Engine hunts - Mercedes-Benz Forum
 
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post #1 of 10 (permalink) Old 02-18-2019, 11:08 AM Thread Starter
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Engine hunts

Hi all,
I have a 1983 380SL if I start from cold it will start immediately and tick over nicely but if I drive straight off it will cough and splutter and virtually die out so I leave it ticking over till to warm then drive off, it drives fine until I get into traffic then if sitting with my foot on the brake and itís in drive it hunts and tries lurching forward a little my wife thinks it me but itís definitely engine related, I can watch the tacho needle goes from approximately 750 rpm up to around 850 900 rpm itís uncomfortable sitting in traffic and I now find I have to put it into neutral every time we are in stop start traffic have checked the obvious had it serviced emissions are very good any one have any ideas on this
Thanks
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post #2 of 10 (permalink) Old 02-18-2019, 05:05 PM
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Where to begin? I have an '83 380sl that does pretty much the same thing, although I don't have any trouble driving immediately after startup on cold mornings.

The things that need to be checked and that all work in concert with each other are numerous but if I had to pick one thing to suspect it would be vacuum leak. There are many hoses and tubes and rubber seals that dry out and shrink or crack with age. They are all suspect.

Next on the list of things to check... in no particular order...

Plugs/wires/cap/rotor

Followed by...

Fuel pump/fuel filter/accumulator

The warm up regulator drops out of he picture once the engine warms up, but that doesn't mean it can't malfunction. Expensive part so I'd check the control pressure before messing with the WUR. If you don't have a fuel pressure tester get one... dirt cheap at Harbor Freight and it has all the fittings you need.

Idle control valve located just behind the thermostat housing... easy to clean as long as the solenoid portion is working. There's a connector on the front of the valve, if you unplug it and the engine dies then the valve is likely working and just needs to be cleaned or its rubber hoses need to be replaced.

Cold start valve. Designed to dump raw fuel into the intake when starting on cold mornings. It's just another injector... it either works or it doesn't. The only other thing it can do is leak and make the mixture too rich after the engine warms up.

And on and on and on...
What have you checked so far?

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post #3 of 10 (permalink) Old 02-19-2019, 01:02 AM Thread Starter
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Hi,
thanks for response, so far we have done plugs, cap, rotor and leads, I thought it could be a vacuum leak as I did have a bit of trouble with the heater vacuum solenoid but I changed this and thought it had cured the fault. I believe there is vacuum to the door locks etc as well but this does not work at all (may be wrong there have not stripped that apart). Last year we took all the vacuum hoses off in the engine bay one at a time and vacuum tested them but all seemed to hold ok perhaps I should just go for renewing them all. I do have a fuel pressure gauge but have not checked fuel pressure yet so will have to look at that and the cold start injector, I am pretty handy around an engine but have not come across this fuel injection system before, fortunately its not my every day car so it only comes out in the summer here in the UK.
The cold start side seems spot on it starts first turn and ticks over lovely but as said if I pull out onto the road and a car is coming up behind it can be pretty scary when it coughs and splutters and limps along (your shouting at it come on! come on!) then it suddenly picks up and flies off. I think the emissions are good at its last MOT test it was reading 0.02 CO when up to temperature.
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post #4 of 10 (permalink) Old 02-19-2019, 01:30 AM
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Quote:
Originally Posted by jensencars View Post
... Last year we took all the vacuum hoses off in the engine bay one at a time and vacuum tested them but all seemed to hold ok perhaps I should just go for renewing them all.
Before you proceed to replace them, make yourself a DIY smoke tester first. Many how to's on Youtube available. Vacuum leaks can arise at many locations unfortunately, not only at the hoses or rubber connections. Just to name a few, throttle body gasket, AFM booth, injector housing seal etc.

I made this one, works fine with air compressor attached at it's lowest possible pressure.

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How hard can it be?
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post #5 of 10 (permalink) Old 02-19-2019, 02:00 AM
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If I was forced to guess based on what you have reported, I'd rule out an ignition issue and concentrate on fuel delivery. It would be very helpful if you could perform a fuel pressure test. There is a fuel line that runs from the top of the WUR to the FD. When cold, the pressure in that line should be about 30 psi (2 bar) as the engine warms it should go up to about 45 or 50 psi (3 -3.5 Bar) and stay there. If it does you can rule out the WUR as the cause and then move on to the next suspect.

Keep in mind... unmetered air into this system will cause havoc with engine performance. The CIS is very sensitive to air flow, it is the basis upon which fuel flow is determined. Any air entering the system that does not pass the air flow plate will result in improper air/fuel mixture.

There is also a fuel pressure valve that lives on the ass end of the FD. It's job is to maintain a constant fuel pressure in the system by dumping any excess pressure back through the return line to the tank. The fuel system pressure can be adjusted by adding or removing washers inside the fuel pressure regulator. It would be best if you first established that your system fuel pressure is correct. The pressure supplied to the FD from the WUR is called the Control pressure, not to be confused with the system pressure. The balance between the two determines the fuel mixture ratio (rich/lean).

The easiest thing to do is first check the control pressure and system pressure. If they are ok then check the idle control valve and cold start valve (if the cold start valve leaks it will cause a rich fuel mixture). If all are fine but you still have the problem, look to a vacuum leak somewhere in the system.

You can watch this animation to get a pretty good overview of how the CIS K-Jet operates.

Good luck.

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post #6 of 10 (permalink) Old 02-19-2019, 03:14 AM Thread Starter
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Hi,
thank you for your assistance, due to work wont be able to get out there till next weekend but I now have a good basis to start working from....
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post #7 of 10 (permalink) Old 02-19-2019, 04:23 AM
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Also, check your Ignition advance in the Distributor to see if it will hold vacuum.

Carl

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post #8 of 10 (permalink) Old 02-19-2019, 07:24 AM
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post #9 of 10 (permalink) Old 02-19-2019, 08:46 AM
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Sounds like several issues.

1. Vacuum leaks (beyond the little connectors on top of the engine). More specifically, the air metering boot, donuts, and more intake issues.
2. Bad idle relay and/or idle air controller
3. Wonky WUR

Smoke test the bugger and see where the smoke leaks out then work up.

I'm impressed your car managed to pass MOT with such issues. Guess those damn tests aren't as comprehensive as I thought they were! :
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post #10 of 10 (permalink) Old 02-19-2019, 11:36 AM Thread Starter
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Quote:
Originally Posted by okyoureabeast View Post
Sounds like several issues.



I'm impressed your car managed to pass MOT with such issues. Guess those damn tests aren't as comprehensive as I thought they were! :
Hi,
I hate the tests itís like going to the dentist lol,, our MOTís tests the structure brakes and emissions steering etc, they donít Road test from cold, emissions are checked with a warm engine, my emissions are spot on when ticking over in park or neutral the hunting issue appears when in heavy stop start traffic the car seems to drive lovely otherwise, must admit I thought it was a gearbox problem Initially but I can see the rpm needle flicker when itís occurring, have ordered a new fuel filter for starters as it looks old and building a smoke tester as well just got to tip something out of one of the wifeís jars
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