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Matt, Engineer
Category: Mercedes
Satisfied Customers: 21711
Experience:  Mechanical Engineer with 23 years experience in the auto industry, 8 yrs in formula 1 engine testing
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Very difficult to start cold or hot, low idle, dies when put

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Very difficult to start cold or hot, low idle, dies when put in gear.
JA: How many miles are on the car? What size is the engine?
Customer: 200k, 3.0
JA: What is the model/year of your Mercedes?
Customer: 1989 300e
JA: Are you fixing your 300E yourself? What have you tried so far?
Customer: Replaced lots of parts. Idle control valve, fuel pumps, accumulator, fuel relay, etc...Had a mechanic do a lot of the work and he can't solve the problem.
JA: Anything else you want the mechanic to know before I connect you?
Customer: No

This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.
As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane or brake cleaner around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Air leaks are very temperature dependent as gaps can open up as things expand with temperature so depending on how the leak occurs ( on a hose joint with a clamp expansion will help seal and on a vacuum hose expansion ill make a leak worse) so leaks can be better or worse as the engine heats up.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.

if the engine condition is the same then chances are the meter or the connection to it is faulty

It’s also worth cleaning the MAF sensor wires, as they can get coated with dirt over time which then offsets the reading
Use some contact cleaner or brake cleaner to spray onto the wires to remove the dirt – on no account touch the wires with anything as they are very fragile and ensure the cleaner is a trichoetylene based one and not a eco type as these can leave a residue

do you still need help?
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Customer: replied 1 month ago.
Hi Matt, I apologize for not responding. I received a fraud alert from my bank stating that they had declined the transaction that would have allowed me to see your reply. I notified them that it was legit. Checking today, I found your reply. If you're still available, I still need help. I had a smoke test performed about 3 months ago, no leaks. The pumps, accumulator and filter were replaced about 3 years ago. The engine runs flawlessly once I can get it in gear without stalling (keep my foot on the gas). Very smooth, lots of power (wants to stall when I come to a stop). The issue is starting cold or hot, I have to crank the engine several times and pump the gas pedal to get it started. When it does start, it idles at around 500rpm and I have to keep my foot on the gas to keep it running. Put it in gear and it stalls. Fuel pump relay was replaced, as was the idle valve and EHA. Fuel mixture adjustment doesn't solve the problem.


OK so there's no vacuum leak but its still want to stall ?

so either the throttle isn't open enough or the ignition timing is out or there's a load of extra drag from the transmission that shouldn't be there

so I'd begin by checking the condition of the transmission oil if its anything other than dark red it should be replaced

and I'd also use a timing light on the crank pulley to confirm that its running the correct 6 degrees of ignition timing

Customer: replied 1 month ago.
The ignition timing is correct, transmission is perfect. The stalling issue is related to low idle and I have to keep foot on the gas to prevent stalling at stops or going from neutral to gear. I'm guessing that it's a fuel/air distribution problem but what part of the Bosch KE system is faulty is the mystery. I've heard from several 300e aficionados that the OVP could be the culprit but I have no warning light on the dash and the car runs perfectly once past 700/800rpm. As mentioned previously, besides low idle, it takes 2/3 minutes of cranking and pumping the gas pedal for the engine to fire up. I'm not certain that the problem can be remedied without the proper equipment and test tools, I'm just looking to see if you might have some ideas I can zero in on. It may be that I might have to transport the car 150 miles to a certified Benz facility, my last resort.


OK 1st try adjusting the fuel mixture

This is best done by running the fuel pump and depress the air flow sensor plate for a couple of seconds to bleed air out of the fuel distributor. Remove one injector line from the top of the fuel distributor, and while looking into the injector port in the top of the fuel distributor turn the mixture adjustment screw clockwise until the port just starts to fill with fuel then turn the mixture screw 1/2 turn counter clockwise. Reinstall the injector line, start the engine and adjust mixture when it reaches operating temperature. This is the Bosch recommended procedure for initial mixture setting and adjustment

if this doesn't help I'd next try adjusting the throttle body to open it a little more, aim for 750-800rpm idle speed on a warm engine

Customer: replied 1 month ago.
Went to my mechanic today and he said that the problem was most likely due to a compression issue. He did a leak down test on one cylinder, opened the oil filler cap and I could hear the leak coming out. Said I would need rings & valve seals. Does this make sense?
Customer: replied 1 month ago.
Forgot to mention that he removed and cleaned the air flow box and found oil in the air intake (bottom below throttle).


poor compression is a possibilty

however no cylinder seals perfectly especially when its all cold so pumping compressed air into 1 cylinder and opening the filler cap you would hear air escaping any way even if the engine was 100%

Can I ask what the leak down figures were? as usually they are expressed in terms of a percentage and up to 10% is acceptable

alternatively what compression pressure figures were measured? as they should be around 130psi

Customer: replied 1 month ago.
Hi, I'm somewhat vague on all the numbers but he claims that I should be at 120 at the altitude I'm at (6,000 ft.) and it's 100.
leak down was 30 I believe. What is strange to me is that the car burns very little oil and doesn't smoke at all. And, once warm, it runs with plenty of power. He suggested that I might want to try a used or rebuilt fuel distributor (only part that has not been replaced) but the real problem was "the heart of the engine." From the numbers you cited, it doesn't look good, what do you advise.


OK sounds like yo may have a bit of an engine wear issue

can I ask if the low compression is on all cylinders or just one?

also if you squirt some engine oil into the plug hole and re-do the test does the number increase?

Customer: replied 1 month ago.
Hi, thanks for all your help. Sadly, I've come to the conclusion that the car is simply very old and not worth sinking more money into it. I bought it new in Germany and I'm attached to the car but it's time to let go.


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