idle hunt for 2mins only when cold 280se 71 3.5 v8??

[body_infinity]

Hi guys hope anyone out there can help as 4 mechanics (experts? here in oz could not work it out??

ANyway my girl starts well in the morning however there is a idle "hunts" for the first 2 mins or so then runs as normal. (ie) rev rises and then drops but doesnt cut out, does this for around 2mins while it warms up ..I guess while untill the aux air unit is closed....I removed the air temp sensor in the air filter and the idle comes down to normal and no hunt but the car doesnt run as well with it disconnected ;-( ( a solution not by a mechanic who charged me for this advice :-(

anyway I have checked the hose from the aux air to the throttle body and found a crack at the throttle body end which I have sealed up but with no luck....can anyone suggest other things to check???

--> Also one more thing while I have you ;-) my fuel flap door vacuum lock is working in reverse I think it locks while the car is running and unlocks when I turn it off...(ie) when the door locks are open .

If I close the door locks it unclocks? is this due to wrong connection of the master door lock valve (three valve VDO unit with three vacuum hoses yellow and two black?) note the boot/trunck lock works fine any tips or if anyone can tell me what sequence the lines need to be connected would be great ;-) THANKS AGAIN

[Tomguy]

Good news: Your throttle position switch is set properly!

Your (cold) idle is too high (possibly warm too). This can be caused by a few things. Mainly a disconnected vac line to the distributor vac retard (it will advance your timing causing high idle) or your idle is just set improperly. It sounds like your idle mix is too rich, and unplugging the temp sensor causes it to be even richer to the point of slowing and choking on too much fuel. I'd check for vac leaks, and verify the vac retard line is connected, then turn the idle air speed screw clockwise about half a turn (if not more) and see if that helps. The air speed screw is the large screw by the aux air valve.

[Jengasan]

Hi

I had this since the day and hour I bought my '71 280 SE 3.5 (coincidence ??)...serves me right for buying it 'warm' . Anyway let me guess, you fire it up and it revs to between 1500 and 1800 rpm then goes back to about 800 or 900 then up again then down again till eventually its levels out at what seems like a normal(ish) warm idle speed. It will only do this when started from quite cold. This drove me insane !

I replaced every rubber hose, checked the brake booster line, the vacuum feed to the tranny modulator, the feed to the MAP sensor, the dizzy advance and of course the feed to the cabin vacuum system....all ok and still the hunting!!! I tried turning the idle down with the adjusting screw but...and I suspect this might happen to you...when the car warms up it stalls cause the warm idle is now way to low, but give it a go.

I also adjusted everything I could think of but to be honest I considered this pretty useless as the idle is so unstable. The good thing is (as Tomguy points out) that your throttle switch is indeed OK, try this, take off the air cleaner and fire it up when cold in the morning then pull the plug on the throttle switch, I bet it stops hunting and simply revs to about 1800-2000 (too much higher and kill the ignition!) and stays there, then gradually reduces to 'normal', in other words the fuel cut off has been working normally so at least that function of the throttle switch is fine (and probably the rest too). Shut off the engine and connect it back up, fire it up and the hunting will begin again......now, that big hole above the butterfly in the throttle body (the butterfly should be totally shut BTW) is the bypass air feed, stick your finger over about half of it and see if the idle calms down, if so you are getting way too much air. The fuel injection system simply tries to compensate for this by feeding the engine more fuel...which causes the high idle till it hits the fuel cut off and...well you know the rest.

Ok, I'm probably boring you by now so.....I'm willing to bet its your AAV. I actually had mine off and in a pot of hot water to check it was working at one point but there's a difference between 'working' and 'in spec'. First try taking the hose off the RIGHT side of the AAV, i.e. the one to the idle air distributor and squirt some WD40 (or ozzy equivalent penetrating oil/spray) in there...you're exhaust might smoke a wee bit but hey that's the least of your problems and it will go away soon enough. With a bit of luck....and if the piston is just 'stuck' that might shift it....a couple of GENTLE taps on the side of the AAV housing might just encourage it too.

If this does not work...have the thing off. Plug the hole or you'll have coolant everywhere. Next you'll want a pan of water on the stove and, if your retentive about these things, a thermometer. The AAV should be fully open below -20 (yeah like you can measure that Mercerdes ???) and fully closed at +65 degrees C. Don't let the brass bulb hit the bottom of the pan or you'll be getting a false reading. If the piston moves at all..........then all is not lost, so if it doesn't try squirting more lube in and poking it with a lolly stick or similar wooden implement that fits the hole...don't be too rough.

If it frees up stick it back on and try it....hopefully it will start working better but you will still have to finally adjust the idle with the air screw (I set mine warm to 800 and it was fine then from cold).

Now for the punch line.......this is not how I fixed mine, but I thoroughly recommend you try all of the above and Tomguy's (who is a lot more learned than myself) recommendations before trying the 'Jengasan Fix'...which, rest assured, I will divulge along with an explanation if said other stuff doesn't work......

Let me know....oh and I do feel for you...it took me nearly a month to fix mine

[kpb]

awesomely instructive post, jengasan.

I just printed it out for guidance since my 4.5 has done this for the three years I have owned it. never bugged me much becuz it is a three season car and about the time it starts doing it in Fall, I take it as a sign to garage it for the winter. Moreover, mine has been sitting dormant since May when I hit a deer. Tried to start yesterday, has spark, has fuel, will not catch but will run momentarily on starting fluid?? Topic for another thread, though...

Exactly what does the AAV (auxiliary air valve?) look like?

And, I cannot stand it -- tell us, tell us, how did you fix it if not by this detailed method????

[Tomguy]

kpb: Simple shortcut method:
1) Turn the air speed screw by the aux air valve (HUGE slotted screw) clockwise 1/2 to 1 full turn. Idle will slow drastically
2) If 1 has no effect, then lift the rubber boot off the ECU (the end of it pointing towards the radiator) and turn that screw to the LEFT about 3 clicks to lean the idle mixture

[230/8]

One item the others did not address...your fuel door sounds like it is operating (locking/unlocking) properly. It should lock when you lock the car and unlock when the doors are unlocked. That way you can fill it at the gas station but the fill door will be locked when you lock the car and left it unattended.

Finally, when you get your AAV sorted, try using a good FI cleaner every couple of tanks...something like a double dose of Techron every now and then. This may keep the AAV free because the cold start injector sprays a hefty shot of gas through it on cold starts.

230/8

[kpb]

Originally Posted by Tomguy

kpb: Simple shortcut method:
1) Turn the air speed screw by the aux air valve (HUGE slotted screw) clockwise 1/2 to 1 full turn. Idle will slow drastically
2) If 1 has no effect, then lift the rubber boot off the ECU (the end of it pointing towards the radiator) and turn that screw to the LEFT about 3 clicks to lean the idle mixture

Thanx Tom. IIRC, I fiddled with the idle air screw and the ecu mix but saw no change in my cold weather up and down idle.

[Tomguy]

kpb: Your throttle plate is probably not aligned properly, then. No changing of the air speed screw will affect idle if the throttle plate is not COMPLETELY closed at idle.

[kpb]

thanx

sorry for the hijack

[Jengasan]

Originally Posted by 230/8

One item the others did not address...your fuel door sounds like it is operating (locking/unlocking) properly. It should lock when you lock the car and unlock when the doors are unlocked. That way you can fill it at the gas station but the fill door will be locked when you lock the car and left it unattended.

Finally, when you get your AAV sorted, try using a good FI cleaner every couple of tanks...something like a double dose of Techron every now and then. This may keep the AAV free because the cold start injector sprays a hefty shot of gas through it on cold starts.

230/8

To anybody that isn't 100% sure, the AVV in the M116 amd M117 V8 engines sits front centre just behind the distributer. If you remove the air cleaner you can trace the idle air line from the top of the throttle body, this will be 'Y' shaped - one branch goes directly into the Idle (or 'bypass') air distributer and the other goes up slightly to the AAV, which is that alloy tower type structure. The AAV its self will have the connection from the 'Y' on the left side (standing in front of the car looking at the engine) and a hose with a 90 degree downward bend on the right side. It is bolted onto part of the intake manifold system and has a brass bulb that sits in the coolant, it requires a gasket if you remove it.

Just one observation 230/8.......the cold start injector is downstream of the AAV and is located on the idle/bypass air distributer so FI cleaner won't have any effect on it, it will clean the rest of the injectors and the CSV nicely though. Only air passes through the AAV its self.

From the reaction to these posts it would seem this is a far more common problem than I thought - good luck to all

[GaryF]

Originally Posted by Jengasan

Now for the punch line.......this is not how I fixed mine, but I thoroughly recommend you try all of the above and Tomguy's (who is a lot more learned than myself) recommendations before trying the 'Jengasan Fix'...which, rest assured, I will divulge along with an explanation if said other stuff doesn't work......

If it's anything like the fix I applied it involves jamming a cork in the air supply to the AAV and setting the idle very slightly higher to compensate.
One thing that did occur to me during the diagnosis is an air lock could form around the AAV bulb as its a high point in the water flow and could cause it not to respond properly. Just a theory before I went to plan B.

[body_infinity]

Ok guys thanks for the tips....one thing I did not mention was that the cold start valve has been disconnected (dont get cold here in OZ ) and have put a perm fuel pressure gauge (best thing for the D-jet) anyway tried/tested the aux A valve all is good...what I have done is also test the Air temp sensor and is around 265 ohm when engine is warm (I think should be 100-200 ohm??) is it out of spec? anyway if the air temp sensor is disconnected no idle hunt and warms up ok just notice the idle is lower..but car never stalls or anything and no drive probs etc...i was waiting to replace this sensor but wanted to test from the ecu first to check the wires(terminal 1?? to ground??) any other tips??

finally re the fuel door
.....kinda bug'ng me it seems to work in reverse opens when doors are closed and closes when door is open then simply stays open once vacuum is lost (5mins or so?) I did notice a valve at the element (fuel door/flap which sits under a black VDO branded cover should this be blocked off??

PS love the punchline re the "Jengasan Fix" any chance of getting this and putting me (and others here in OZ) out of missery every morning ..hehehe

cheers B_I

[GaryF]

So your saying when you lock the doors the filler cap is released and when you unlock the doors the filler cap is locked ?
Vaccum should last for more than 5 mins so you have a leak somewhere. The filler lock is a single vacumm element so will only hold the flap locked while a vacumm is present and, at least on my car, it just T's off one of the lines to the trunk lock. It sounds like its just hooked up to the wrong line.

[body_infinity]

Originally Posted by GaryF

So your saying when you lock the doors the filler cap is released and when you unlock the doors the filler cap is locked ?
Vaccum should last for more than 5 mins so you have a leak somewhere. The filler lock is a single vacumm element so will only hold the flap locked while a vacumm is present and, at least on my car, it just T's off one of the lines to the trunk lock. It sounds like its just hooked up to the wrong line.

Hi thanks mate, a pic may help explain better, any tips would be great will check the connections again but the trunk lock works like it should only the fuel flap lock is working like crazy? re vacuum i was told the flap should still be locked even if vacuum is lost from the door locks as it traps the vacuum via the master door lock valve (see pics hope these are ok for the site as per the photo rules :-))?



master door valve:

[GaryF]

My understanding is that nothing connects to the right hand side, this is at atmospheric pressure and vacumm to the left hand side simply works against the spring to lock the cap. A continous vacuum must therefore be applied to keep the cap locked, this should come from the reservoir when the engine is off and providing there are no leaks hold it for a few weeks.