1977 300D Vent tube
 

I am still having starting issues on my 300D. Fuel lines, filters, primer pump are all new. Air is getting in somewhere. I'm going to replace the seal at the lift pump and some of the metal seals this weekend. I have been sitting here at work trying to focus on actual tasks at hand but keep thinking of the Benz.

Let me run this by you guys and tell me what you think? My car wouldn't start after work last night. Got it going after pumping the air out. Made several stops and all was well. It sat overnight and started right up this morning. I went out at lunch time and no go. The outside temp changed about 20° or so. So my question is could a clogged tank vent tube cause that with the temp change? Where is the vent tube on my car?

Thank you all!

The vent is for the tank itself, if it's cogged up you may end up with a collapsed fuel tank, but it'll pull fuel.
More history is needed. How long have you owned the car? Has it sat around for a long period of time? Does the primary, clear inline fuel, filter appear clean?

I would, before digging into anything, make absolutely sure you GPs are all working, NG GP, biggest cause of "no start" situations. A clogged tank vent (or tank screen) is likely not your problem but checking it is a good idea. Is your primer pump the "screw in handle' type or the "just pump' kind?

If there is a fuel line air leak, whether it leaks or not might be determined by whether the car is sitting nose uphill or downhill. That difference could account for why sometimes it starts and sometimes it doesn't.
I second Stevo's comments about the glowplugs.

I've owned the car for around 8 weeks now I think..

I am on my third inline primary filter(have 5 spares in the trunk) and my second secondary filter. The GP's are all new Beru's. I have verified that they are in fact working properly. The primer pump is a new style which replaced a new style that the PO installed. I have also stretched the spring back to original length in the fuel return valve on the IP.


After a few days I literally have to pry the fuel cap off. It is sucked on. Is that normal?

No. sounds like a plugged tank vent. My recollection is that vent is in the wheel well. Bell shaped thing. You could also leave your fuel cap partly open to see if the problem is resolved.

If you can't get the cap off, and when you do you get a big sucking sound, then the vent it likely clogged. As stated above it's a bell shaped piece of rubber that hangs down under the car. I can't remember which side, but follow the hard lines around and you'll find it.

That would have changed my GP diagnoses:D:o

tried the fuel cap half off thing. No dice ,same issue this morning after starting fine yesterday evening and making several stops and starting right up like normal. I'm going to try and find the vent at lunch today and verify if it is clogged or not.

I am also going to order another hand pump and recheck all the fuel lines. :mad:

next time it wont start try pulling off the brown vacuum line going to the shut off switch on the rear of the IP.

the vent valve is on the left side of the car underneath the car. To properly diagnosis check your glowplugs with a multimeter. Do you have loop style or the upgraded plugs?

How old is the fuel? have you replaced the inscreen filter?

Thanks for the location. Yeah used my MM on the glow plugs. They are still the old loop style. It isn't the GP's I can see evil air bubbles in the clear fuel line.

Fuel is fresh, topped off a few days ago. By "inscreen" you mean in the fuel tank? No I have not checked that. Both the primary and secondary filters are fresh.

do you check the hoses at the fuel tank ? make sure they arent cracked. The inscreen filter in the tank can be clogged and causing air to enter the system

How did you use the MM on the plugs? You should see the heavy squiggly wires glowing red with the key in the glow position. Are they glowing red when the car wont start?

Which clear line has the bubbles?

I followed the myriad of GP testing threads on this site. Yes they glow red when the car wont start.

I'll get under the car this evening and check the hoses at the fuel tank.

As far as the hose, the PO replaced the "cigar" fuel return hose with a clear fuel line and that is where I can see the air bubbles.