300CE 24V Poor fuel economy and intermittent tick-over problem

[TonyVaughan]

My car runs well most of the time but goes through bad patches when it behaves very badly with lumpy tick-over, sometimes stalling and decelerates when pressing the accelerator, and terrible fuel economy (10-15 mpg).

I can reproduce these symptoms by:

• Leaving the car unused for about two days.
• Driving it for about half a mile (basically warming it up).
• Leaving the car and coming back to it after about 15 minutes.

The car will then start badly and run with a lumpy tick-over. Sometimes, apparently when the weather is hot (but I haven't proven that yet, still testing), the car keeps stalling when the accelarator is pressed and runs very badly indeed. If, instead of starting the car after 15 minutes, I come back after over 30 minutes (presumably when it has cooled down), the car starts and runs perfectly again, although the tick-over will occassionally hick-up. This is a nightmare when I'm just popping out to the shops and only stopping for a short time.

I have replaced numerous parts, but no-one seems to have an explanation for this behaviour.

Does anyone know what might be causing this?

Thank you in advance.

Tony

 

[carat 3.6]

Welcome to the forum.

It sounds like moisture in the distributor cap to me, that and the rotor arm would be the first things to check on these engines. If in any doubt it is best to replace them.

What parts have you changed so far?

 

[TonyVaughan]

Thank you for your reply

I have changed the distributor cap, rotor arm, plugs, plug leads, and coil.

Sadly, no change.

Thanks for the thought though.

Tony

 

[TonyVaughan]

Sorry, I should have said that the car is 1992 with a MAS Fuel Pump Relay, also replaced.

Too many parts replaced to mention here.

I will confirm any suggestions as to whether that part has been replaced.

Tony

 

[BlackC55]

Have you replaced the ovp?

 

[bolide]

You'd be better off saying what has been done to date rather than starting a game of twenty questions. There are plenty of 300E / 300E-24 bad running threads on here already

Nick Froome

 

[grober]

Welcome to the forum.

It sounds like moisture in the distributor cap to me, that and the rotor arm would be the first things to check on these engines. If in any doubt it is best to replace them.

What parts have you changed so far?

Echoing what carat 3.6 has said symptoms sound like the dreaded moisture in the distributor body/cap If moisture has collected in the body of the distributor the car will start when stone cold however once the engine warms up a bit--- water vapour is produced which volatilises up from the body into the cap/rotor arm area where it causes HT leakage. Try taking the cap+rotor arm +shield off and drying off the "innards" of the distributor with a hairdrier and see if that improves things. best would be to remove entirely and leave it a warm dry area like a airing cupboard or oven [ not too warm!] for couple of days- but only do this if you are happy removing the entire distributor!

Although you have replaced many of the HT components I hope you have used quality parts such as Bosch or BERU because there are lots of tales on here of cheap pattern parts continuing to give HT breakdown problems even when replaced.

 

[TonyVaughan]

Hi Olly

No, I haven't replaced the OVP as no light on the dashboard comes on when my problem happens. I've asked my garage about this, but they don't think it is the OVP.

Thanks for the suggestion anyway. I need to check how much it costs with the possibility of replacing it anyway.

Tony

 

[TonyVaughan]

Hi Nick

The main parts relevant to this issue have been mentioned and recently the crankshaft sensor was replaced. The problem is that, although there are many other threads reporting similar problems to mine, I haven't found one where the initiator of the thread actually reported a solution matching my issue. What I am seeing are many threads with no solutions which makes me growingly lacking in confidence that there is a fix.

I would be most grateful if you can point me to any threads with a solution.

Thanks again.

Tony

 

[TonyVaughan]

Hi Graeme

Interestingly, the problem started when the distributor cap disintegrated owing to case hardening. I had the rotor arm changed at the same time. Therefore, I don't think moisture would have had time to build up before the problem revealed itself.

However, I will check this out in case I actually have more than one faulty part.

Does anyone have a view about whether the fuel pressure regulator would be a likely candidate? Any experience in this area would be most welcome.

Thanks in advance.

Tony

 

[carat 3.6]

Have you confirmed if it is ignition related or fuel related? Checked for vacuum leaks? Read the fault codes with a flash code reader?

As olly said, the ovp relay (or the fuse in it) is also worth a look.

Just keep your fingers crossed that it isnt the ezl module...

 

[TonyVaughan]

Hi Graeme

Just to check, my distributor is located at the front of the engine, which is a 6 cylinder DOHC. It doesn't have a distributor body. Is that the same as the one you are talking about (KE-Jetronic)?

Tony

 

[TonyVaughan]

Hi carat 3.6 amg

That is the crux of the problem. The symptoms look like fuel starvation. I started off assuming the issue was electronic and changing parts related to that with no joy (all Bosch or equivalent). I am now thinking the issue is fuel related.

However, I don't want the garage to replace any further parts unless they can provide a logical explanation for why it would fix the problem - no hunches. I am able to do some of the work myself, but not everything - especially fuel related bits.

We have also checked for vacuum leaks and there don't seem to be any.

Thanks for your input.

Tony

 

[nevilledoreen]

Hi I have a 300 sl with the 24 valve engine with the same problem as you I chased it for about 8 months until I fixed it myself with Grobers help .It was the distributor cap
an after market one had been fitted by my mechanic .
Take the cap off and see if is sweating inside if so dry the distributor with a hair dryer replace with a genuine Bosch distributor cap and rotor button.
Also the seal plate under the rotor button my need to be replaced but this is not always necessary.
An easy check to see if the cap is sweating is to remove the plastic cover over the distributor ie the one with the Merc symbol on it if the car the plays up switch on the air conditioner the electric radiator fan will come on and clear the air circulation over the engine will temporarily clear the problem.
Good Luck
Neville

 

[TonyVaughan]

After taking the car to three garages, I've finally got the answer to my problem and it was not expected. The good thing is that it had nothing to do with the Mercedes. It was the alarm's immobilizer system which had been wired into the ignition system and was clearly not rated highly enough to handle the current. The final garage removed the immobilizer from the circuit and all magically returned to normal (after 2 years!!!).

The alarm is a Cobra 4138HF. I was flabergasted as I thought Cobra was a reputable maker of alarms. I have to say that the wiring job done by the installer was very poor (bare wires in abundance). This has cost me a lot of money.

Now, driving the car is blissful and I am back in love with my motor (I was rapidly falling out of love over the two years).

The distributor cap did have some corrosion, presumably owing to the sweating mentioned by some contributors and the cap and rotor arm have been replaced. I'll keep a close eye on that in future. But the car was still running okay before they were replaced.

I guess the lesson here is to be skeptical of anyone's work when you haven't had a strong recommendation.

 

[trapperjohn]

Glad its sorted. And thank you for the feedback.

 

[grober]

Its an amazingly frequent occurrence on the forum. Someone has say--- an ignition problem. BUT the car has recently had several ignition/electrical components replaced on the car--- by persons unknown [ previous owner/new garage] SOOOOOO--- it couldn't possibly be them---- they're new------ and everyone goes on a wild goose chase looking for some obscure fault. Later it emerges that the " new parts" were second hand and that they were fitted by the previous owners's mate--- who "knows something about cars" and it turns out to be something fairly basic after all. As you so rightly surmise it pays to be suspicious of any repair you don't have first hand knowledge of--- and even then it should never be ruled completely out.

 

[TonyVaughan]

I know this is a long time since I started this thread. I am adding this post to say that the problem occurred again after about a year and I am now convinced that moisture in the dist. cap is the root cause, (though the alarm immobilizer was related in that it caused the car to fail to start).

Can anyone explain where the moisture is coming from and why some people get this problem while others don't?

I previously had a 300CE and it never happened on that car - are the distributors significantly different?

Thanks especially to carat and grober.

Tony

 

[1001ab]

This sounds very similar to an issue with my 300CE-24 that was diagnosed the other day as a faulty cold start relay of some kind connected to the FI system.

I'd previously tinkered with the dizzy, the HT leads and pursued it from that angle though with nothing like as methodical an approach as you have documented.

My fix came by accident. It wouldn't start after a gearbox rebuild and, even after I explained that it does that sometimes, the gearbox specialist passed it onto a Bosch specialist he knew.

 

[TonyVaughan]

You mention

"a faulty cold start relay of some kind connected to the FI system."

Do you know which part it was, so I can check with my garage?

Also, did anyone explain how this faulty part might lead to moisture in the dist. cap?

I wonder if this might explain why some people get the moisture and others don't. Sounds interesting. It would be good to check this out.

Tony