1974 280 SE Cold start issue. D Jetronic

[carl888]

Haven't been around much, mainly working on the 380SE money pit however, I have managed to put 3,000 kms on the 280 SE in the last two months, its been fabulous, apart from the differential, more about that later.

I do have a cold start issue that's cropped up in the last month though and I'm a little bewildered.  When cold, I need to crank the car twice to get it to start.  When cold, if I crank the car after two seconds it fires as normal and runs at a very low rpm then dies.  I then crank again for 2 seconds and it fires, idles roughly for a second or two and then runs beautifully at about 1,100 rpm until the AAV closes and brings the revs down to 800.  There are no other issues from then on either hot or cold.  In fact, the idle is so smooth it is approaching the K-Jetronic 280 I have.  The car used to start very well, just a second of cranking when cold and off it went.

I can confirm the following:

-Nice juicy spark
-Ballast resistor functional
-Valves adjusted
-Ignition timing correct
-AAV funtional
-Temperature sensor 1 + 2 functional
-Cold start injector functional. (In fact, the car will not start when cold with it disconnected)
-Fuel pump priming the system in ignition position II
-TPS correctly adjusted

I'm running out of options so my next step is to check the following:

-Battery under voltage scenario if the car has been standing over night. 
-Rest position of the TPS, although I have checked this, there is an idle position.
-Standing fuel pressure.
-Flow test of the cold start injector. 
-ECU test with another unit and/or check voltages as per manual
-ECU trigger points (Replaced 13,000 kms ago)
-Points gap/dwell

Anything else I should add to the list?  Thanks for any suggestions.

Happy New Year to all.

CJ

[KenM]

Hi Carl, only thing I can think of is the fuel pressure damper, if it's not holding fuel then you may have vapour lock temporarily, this happens with my 450 D Jet but I must admit only after a couple of weeks of sitting.

[carl888]

Ken, thanks, and absolutely, that's what I think it is too.

[carl888]

OK, update.

This car has been giving me the shits!  Been driving it most days over the last month and the cold starting issue is just painful in the extreme.  The problem with the diagnosis is that hot starting is a dream, fires up immediately.  This suggests that the damper/non return valve is functioning fine.  Fuel pressure spot on.

The car is due for a major service so I've used that as an excuse to pull bits off the engine in an attempt to find this gremlin, sigh...

Found some items of note, however.

1. The TPS had an intermittent contact at idle i.e open circuit.  I pulled it apart and toyed with the idea of moving the wiper sideways on the track so that it runs on a "Fresher" section of the track however where the track meets the substrate it looks a bit worn.  A bit like a very small ploughed paddock!  Some years back, Oscar gifted me a new TPS (Thanks Oscar!!!!!) so I'll be fitting that this afternoon.

2. The ignition points gap had closed up 0.05mm in 7,000 kms.  I've reset that to 0.30mm and used the required grease on the distributor cam though where that places the dwell I won't know till starting.  Having said that, I have never had an issue with dwell on any points equipped vehicle provided the points gap is correct in the first place.  16,000 kms ago the injection trigger points were replaced and these test fine.  Additionally, at this time, the fuel system was overhauled, all hoses, the filter and strainer replaced.  System pressure as per the book.

3. Today, out of frustration I measured the resistance of temperature sender I + II (The engine induction air and engine water  temperature senders and they previously tested fine) and was surprised to see temperature II (Water) record 32.6 Ohms at 19 degrees C.  According to the WSM chart on page 07.4.2 of "Service Manual Engine M110" it should read 2,500 Ohms at that temperature.  In fact if you read the chart, it should read 190 Ohms at 100 degrees C so I'm not sure where 32 ohms will leave it!

Tomorrow I will substitute a 2.5K resistor for the sender to see if that improves the cold starting.  What I find odd, is that that these variable resistance units with temperature rarely fail by giving bizarre resistance readings, they either go open or closed circuit. 

Question, does anyone know the Bosch part number for this sender?  I can't for the life of me see any number on the side of the unit.

[kastad]

Temp sensor II is 0280130014

It's easier to just unplug it for checking. Unplugging = a zillion ohms 

Did you find out about your fuel pressure? I'm thinking it may run a bit lean, that will cause hard starting when cold, but not a problem when warm. You may use an injector cleaner to help for partly clogged injectors. Checking the spray of each injectors is tiresome, and should only be done when you're more or less sure the injectors are causing your behaviour.

One thing you can check is to verify that all injectors are opening, either by listening to the engine, or by holding a screwdriver to each injector and "listen" to it. You'll hear a nice ticking. In addition, you can disconnect the cold start valve, and crank the engine without the coil connected. Then you should find wet plugs in all cylinders.

[carl888]

Thanks for the part number.  Quite a few listed on eBay it seems:

http://www.ebay.com/ctg/Bosch-0280130014-Engine-Coolant-Temperature-Sensor-/77052430

During checking, the sensor was disconnected   Also, the fuel pressure is 2.0 bar, which is what the book says.  Initially, I thought the problem may have been the non return valve at the tank end bleeding off the excess pressure to zero after shut down, but tests reveal this not to be the case.  It probably would have also been difficult to start when hot if this were the case I suspect.

Anyway, I think I may have found the problem.  Unfortunately I cannot say for sure, because yesterday I removed the D-Jetronic loom from the engine, cleaned the connections, including those on the ECU in the footwell and replaced the TPS before I noticed what may have been wrong!

When I measured the resistance of TS II, I recorded 32 Ohms.  Looking at the sensor itself revealed the wiring loom boot to be torn.  When I looked inside the spade terminals of the sensor, they were very dirty and oily.  As it faces upwards, dirt can accumulate into the body of the sensor if the boot is damaged.  I cleaned the terminals and blasted the area with compressed air and brake cleaner.  After letting it dry overnight, I now record 2,747 ohms at 18 degrees C.   I also cleaned all the terminals on the loom and replaced it.  My only conclusion is that what ever liquid was in the terminal receptacle, or even perhaps the spade connector on the loom had very poor dielectric strength.

This morning, the engine started first time after one second of cranking (I am not sure if one second it too long??).  A road test revealed much improved driveability off idle as well.  This may be due to the clean wiring, re-set point gap, new TPS, clean sensor, cleaned plug leads, cleaned trigger points, I really don't know.  Tonight I will check again and hopefully it should start first time.

I will check the idle mixture with the gas analyser as well.

 

[kastad]

Nice to hear!

Funny thing when trying to solve a problem, we tend to fix many other things before we find the fault! That's good when we don't send our cars to regular services! 

[carl888]

Tested the car first thing this morning, it started first time, what a relief! 

A question about the points.  I see in the maintenance schedule that the ignition points are supposed to be replaced every 15,000 kms. Does anyone actually do this?  Mine are due to be done, but when I had them out for this job, they looked almost perfect, with no pitting or damage, just a small circular mark where they have been making contact.

[carl888]

A few pics of the goings on...

[arman]

Well carl888, you probably cleaned out the last of the dirt to be found in your car when cleaning that sensor boot That so little dirt can cause so much trouble!

What an awsome sight your engine bay is!

My 450 Djet has also trouble starting after some time sitting. So I prime my pump several times before I start, then it starts within a second, but I also have to push down the accelerator for a short moment. I don't think my fuel system holds pressure for more than a day.

BR Arman

[carl888]

Update, the car required two cranks to start yesterday.  I cranked it for about 5 seconds, nothing.  Waited 5 seconds, turned the key again, heard the pump prime, then hit the starter, fired right up and idled somewhat roughly for a few seconds, then caught on all 6, and all was fine.  Even with the above disappointing episode above, it still started better than it had been over the past few months.

Last night, the car was fine after standing for 14 hours.  Fired up right away and idled nicely. 

If I crack the throttle wide open quickly however, the car also misfires in the 1,000-2,000 rpm range.  If I open the throttle progressively, ie, normal driving, it is fine.  This test is obviously only done when the engine is hot.  I don't know what could cause this.

I can't help thinking that there is a fuel issue somewhere.  I might reconnect the pressure gauge and watch what happens at start up.  Grrrrrr.  More work is required.

Thanks for the comment about the engine Arman, it doesn't look that good in real life, I've not cleaned it since the concours 3 years ago.  It did look good when it was first back together though!

[carl888]

Update, it's still playing up.

Not every time, like it was before, but most times.  Next problem, is the car sailed through the fuel pressure test.  The residual pressure is very strong, and after the car sits, it still has 1.0 bar after 12 hours.  Switching on the ignition to position II (To allow the pump to prime) brings the pressure immediately to 2.0 bar exactly.  Grrrr.

I thought the cold start injector may have been playing up, but it's firing, and, not leaking after shut down.

Bearing in mind the car has good cranking voltage, fuel, spark and air, the only possibility is starting mixture, the loom or the ECU.  Unless I've missed something.  Next option is to try a replacement ECU. 

[carl888]

Another update...

Yesterday car fired up most painfully.  Took some 6 cranking episodes to start.  Took it for a drive, let the car sit over night, then this morning it started much better.  Not perfect by any means, but after 4-5 seconds it fired......grrrrrrrr

Question, I'm going to start poking around the ignition and fuel injection ECUs now.  Today's test was checking the ignition, again, connections, coil, ballast resisitors and timing.   Does anyone know if the following ignition ECUs are interchangeable?  The book only mentions the changeover in December 1974, but doesn't mention any other changes with the ECU update.  Original to the car is the version with the external heatsink.  I opened the spare I had, not much inside!

[carl888]

I can now confirm the above two ignition ECU units are interchangeable.  No change with the manner in which the car runs with the new unit fitted.

Starting issue persists...

[arman]

Bugger! I'm out of ideas...

Good luck!