Hi all,

I'm slowly going through my revamp of a 16v, but two problems persist. One, the car takes forever to start if it's been more than about 15 mins since it was last started. It will crank 10-15 times, try to catch every once in a while and finally start. When it's been sitting for 5 mins or so it will start much quicker. 2. when idling, the motor vibrates so much the car shakes. If my hand is hanging out the window it visibly shakes up and down.

Now, things I have already done, all of which have helped power and smoothness but those two problems persist (with the starting problem getting somewhat worse):

1. Valve Adjustment
2. Pulled fuel distributor, lines, injectors, idle valve and gave them an ultrasonic cleaning. I did not clean the EHA valve, I will do this next.
3. New, proper, non-resistor plugs. (NGK BP7EFS)
4. New motor, trans and subframe mounts
5. New cap and rotor

Motor is smooth as silk once I put my foot down, even if I keep it just above idle. It pulls smoothly to redline and will burn the rear tires easily. Sounds demonic at WOT.

Fuel pump is of unknown vintage, fuel filter was replaced within the last 3 years and I have a new one ready to go in.

Any thoughts?

How about the overvoltage relay, do you have power to the ECU?

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I'm thinking it could be a compression issue. I have similar symptoms and found it was a couple of toasted exhaust valves in one cylinder.

if I did not have power to the ECU, the car would not start at all, yes?

I'm not super familiar with the CIS-E system or the ECU that MB used in this era... I'm much more familiar with EFI BMWs. How would I go about checking this?

MTI - responded to you on mbworld as well, but wouldn't a compression problem be evident in the entire revband?

The valve clearances were WAY off when I did the adjustment, but all adjusted to within spec with very similar sizes on intake/exaust, respectively, indicating that the seats are wearing evenly...

Yes the car will start without the ECU but it can run poorly and be stubborn to start depending on the basic setting . The car has a basic mechanical setting that will allow the car to run, the ECU just fine tunes it for the operating conditions. When the overvoltage relay doesn't power up the ECU its common to have the symptoms that you describe.

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90 300CE 104.980
Milled & ported head, 10.3:1 compression
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It won't be evident using the "seat of the pants" dynomometer.

Here's a picture of one of my damaged exhaust valves



Despite low compression values in that cylinder, the car ran like a champ when warm, but had a tendency to hesitate on cold start, vibrate and not hold idle. When warm, the idle might drift upwards, but a throttle blip would settle it back to 900 -1K RPM.

Yikes. I hope that is not the case, I don't want to have to remove the head.

Just checked the OVP relay. It looks original, although it is no longer bolted on to anything... just flopping around in the battery try. The fuses are fine, and when I unplug the unit it absolutely will not start, so it seems to be functioning.

Guess I'll go buy a compression tester tomorrow and hope for the best.

edit: relay has the newer part number, so it has been replaced at some point.

2nd on testing the compression. My 16v had many of the issues you describe (difficulty starting, erratic idling, occasional stalling, etc.) yet seat of the pants the car seemed ok.

A bad head gasket necessitated head removal this year, and all exhaust valves were bad (not cracked like MTI's, but replaced). With a full head rebuild the car now runs perfectly - smooth idle, starts every time - and the car is clearly much more powerful.

Hopefully this is not your issue - but don't ignore the fact that it well could be.

i will compression test today - but there are no erratic idle or stalling issues. It holds a steady 900-1k RPM idle, it just shakes like a blender while doing it. I've never seen any smoke from the car; headgasket should be good.

Hopefully I don't have to pull the head, but thankfully I have a second car to drive around in the event it needs to be pulled this winter. How big of an undertaking is it? Assuming I keep everything marked at TDC I won't need to retime, yes? I loved being able to easily remove the cams and keep timing when I did the valve adjustment... my S54's timing is much, much more complicated...

So i was poking around EPC trying to grab the OVP relay part number to replace, just to be thorough. I notice that there is a different part number for cars to up '86 and one for 87+. the '86 is 6 pin, '87 in 9 pin. My car is an '86 and currently has a '87 9-pin relay jammed in the 6-pin harness connector.

Love used cars, hate previous owners.

Still compression testing tonight, but I'd be willing to bet the incorrect relay is causing issues.

looks like I'm rebuilding the motor. Just took compression numbers...

cyl 1 - 90psi
cyl 2 - 115psi
cyl 3 - 110psi
cyl 4 - 125psi

engine was cold (hadn't been started in 10 hours) but those seem awfully low and varied. All plug wires were disconnected and the other 3 plugs were installed. Fuel relay was disconnected (if it makes a difference)

Any rebuild guides out there? I've never removed a head before.

edit: also, the plugs indicate the motor is running SUPER lean.

Why would you leave the other 3 spark plugs installed when compression testing a particular cylinder? Makes more sense to remove all spark plugs before performing compression tests.

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Fred Hoelzle

Gah, what a bummer! Unfortunately these heads are super sensitive to valve adjustment, and I think most cars have been ignored for a good part of their lives. You may want to do a leak-down to confirm the source of the low compression before diving in, but they are the likely candidate on this motor.

If you are going to do it, there is a lot of auxiliary work that should be done at the same time unless you have rock-solid proof that it was otherwise done recently. That's what makes it get pricey. You should do the timing chain ($60+), guide rails (cheap-ish) and tensioner (and it's own guide - ~$200 - this is an updated part and must be done if it hasn't been done already), timing chain sprockets if things are badly worn ($350+ each!), head gasket ($150). Then you still have to deal with the head. Valves and seats should be ground and/or replaced. $300-$500 to the machine shop. Exhaust valves bear the brunt of the wear, and of course those are spendy - $100 each and hard to find. Couple hundred bucks for valve guides. Presumably you'll need at least a selection of shims to get valve clearance right.

As you can see, it adds up. You might want to consider a used head - there is one on 190rev right now, fully rebuilt for about the cost of a set of exhaust valves...

Valve adjustment was awful when I bought the car - the exhaust side was all sitting at around .15-.2mm clearence - bringing it back up to the .3 spec gave it more power and made it run smoother above idle. Cam sprockets are fine. Shims aren't a big deal - have a nice Kawasaki dealer down the street

PO Claimed "it's always run like this" so chances are it's had these faults for a while, he owned it for 3 years. I have no idea what kind of oil is in the thing right now (probably too thin) and it's due for a change so I'm gonna throw some rotella in there and see if it smooths things out a bit. PO did the timing chain and tensioner within the last 20k (he also gave me a baggie of brand new MB parts for the next change). He really seemed to be enthusiastic about the car and MBs in general (he did rebuild the SLS system!), but I think he was in over his head.

realistically, what kind of damage will the motor sustain if it keeps on running like this? I'm thinking of a rebuild this winter, but it would be nice if the car didn't have to sit until then, the M3 needs a clutch job and some other little things and I HATE rushing that kind of stuff.

I'm gonna take it out and run it up to operating temp today, change the oil and re-take compression numbers with the plugs out.

when I find and call a machine shop, what do I ask for? I've never done this kind of job before and I don't want to get taken.

Those numbers are not accurate. You can use them in comparison with a WARM compression test to verify suspicions.

Remove ALL the plugs when testing. You need as many RPMs as you can get.

You might be jumping the gun on a full rebuild. Its probably just time to do the head like MTI mentioned.

Damage?... running lean is certainly not good for the cylinders and a shaking engine is bad for bearings. But both of those take time to do severe damage.


edit: I see you are planning on re-doing the comp test.

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