W201: Hot Stall Problem

There aren't many pics to this post, sorry. Down the road I'll try to post some pictures of the processes.

Customer came around with a 1991 190E with a 2.6 M103 in it that "stalls after driving for a bit". Nothing like a vague description to get the ball rolling. After being a bit more inquisitive, the car hasn't had anything done to it in a while (about 2 years) - recipe for disaster when it comes to these older CIS cars. Over the years I've got a formula for reliable, non stranding CIS cars - if you're not spending atleast $300-$600 a year in strictly maintenance, you're going to get stranded. Find something to spend money on and your car will love you.

Anyway, after going through the car a bit, it was very evident that it was in desperate need of a general tune up. A pea shudder and a random miss when running, Cyl 2 and 5 had plug wires that deteriorated and split from the boots from someone yanking on the wire rather than boot, had Autolite resistor spark plugs, rotor was white from arcing, cap seal bone dry from never being changed, and the engine bay was in overall disrepair.

So, starting with the basics. Rockauto and AutohausAZ get most of my regular repair business. I use the dealership mostly for hard rubber parts like bushings and the oddball fuel lines.

So what did I buy to get this car back in tip top running shape?

Denso T20EP-U spark plugs
Bosch Cap (and new cap seal)
Bosch Rotor
Mann Air Filter
Mann Fuel Filter

It's not much, but that's generally what will make these cars run like pigs, or take them out of commission. There's more to go through, as outlined in this blog post I wrote a few years back: http://artisanexcite.blogspot.com/2013/12/what-to-do-when-my-m103-is-giving-me.html

While still serviceable, it's been far too long since this air filter has been changed out. It's darker than I'd like it to be.

Here's a look at the distributor rotor, new on the left, old on the right. Not sure if you can make it out too well in the picture, but the white haziness on the otherwise black rotor is from arcing. That's usually from water intrusion from a poor cap seal. Water gets in, causes arcing, which is what feels like misfire in most cases. If your rotor looks like this, it's time to replace it. It's not as effective in spark transfer as it used to be.

 New cap seal on the left, old on the right.

Not many people know to replace this. All your brand new ignition parts will most likely fail prematurely from water intrusion as a result of this faulty seal. It's cheap ($6 at most), just replace it.

So with the spark plugs, two plug wires replaced, cap, rotor air and fuel filters replaced, NOW I can go about figuring out what is causing this car to stall. I went through all the cars systems with the engine at operating temperature and a couple trips around the block to heat up the O2 and cat a bit.

Set lambda to 47-51% as it was very rich at near 70%, idle smoothed out and throttle became much more responsive. The adjustment tower was never molested, so out of all the cars I've had to touch, this was only the SECOND that I had to pop the blocking ball bearing out of it!
Set the air flow meter potentiometer to as close to .70 volt as I could get. It was slightly out of adjustment and the idle settles much smoother with this set this way.

Now I've got this engine running nearly smooth enough to balance a quarter on it. So now I have to try and replicate the stalling.

I drove the car nearly 30 miles and no stall, but I'm hesitant to call it done. I let the car idle in the sun for about 15 minutes. It got hot enough to finally kick on the auxiliary fans in front of the radiator and after the fans shut off is when I got some action.

A minute or so after the fans shut off the car seemed to misfire just once. When I finally got up off the couch in the garage, the engine instantly shut off, like someone turned off the ignition. I tried starting it up and nothing, not a hiccup. I went over to the toolbox to get my test light, starter jumper, timing light and multimeter, time for some diagnosis.

Since the car still wouldn't start, it gave me an opportunity to check for spark. I pulled a plug, jumped the starter and got nothing. Cool, I directed my attention to the coil. With ignition on, both terminals on the coil should have 12v - with engine cranking the negative side should be pulsing. Check with test light while cranking engine for the light to pulse - if it's not pulsing, we have to move down the line. If it IS pulsing, chances are your coil isn't pushing anything out.

Next step for me was crank sensor check - by this time, the car cooled down some and the engine started. So I shut her down and tested the crank sensor, which checked out okay at 980ish ohms. Spec is around 650-1200, if it's within that, your CPS should be in decent shape. BUT, there's a huge BUT, read on..... With that, started the engine again to let it heat up. Same story, fans shut down, couple minutes later the engine completely shut down. Since I couldn't get the engine started after it shut down, I retesting the CPS. Guess what? OPEN CIRCUIT. No resistance what so ever. Well, I think I just found why this car was stalling. I left my meter on the sensor to see what happens when it cools down some, after about 5-10 mnutes, resistance returned and was around 1.1k ohms. Well I'll be damned, I've never seen a CPS fail this way, usually they'll just fail outright and give me a no start.

CPS is on it's way, and will update this post below when it arrives. I'll have pictures to add to the post then too.