in nuetral I revved the engine and held it to 1500rpms (or somewhere above idle) and it still 'hunted', would the TPS still be a suspect?
I haven't tried this, but will later.

I put a variable resistor on my head temp sensor and dialed up the resistance until the hunting went a way. This has the net effect of making the ECU think the engine is colder than it actually is so it fattens the mix. I'm not going to promise you this will stabilize your idle hunt, but it made mine rock steady. And, I thoroughly went through my FI before I did this. This was a last resort for me.

BTW mine is 73 2.0 which already has a resistor on the head temp sensor, but the logic still applies to any car if it has no resistor. The setting would just be at a lower resistance than mine.

If the ECU knob doesn't change things, id suspect the TPS. Something is not letting you adjust the idle mixture then.

It changes the overall idle speed (if memory serves), but it does not change the idle drop.

You need to make sure your valve adjustment is spot on. Mine was hunting as well. I went through the normal checks (vacuum leaks, TPS setting at idle, fuel pressure) but the it did not go away until my valves were adjusted properly. I found that a few were too loose.

It seems to make sense (and some of the more experienced mechanics could hopefully verify this) that if you change the valve gap you will change the vacuum signal that the MPS sees. I guess as the engine warms up the valve gaps change as well and can impact things more.

Record your valve gaps before and after.

Brian

Good suggestion, but Ive done two valve adjustments without it changing things. I have hydro lifters, btw.

Reviving this thread I started a long while back because I still haven't been able to fix the issue. Would love some additional input.

Scenario is that when at idle, there is a patterned RPM drop of ~300, sometimes more, in a patterned manner. The drop also happens if I rev and hold at higher RPMs, although the drop is much less. Less likely to happen if engine is cold.

When coming to a stop after driving and putting the car in neutral, the drop in RPMs can be enough that it feels like the engine may stall, although that's never happened.

This has been going on as long as I've owned the car (6+ years) and continues after verifying valve lash (hydro lifters) and full tuneup (plugs, wires, timing, dwell, replacing points, etc).

Someone suggested the TPS, but if it happens at higher RPMs, doesn't that make the TPM a less likely candidate?

Have you done anything to the CHTS yet?

Wouldn't that only come in to play when the engine is cold, or if it were increasing in RPMs vs dropping? Drop doesn't happen when engine is cold.

-nevermind, just re-read the writeup on the pbanders site. I'll check it and post results.

I notice others keep mentioning the hunting issue. Is that what you are describing or is it simply an idle drop that does not come back up once it has dropped? If it is "hunting" I would look for a lean condition.

It always comes back up.

This one sounds like its definitely fuel related but if it's been going on for 6 years, then it's time to give to a new set of eyes.

I replaced the fuel pump a while back when it failed and cleaned the gas tank/replaced filters (internal and external), too.
Could a bad injector cause this, or would that also cause bad throttle response? (which I don't detect).

I've seen the surging at rpm twice. Vacuum leak both times. Once was intake, the other was throttle body gasket

if you suspect lean conditions, why don't you temporarily increase the fuel pressure at the fuel reg to see if this hunting goes away?

Do you know the fuel pressure? I see in your signature it's a 75-76 2.0. What color are your injectors. Has the smog pump been removed?

Fuel pressure was verified factory spec when the new fuel pump was installed and it was doing it then.
Green tip injectors. 1 new, (replaced one when a visible inspection showed it to be cracked). The other 3 haven't been tested. Smog stuff was all removed during an engine rebuild by the prior owner. Hydro lifters were used during the rebuild.



It's not surging, but dropping out. Timing and dwell is set correctly. Idle speed would increase if there was a vacuum leak.