450SL difficult to start but narrowed down. - PeachParts Mercedes-Benz Forum

Genbiltstein · #1 08-24-2009, 05:48 PM

I have posted before and wrote about working on my '74 mercedes 450SL. the problem she has is.
Intake Air Temperture Sensor: when I unplug it the car starts up perfectly. I suspect it is because the injectors are putting more fuel during start up.
When I reconnect it she gets a bit on the stubborn side when starting and stumbles on acceleration until she warms up. Idles rougher.
When disconnected she starts up fine.

I can assume there is something going afault here. Am I bypassing the manifold pressure sensor when the Air Temp Sensor is disconnected? Is the Air Temp sensor unserviceable?
Is the dial at the bottom of the D Jet computer misadjusted from a previous owner? She runs like a scalded cat without the Intake Air temp sensor connected as it is.
GB

meltedpanda · #2 08-25-2009, 01:45 PM

may just have a bad sensor, replacements are less than 20$. also check wiring in connector, I know on mine one wire was broken inside the connector.
The D jet ECU pimary function at is start up

Genbiltstein · #3 08-25-2009, 06:19 PM

I'll try it Panda thanks for the advice.

gtwendt · #4 08-27-2009, 07:46 AM

I had the exact problem with my 76 450SL. One of the wires were disconnected to the air sensor. However, I think that my largest problem was in the warm up regulator. There is a small screen in the WUR at the upper fuel line connection. I cleaned mine out and blew the WUR out good with my air compressor. Car started right up and has ran smoother than it ever has. I picked up this excellent guidance from this forum!

meltedpanda · #5 08-27-2009, 11:00 AM

does the 74 have the WUR

UT_Tech · #6 09-02-2009, 11:16 PM

No, the WUR is part of the later K-jet system, not D-jet.

ctaylor738 · #7 09-10-2009, 05:09 PM

When you unplug the air temp sensor, the brain throws the mixture to full rich (12% CO). So you are lean when cold and unplugging the sensor provides some extra fuel, engine runs better.

You should simply check the air temp sensor. There should be 300-400 ohms across its terminals around 60 degrees.

However, the the water temp sensor is a more likely culprit, since it has a larger influence on how much the brain enriches the mixture during warm-up. It is in the front of the intake manifold and should show about 325 ohms with the engine warmed up, much higher with the engine cold.

Genbiltstein · #8 09-17-2009, 07:03 AM

CTaylor:

Quote:

When you unplug the air temp sensor, the brain throws the mixture to full rich (12% CO). So you are lean when cold and unplugging the sensor provides some extra fuel, engine runs better.

You should simply check the air temp sensor. There should be 300-400 ohms across its terminals around 60 degrees.

I thoroughtly agree. Did that. Put the temp sensor in the freezer and ohmed out quite well. The contacts were corroded. I also found that the cylinder head engine temp sensor was disconnected. This temp sensor is used in conjunction with the air temp sensor.
The car drives like one heck of a champ now. Question is when she cools down will she start up as well as she did when the air temp sensor was disconnected.
Before I put my foot in my mouth I will say this. Corroded connections plus a disconnected engine temp sensor played into poor driveability and terrible startups when I wanted to start the car returning from work.

As said before this engine runs like a scalded cat and I will be doing some looking into her to see what has been done if anything. I read reports that some people were dissatisfied with performance of the '74 D jet 450sl.
No Smoke out the exhaust. Runs smooth as hot butter sliding on a grill. Shifts firm but has an input seal leak.
The brake lines were contaminated beyond belief. Repeated bleeding and time is the only cure.

Genbiltstein · #9 09-17-2009, 07:55 PM

Quote:

However, the the water temp sensor is a more likely culprit, since it has a larger influence on how much the brain enriches the mixture during warm-up. It is in the front of the intake manifold and should show about 325 ohms with the engine warmed up, much higher with the engine cold.

I checked out the water temp sensor. I will never rule out checking out the wiring all the way back to the computer. I have found hard and stiff wiring. breaks in the sheathing.
Thanks for the tip

Genbiltstein · #10 09-19-2009, 09:30 AM

I found out that the lead on the water temperature sensor is old and moveable. Intermitant. The crown around the brass has loosened a bit as well. This is the failure.

Thank you

Graham · #11 09-19-2009, 06:40 PM

Quote:

Originally Posted by Genbiltstein

I found out that the lead on the water temperature sensor is old and moveable. Intermitant. The crown around the brass has loosened a bit as well. This is the failure.

Thank you

Considering these sensors are not overly expensive, it seems like an idea to buy new ones. (air and water). They provide key input to the injection system.

Bill Adkins · #12 09-20-2009, 11:31 PM

Actually the air temp sensor in a 74 is in the air filter snorkel and unplugging it will cause an enrichment of .4 percent CO. The water temp sensor will make it really rich, it's in the manifold.
Bill

ctaylor738 · #13 09-21-2009, 09:37 AM

What I have observed about D-Jet problems is that more often than not the problem is one of the inputs to the system (temps, throttle position, trigger points) rather than the more exotic pieces like the MAP sensor or the brain itself. Another problem area, noted in this thread, is old, brittle, and corroded wiring.

So I hope that in the future, people troubleshooting D-Jet will see this post and check out the basics before they start replacing injectors, adjusting the idle mix or screwing with the MAP sensor!