Engine idles rougher when engine temp is hotter...

[rothsean]

Engine idles very smooth when engine temps are 140 or below...

Once temps reach 170 or so the idle runs rougher.

If I let temps reach 200 the idle is the roughest and seems to miss a cylinder (or at least act like it) every 10 seconds or so.


When it cools down again, like on the highway, it runs smooth again at idle (when exiting highway and stopped).


Any ideas?

[Bitsyncmaster]

Is it just idle speed the problem shows up?

[rothsean]

Is it just idle speed the problem shows up?



Yes, it only has the problem at idle.

[David T]

Sounds like a sticky valve, intake or exhaust. Try an engine flush and follow it up with an oil change.
David Teitelbaum

[Bitsyncmaster]

Yes, it only has the problem at idle.

So is it just a "hunting" idle or are you hearing a miss in the ignition?

[rothsean]

So is it just a "hunting" idle or are you hearing a miss in the ignition?


It idles at 900 rpm and will do a quick drop to 700 RPM (-ish). Then right back up to 900 RPM. Stays low for a half second. Then comes back up to normal idle, then drops again in 7 seconds or so.

I change my oil every 6 months. I have run a can of seafoam thru the car.

What other cleaners should I use?

[Bitsyncmaster]

It idles at 900 rpm and will do a quick drop to 700 RPM (-ish). Then right back up to 900 RPM. Stays low for a half second. Then comes back up to normal idle, then drops again in 7 seconds or so.

I change my oil every 6 months. I have run a can of seafoam thru the car.

What other cleaners should I use?

A common problem that sometimes will be fixed with some adjustments of the idle screws. You should take the idle motor out and clean it up with solvent first. Then I oil the valve with 3 in 1 oil.

[rothsean]

A common problem that sometimes will be fixed with some adjustments of the idle screws. You should take the idle motor out and clean it up with solvent first. Then I oil the valve with 3 in 1 oil.

Why would the intesity of the RPM drop be directly related to engine temp? It barely does it when cooler, even after driving a long time on a highway it is fine, but gets worse when in city driving.


What is 3 in 1 oil? What kind of solvent should I use to clean the idle motor? Throttle body cleaner?

[Bitsyncmaster]

Why would the intesity of the RPM drop be directly related to engine temp? It barely does it when cooler, even after driving a long time on a highway it is fine, but gets worse when in city driving.


What is 3 in 1 oil? What kind of solvent should I use to clean the idle motor? Throttle body cleaner?

3 in 1 oil is a very light machine oil sold under that name.
Gasoline, paint thinner, etc. are all solvents.

When the engine heats up the idle fuel/air needs much less to maintain the idle RPM. So that holds the idle motor in a less open location. Your idle motor may be sticky at that location and not at other locations.

[rothsean]

3 in 1 oil is a very light machine oil sold under that name.
Gasoline, paint thinner, etc. are all solvents.

When the engine heats up the idle fuel/air needs much less to maintain the idle RPM. So that holds the idle motor in a less open location. Your idle motor may be sticky at that location and not at other locations.

Ok, that makes sense. Can you send me a photo of it so I can see what the idle motor looks like?

I am assuming it is the one jsut in front of the cold start valve controller??

[David T]

First you posted it idled "rough" and now you say it hunts. If it is a hunting problem one or more cylinders are not balanced, ie, making as much power as others so when the bad ones have to fire they do not produce as much power so the motor slows down till it hits on other more powerful cylinders and then the motor speeds up. The idle control system can't adjust fast enough to make the motor run smooth. Anything that can unbalance a cylinder can cause this including:
Leaky valves/misadjusted valves/sticky valves
vacuum leaks
bad, dirty, misgapped spark plugs
bad, dirty injectors
Bad ignition wires
leaky head gasket
Among the more common causes are vacuum leaks and dirty injectors.
David Teitelbaum

[rothsean]

First you posted it idled "rough" and now you say it hunts. If it is a hunting problem one or more cylinders are not balanced, ie, making as much power as others so when the bad ones have to fire they do not produce as much power so the motor slows down till it hits on other more powerful cylinders and then the motor speeds up. The idle control system can't adjust fast enough to make the motor run smooth. Anything that can unbalance a cylinder can cause this including:
Leaky valves/misadjusted valves/sticky valves
vacuum leaks
bad, dirty, misgapped spark plugs
bad, dirty injectors
Bad ignition wires
leaky head gasket
Among the more common causes are vacuum leaks and dirty injectors.
David Teitelbaum



I don't really consider it hunting much because when the RPM does drop for a bried second it shakes the car slightly. If I did not know better I would say it has a bad spark plug, but I have verified that they are all working. It only does this when warm.

I have new spark plugs, new wires, rotor and cap, new injectors.


The valves may be the issue, but wouldnt it behave that way even when cold if it was the valves?

Leaky head gasket, maybe. I did put new intake manifold o-rings in when I had the manifold off to change injectors and plugs.



My biggest questions is why does the symptom only happen when the engine warms up? Also, it is really dramatic if I have the A/C selected ON and working.


I also noticed today that if I shift to neutral and I am still moving, like rolling to stop light, the engine idles smoothly at 900RPM. Once the car stops is when the rough stuff starts.

[David T]

Idle quality is most noticeable at the lower RPM's and when warmed up. At higher speeds it evens out and when cold it is running a little richer, both tend to obscure vacuum leaks or any other cause that will unbalance cylinders. Adding a load (A/C) just makes it more noticeable. Anything that makes one cylinder work a little better or worse will make the motor tend to hunt. The more even every cylinder works compared to the others, the smoother the motor will idle.
David Teitelbaum

[Elvis]

Do you have an O-scope or something to measure the PWM of the pressure control valve ?
(dwell something meter ? )

I bet the mixture is off and the ECU tries to set it but can't as it is out off reach.
The PWM then jumps from max to min like some kind of reset.

I know I'm the only one who has ever observed this and the only one suggesting to
measure it but hey, that's me.

A vacuum leak, misadjusted/dirty CPR or faulty injector could lead to this.

Why do I think it could be this ?
Because when the engine warms up the lambda unit starts to work and tries to control
the air mixture. And when it does this jump from max to min (or the other way around ? )
this means a sudden change in mixture that will make the idle drop and shake the car a bit.

When OK the PWM should vary between 40...60%.

Elvis & 6548

[Bitsyncmaster]

As Elvis pointed out the mixture changing will change the idle RPM. That along with many other things not perfectly working will often lead to that hunting problem. There are some "dead" positions with our stock idle motors and if your adjusted in those areas it will cause the hunting. Most often just adjusting the curb idle screw will move the motor off those dead areas and help reduce the hunting.

[rothsean]

As Elvis pointed out the mixture changing will change the idle RPM. That along with many other things not perfectly working will often lead to that hunting problem. There are some "dead" positions with our stock idle motors and if your adjusted in those areas it will cause the hunting. Most often just adjusting the curb idle screw will move the motor off those dead areas and help reduce the hunting.

My idles screw is set at the lowest idle position, it won't go any lower because it rests on the metal stop.

[rothsean]

A common problem that sometimes will be fixed with some adjustments of the idle screws. You should take the idle motor out and clean it up with solvent first. Then I oil the valve with 3 in 1 oil.

When you say "idle motor" is that the name of it in the workshop manual or is that the same thing as the idle regulator?

[Bitsyncmaster]

My idles screw is set at the lowest idle position, it won't go any lower because it rests on the metal stop.

The lower (curb) idle screw is normally set about a half turn in after it just contacts the plate. So you back it out until you see a gap (off the stop), then turn it in until it just contacts, than a 1/2 turn more. You have to be sure your idle switch screw is not stopping the curb idle screw function.

[Dangermouse]

When you say "idle motor" is that the name of it in the workshop manual or is that the same thing as the idle regulator?

Idle motor = idle speed motor = idle regulator.

Though it isn't really a motor. More of a quarter turn valve.

It is located on the drivers side of the engine between the oil filler and the starting resistor on the bulkhead. It has 2 vacuum hoses and an electrical connection. take it off and clean it out with carb cleaner.

[DMC5180]

When the engine is up to Normal operating temp idling. Confirm that the ISM micro-switch is tripped. Depress the switch manually with a screw driver and hold it. Does the RPM still jump between 700 and 900 as you've described or does it stabilize just below 800 (775+/--25). Just trying to eliminate the possibility of worn linkage before moving into other possibilities.