Warm start issue - just checked fuel pressures
 

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

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

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.

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?

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

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...

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

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.

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!

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

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

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.

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?

It's been a long time since I replaced injectors, but as I remember there is a rubber seal, then a plastic holder, then a rubber 0-ring. I think I replaced both rubber pieces, reusing the plastic holder, which seemed fine.

If you are pulling the injectors, which seems like a good first fix attempt if spraying around them definitely causes changes in the idle, then you might try to move the hard fuel lines around, and hook up the injectors outside the engine, where you can check for leakage. Just turn on the pump for a few seconds and see if one drips gas. Obviously use precautions like a cool engine, fire extinguisher handy, etc.

So, it seems the idle starts out okay, then goes so lean the EHA can't compensate, then catches and goes back to normal.
Can't come up with a way an idle leak could cycle like that. And since pulling the EHA allows it to still happen, but not heal, then maybe it's a fuel delivery problem in the fuel distributor. Maybe a leak around the air valve piston that leaks down to a certain point, then the engine tries to recover and opens the IAC further, forcing the air valve to change position and clear it up awhile, then it starts all over again.

Just guessing of course, but it might point to trying to measure the EHA current as the cycle happens, and also measuring the low=side pressure to see if it drops during the go-lean cycle.

DG

With KOEO, EHA current reads .89 mA's. That's not right - I think it should be around 20 mA's...? Pulled the OVP and mA's are now 0 with KOEO - which if I understand the wiring properly, is correct. BTW, the OVP rattles some and is the original, so I'm going to replace it.

My '91 103 reads 20 ma with KOEO, so you may be on to something.

An interesting EHA observation I made one day: when warm, as soon as the engine is started, the EHA reading then jumps to the last setting it was at before it was turned off (in the 1-2 ma range, if Lambda is set pretty well). So it seems to have a mamory of where it should be with a warm engine, even before it goes closed-loop. From that, I deduced that setting the lambda either rich or lean to aid warm starting may not help much, because the EHA cuts right in immeditely on a warm start and mimics the previous closed-loop operation.

DG

Could the air intake temperature sensor in the air boot (late m103 only) be the cause of the intial startup hesitation? Never been able to figure out exactly what the AIT influences (ICV, EHA, ???)

OK, some updates..

I used another multimeter today and checked the EHA current again - since I have a '91 300CE with basically the same system, I was able to compare results.

Using an older Sears multimeter - model 82015 - I was able to get 20 mA's at the EHA in my 300CE with KOEO. For some reason, I guess I can't figure out my newer Sears that I bought with the duty cycle feature - model 82139. With it, I initially got 2.72 mA's - I think...?

Using the older multimeter with my 560SL, I get 75 mA's with KOEO (According to the FSM 07.3-121, 75 mA's is correct with KOEO) - once the engine starts to warm up it drops back down and hovers around 0. I have noticed a hissing noise, air escaping from around the air sensor plate. I decided to try and adjust the mixture - after some fiddling I got the duty cycle to hover around 50% and the EHA current to center around 0 mA's. The idle evened out and isn't nearly as "rhythmic" and the duty cycle does not go to 90%+ now - BUT, the idle speed has increased some, to about 900 rpm in park and 750-800 rpm in gear.

Am I getting closer?

OK. Sounds like the EHA is doing OK.
If it won't idle down, then it would seem the IAC is not adjusting properly, or it's turned all the way down and you have a stuck throttle blade or a bad vacuum leak. There is a simple way to get the EHA to close all the way, disconnect it while it is running, or not running or something - I can't remember right off. But I have done it, and then checked the idle speed from just the throttle blade opening. It idled about 500 with the IAC closed, so my throttle blade must be about right.
There is an adjustment screw on the throttle blade, just like an old carb.

To idle fast, it has to be getting air somewhere: throttle blade, IAC, or a vacuum leak are the main suspects I suppose.

But, all you did to get the fast idle was mess with the air meter arm, right?
So, maybe it leaned out and sped up? but the EHa says it's pretty right according to the signal from the O2 sensor.

hmmmm. more stuff to try.

DG

I turned the mixture screw using my 3mm wrench - mostly CCW, which dropped the duty cycle down in the 30-40% range. This effected the idle too much - too slow...so I adjusted it CW. Got it to even out with duty cycle in upper 50's, but idle was too fast - probably 1100 rpm's. So I brought it back down - CCW - to where I am now.

I don't know if I can adjust the IAC valve - I did unplug it and idle really speeds up and I hear a serious air leak. What do you mean by the throttle blade? I am thinking I need to adjust the air sensor plate...

The throttle blade is underneath the air sensor plate, and controlled by the throttle linkage. It has a stop screw on it that allows a small bit of air to pass thru at idle. Just like an old carburator. The IAC adds the rest of the air, depending on need. If the throttle blade is too far open when the accelerator pedal is closed at idle, then the IAC valve cannot adjust the air flow down enough. With my IAC completely closed or blocked off, the engine idles at 500 RPM with the air sneaking past the barely open throttle blade.
then the IAC comes on and adds air to achieve the 600 RPM idle with the AC on and the tranny in Drive.
may not be it, but worth a quick check.

DG

Is this the "lambda tower" as some refer to it? My SL had a COLD starting issue and a stumble for a few secs after start. Cut out the "plug" and it's a new car now

I had simlar issues with the idle rhythmically going up and down after starting. The problem truned out to be the idle control valve was bad even though the reference check was good. If I jumped it with battery voltage and all those amps the valve could be geard clicking with on and off as I touch the battery positive post with jumper cable. My car was having difficult start cold or hot. Try used ICV.

Will, sorry for late reply.
no, the lambda tower is the projection with the plug in the top that houses the adjustment screw for the big air valve in the top of the throttle body.
This throttle valve or valve is not connected to the accelerator pedal, but is moved down by the air being drawn around it. The actual throttle blade or valve is directly nuder it about 4 inches, and is totally controlled by the accelerator pedal cable and your right foot. Thats the adjustment I was referring to above.

Oh, IAC can certainly affect uneven idle speed if it is not functioning freely.
It's just a little rotating cylinder with a slot in one side, turned by a stepping motor.
A quick fix might be to pull the IAC unit off and spray it liberally with a cleaner or even WD40. I actually blast mine with WD40 and then drip a little oil on the rotator every couple of years; so far so good.

DG