86 560SEL cold start/rough idle

[algae1]

I continue to frustrate myself trying to find the source of this annoying cold start problem. I have diagnosed to the point where I observe the following:
-Engine cranks hard (about 7-10 sec) and then requires throttle blips to keep running first 30 sec or so.
-Idle is very much slower than spec at about 400 rpm and rough UNTIL the smog pump relay kicks out and the pump shuts off --as I understand it this is when Lamda takes over and at that point the idle smooths out (slowly) to about 700 rpm or normal.
-Car runs fine at low and high speed once "warm" and there is no smoke from exhaust at any time.
- I have checked EHA and Idle Air Valve for basic resistance and they are fine. I took the idle air valve out and checked it on the bench and it operates at 12 V. I put the VOM on the disconnected two pin conn to the idle air valve and started the car (it starts just the same as above) and the voltage (DC) bounces around intermittently at a peak of about 3 volts-- is this normal or should the idle speed controller be putting out a constant voltage level??

A few more questions for u experienced wrenches:
-Per SB's ground braking article in DIY can i get fault codes out of my 86 126 chasis--USA version??? factory docs are confusing on this point. If so, can i use Steve's technique on pin 3 of X11 and meausre the pulse widths??
-Since I have not been able to find any sensors bad is it possible that basic engine timing is so off that the car wont idle until the lamda takes over???If so, can that timing be adjusted???

Since my problem is at start up before Lamda - do the codes tell me much (if anything???)..

Any help appreciated.

Have you checked your fuel line pressures?Have you checked the fuel pressure regulator? Sounds like you are getting bleed down when you shut the car off and pressures have to build up for car to start. As a starting point - check the one way valves on your fuel pumps to make sure they are not leaking fuel back to the tank by losing pressure. A pressure test at the fuel distributor will tell you if it is a fuel pressure loss problem. Haven't had that problem on my 560 but reading a lot of posts on this site it seems to be pressure related.

[hbofinger]

I agree with moedip. I'd suspect fuel pressure. Especially if it runs really rough a short period of time after starting (you said 30 secs), and reving the engine helps.

Also - how about the cold start injector? If you shut down the car after driving it (i.e. fully warmed up), and you restart it, how bad is it? If it only has trouble starting cold cold cold (i.e. you drove it to the supermarket, went shopping, came back, and there is NO problem after sitting 45 minutes), then you don't have a leakdown, and I'd suspect the cold start injector.

[algae1]

moedip-hbofinger
Many thanks for your reply. I had the local wrench check the fuel leak downs and we verified at the ditributor the pressure was ok. That was our initial thought as well.

Cold start injector does not enter into the equation as I understand it becuz it is inop above above 40 deg. I replaced it anyway and tested operation.

I do observe that when idling rough if i leave it alone -- once the engine goes into lambda (the smog pump shuts down) it smooths out.

Still searching.

[hbofinger]

Keep us informed - we'd like to hear how this resolves itself.

What I find confusing is that if the fuel pressure is really OK then why the hard cranking? You need compression, gas and spark for the engine to start. One of the three has to be missing. Have you done a compression test on a cold engine? If the compression is good (this will rule out valves and rings) then that leaves gas and spark. If the gas pressure is there - how is your electrical? Do you have a bad spark plug wire? check ohmage on each. Check your distributor cap and rotor for signs of arcing. Are you using platinum plugs - if so - change back to recommended copper ones.While you are at it check your plugs for signs of fouling. By process of elimination the problem will be fixed. The worst case I am thinking of is the fuel distributor or associated component.

[algae1]

Thanks Moe-

Yes this one has me stumped. Wires and plugs are fine and i put new cap and rotor on. I suppose fuel dist is possible like u say -- i did see on a search that there are screens that get clogged. mebbe need to go there next.

Thanks

Forget the screens - if they are plugged that bad - the car would never run. Have you checked the spray pattern of your injectors? Bad injectors could squirt raw fuel in so bad that the engine would have to clear it to run and it would run rough until the extra was cleaned out - similar to your scenario. Do you have any black smoke out the tailpipe when it is cold? Is the inside of the exhaust tip of your tailpipe coated in black soot? If the exhaust is good then it must be fuel starvation - I pulled up an older post about fuel pressure on a 300E -principle should be the same. I am pasting the response from Steveflb here in the hope it may help you:
You need to have fuel pressure for the system to work. You can't condemn the distributor till you have checked fuel pressure.

Does the airflow meter flap offer resistance when the pump runs compared to when its not running. The pressure drops in half instantly on fuel pump shut-off this is a distinct difference in the feel of the flow plate. Jump the fuel pump to test the feel with pressure (?). Also while cranking try pushing down on the plate massive misaddjustment can keep fuel from flowing till large plate movements.


__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
27 years MB technician

Hope this is of assistance - keep us posted

[algae1]

This situation was resolved last week with the replacement of the main vacuum hose. The idle problem worsened to the point where the engine almost would not start, and the brake booster was not funcitoning when the hose went. Replaced the hose and all is well.

Many thanks to responders. HTH others.