300TE hard starting and hesitation problem – Solved

[rdw]

My ’88 300TE with 90,00 Km was running fine. It got quite cold here and they spread a lot of salt and gravel on the roads so I parked it at the beginning of November. Com mid December we got some nice dry days so I went to fire it up and do some errands. It was harder than usual to start, but it started. It exhibited a hesitation when you touch the gas. After driving for 10 minutes the hesitation became a stall and it became increasingly hard to start each time it stalled. I got it back home, into the garrage but by then, it was next to impossible to start.

I took out the plugs (with about 1000 Km on them) and found them soaked with gas and carboned up. I found that if I cleaned them and it was a fairly warm day (0 degrees C or warmer) you could start the car, but only once. Next day there would be no chance of starting it again unless you take out the plugs and clean them.

Here are the things I did in sequence:

• Checked the spark by using a sparkplug outside of the engine. It looked nice and bright to me.- Measured the battery voltage (over 13V).
• Replaced the OVP relay. A previous owner had pried mine apart and the contacts looked black (the new one made no difference).
• Since the plugs were soaked, and I had put in a bit of gas line antifreeze so I took off the fuel distributor to make sure the fuel control plunger was not stuck – it was fine.
• At this point I towed it over to the local German Car garage (mostly VW/Audi, but the sign outside says, VW, Audi, Porsche, Mercedes, BMW). They put in new plugs and roter and charged me about $200. I went to pick it up, and yes it started but when I touched the gas it died. I told them about it and they made a good try to further diagnose the problem the kept it for a month and did the following:
  • Checked the fuel pressure and made sure that both fuel pumps were working.
  • Replaced the fuel filter
  • Did a smoke test to look for any vacuum leaks.
  • Replaced the idle change over valve.
• None of these things helped, it was still exactly as it was when I brought it in, so they told me to come and get the car, I had it towed home.

Over the next little while I looked at various things that could be a problem.

• Since the plugs were getting wet, I replaced the cold start valve and put in a switch to disable it and a light to tell me when it was on or not. It was getting power exactly as the manual says it should. The car didn’t start with it turned off either.
• My Fuel pressure regulator was all covered in silicon by the previous owner to cover a break in the rubber hose, so I replaced that. No difference.
• I used an oscilloscope to check the coil drive signal coming out of the EZL ignition control unit. It went from about 0V to about +10 so I figured that was working okay.
• Today, I replaced the coil with an after market coil from Hqamburg-Technic and Voila! it started and ran again so I’m hoping that my problem is finally solved.

I’m quite happy it was not the Bosch Ignition control unit, as these are rather expensive. I also hope that this Hqamburg-Technic coil is as good as the Bosch. The old wan was Bosch part # 0 221 501 007, Mercedes part # 000 158 5003; the new one is # 000 158 6200

This motor must need an awfully strong spark to run. I remember my friend bringing over his old VW Rabbit (about an ’81) saying that it didn’t start as good as it used to. I measured the voltage on the +side of the coil and due to a bad connection at the back of the fuse box it was only getting 2 Volts!!. The VW still ran fairly well. I can only imagine what the 300TE would run like with that kind of voltage. These hard to start symptoms and hesitation seem to be common on this motor whenever the mixture is not quite right or the spark is not absolutely top notch. I wonder why?

 

[television]

Coils do fail in this way and is the most common way to go, it only takes 1 short turn in the secondary winding (15,000 turns) to render it U/S. This kind of short cannot be measured with an
Ω meter as with any coil the battery in the test meter will set up an opposing field. The only way is with an inductance meter, but since we do not know what the correct inductance should be ,that is of no use.

In practical terms the one short turn in the secondary apposes the out going high voltage, the coil will heat up further reducing the output to the point where it drops to zero. Once cooled down the same thing will happen all over again. though the coils do run warm in the fault condition they will be very hot, often masked by the large clamp that acts as a heat sink.

Malcolm

Sorry about it going bold, it was that ohm simbol

 

[Dieselman]

Reading your symptoms made me think of weak spark straight away.
Good fix though.

When you drove the car was it spluttering under accelleration then cleared as the revs rose to meet the throttle position?

You can buy a cheap flash tester to check the output of the HT spark. You need about 25kv to jump an inch of air and about 15kv to fire the car up from cold. 30kv is better though.

A good addition to a DIY toolbox as it will find all manner of HT problems BEFORE they become a problem.

Thanks for posting this and welcome aboard..

 

[rdw]

Thanks Television and Dieselman. I like that Flash tester. It could have saved me a lot of trouble. Yes it would sputter and often stall when you try to accelerate from idle. It idled fine and ran fine above 2000 rpm or so.