Cold start question - PeachParts Mercedes-Benz Forum

VA-Merc · #1 05-14-2013, 01:28 PM

Hey guys,

I recently completed a head replacement (with a used no-crack #14 head) on my 87 300d as I mentioned in another thread. Everything seems to be working well! I have noticed that on a cold start (keep in mind cold engine not cold outside...about 40-50 degrees at night right now) it will misfire even after letting it go through a full glow cycle (about 20 seconds or so). If I rev it up to about 1k rpms for about 10 seconds and then let it back down to idle the miss and rough idle are gone.

Here is a list of what was done when replacing the head:

1) Replaced all rubber fuel lines, primary and secondary filter, braided return lines, and clear lines.
2) Replaced injectors with freshly rebuilt and pop-tested units
3) Replaced glow plugs with 6 brand new monark units and reamed out glow plug holes
4) Replaced the seal in the fuel pre-heater...no fuel leaks can be seen from the front of the engine (the PO replaced the delivery valve seals just before I bought the car a year ago)
5) Removed the injection pump, replaced all oil seals, and reinstalled to the 15 degree spec (used the tool to lock the pump when installing)...did not use a light to adjust timing since I don't want to buy one lol

When I had the head off I checked for timing chain stretch and wear by comparing the crankshaft and camshaft marks...not even a 1 degree difference so I left it alone. The car runs perfect after the initial 10 seconds...and any subsequent warm starts exhibit no issues.

So...am I just being paranoid? I have used the search feature and found that alot of people say not to worry about it...but I am kind of anal about how my car runs lol. By comparison, my 300sd with the om617 and 343,000 miles purrs with no miss at cold start-up.

I did notice that the clear primary filter looks about half full of air...could it be a flow issue from the tank strainer (I've never checked that)? The car runs perfectly otherwise, is not pressurizing coolant and has a ton of power (doesn't seem like it's starving for fuel).

Thanks for any advice!

Diesel911 · #2 05-14-2013, 04:18 PM

Is there any difference when the Tank is full or close to empty? If so that could indicate a Fuel Supply issue.

If the Fuel Supply/Lift Pump Valves are not seating well Fuel can migrate back to the Fuel Tank.

Then there is the Fuel Pressure Relief/Overflow Valve. Some Members have said there is some small Plastic Piece inside that can cause issues.

If you are able to access the Fuel Line that goes back to the Fuel Tank you could get a longer Hose and submerge it in some Diesel Fuel and see if you are getting Air Bubbles when you first start.

VA-Merc · #3 05-14-2013, 05:00 PM

Thanks for the reply!

So for this test I would disconnect the line from the tank in the engine bay and attach a longer hose and put it in a bucket of diesel fuel or something like that? What would air bubbles tell me? Sorry I'm just making sure I understand what I'm testing for.

Tim

jay_bob · #4 05-14-2013, 05:10 PM

I'm having the same problem as you, runs crappy on cold start then smooth as butter.

I am getting air in the fuel system somehow when it is shut down (i.e. really small leak).

I have already swapped the rubber lines between the tank supply/return hard lines, primary filter, lift pump, etc. Also I redid my return lines.

I have the 3 clear hoses (well they are supposed to be clear) between the IP and filter canister holder on order from M-B dealer and will be swapping them this weekend. Also I ordered a new rubber cap for the return line on #6.

If it helps I will report back.

Diesel911 · #5 05-14-2013, 09:13 PM

Quote:

Originally Posted by VA-Merc

Thanks for the reply!

So for this test I would disconnect the line from the tank in the engine bay and attach a longer hose and put it in a bucket of diesel fuel or something like that? What would air bubbles tell me? Sorry I'm just making sure I understand what I'm testing for.

Tim

Since you would be using the Fuel Return Hose going back to the Fuel Tank if you see Air Bubbles it would tell you if you are getting Air into the System.
It will not tell you where the Air is coming From.

barry12345 · #6 05-15-2013, 03:47 AM

Depending on how you checked the valves on the replacement head for sealing. If you cannot find an air issue you might consider doing a cold compression check.

A low pressure cylinder could be slow coming on line. Also even though you are glowing are all the glow plugs really heating well? On the 603 engines many are a little slower coming fully on line by nature when cold. So it would not take a lot to make it worse.

cho · #7 05-15-2013, 03:59 AM

Quote:

Originally Posted by jay_bob

I'm having the same problem as you, runs crappy on cold start then smooth as butter.

sound like school book example for weak glow plug/s or lazy cyl.

cheers
ChO

.

compress ignite · #8 05-15-2013, 04:04 AM

What Cho said.

VA-Merc · #9 05-15-2013, 09:21 AM

Thanks Diesel911...that makes total sense...my brain wasn't working yesterday. I'll check that tomorrow since I won't have any time this evening. All of my glow plugs are brand new, but I guess that doesn't rule out the possibility. Also Jay_bob...I did all of the same things as you and also replaced all of my clear lines with new ones from the dealer...no difference for me yet.

I think the air bubble check will be a good first test. I don't have a diesel compression tester so I will have to purchase one I guess if I need to check the compression when cold.

For history, even before I put on a replacement head it has always started a little rough when cold. I honestly thought a new round of glow plugs, injectors, and a low mileage head would make a huge different on the cold starts but it is the same as it used to be.

I'll report back with what I find tomorrow...thanks as always for the advice!

jay_bob · #10 05-15-2013, 10:42 AM

Quote:

Originally Posted by cho

sound like school book example for weak glow plug/s or lazy cyl.

cheers
ChO

.

I changed the glow plugs for new (Bosch) plugs last fall.

Last time I was up at vstechs, doing the front suspension work, he saw air bubbles in the fuel lines on startup. I'm hoping new hoses fix the problem, it can't hurt to replace the old ones since they are original and look very brittle.

Thanks for the info, if the hoses don't fix it then I will start looking into the other two things you mentioned.

eatont9999 · #11 05-15-2013, 06:28 PM

Try letting the glow plugs go a little longer after the light goes out. Sometimes if I hit the key right after the light goes out, the engine runs a little rough for a few seconds. I usually wait 3-5 seconds after the light goes out to start the engine and it will run just fine on startup. I also have new Bosch plugs but the controller is the same that was on the car before.

OM617YOTA · #12 05-15-2013, 08:57 PM

Double check the glow plugs. Just because they were new a year ago does not mean they are still good. Even new in box is not a guarantee of a functional part. Double check that all the glow plugs are receiving power, and double check the resistance of each glow plug.

After that, compression test.

Diesel911 · #13 05-15-2013, 10:14 PM

"3) Replaced glow plugs with 6 brand new monark units and reamed out glow plug holes"

The Forum Members only recommend Bosch or Beru.

I have a different Year and Model but before I was a Member I had bought a bunch Autolite and Monark GPs on eBay.

6 Months later when I acquired a Glow Plug Reamer I removed the Plugs to ream the Holes only 2 find it was difficult to get 2 of the Autolites out due to swollen tips.

I replaced the Autolites with Monarks because I already had them. I had one Monark fail in slightly over 1 years use.

Replace those with Bosch Plugs and have had no further issues.
Also it is not cold here where I live in S CA.

VA-Merc · #14 05-16-2013, 03:03 PM

Well I may have stumbled onto something...I was getting air bubbles at cold start when I checked it last night. I looked very carefully over everything, and noticed one of the braided return lines was weeping through the casing (I must have flexed it too much when putting it on). I put another length on and ran the car for a few minutes to get any trapped air out. I let it sit overnight and when I started it this morning it didn't miss at all. I'll monitor it the next few days and see if that was it, but I would be pretty happy if it was that easy of a fix lol.

jay_bob · #15 05-26-2013, 02:14 PM

Quote:

Originally Posted by jay_bob

I'm having the same problem as you, runs crappy on cold start then smooth as butter.

I am getting air in the fuel system somehow when it is shut down (i.e. really small leak).

I have already swapped the rubber lines between the tank supply/return hard lines, primary filter, lift pump, etc. Also I redid my return lines.

I have the 3 clear hoses (well they are supposed to be clear) between the IP and filter canister holder on order from M-B dealer and will be swapping them this weekend. Also I ordered a new rubber cap for the return line on #6.

If it helps I will report back.

Closing the loop on this issue:

Replacing the 3 fuel hoses from the IP to the filter made a huge difference in the idle and the cold starting. Now the car starts up smooth and idles much better. I used to have to run a 5 or 6 on the ELR now I have it set on 3 and it is smooth.

Also I used to see bubbles in my clear injector return lines at idle and now I don't. The new hoses are solid full of fuel like they should be at all times.

It was very easy to do this. Hardest part is you will have to pull the IM in order to get to the return hose on the inboard side of the IP.

I only lost about a 1/2 cup of fuel between the 3 hoses and it took only 2 attempts x 10 sec to refill the lines and get her going again afterward. I did not have to crack the injector nuts.

If you do this repair you will need the 3 hoses and a total of 10 crush washers. It's all on EPC or if you want the bill of material let me know.

Important safety tip: The return line that connects to the inboard side of the IP has a banjo bolt that is not like any of the others. It will fit in the filter but it does not belong there. Do the lines 1 at a time and do not confuse the banjo bolts and you will be fine.