Intermittent no-start issue '89 300SEL
 

Twice now my '89 300SEL has cranked fine, but showed no sign of catching when I tried to start it. Once was in the morning after a cool night. The next time was on a hot day after a long drive (so engine temperature seems irrelevant). For 20 (or so) starts in between, it was fine.

Both times I had the starting trouble, I first checked spark. To do this I used a screwdriver inserted into the distributor-cap end of the coil wire, and held the metal of the screwdriver close to metal on engine. Spark was great. After I performed this check (at the first incident) I put the wire back on, and the car started right up.

The next occurrence I pulled the wire, then just plugged it back into the distributor cap. There was no change (still didn't start). Then I did the spark check (like the first time). Plugged the wire back in and the car started right up again.

I checked the resistance in my coil wire and there is pretty much none, and it didn't change at all as I bent and wiggled the wire (I thought it might be a faulty coil wire).

Now I'm stumped. Is it, perhaps, a sensor starting to go bad?

Any thoughts would be appreciated.

Brett

Sounds fuel related....

Try turning the key to run a few times prior to cranking, maybe then it'll start right up.

Pump weak. Bad fuel pressure regulator. etc. I'd go there next.

that is electrical....it shows up as a fuel delivery issue but it's really electrical.....

Take the air cleaner off and check to make sure that the air meter plate is able to move freely and is not binding against the housing.

Does sound like fuel delivery issue though..... Could be mechanical or electrical in nature.

Is your fuel pump original? When's the last time the filter was done?

Thanks guys.

The pump is most likely original (and the car has about 240,000 miles). I've had the car for about two years and haven't changed the fuel filter.

I do have a 420SEL and a 190E sitting in my collection of cars ready to be fixed or used for parts. I'm thinking the pump from either of those should swap right out. Correct? That way I could test without having to buy a new pump. Or is there a way I can test my current pump?

If the problem is electrical, I might just start driving a different car. Oh man I dislike trying to find electrical problems.

Brett

How is a bad fuel pressure regulator electrical? :confused: Seeing how it is vacuum referenced.



Anyway, with a pump that's 20 years old I'd change it to be safe. Check PNs to see if your 420 pump will swap, I don't think as V8s use dual pumps. But remember, the newest 126 is from 1991 and is 18 years old.

My 300sel's pump was VERY loud before it about kicked the bucket. It was in the $100 range when I bought it, cheap all things considered for a Bosch pump. I'd do it and a filter.

You have testing to do.

I was agreeing with the potential of it being electrical because the fuel computer could be acting up. The regulator certainly isn't electrical. It's also as far as I understand not vacuum referenced like other MBs. On this setup that line is just there to direct fuel into the breather in the event of diaphragm failure.

Couldn't agree more. Just went through that with my car, and although the new pump didn't completely cure my issues, it restored quite a bit of power to the motor, and cured other issues.

Yeah, I just did both of mine and although I still have issues, they really did need to be replaced. The new pump was quite a bit quieter.

Me too, unfortunately......

It took a week to get the pump (internet was ~$110 and local parts store was ~$240). I put it (and the new filter) on yesterday. I'll be using it tomorrow, so hopefully that fixed the problem.

I did notice that the new pump is much quieter than the old one was.

Brett

It's been over a month now since the fuel pump and filter replacement. In that time I've had no problems with starting. Thanks for your help.
Brett