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D-jetronic: How do I adjust cold engine fast idle?

15K views 41 replies 9 participants last post by  berntd 
#1 ·
Hello

The idle on my 450SL while the engine is cold is way too high. This makes it difficult to control the car as it wants to zoot off.

How do I adjust this?
Not a single D-jetronic manual manual mentions this.

The idle after warm up is fine.

Regards
Bernt
 
#3 ·
How do I adjust this?
Not a single D-jetronic manual manual mentions this.
The shop manual has a whole section on subject (07.4-110) including a picture showing idle screw location.
The link that Ianwright posted has a link to shop manual as well as a picture a little way down showing the idle screw location.

If you go for a drive and get engine warmed up but not overly hot, you adjust idle rpm to spec. Should be in 750-800rpm range. Then next time you do cold start, check rpm. It should start up quite high - maybe 1100-1400rpm but then start to drop back as engine warms up.

If idle is very high after doing this and engine surges, then your AAV could be faulty. Sticky AAV is main cause of high idle problems. Do a search including the Comprehensive Guide link - it has section on AAV.
 
#4 · (Edited)
Hello everyone.

I must be missing something.

The idle problem I have is only when the engine is COLD not warm /hot.
Doing that procedure will not make any differentce as the idle is correct at 800rpm when the engine is warmed up.

Are you saying that the AAV opens too much to result in such a high cold idle?
How can that be? How to I adjust it?

The fast-idle (cold) is dangerously high at around 1800rpm.


Regards
Bernt
 
#6 · (Edited)
Yes, I have gone through all of it.

The first item is:

"The AAV (Auxiliary air valve) Diagnosing/surging idle, high idle, poor performance when hot."

I do not have a surging idle, poor performance, high idle when the engine is hot.

I have high idle when the engine is cold - which is not mentioned anywhere in that whole thread.

All of the things mentioned there have to do with the valve either not openeing when cold (result = no increase in idling when cold)
or being stuck open when hot (result = high idle and performance issues when hot)

Does the fact that my idle normalises from cold to hot, not indicate that this valve is closing correctly?



Regards
Bernt
 
#7 ·
Does the fact that my idle normalises from cold to hot, not indicate that this valve is closing correctly?
No, not really. If your rpms get to 1800 you should get surging idle because the djet system is designed to do that. Happens at about 1500rpm.

The link suggests screwing the idle screw all way in or squeezing the AAV outlet hose. Did you try those? What was the effect?

Have you removed AAV and tested it? It should open/close smoothly and be fully closed at 150F.

If not AAV, then you must have air leakage elsewhere.
 
#8 ·
Thanks for your kind help.

I have to admit that I am not sure sure what surging means. Dictionary suggests gushing ?

I have smooth non hunting / sawing idle both cold and warm. It runs like a seweing machine.

With the idle being correct at warm engine, are we suspecting that someone has 'corrected' the idle adjustment although there might be an air leak or faulty AAV ?

Like in 2 wrongs make a partial right?

I will do the hose and AAV test asap and report back.

Regards
Bernt
 
#14 ·
But just to confirm: There is no adjustment for the fast idle speed?

Regards
Bernt
I thought we covered that before? You can adjust idle speed with the idle speed screw. Screw it in and the idle speed may not be as fast. In fact engine should stall if it is warmed up. Screw it out and idle speed should go up. If idle screw does not have much of an effect, then you are leaking air in. Then you "adjust" high cold idle speed by finding where air is leaking into intake manifold:
- through AAV that is not fully closed or
- through several hoses that could be disconnected or
- Connections between upper/lower 1/2 of intake manifold (harder to detect) and
- make sure throttle plate is fully closed at idle. If someone messed with linkages to throttle plate thinking that was how idle was adjusted, that could leave throttle plate slightly open. That could also mean that the TPS is not signalling to the ECU that car is at idle, which would affect idle mixture. (As Trog suggested)

Read everything in the section in EGV107 on Djets. It has a section on rebuilding a TPS. It explains how to adjust TPS. May not be your problem. It also has an original Bosch trouble shooting guide for EFI that includes explanations of how some things work.

Get your car running "like a sewing machine" again at 800rpm.
Disconnect the linkage to the throttle plate and see if rpms change. They should not if throttle plate was closed.
With engine idling, screw the idle screw all the way - engine should stall. If it does not, air is leaking in.
Try squeezing the inlet or outlet hose from the AAV so as to prevent any air flow through AAV. If rpms drop, then AAV is not closed fully.
Check for disconnected vacuum hoses at back of engine (to locking system and transmission) as well as one that starts at throttle body and goes to distributor via a switchover valve. Also check the hose that goes from back of engine to MPS.

Read everything you can on Djets! Lots of stuff on net. Acronyms are described somewhere in EGV107, but here are some:

AAV - Auxiliary Air Valve
TPS - Throttle position switch
MPS - Manifold pressure sensor
ECU - Electronic Control Unit? (The computer!)

Surging also known as sawing occurs when rpms go above about 1500rpm at idle. ECU then cuts off fuel through injectors until rpms are below about 1000rpm, then switches fuel back on and keeps repeating this, but only at idle. If you don't see this, as Trog suggested, linkages and TPS may have been messed with and TPS is not signalling to ECU that engine is at idle.

Get back to us once you have done some testing along the above lines.
 
#13 ·
@berntd
Nowhere in this thread do I see where you state the RPM that you are getting when cold. If it's around 1500, then all is fine and normal. What are your cold RPM? I think I recall getting my cold idle to drop by hitting the gas pedal in park after it has warmed up a little.
 
#16 · (Edited)
Morining all

An thanks for all that info. It is greatly appreciated and I will do all of the adjustments in time.

This D-jetronic system does not seem to be very complicated but of course, some tricky things can be wrong and fiddled with.

I just need to clarify something:

"Fast-idle", as I as asked about, is in the industry known (at leat where I learned) as the increased idle speed when an engine is cold.
In case of the D-jetronic, it seems to be controlled by that AAV.

My question was purely whether the "fast-idle" ie the increased idle speed is adjustable.
Since nobody has mentioned any adjustment for it, I have to conclude that it is not adjustable but purely based on the size and opening of that AAV.

Last night, I was quickly going to just pinch the hose on the AAV to see if it at least does something, only to find that all those hoses are rock solid and cannot be pinched. :-(


Best regards
Bernt

BTW: I have actually read all that stuff on this EFI. I have also printed off the schematics and the Bosch explanations etc.
It was just that not a single one of all those guides mentioned that the idle speed could be too high when the engine is cold and how to adjust / remedy just that problem.
 
#22 · (Edited)
With AC on, the vacuum retard on a 1973 450sl is turned off. So the idle may actually increase a bit. If your AC is on, it may be possible that the cold fast idle is even higher. So you might say that turning on or off the AC is one way to adjust the idle.



Also, the AAV bulb is crimped at the factory to make it open and close at the right temperatures. I guess it could be possible to crimp the bulb more, or if severely crimped, maybe opened up a little by trying to make it less flat. I'm not sure that would work. But that's the only way I could imagine altering the idle speed via the AAV.



Maybe there are other things you can do like block off some of the passageway in which the AAV gets its air.



Another thing that could allow the idle it increase is excess air from vacuum leaks.



And also, the emissions plate on my 1973 said 750rpm idle I believe, and that's with AC off. If the timing is more advanced than spec, you will likely get a higher idle.

Edit: oops. I don't know if the standard model (not US) even has the vacuum retard.
 
#24 · (Edited)
Berndt,
Today, I let my 350SL warm up at idle while in Park. It is cool here, about 18C. Started up at 1200rpm, but gradually increased to about 1750rpm. At that point, the "sawing/surging" started. Every few seconds rpms went between 950 and 1750. I have AFR gauge and it showed mixture as very rich (AFR-10)

I have idle set so it is about 950 when in warmed up and in Park/Neutral (which is where it ended up once I had gone for a 20 min drive). I set it this way so that when I put car in D or R, rpms don't drop too low and cause hot start problems in summer.

Back to your problem. Firstly, you should get sawing if TPS is properly adjusted. Could be it would happen if rpms got just slightly higher. You could check this by just screwing out the idle screw a bit.

What happens if you put car in Drive? (assuming your car is an automatic). On my car, with cold idle at 1750rpm, in drive it dropped to 1000rpm cold and about 750 after good warm up. I seldom let car sit idling in P when cold, so I don't notice the surging.

I am happy with my car with these rpms - not much different than yours?

Just some thoughts on Djet warmup:
Bosch designed system with two coolant temperature sensors close to each other . One (T2) is an electronic sensor that provides input to ECU, and one a mechanical device (AAV) that needs to work in unison with ECU but sometimes seems to act independently ??
Bosch must have designed the AAV to close at some rate that would be dependent on coolant temperature effect on bulb fluid plus the friction and spring inside the valve. The mixture control in ECU would have had to have those characteristics built in.
That is fine, but when our AAVs get old and, like some of us, no longer function like they did when young, we have these warmup problems. A good cleaning sometimes helps.
Not such a big deal, but too bad new AAVs cost so much that we don't replace them. They are no more than a thermostat.
 
#25 ·
Hi Graham

I have nor done anything to it yet but your post is very interesting and helpful.

I had a look on ebay, just to see what a replacement valve would cost and I was suprised that they go for so much money, used!!!

How long does your car fast-idle?
Or how long till it gets to operating temperature and idles normally?

Regards
Bernt
 
#26 ·
Hi Graham

How long does your car fast-idle?
Or how long till it gets to operating temperature and idles normally?

Regards
Bernt
Bernt,

I don't really know, because I just don't just leave it in park and wait for it to warm up. I put in drive and go for a drive. Today, I waited about 5 min and it went from 1200 to 1750rpm and started surging. But then I put it in Reverse and rpms dropped to 1000 then backed out of garage, put her in Drive and drove for about 15 to 20min. When I stopped, rpms in Park were just under 1000 (say 975). This in cool weather. Later after I had driven further I got home and rpms in drive were about 750. Which is where I would like them to be.
 
#27 ·
Hi Bernt,

I'm new to D-Jets, certainly no authority, and defer to MBGraham's advice. His posts have greatly helped me trouble shoot many areas!

I wanted to suggest you could remove and likely clean/repair your AAV. There are threads about it on this board.

Post #2 http://www.benzworld.org/forums/r-c107-sl-slc-class/1600137-comprehensive-illustrated-djet-72-75-350-a-2.html

http://www.benzworld.org/forums/r-c107-sl-slc-class/1592399-aav.html

I also found this link which might be of interest.

Auxiliary Air Valve Repair ? Paul Noren's Personal Blog

Hopefully that will save you some $$$ from having to purchase one.
 
#29 ·
You can get your car to operating temp. turn the car off, remove the AAV hose and then use something to plug the hole at the idle adjust (3 layers of Duct Tape works good) then restart the car and get the RPM down to 750 or so then start looking for air leaks using carb cleaner spray. There are two ways to adjust AAV if you have no vac leaks.
 
#32 ·
Just for some feedback...

Before I stripped the car and removed the engine, I did spend some time on the fast idel problem.

I bought a replacement Aux air valve as a non working used for parts unit off ebay.

It sort of worked but needed 80 degress before getting even close to being shut off.
I took that apart, cleaned it out and aadded some whashers to the actuator.

I tested it again and then squashed the bulb a bit so it was fully closed at at 65 degrees.

I put it in and adjusted the idling and I also corrected the throttle switch adjustment. It was not right as mentioned.

I had the car purring nicely at 750rpm and was so happy.

Then I drove it out the garage and when I braked, the engined died.
It did this every time and so I discovered that the brakebookster was totally shot.

I posted a thread on the brake booster rebuild here:

http://www.benzworld.org/forums/general-mercedes-benz/2753073-brake-booster-what-wrong-when-leaks.html

Anyway, after this, I did not do anything apart from stripping the car out to get to the rust.

If I ever put it back together, I will order a new bulb for the auxilliary air valve and install it into the valve I removed.


Best regards
Bernt
 
#40 ·
So are you going to order a bulb?

I did, just in case.

I have also learned from my own test as well as the Dr D-jet site that the element seems to get tired and collapses.
This means the things starts off at something like a -20degC position and takes much longer to close.

I personally have only looked at the one valve and the changes I had to make to get it to close at 65degC indicated that the bulb was tired.
The valve had no corrosion or slider issues.

I think fitting the new bulb may require a bit of machining.
We shall see.

I like doing that stuff much more than the rust repairs I now getting myself into.


Regards
Bernt
 
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