Warm start issue - just checked fuel pressures - PeachParts Mercedes-Benz Forum

rgnprof · #1 08-16-2011, 11:15 PM

New thread, but an old problem. My '86 560SL has developed a warm start problem...after sitting for 30 minutes to a couple of hours, especially in the heat (routinely above 100 here for the last month or two) the car starts but it won't 'go'. It will chug roughly at idle, but if I try to give it gas it just doesn't go and if I don't let off the gas it will stall. After a few minutes, it starts running normally and then runs great. Drove it to Tulsa and back this weekend and it ran great on the highway.

Replaced the fuel accumulator to no avail, so I checked fuel pressures tonight and they are all within specs - engine cold and running 6.4 bar, lower chamber about 6.1. Pressure drops to 3.5 bar when I shut off the car. I have left the gauge on to check any leaking off of fuel pressure and after approximately 2 hours, gauge still reads 3.1 bar (FSM indicates that pressure should remain above 3 bar for 30 minutes after shutting off the engine).

So, I am more confused now. Anyone have any ideas?

ryan

porkface · #2 08-17-2011, 01:08 AM

sounds like the fuel dist and/or injectors are leaking. runs bad until the extra fuel is burned off. good luck, chuck.

Hirnbeiss · #3 08-17-2011, 08:21 AM

Doesnt your car have a cold start valve? IIRC it operates off coolant temp, and it may think the temp is colder than it is and is incorrectly engaging. That would say bad valve or bad sensor.

rgnprof · #4 08-17-2011, 12:51 PM

I'm having trouble figuring out the cold start valve. It seems that it only runs - and squirts fuel - when the starter is running. I don't know what shuts it off...Wiring diagram in the FSM is not terribly helpful here???

So, if my cold start operates when the engine is actually warm - squirting fuel would flood my motor out - right?

S-Class Guru · #5 08-17-2011, 01:15 PM

Ryan, I have a very similar issue on my 300SE that's been driving me nuts for over 10 years. Mine will start, then chug for about 5 seconds, and suddenly clear right up. So, it's not as bad as yours. My pressures hold well, just like yours, after shut-off.
Over the years, I've checked everything, and just can't find it.

From your pressure readings, it appears your system pressure may be too high. For my 6-cylinder, the system pressure is 5.5 Bar, and the pressure differential is 0.4 bar. It may be different for the V8, but you can do a search here to verify those numbers. The pressure controller would be the pressure regulator, but it doesn't seem likely one would read higher than spec as it got older???

You might also try a few of the things I've done (but found nothing wrong).

Disconnect the electrical connector on the cold start valve, or even unhook the line at the fuel distributor and plug it with an appropriate short bolt. This will eliminate the cold-start valve as either leaking or firing when it's not supposed to.

Check the mixture % duty cycle. There are several good tutorials herein.
basically get a cheap Sears meter with a duty cycle readout, and plug it across terminals 2&3 on the round diag connector. Should read close to 50% when warm and idling.

Make a jumper wire setup for the EHA plug, so you can patch in a current meter, and measure the EHA current. EHA should read about +-2 to 3 milliohms when idling warm. There are some good discriptions on doing this in the archives here also.

Check the engine vacuum when warm and idling. Should pull 15-20 inches.

None of these tests, and everything else I could think of in the last 10-15 years, have fixed my problem, but you might be luckier than me.
My engine runs absolutely perfect except for this, and gets great mileage.
All tests indicate spot-on, and everything seems to work just as it should, but it still chugs after a heat soak.

DG

rgnprof · #6 08-18-2011, 12:19 AM

Does anyone know which switch controls the cold start valve? I keep reading about a 'thermo time switch', but I'm not sure I have this switch on my '86 560SL KE fuel injected model? If not, what coolant temp switch am I looking for?

I have checked and re-checked mixture and EHA (see previous posts)...my mixture is around 50%, but my car idles in surges - as the idle surges up the mixture will move from 48% to 60%, or even up to 90% - then it drops back to 50%. I might have isolated my vacuum leak that is effecting the idle - brake cleaner around the several of the injectors caused my idle to even out, increasing some.

My fuel pressures are spot on for a dual fuel pump system...

S-Class Guru · #7 08-18-2011, 06:37 PM

I'm looking at an '86 560 wiring schematic. It's pretty hard to track everything down, but it appears there is no thermo time switch on this later model. The temp sensor you want is a dual unit, with one input going to N3 - the kE control unit, and the other going to the ignition module. It may be the same one as on my 103 engine. On mine, the sensor sits at the back of the head, it's round, with 4 terminals. It is tested by checking resistance across opposite terminals (1st-3rd base, home to 2nd base). Should read about 1100-1200 at room temp, about 300-400 at 80C. There are two other single sensors, one for the fans and one for the gauge.

Tracing around further, I think I found the "starting valve". It's tied to a 3-way relay N16-2, the fuel pump relay with cold start valve control and kickdown shutoff. It actually appears the cold-start valve is tied thru this relay to the starter, and may only run when the starter is engaged, as you said.

Don't quote me on this, this schematic is really small and complex, but this may help.

Good deal on the pressures; I guess the V8 would logically need nore gas than the six.

DG

JohnM. · #8 08-18-2011, 07:57 PM

My M103 has had the same exact "chugging" after heat soak, goes away after one or two shifts completely. I've replaced a LOT of things since owning the car and the chugging hasn't changed. My current best guess is fuel injectors leaking a bit of gas after shutoff, causing a semi-flooded/chugging condition on restart.

Performance and gas mileage are above average on my car too, so I'm not too worried about it.

ps2cho · #9 08-18-2011, 08:46 PM

Quote:

Originally Posted by JohnM.

My M103 has had the same exact "chugging" after heat soak, goes away after one or two shifts completely. I've replaced a LOT of things since owning the car and the chugging hasn't changed. My current best guess is fuel injectors leaking a bit of gas after shutoff, causing a semi-flooded/chugging condition on restart.

Performance and gas mileage are above average on my car too, so I'm not too worried about it.

Regarding yours, IMO, the system pressure to open the injectors at idle is much higher than the 3.1bar the accumulator holds it at. If they were leaking with the car off, they would be leaking more fuel across the driving range. So unless your fuel economy and power is weak, I would look elsewhere -- Possibly airflow potentiometer, or the EHA.

rgnprof · #10 08-18-2011, 09:42 PM

I also tracked down the same wiring diagram...I have the following 2 sensors on my car : the 2 pole one is on the left rear cylinder head (driver's side) and the bigger green 3 pole one is on the front of the motor. I know the 2 pole one runs to the EZL and the CIS-E control unit (B11/2 on the wiring diagram), but I'm not sure what the other does...

I didn't think I had the time switch, but that one of the 'coolant temp sensors' controlled the cold start valve. Although, I am now pretty sure that it is only activated when the starter is running - and only if temps are below 50F (which hasn't been the case here for months!). So I don't think my cold start valve is causing my problems unless it's leaking all the time...which I need to double-check.

Some observations: My idle surges up and down and during this my mixture runs from 48% up to 90% (using my Sears multimeter on pins 2 and 3 of the diagnostic socket) - almost rhythmically . Sometimes it gets stuck at 98.3% and stays there until I bump the gas...

I didn't think my idle control valve was working, so I pulled it off and cleaned it with brake cleaner and I think that helped. But my idle is still an up and down rhythm...I don't know if this is related or not to my heat soak problem, but I don't know what is!

S-Class Guru · #11 08-19-2011, 01:28 PM

RG, I think your B11/2 is the same dual sensor as my 103, it just has one common ground pin, thus the 3 poles vs my dual-ground 4-pole.
So, it would be the one controlling the KE unit with the cold-start injector, and the ignition. But, i agree, very likely not the cold start injector. I unhooked the entire fuel line to mine and plugged it with a cut-off bolt, and ran it that way for awhile - no difference.

The duty cycle readings will indicate correction for rich below 50%, and correction for lean above 50%, so a 90% read would indicate it's trying hard to richen up, maybe correcting for a vacuum leak as you suggested.
one thing, on a hot restart, the EHA remembers where it was running at idle before it was shut off. So, if it was really working to control a lean condition it would be enriching the mix a lot on open-loop start.
You may need to build the little harness to check the EHA values.

But, be careful on the connection to pins 2-3. on my Sears meter, the RED lead goes in pin 2, just backwards of what you would think. You can check it by looking at the % before you crank it, should read + 70% or some other positive % depending on your specific car.

The fuel accumulator has no prevision to control pressure at some value, it just stores what is there. I have always read in the manuals that the initial injector release pressure is around 3.2 BAR. I would think that if an injector is leaking a bit, and the pressure is slowly dropping, that the chugging would clear up pretty quickly after hot start, like mine does, within 5 seconds or so. But, I have replaced all injectors twice with no improvement in the chugging. so, I wrote that off. Maybe I'll pull them all out and reconnect them above the head and pressure the system and look.
but, once i actuall pulled all the plugs and blew out all the combustion chambers with compressed air, then quickly reinstalled the plugs and fired it up - chug, chug for about 5 seconds. Ho Hum.

I'll hang with you on this, you're a good guinea pig for more attempts in solving the mysterious problem.

DG

rgnprof · #12 08-19-2011, 01:56 PM

I actually have the Mercedes EHA harness and have tried to figure out the readings on several occasions - not sure how to interpret these mA readings...

The EHA valve is brand new - well, within the past few months (old one was leaking). I am well aware of the problems with the reading the duty cycle (if you read through my previous posts you'll see numerous questions on this very topic - both here and on the SL forum). I also have a '91 300CE (M104) and it reads just the opposite (red and black leads have to be connected the other way to get the initial 70% reading - CONFUSING!).

Since I have no fuel pressure leakdown, as observed with my pressure gauge, I don't think my injectors (or even the gas cap) are the problem - right? I am maintaining above 3.0 bar pressure for hours after shutting off the car...

As far as vacuum leaks, it seems that this is effecting my idle and hence, both the duty cycle mixture readings and the EHA miliamp readings. I am thinking I need to get this idle evened out first...but I'm not sure? As far as that problem, in spraying carb cleaner around different areas, the idle noticeably changes when I spray around some of the injectors. I don't know if this is enough to cause all of my problems, but I don't see any other vacuum related issues. I am thinking I will replace the seals around the injectors, but I am a little confused as to what all I need to replace. I show o-rings, seals, and fuel injection insulators - do I replace all 3?

ryan

LandYaghtLover · #13 08-19-2011, 02:41 PM

I had a simliar, if not same issue, with my 560SEL. In my case it was the fuel tank purge system. When I was doing engine lambda adjusting I had to remove the purge line. The car would idle great. But once I hooked the line up my reading were all over.

Basically it was a vacuum leak that was "controlled" by one of those temp sensors things that the vac lines go through. When cold, that line was closed off. But when warm, it would usually be open and I would have the issue.

Anyway, once I plugged the black line with white stripe (larger tube by the way) my issue was gone. And my milage is way up. And I feel warm and fuzzy inside. But now my charcoal can might bite the dust.

JohnM. · #14 08-19-2011, 03:04 PM

Quote:

Originally Posted by LandYaghtLover

I had a simliar, if not same issue, with my 560SEL. In my case it was the fuel tank purge system. When I was doing engine lambda adjusting I had to remove the purge line. The car would idle great. But once I hooked the line up my reading were all over.

Basically it was a vacuum leak that was "controlled" by one of those temp sensors things that the vac lines go through. When cold, that line was closed off. But when warm, it would usually be open and I would have the issue.

Anyway, once I plugged the black line with white stripe (larger tube by the way) my issue was gone. And my milage is way up. And I feel warm and fuzzy inside. But now my charcoal can might bite the dust.

Very interesting.

rgnprof · #15 08-20-2011, 06:55 PM

I checked the fuel purge system again - pulled the hose and plugged it and it made no difference in my idle surging.

I swear, this idle surge is so rhythmic, you could set your watch by it...mixture readings start out at 49% and gradually increase to the low 90's%, and then jumps back down to 49% and starts over...idle increases about 100 rpms during this - drops back then slowly increases 100 rpms, drops back and it all repeats.

When I disconnected the EHA, it steadily increased to 98.3% and stayed there...idle actually seemed smoother!

Which comes first - idle problems causing fluctuating mixture readings - or the other way around?

Anyone have any ideas?