1973 450SEL engine/RPMs surging - Mercedes-Benz Forum

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post #1 of 13 (permalink) Old 08-27-2015, 05:57 AM Thread Starter
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1973 450SEL engine/RPMs surging

Hi,

When the engine is cold and started it surges in a cycle lasting 2 to 3 seconds whereby the RPMs go up then down then up then down. Once the engine is warm after a few minutes this stops. What should I do to fix this?

Thanks.
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post #2 of 13 (permalink) Old 08-27-2015, 07:31 AM
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Hi,

When the engine is cold and started it surges in a cycle lasting 2 to 3 seconds whereby the RPMs go up then down then up then down. Once the engine is warm after a few minutes this stops. What should I do to fix this?

Thanks.
Common djet problem. Usually means that AAV (aux air valve) is sticking. They are quite expensive to replace, but can sometimes be cleaned if removed.

The other thing to try, is go for a drive to fully warm car up. Then set the idle speed (using the idle adjustment screw located near the AAV) to 700rpm. Next time you start car cold, check how high the rpm goes if it is still surges. The ECU cuts the fuel if it goes over about 1500rpm, then opens it again at about 1000 rpm (numbers from memory, but approx correct). As a result you get the up and down sawing effect that you have noticed. Sometimes setting idle rpm just slightly lower may avoid the sawing effect. But then rpm when engine is really hot may be too low.

In case you need to know where AAV and idle screw are:



1972 350SL 4.5L - MBGraham, near Kingston, Ontario

Last edited by MBGraham; 08-27-2015 at 07:39 AM.
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post #3 of 13 (permalink) Old 08-27-2015, 07:32 AM
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Originally Posted by rwdnp View Post
Hi,

When the engine is cold and started it surges in a cycle lasting 2 to 3 seconds whereby the RPMs go up then down then up then down. Once the engine is warm after a few minutes this stops. What should I do to fix this?

Thanks.
Let's start with what HAVE you done?

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post #4 of 13 (permalink) Old 08-27-2015, 07:42 AM
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X2 on the AAV. I had the same symptoms on my 450 SL

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post #5 of 13 (permalink) Old 08-27-2015, 11:06 AM Thread Starter
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Very helpful. Thanks so much. I will try this. I didn't know where to start. A mechanic had advised perhaps a problem with vacuum hoses but I hoped I could get more precise response here which I did. Thanks again!
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post #6 of 13 (permalink) Old 08-27-2015, 11:35 AM
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Very helpful. Thanks so much. I will try this. I didn't know where to start. A mechanic had advised perhaps a problem with vacuum hoses but I hoped I could get more precise response here which I did. Thanks again!

The mechanic could be correct. In either the case of the AAV or the vacuum leak, I believe the scenario is the engine is getting too much air which causes the engine to race. An AAV that is stuck open is similar to a vacuum leak or the idle screw being wide open.

I think on the d-jet the AAV gets it's air from a tube in the front if the throttle body above the throttle plate. If you partially block the off, does the swinging idle come under control? BE CAREFUL though. The suction on that tube is very strong. I think I've gotten my finger caught on it (and stalled the engine) before.

.

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post #7 of 13 (permalink) Old 08-27-2015, 06:19 PM
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The mechanic could be correct. In either the case of the AAV or the vacuum leak, I believe the scenario is the engine is getting too much air which causes the engine to race. An AAV that is stuck open is similar to a vacuum leak or the idle screw being wide open.

I think on the d-jet the AAV gets it's air from a tube in the front if the throttle body above the throttle plate. If you partially block the off, does the swinging idle come under control? BE CAREFUL though. The suction on that tube is very strong. I think I've gotten my finger caught on it (and stalled the engine) before.
Fonzi is right. A sticking AAV is essentially a vacuum leak. There are several potential leak points:
- hose from back of intake manifold to Manifold Pressure Sensor
- connections at rear of intake manifold, passenger side that go to door lock system and transmission.
- small plastic line that runs from throttle body to a valve on passenger side fender and on to distributor (controls distributor retard)
- short hoses that connect upper and lower halves of intake manifold internally (internal, can't be seen)
- hoses on either side of AAV.
- any others Fonzi?

Regarding blocking the port on throttle body. That port supplies idle air to both the AAV and the idle control valve. If you block it, engine should stall if there are no vacuum leaks because that would block total idle air supply when throttle plate is closed.

Generally, if you have serious vacuum leaks you cannot adjust idle speed using the idle screw. That is why I suggested getting car well warmed up, then set idle speed to 700rpm. If your idle screw has no affect, then start looking for vacuum leaks (AAV not closing or elsewhere)

I know I was advised many years ago by an MB mechanic to search for vacuum leaks to solve a similar surging problem. Even to use a smoke machine to find the leaks. But it turned out to be a sticky AAV and in time I found that many others had same problem.

1972 350SL 4.5L - MBGraham, near Kingston, Ontario
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post #8 of 13 (permalink) Old 08-27-2015, 07:34 PM
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Originally Posted by MBGraham View Post
Fonzi is right. A sticking AAV is essentially a vacuum leak. There are several potential leak points:

- hose from back of intake manifold to Manifold Pressure Sensor

- connections at rear of intake manifold, passenger side that go to door lock system and transmission.

- small plastic line that runs from throttle body to a valve on passenger side fender and on to distributor (controls distributor retard)

- short hoses that connect upper and lower halves of intake manifold internally (internal, can't be seen)

- hoses on either side of AAV.

- any others Fonzi?



.

I had a bad intake manifold gasket on my black 1973 450sl. A vacuum leak could be anywhere really.

A big host that cracks is the brake booster hose.

.

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post #9 of 13 (permalink) Old 09-03-2015, 01:28 PM
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Any info on a part number for the AAV. Im looking at a 76 450 SEL that I believe needs one. MB dealer had no idea what I was talking about. He quoted me $875 for something, but I believe it was the idle control valve he was thinking of.

How much is a new AAV?
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post #10 of 13 (permalink) Old 09-03-2015, 07:07 PM
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