Idle hunting when cold

[SIMid]

Hi everyone,

Had a search on the forum but couldn't anything relating to this (or I didn't search hard enough?).

Whent he D is cold and idling, the revs "hunt" a little, bouncing up and down slightly. Once she's warm, she's ok. Is this normal with Volvo engines or something needs to be looked at?

Thanks D-owners! :)

[Farrar]

Legend has it that this is caused by the system waiting for input from the oxygen sensor. It stops hunting the the sensor is warmed up and starts "talking" to Lambda.

Farrar

[Bitsyncmaster]

Often it's just a little sticky idle motor so cleaning it would be the first step. Then if it still does the hunt you can adjust the curb idle screw a little so the idle motor will be running more open or more closed. The curb idle is the screw below the idle switch screw.

Of course you need to be sure you have no vacuum leaks and your distributor advance is shut off at idle.

[DMC3165]

I had this problem for over a year after I put 3165 back on the road. About a month ago I had a "high" idle issue. So I decided to get a can of throttle body cleaner and squirt a little in there. Honestly it cleared both issues right up. I'd say give that a shot and see what happens. It's a quick cheap option to a nuisance problem.

[SIMid]

Thanks for the replies everyone.

Couple of options to dig into, will see how I go.

Thanks! :)

[jawn101]

Did you ever get a resolution on this?

[Mark D]

I had a similar issue with idle hunting and after replacing my fuel lines the problem disappeared. One of the hoses to the cpr was apparently bleeding off pressure due to a leak and was the cause of my problem.

[jawn101]

Fuel lines are all brand new stainless ones. Car runs and idles great once warmed up, but has the hunt when cold. Only lasts about 60 seconds, less if it's warm outside. Idle screws are all closed all the way, I think. Will triple check this evening.

[SIMid]

Did you ever get a resolution on this?

I haven't had the opportunity to have a look (with my mechanic) regarding this. Being a wamer climate now (summer), she seems okay. Also going through a seperation so kinda tied up moving my shit out of the house. :(

[Bitsyncmaster]

What I've found with my work on my idle ECU is: When the engine is cold, the idle motor is running at a wider opening. It seems at the wider opening, changes in the movement of the opening has more affect on engine RPM. So the ECU would need to use less movement in adjusting the RPM.

Not much you can do about it with the stock ECU. My only suggestion is to back out your curb idle some more so the idle motor is not running so wide open.

[Jonathan]

I had this problem for over a year after I put 3165 back on the road. About a month ago I had a "high" idle issue. So I decided to get a can of throttle body cleaner and squirt a little in there. Honestly it cleared both issues right up. I'd say give that a shot and see what happens. It's a quick cheap option to a nuisance problem.

Mr Chris... hmmm... Me likey :). I'm all for easy and cheap fixes and a can of throttle body cleaner is right up my alley. My idle is generally pretty good but every so often I get the feeling something is sticking. How, where and how much did you spray? Is it ok to just give the whole throttle spool a good little soak down? Thx!!

[DMCTek]

I would not consider a idle hunt cold a cause for concern. It should stop after the first minute or two.
If this hunt bothers you it can usally be eliminated with a C.O. adjustment.

[DMC_PRIME]

I would not consider a idle hunt cold a cause for concern. It should stop after the first minute or two.
If this hunt bothers you it can usally be eliminated with a C.O. adjustment.

Completely agree. My car has been doing that ever since I have owned it 10 years ago. PJ Grady had it right before I bought it and said perfectly fine, which has been.

So I wouldn't touch it.....

[jawn101]

That's what I thought too. It only does it for about a minute until warmed up, then never does it again. Idle is perfect otherwise. Thanks for the tips all :)

[Elvis]

This has been up many times. I really wonder why nobody mentioned it ?

Idle hunting when cold means the engine reacts slower to changes of the
ISM. Slow reactions or dead-times lead to swinging control loops of the idle ECU.
Most likely the mixture is a bit off (too rich) with the WUR still heating up.

[Spittybug]

As I play with the timing advance and the fueling level at idle (I'm EFI, not stock), I have found there is much more tendency to hunt when the engine is cold than when warm.

Using our tuning tools we can watch the "bouncy blue ball" jump from cell to cell on the ignition advance table or the fueling table and see it oscillate. Each cell is the intersection of an RPM and measure of the engine vacuum (load). Even a very small amount of fuel difference can change the oscillation when cold. When warm, the minimal difference in fuel doesn't make the engine oscillate much if any at all. Solution appears to be hop in and drive it while it warms up!

[jawn101]

As I play with the timing advance and the fueling level at idle (I'm EFI, not stock), I have found there is much more tendency to hunt when the engine is cold than when warm.

Using our tuning tools we can watch the "bouncy blue ball" jump from cell to cell on the ignition advance table or the fueling table and see it oscillate. Each cell is the intersection of an RPM and measure of the engine vacuum (load). Even a very small amount of fuel difference can change the oscillation when cold. When warm, the minimal difference in fuel doesn't make the engine oscillate much if any at all. Solution appears to be hop in and drive it while it warms up!

Well, right in the owners manual (page 15) it does say this:

Warming Up
When the engine is cold, drive the car as
soon as the engine has started, Do not
warm up the car by allowing it to idle for
long periods with the car stationery.

Guess I could pay attention to a manual for once... :)

[Bitsyncmaster]

I agree that a little idle hunt when your engine is cold is no big deal. You are the only one that will see it. When the engine is warmed up, it's nice not to have ANY hunting but that's only to show off how smooth your engine runs.

Looking at manifold vacuum shows RPM variations more so than the dash tachometer. The oil pressure is also sensitive to idle RPM.

[jawn101]

I agree that a little idle hunt when your engine is cold is no big deal. You are the only one that will see it. When the engine is warmed up, it's nice not to have ANY hunting but that's only to show off how smooth your engine runs.

Looking at manifold vacuum shows RPM variations more so than the dash tachometer. The oil pressure is also sensitive to idle RPM.

Agreed 100%. It's just the character of the car to do that at warmup, but I was curious if it could/should be eliminated. I know I have no vacuum leaks and have no hunting whatsoever after warm-up, the idle is smooth as can be. But reading this thread has actually taught me an amazing amount about how this motor works.

[DMCTek]

Agreed 100%. It's just the character of the car to do that at warmup, but I was curious if it could/should be eliminated. I know I have no vacuum leaks and have no hunting whatsoever after warm-up, the idle is smooth as can be. But reading this thread has actually taught me an amazing amount about how this motor works.

Again not a cause for concern but can usually be eliminated with a proper C.O. adjust. A rich mixture is the usual cause (counter clockwise turn of adjuster).

[jawn101]

Again not a cause for concern but can usually be eliminated with a proper C.O. adjust. A rich mixture is the usual cause (counter clockwise turn of adjuster).

As in a counter clockwise turn will lean the mixture, or a counter clockwise turn is the richening culprit, and I should turn clockwise? :)

[DMCTek]

As in a counter clockwise turn will lean the mixture, or a counter clockwise turn is the richening culprit, and I should turn clockwise? :)

Counter clockwise turn should eliminate your hunt. Adjust in small increments. You can try tweaking and see what happens but mark where you began. This should really be done with a dwell meter reading the frequency valve signal.

[jawn101]

Counter clockwise turn should eliminate your hunt. Adjust in small increments. You can try tweaking and see what happens but mark where you began. This should really be done with a dwell meter reading the frequency valve signal.

Absolutely, just trying to find a proper one... thanks!

[Elvis]

Don't mess with the CO screw if you can't measure it !!!!!

1/16th of a turn already changes a lot !

Warm up is no regular running condition !
The WUR can be out of adjustment but still when warm the
car can be set correctly.
Before you mess with it be sure your CO-Sensor is OK and you can
measure the PWM of the frequency valve somehow.
I get and use a O-scope before even touching that screw - and then I rely
on an old used uncalibrated CO-sensor. can't really be compared to a calibrated
gas analyzer.

[David T]

Elvis is correct, you should not be messing with the mixture screw. The only time to adjust the mixture screw is when you KNOW there are no vacuum leaks, the parts are all working correctly (spark plugs, ignition wires, injectors, etc) and you have a dwell meter or combustion gas analyzer. NEVER try to correct an idle quality issue by twiddling the mixture screw. 99 out 0f 100 times you have a problem and adjusting the mixture screw WILL NOT fix it.
David Teitelbaum