87 300E DIY diagnostics?
 

Is there anyone out there who knows how to do DIY diagnostics on a 87 300E W124 M103 engine? My diagnostic plug is a 9 pin plug that sits in the engine compartment just above the ignition module.
It won't start. It was running well and then dying out when it got up to running temps and then won't restart. It was only restarting after cooling down for a bit. But just recently, it won't even start anymore. The starter turns the engine over but it won't catch. Thinking it was the CPS, I replaced it but there weren't any changes.
I have fuel, I have spark.
Any ideas anyone? Ignition module?
I'd love to run diagnostics but would like to avoid the unnecessary $105 at the local benz shop...thanks guys.
Cam

the 9 pin connector won't do you any good on a no start.


if you have spark, you shouldn't need the ignition module. question is, do you have ENOUGH spark. cap, rotor, wires, plugs and/or coil are real possibilities.


you have fuel, but do you have ENOUGH fuel, as in pressure and volume. you need a specialized pressure tester for this system. spend some time doing searches, especially here, then make up your mind if you want to spend the 105 on tools to diagnose it correctly or let the pro do it.

there's a remote possibility the engine slipped it's timing chain.

the fuel pump relay could be quitting on you. lotsa variables.

we can't see under your hood, the light here sucks. you have to check basics and get real numbers. basic timing, spark kv, fuel psi, fuel volume/minute, etc. good luck, chuck.

Replace the ignition coil.

Diagnostic (specific enough to be worth a grain of salt) didn't really come into play on these cars until the early 90's (from what I understand). I would do a THOROUGH inspection of you ignition system, especially you cap and rotor. Pull it all aprat, and replace what is nessecary, or just replace it all. You don't want to have to replace the ignition module (=$$$). I had the symptoms you describe earlier in my cars ownership, and once I pulled everything aprat, the rotor button was broken of inside the cap. Somehow it would star cold, run fine, but if I shut if off (taking it to a store or something) and tried to restart it, it would just crank. After seeing that, I was baffled that it would even start at all. Good luck, and post research here, so that we can better help.

Check your idle air control hoses. Have they split or come off? Any vacuum leak can cause this symptom as well. As it gets warmer the cracks open up wider and can't keep steady air pressure. If these are as hard as plastic, they are probably cracked somewhere...

Wow, great feedback everyone, I'll keep you posted on what happens.

I would like to sugguest that you start from the flowchart and go through it. It is a very good way to find the trouble on them I have found. It eliminates the guess work & a lot of the unnecessary expense. It's better if you have the bosch analyzer, but it is not really necessary. That way you can rule things out as you go! Good luck!

Ah, yes, start with the flowchart, great idea. What flowchart?

Vacuum leaks must be addressed before anything else. Period. If they are not soft, replace them. Don't mess around over $40 in hoses. They develop cracks in places that are completely out of sight....

....hours and hundreds of dollars later you end up having adjusted multiple items only to find you have to "attempt" to reverse it all because vacuum leaks where throwing you off the entire time.

Here is a DIY on the idle air hoses since they can be tricky:

I was referring to the engine diagnostics basic troubleshooting flowchart. It takes you from start to finish on the start-up capabilities & necessities. One thing you might try thats easy first off beings you changed the CPS etc. Take it out of closed loop...like pull the fuse in the OVP. Crank on it & see if it starts up. If so let it run a short bit, then shut down & replace the fuse. Then try to start up again. They can get cantankerous & get locked out. If it starts then it will give you a run anyway to go from there. Usually, there is only one thing that quits, that leaves a car where it won't start. Lots of things wrong is only from a tragedy...like a wreck, fire, human intrusions. You may have disturbed the old wiring when you replaced the CPS??