Poor running when hot?

[Kristan]

I seem to have got the 300 to a point now where it will start and run fine when cold, but after 10 minutes or so of driving, it starts to missfire under acceleration, stall at junctions and have lots of difficulty restarting. The flasher diagnostics are still reporting the CPS as faulty, despite the car dying instantly if I pull the CPS sensor. Regardless I have one on the way, but I thought I'd try to canvass other opinions - cap, rotor and plugs have been very recently changed, so I doubt it's any of those, but I was thinking perhaps HT leads or the coil?

A faulty fuel pump or blocked line/filter were suggested by a garage, but I'd have thought the fine running when cold (and she does run well, a bit of poke in those the big M104 3.0 24v ) kinda rule out fuelling.

Any suggestions?

Ta

 

[ray_hennig]

Same as my 1991 300CE-24

What make rotor arm did you install? I would recommend genuine MB as it is an active component (well, it's got a 1K resistor in it) and things get pretty hot up there at the top of the engine. My resistor was failing when hot.

Your new CPS may solve things but failing that I would want to complete the electrics overhaul with new HT leads.

Other things to do include removing the ignition controller and ensuring that it's clean and thermally connected to the body with nice new heatsink compound. I'm not suggesting that this is a cause of your problem, just a good thing to do. I also landed up replacing a cracked and aged looking plastic screen behind the rotor arm. Sadly, it was jolly costly. I make a point of cleaning all electrical connectors when I find a new one - unplug, spray with contact cleaner, plug/unplug/plug several times to clean the contact.

Keep up posted.

RayH

PS: Oh, and don't let the thing run out of petrol - it took about 20 kms of spirited driving to stop it mis-firing afterwards. What a dolt I was!!!

 

[Kristan]

Thanks for that - I'm not sure what brand rotor was installed as it was done prior to my ownership or the car. I'm fairly certain it's MB thou, as it was done the same time as the cap and I seem to recall an MB part number on that..

HT leads are something i'm considering, thou like you say, an electrical overhaul isn't cheap!!

 

[grober]

HT lead problems would be constant even with the engine cold. Sometimes distributors can have internal condensation problems. The car runs fine when cold then the engine warms up water vapourises inside the distributor and misfiring occurs. Cure is to dry off the distributor body-not cap! with a hair drier being careful not to melt anything. Could also be the EZL ignition unit as suggested or possibly a bad coil [altho again a coil fault would probably always be there]. If its the EZL be sitting down when you hear the cost of replacement-if you can locate one.
Other possibility is a inlet /manifold leak leading to weak mixture. This is masked by mixture enrichment by the injection system during cold running. A quick look at the spark plugs might tell you if the car is running weak.

 

[FazMan]

Can also be caused by the distributor failing when hot.
Caused me a lot of problems on a mazda and was very difficult to find. Basically the ignition became too advanced. Some of the scary suggestions i had included cambelt jumping a tooth, head gasket, etc, etc. A reconditioned distributor solved it.

 

[Kristan]

Thanks guys, that's some good advice there. I'll take the distributor apart and give it a goos clean and check for moisture when I fit the CPS - hoping that will arrive soon!

 

[Kristan]

Btw, sparks are pretty sooted as it's already running rich as I think the previous owner turned the mix up to mask the problems!

 

[ray_hennig]

If plugs are sooted ....

You should have said that first!

My problems were frustrating because the plugs looked and have always looked text-book-good - brownish etc. etc.

If there's sooting, something else must be amiss. I have little experience of such a problem so I hope some brainy boys come to the rescue.

Have you done a duty cycle check? - sooting suggests rich fuel mixture.

Are the plugs sooty after a thrash on a motorway? - lots of 5-6K revs.

You say that it runs fine warm but then things go awry. Is the cold rich switching off? - temp sensor? leaking aux injector?

What about the air filter?

Come on you brainy boys!

RayH

PS: Also, a few glasses of red have fogged my perceptions a mite!

 

[Kristan]

It's hard to check the mixture as it hardly idles while warm and that's kinda a requirement for checking the afr! I've replaced the temp sensor and air filter as they were knackered already I have to admit I've not taken it on a long run as it's a nightmare when I need to stop so I'm not sure if the plugs would be ok after a long run. Fingers crossed the CPS will cure the stalling when warm/hot problem and I can give her a proper blast and clear out the cobwebs!