Warm idle too slow - occasionally stalls 300E

[Brabus3.6]

Hi,

I have a Brabus 3.6 based on a 300E (M103) which has just had it's head gasket changed, along with new valve guides. The car starts first time from cold as it should holding a high 1200 RPM idle getting progressively slower as expected. The problem is that seemingly the warmer the engine gets the slower it tries to idle. By 85 degrees it wants to idle in drive (i.e. in
traffic) at 500 RPM where it's just about alive but very uneasy, sometimes
dying. Also at this slow RPM oil pressure can dip below 1 bar. I don't
believe there is a prolem with the oil pressure because by 750 RPM it's at
~1.5 bar and at 3 bar by 1500RPM. Additionally the engine idles more
erratically the hotter it gets, lumpily. It does have manual lifters so will
never be silkily smooth I know. If I raise the revs to say, 750RPM the idle
is noticibly smoother & do not believe it would cut out at this speed.
Indeed when it warms I can drive it by left foot braking in traffic if
necessary Engine power does not seem to done although the engine is still
being "run-in" so I dare not stray over 4000RPM.
I have searched forums for answers and the two most prominent are the
overvoltage protection relay (OVP) & the idle control valve (ICV). I rempved
the ICV today, it was clean but I sprayed some WD40 in it none-the-less. I
turned the ignition on and saw it open then shut so it is functioning
properly. The fuse on the OVP is also fine but I'm not sure how to check the
OVP itself. I have also recently changed the air filter, the old one looked
fine anyhow, no signs of the oil I'm used to seeing from the M110 motors.
Any other possibilities? I fear it may be the bottom end, piston rings or
cylinder bore causing a loss of compression but I would have thought this
would be tagged with a loss of power and "smoking" both of which there
aren't signs of.

Thanks for reading & helping if possible. Merry Christmas to all!!!

Mehmet

[KLP 92]

I have a Brabus 3.6 1989 Coupe.

My Brabus

What sort of mileage has the car got on it?
Mine is a 100k example and it idles at 750 rpm, with the oil pressure between 1.5 and 2.
Where abouts are you based? I've had some work done on mine by George Fraser in Wembley, London. There's a chap there that know the Brbaus inside out.
There is a sensor on them which is notorious for playing up which can give similar problems to yours. I will try and find out which one it is.
Just out of curiousity how much did the head gasket cost? Are the rumours of £500 for just the gasket true??

[Brabus3.6]

It's a 1989 black (of course!) Brabus 3.6 saloon. It's done 121k miles. It is an auto trying to idle at 500RPM in drive when warm. Trouble started last year when my uncle was driving the car and the water pump blew. Subsequently blowing the head gasket. I did a head job on it in October last year, HG £564.87!!!!! But the engineers I entrusted the head can't have been great because the valve guides were worn, causing the valves the bend again. It lasted 6 months before what I believe was combustion chamber to coolant passage head gasket failure with coolant once again disappearing. This time I entrusted the head to a firm in Romford who did the valve guides too. They can make a head gasket for less than Brabus they say. This time Brabus charged me £440. The power seems to be all there.
I may visit George if he opens between Christmas and New Year because I'm off skiing on 3rd Jan and off to Uni on 16th Jan! Today while fiddling I observed that placing load like headlights and wipers on the electrical system slows the car down leading me to believe the alternator may be on its way out. Could this cause such a problem? I believe the car may be misfiring at idle as the revs and oil pressure peak and trough slightly but audibly. I may readjust the tappets although I'm sure they're spot on as I've got the car idling more quietly then it ever has before. Any suggestions? Thanks

[Ian B Walker]

Just wondering if your tappets have been tightened down too much? or do you have an air leak somewhere in the pneumatic system?

[Dieselman]

Originally Posted by Ian B Walker

Just wondering if your tappets have been tightened down too much? or do you have an air leak somewhere in the pneumatic system?

Or inlet manifold.

[Robby]

Could be as simple as battery on the way out - will give those symptoms. When my old golf had a knackered battery it started and ran, but would stall sitting in traffic after about 5 minutes of idling.

[Brabus3.6]

How could tightening down the tappets too much cause such a problem? I set the inlet and exhaust clearences to there correct values when cold then tighten the nut fairly strongly to ensure the vibration of the tappet doesn't cause it to loosen. I then recheck the clearence to ensure the tightening hasn't adjusted the tappets further.
How do I check for air leaks in the vacuum system?
I'll get hold of a battery from another car to test that theory as soon as possible. Today I noticed it trying to idle below 500RPM consequently cutting out, this is out of spec as the ECU should keep the idle in drive within the 550-650 RPM range. Perhaps a sensor is too blame?

[Ian B Walker]

Dont forget that metal expands when hot. When they expand the valves can remain open a little on the compression stroke. It may give you a slight missfire and defo reduction in power while ticking over. Checking for system air leaks, hot soapy water brushed round the joints will create bubles and show you where the system is leaking. Its just a thought, would love to hear that you get your problem resolved asap as such a car must be a joy to drive.

[KLP 92]

Originally Posted by Brabus3.6

How could tightening down the tappets too much cause such a problem? I set the inlet and exhaust clearences to there correct values when cold then tighten the nut fairly strongly to ensure the vibration of the tappet doesn't cause it to loosen. I then recheck the clearence to ensure the tightening hasn't adjusted the tappets further.
How do I check for air leaks in the vacuum system?
I'll get hold of a battery from another car to test that theory as soon as possible. Today I noticed it trying to idle below 500RPM consequently cutting out, this is out of spec as the ECU should keep the idle in drive within the 550-650 RPM range. Perhaps a sensor is too blame?

Did you set the valve clearance to the 300E spec or the 3.6 Brabus spec. On my front cross member there is a Brabus plaque stating the correct valve clearance gap and spark plugs as these values are different from the standard engine.

[Dieselman]

What injection system does it have? If it's a Bosch K-jetronic the static idle has to be set before dynamic idle control can come into place.

Goes something like; disconnect the engine temp sensor and rev the engine over 3k to engage limp home mode and park idle valve, set the idle bypass to the correct value, reconnect temp sensor. Idle speed will then be controlled.

[Brabus3.6]

Originally Posted by KLP 92

Did you set the valve clearance to the 300E spec or the 3.6 Brabus spec. On my front cross member there is a Brabus plaque stating the correct valve clearance gap and spark plugs as these values are different from the standard engine.

I used the Brabus plaque. I believe that the M103's that these engines were based on used hydraulic lifters. I believe it has a Bosch K-jetronic system, the basically mechanical one with a touch of electrocal control. Any details of where I can find out how to set the static idle in detail Dieselman?

Ian it is a joy to drive when it's 100%. I think I'll pay George Fraser a visit if he's working.

Today I got my multimeter out & checked the voltage across the battery, it was ~12.35V. I started it and at idle it read 13.8V, both figures seem fine to me. However the car wasn't used at all yesterday & I noticed that the engine turned over rather more slowly than it should. The starter seemed to be struggling which doesn't make sense to me since the battery was holding 12.35V. My uncle returns from his Christmas break today so I'll swap out his battery to see if that improves matters, although I can't envisage how a battery could cause this problem. I always thought that once the engine was running the alternator basically took over.

[Dieselman]

If your car has mechanical FI with a cold start valve and injector then there is an air bypass screw on the throttle body IIRC, The mixture screw is then vertically in the top of metering unit.

[Brabus3.6]

An update. Over the last couple of days the car has worsened, it will definitely cut out if I don't right foot accellerate and left foot brake. I decided to buy a cylinder compression tester. The procedure I followed was remove all spark plugs, coil lead and fuel plug relay, then screw in and test cylinder compressions.
The results surprised me as I expected low cylinder compressions. The specced cylinder compression for my engine is 10.1:1 up from the 300's 9.2:1. The actual compressions I read from cylinders 1-6 were: 12.5, 13, 12, 11.5, 12, 10.75. So hopefully Ian was correct when he mentioned the valve clearances as excessive clearences will give higher compression. After the valve's were re-seated the all the clearances became too small so I had to loosen all of them, perhaps all too much. I'll re-check them tomorrow. I would have thought if the valve clearances were out the engine would be noisy though as remember this happening on the M110 engines, my 103 runs fairly quietly. Does anybody know of a site that explains clearly how to get valve clearances 100%?!
I have attached a picture of a spark plug that just came out. Unfortunately the picture isn't great but the plug tips look fine however the base of the plug, bottom of the thread is blackened, its smell I cannot make out. Could be carbon or oil fouling. The valves have just been re-seated etc. so I guess it's either incorrect valve clearences or an overly rich mixture?
Thanks & enjoy yourselves tomorrow night!

[Brabus3.6]

Yesterday I removed the distributor in the process of removing the valve cover to reset the valve clearances. I then saw that the rotor arm was badly burnt & the centre electrode of the distributor cap had burnt "singed" in half. I duly replaced these items only to find a temporary (few minutes) better running & then the same problem again. Tonight I took the car out for a spin & gave it a quick blast. On my way back home I was coming to a standstill expecting RPM to drop to 500 & so engaging my right foot when I noticed it idling happily in drive at 650 RPM. So I took it for a short drive and it was fine. I left it idling for 5 minutes outside my house & when I returned it was idling in neutral at 550RPM, sure enough I dropped it into drive & it sank to 500RPM. It's like the car's teasing me!
An OVP question. Today I removed the OVP to find it had 89 stamped on it so it's the same one since the car's inception. With the car cold started I removed immediately the idle slowed, roughened and died. Does this mean it's okay or could there be a problem with it which shows up when warm?
Okay, I think I've just isolated the problem. I was driving in daylight today hence no need for lights and I didn't have my radio on (two amps, components & a sub, it's juicy on the current), the car was working well. Then while in traffic I put he radio on with a song that was bass heavy, drawing lots of current to drive the sub. Idle immediately dropped. So I put the radio off, blasted home, idle outside my house was fine. I put the lights & radio on, idle dropped to 500 again. Switched them off, revved it, let off the gas & idle increased to 600. So I believe this to be the alternator. I'm off skiing in France this morning but as soon as I get back I'm on it. I need to upgrade the alternator anyway since it's still the original 70amp unit. I'm thinking the 115/120A units should be sufficient but I'll grab a 143A or 150A if it's not too hard to source.

[Brabus3.6]

I'm back from Vald'Isere ready to face the music . Today I replaced the alternator with a 150A model, upgrading the wiring to 2 4AWG cables etc. The underlying problem still seeems to be there although it didn't cut out tonight. When engaging electrical components the RPMs only deflect slightly, not as bad as before. However the idling is still inconsistent. Sometimes at 600, then 500, then later 600 again etc. Very strange. Tomorrow I'll grap an OVP and replace that. It could perhaps be the battery not holding charge as Robby suggested although I'd have thought that with the car running the alternator, or at least a 150A one, would take care of everything.

Today I tried an updated OVP, p/n end 37 45 & also a different ICV. Both didn't make a difference. However I noticed that when it is idling slowly, if it is given 3000RPM or so for a few seconds it will then rest at a higher idle and be okay for a while. I'm going to remove the air cleaner assembly & clean out the part below it now in case all that crud is causing a problem. After revving it idles perhaps a little too high, then slows down bit-by-bit suggesting something is sticking perhaps.