adjusting the idle on 308qv

When starting the engine from cold my 308qv idles at around 2000 revs and continues like this when stationary at lights etc for the duration of the drive. However when I turn the engine off after say 20 minutes I can restart it and it will idle at nearer 1000 revs. I'm guessing the choke is not adjusting itself once the engine has warmed up? Anybody got an idea what needs to be done to resolve this?

Cheers

 

Assuming that you have a euro/standard version 308QV (please tell us if you have some other version -- e.g., CH/Swiss), an easy first thing to try is "pinching" hose #35 or hose #42 closed and hose #69 or hose #72 closed -- if the idle is high and the water temp is ~65 deg C or higher. Item #30 (shown two places, but it might be that you only have one or the other) is the "additional air valve", and should be open when cold -- which lets extra air (and fuel) into the engine for cold-running. Blocking those hoses simulates a closed condition so if the warm idle RPM drops when doing that, it indicates that the corresponding AAV is not closing properly. There are some other possibilites, but try that first.
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Thanks very much for the comprehensive answer Will give it a try and report back!

 

Nice info. Thanks!

 

Tommy -- Is your 308QV also a euro version? If so, do you have the AAV under the coolant tank, on top of the engine, or both?

 

Steve, do you know about the 308s so much because you had one OR do you work for Dave up there in Boulder ?

Quick question on the idle, whats the idle supposed to be at normal operating temp (warm) on an 84 US QV? On cold start, mine is about 1800 or so, then after about 5 mins when it gets warm, it drops to around 1300 rpm and stays there at idle. 1300 at idle seems high to me, should it be 1,000 +/-? If so, where is the "normal" idle adjustment screw on that diagram?

James in Denver

 

I had the same problem on my 85 euro. I just turned down the idle screw until it got to 1000 RPMs. Im not sure if this is the correct fix, but it worked.

The idle screw was located on the left side of the throttle body. Bout half way between the front and back. It was a pencil tip like screwing pointing up.

Aaron

 

There are actually two adjustment screws controlling how much air enters the 308i/308QV/328 engine at idle (and they aren't shown on that SPC diagram -- see the jpeg). The strategy is that you really want to achieve two conditions to make sure that you've got the right amount of air flowing in each path at idle:

Condition #1 (normal warm idle with the O2 sensor unplugged) -- When things are right, the throttle plate is a little bit open and the air bypass screw is a little bit open, the engine RPM = 1000 +/- 100, and the mixture is set for something like 1.2~1.5% CO in the exhaust.

Condition #2 -- When the air bypass screw is fully closed, the RPM is something like ~500 RPM (someone please post the correct exact spec), the mixture might be off -- but don't worry about this, this condition is only used to adjust the throttle plate adjustment screw. You then open the air bypass screw to get back to condition #1 -- i.e., 1000 RPM and reset the mixture, if necessary).

Of course, your tach is really just an analog voltmeter so not too sure you should trust it too much, but just fully close the air bypass screw at warm idle and see how low the RPM goes. If it falls down to the ~500 RPM (or whatever the true exact spec is for condition #2), this would indicate that the throttle plate adjustment is OK. Then open the air bypass screw back up to achieve the 1000 RPM. Of course, you should probably check/reset the mixture too since you've altered the airflow -- also, since your RPM seems off now, not sure that you could be sure that the mixture adjustment was OK before either.

Since I've owned a carbed 308 and a CIS-injected TR that's made me quasi-familiar with the CIS-injected 308 (even though I haven't had one)
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I think the main purpose of the throttle plate adjustment screw is to just barely keep the throttle plate open so it does not bind/get stuck in the throttle bore. There is a fair bit of spring power pulling the throttle shut and I think if you did not keep it from closing as far as possible there would be problems. Obviously having the trottle plate just cracked open is going to allow some air into the engine and will affect idle. I would think it should be adjusted so it just holds the throttle plate very slightly open. Of course it you can't get up to 1000 rpm idle on a warmed-up engine with the air bypass screw all the way open it would indicate you need to adjust the idle stop screw to open the throttle plate a wee bit more.

 

Have a look in the coolant overflow/circulation tank. Remove the cap with the engine cold then start it up and let it idle until it reaches temperature (radiator fans cycle on and off). While it's warming up have a look in the overflow/circulation tank, with a flashlight, and see if there is strong movement of the coolant (there should be). The coolant should be shooting in from the small top hose and going out the larger hose that is lower on the tank. If flow is restricted because of a partial blockage elsewhere (as I found on my car) your average temperature in the overflow/circulation tank may be somewhat lower than it should be. Since the auxilliary air valve is heated by the tank (by being screwed to the bottom of the tank) the heat from the tank is what makes the auxilliary air valve close, thus reducing idle. If the tank is not getting hot enough there will not be enough heat to completely close the air valve.

I'm assuming you have an engine thermostat installed and it's the correct temperature rating. Does your coolant temp. guage give normal readings when the engine is warmed-up?

My auxilliary air valve is completely mechanical. Yours might also have electrical heating to supplement valve closure.... not sure.

Do you have a thermometer that will read up to about 250 F? Hold it in the tank when the engine is fully warmed-up and see what reading you get. It should be close to the rest of the coolant temperature in the engine. If not, then once again, you may not be getting enough coolant circulation through the tank which will keep the average temperature too low.

I just happened to check mine yesterday. When the radiator fans turned on for the first time my instrument panel guage read exactly 90 degrees celcius and the thermometer in the coolant tank read 88 degrees celcius.

My auxilliary air valve is removed now but I had a similar problem when it was connected. Fast idle seemed kind of random at times. I found "crud blockage" in the small coolant pipe that sends water through the small hose going to the tank.

 

My 84 QV US has one AAV under the expansion tanks. I didn't know some had two, what is the purpose for two?

 

Hi Mike,

Where did you see two AAV's mentioned?

 

Erich, I haven't actually seen one but what does he mean "or both"?

 

And dont forget to make sure you dont have the cruise control on (carpet pushing on the accelerator).

 

Once the throttle plate is set, it should not be touched. That's why it came from the factory with a protective cap.

To adjust idle speed, use the bypass screw/bolt only- as others have already said.

I'm betting you have an air leak or your AAV is inop.

 

Ok, I went for a drive this afternoon and once again the idle was around 2000rpm. I stopped and with the engine still running gave hose #42 a pinch for a second or two (not sure how long would be required?) Anyway it didn't have any effect on the idle speed. I switched off the engine, restarted a few minutes later and as usual idle was 1000.

Will try again tomorrow but this time try pinching #69 or #72 at the same time as #42 which is what I think you intended?

Thanks.

 

After driving the car today I left it for around 30 minutes before returning and restarting the engine. I checked the overflow and found there was a good strong flow of coolant as you describe meaning that the temperature would be normal I suppose. Incidently once restarted the engine idled at around 1000rpm, it really does seem to be that first start from cold that is the problem, once warm it appears that all future restarts have normal idle (1000rpm).

Thanks for your help.

 

Doesn't have to be done at exactly the same time -- i.e., if you pinch hose #42 shut and the idle doesn't change, that says that the AAV mounted on top of the engine is properly closed. If you follow that by pinching hose 69 or 72 shut, that would check whether the AAV mounted under the coolant tank is closed or not.

Do you have the two AAVs?

 

I missed that in the previous posts. Now I see the second AAV on the parts drawing. I never knew there were any versions with two. Talk about adding more complications!

 

I think I only have the 1 AAV. I've taken a couple of photos... photo one below shows what I think is the AAV mounted below the coolant tank, photo two shows where the second would be to the right of the plenum as shown in the diagram.

I tried pinching #72 yesterday after a 25 minute drive but unfortunately the idle did not drop. I spoke to a neighbour who thinks the AAV could be gummed up but is freed on restart by the sudden bolt of electricity, in his experience with mercs a blast of wd40 into the AAV usually does the trick?

Thanks.
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Yes, you definitely have the AAV under the coolant tank, but the other one (if present) would be further forward on the engine (i.e., forward of the big coolant tubes above the top of your pic). In your prior post, you indicate that you have a hose #42 -- can you show it in the pic and what it connects to?

 

Sometimes the valve inside the AAV can get stuck, and a little WD-40 or some carb cleaner sprayed into it will free it up. To be sure if the AAV is working, remove it from the car and put it in the freezer for about 15 minutes (make sure any cleaning liquid you sprayed in is removed) you should see the little valve most of the way open. Then put it in the oven for another 15 minutes at about 200 degrees and you should see the little valve most of the way closed. If the valve is working and you still have the problem then mabe you have to look at the warm-up-regulator or the cold-start injector. Good luck.

 

I don't think it is going to an AAV (I can't see what it connects to as it goes back underneath the plenum out of site) but below is a pic of what I think is hose #42. Seems strange that it would have 2 AAVs. Will take off the one under the coolant take and try some WD-40.
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Thanks may give that a try at the weekend.

 

ok, i dug this up for a good reason:

my car ( 1983 308 Qv, euro) is doing just the OPPOSITE of what the thread starter experienced.


my QV seems to need the throttle opened while starting to start cold or hot.

car idles fine at first, cold.

when warm the idle creeps to 2000 rpm, and stays there.

car is not hard to start (HOT or COLD), as long as you crack the throttle a bit.

i have checked all the hoses, and will remove the air filter box and check that the throttle plate is clean and opening as it should.

with CIS, i am not sure why i would have to open the throttle to add more air on an injected car.

i did not want to start another thread, given the great info in here. i will also look at my AAV and see whats up.

we did the belt service complete with new water pump this week. car runs nice and cool with no issue other than as stated above.

car is new to me and otherwise unknown as far as this issue goes.

any help as to a direction to go in would be great.


michael