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Discussion Starter #1
Hello,
I've a following problem (I've spent a dozens of hours in garrage to solve it...).
A story is:
Some months ago I’ve bought a 600SEC ’92 from Japan. A mileage is about 80 000km
I have a problem with it and maybe you can help me.
A problem is as follows:
  • a motor starts w/o any problems, runs smoothly until about temp. of 80C.
  • when it exceedes 80C (motor warm) motor starts to shake, but it drives ok. It seems that a problem is only at idle speed (about 500-600rpm). I see that a right row of cylinders (all 6) do not work at idle when hot.
I’ve mesured, that on a right row of cylinders a fuel mixture is too lean. Both Oxygen sensors and both exhaust temperature sensors in exhaust works properly.

I’m trying to find a reason from months w/o result.
What I’ve already done:
  • no vaccum leackages, a vaccum is above 500mbar.
  • flow metres - both works properly, if I change from L to RH – no change
  • fuel injectors – checed and cleaned. Works properly, BUT I’ve mesured with osciloscope, that at idle speed one injector (6th cylinder) doesn’t work when exceeds 80C - a control signal to this injector disapears. Until 80C everything is ok.
When I disconnect one of temperature sensors motor runs on (from next start). I don't know why... I've also swtched an LH SFI units but w/o result. The same situation with ignition modules.



  • all wirings – rapaired / replaced
  • spark plugs – replaced
  • spark wires – replaced
  • spark dstributors – ok – I’ve changed them from left to right – no result
  • ignition coils - replaced
  • distributors caps – replaced
  • cooling water temperature sensors – replaced (both) and many more…
  • throttles – both works properly,
-

There is no mechanical problems – valves, compression, heads, etc. – no problems. Timing chain, ok – everything seems to be ok….

Please, send me your suggestions what to do.
I really wait for your answer – I’ve no more ideas how to have a proper motor work (I’m sorry for my English…).
Greetings from Poland!
 

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Check the ground connection for that injector. Especially, check the wire grimo in the terminal at the ground point. Sounds to me like a connection is getting hot, increasing resistance until it kills the signal.

Jon
 

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get a dedicated scanner like a Snap-On 2500, and read the ignition voltage. if it's above specs, there is an excessive cap/rotor gap between contacts. even the stock new cap/rotor had excessive gaps. and I think that's why Mercedes replaced it with a new versions or cap/rotor in mid-1993. high gaps, lead to unburned fuel in exhaust stream, lead to computer leaning mixture out to compensate, lead to a lean condition. I know it sounds counter-intuitive, but that's what the early OBD1 and OBD2 systems did, especially when they had distributor ignition. they were VERY sensitive to cap/rotor and spark plug gap, and contact fouling or corrosion.

pull the vacuum hoses off each side of the motor, that supply vacuum to the right/left control valves, that control the EGR valves, and AIR injection pump bypass valve. the control valves are located on sides of intake manifold, about midway back, bolted to side with electrical connectors underneath, and small vacuum hoses feeding them from small nipples on front sides of intake manifold. pull that main feed hose from front, and block it at intake. does the problem then go away ? if so, leave this emission equipment disconnected.

if there an exhaust leak ? it will lean out the o2 sensor and drive the voltage low on that side, if located ahead of o2 sensors.

are the air filters clean ? what is the condition of the intake air temperature sensors ? they are relatively cheap to replace. they are located in front of the air filter housings on the intake tubes.

are there any codes in computer ? you need to put it on a scan tool and read ALL the live data while the engine is running. what is the fuel pressure with key on/engine off, and key on/engine running ? check it at the rail on regulator, back of engine, top of intake.

what is the condition of the fuel filter at back of car near tank ? a fuel pressure/volume test must be done.

what is condition of the catalytic converters ? pull the o2 sensors, insert a backpressure gauge, read the pressure. if it's above 3 psi, you need new converters. it should be zero, but I see new converters with 3 psi.

what's the condition of the mufflers, any chance they're blocked ? there are vacuum hoses that go down behind engine at back of intake on top, very hard to find a leak there. also hard to find a leak where vacuum hoses run under windshield wiper area cowl cover.

are both sides of motor staying in closed loop when warmed up, with o2 sensor output voltage varying from 50 to 900 mv ?? if it's staying near the high side, or staying near the low side, there is a mixture problem, or the o2 sensors are bad, or there is a wiring problem.

pull the hoses off both sides of gas tank vapor system, located on top of each throttle body. block it with caps on throttles. does the problem go away ? if so there's a leak in vapor system.

pull the hoses off engine crankcase feed, on BOTTOM of each throttle body. block at throttle body. does problem go away ? if so the crank evacuation lines under intake are leaking.

fwiw you can block the egr valves at the intake as well, remove the metal lines under the the throttle bodies, insert a thin .030" piece of sheetmetal cut to proper size, retighten flange- it will block the egr valve permanently, from any leaks in the metal feed lines.

keep in mind a hole or leak in the air injection lines, that run on both sides of engine, and under the engine, will suck fresh air INTO the exhaust, and lean the o2 sensors out as well. you may or may not even hear it. the only test if block it somehow see if problem goes away.

last it may be egas or lh injection modules acting up. but I'd look at all the easy cheap stuff first.

I've noticed these engines tend to act up when they get hot, rather than cold.
 

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when you say "a fuel mixture is too lean" do you mean one single cylinder, or a number of cylinders ??
it reads like it can be either one, or more, or the entire bank of cylinders ?

be sure the intake valve camshafts are not advancing prematurely at idle. when I did an actuation test on the cam advance mechanism on mine, it caused the engine to run rough and shake, when actuated while sitting still in the driveway in park.
 

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Discussion Starter #5
Check the ground connection for that injector. Especially, check the wire grimo in the terminal at the ground point. Sounds to me like a connection is getting hot, increasing resistance until it kills the signal.

Jon
Thanks for reply. As I know. an injector termial has two pins - 12V (continiously) and a ground given by "computer". I think, that I still need to search. Your theory would be ok, but why when I'll take out a temperature sensor signal motor runs ok? A real temperature remains the same. A reisistance of sensor is proper acc. to STAR.
Please look below - first image - motor below 80C (or temp. sensor unplugged). Second - motor is shaking.
6th -1.png
6th -2.png
 

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Discussion Starter #6
when you say "a fuel mixture is too lean" do you mean one single cylinder, or a number of cylinders ??
it reads like it can be either one, or more, or the entire bank of cylinders ?

be sure the intake valve camshafts are not advancing prematurely at idle. when I did an actuation test on the cam advance mechanism on mine, it caused the engine to run rough and shake, when actuated while sitting still in the driveway in park.
Fuel is too lean - I mean one row of cylinders - I measure fuel / air ratio by Lambda sond.
A camshafts variators works properly.
 

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it runs smoother when you unplug temp sensor, because temp is reading at like -72 F with it unplugged, making engine go into rich cold startup mode. but if you try to drive it that way, it will be way too rich and run lousy, usually. at least mine did. what you can try is, unplug both MAF mass airflow sensors, it will then revert to speed density mode, running on the MAP sensors, what happens then ?
 

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Fuel is too lean - I mean one row of cylinders - I measure fuel / air ratio by Lambda sond.
A camshafts variators works properly.
ok that means an entire BANK of cylinders is lean, which is usually what happens with these cars, being it's 2 different systems controlling each side. now you have narrowed it down. it's not a single injection plug, it's a sensor or leak or pressure affecting the entire bank of cylinders, or an air filter blocked ? pull both air filters out, replace the lids, see if it goes away. if it does you have a blocked air filter. with my snap on 2500 I can actually see each cylinder separately 1-12 and locate a misfire exactly on the ignition. it will also show each injector firing or not. you need that capability. if you pull the plugs you can read them, are they all white, or just one white ?
 

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unplug the MAF located on driver's side, if it goes away, it may be the MAF sensor, remember the MAF and intake air temp sensor and throttle body, etc. on LEFT side, is what feeds the RIGHT side bank of cylinders. so if you have a leak in the vapor cannister or crank ventilation system on the left side, it leans out the right side cylinders too. and vice versa. EGR valves by contrast, affect the side they are located on.
 

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Discussion Starter #11
get a dedicated scanner like a Snap-On 2500, and read the ignition voltage. if it's above specs, there is an excessive cap/rotor gap between contacts. even the stock new cap/rotor had excessive gaps. and I think that's why Mercedes replaced it with a new versions or cap/rotor in mid-1993. high gaps, lead to unburned fuel in exhaust stream, lead to computer leaning mixture out to compensate, lead to a lean condition. I know it sounds counter-intuitive, but that's what the early OBD1 and OBD2 systems did, especially when they had distributor ignition. they were VERY sensitive to cap/rotor and spark plug gap, and contact fouling or corrosion.

pull the vacuum hoses off each side of the motor, that supply vacuum to the right/left control valves, that control the EGR valves, and AIR injection pump bypass valve. the control valves are located on sides of intake manifold, about midway back, bolted to side with electrical connectors underneath, and small vacuum hoses feeding them from small nipples on front sides of intake manifold. pull that main feed hose from front, and block it at intake. does the problem then go away ? if so, leave this emission equipment disconnected.

if there an exhaust leak ? it will lean out the o2 sensor and drive the voltage low on that side, if located ahead of o2 sensors.

are the air filters clean ? what is the condition of the intake air temperature sensors ? they are relatively cheap to replace. they are located in front of the air filter housings on the intake tubes.

are there any codes in computer ? you need to put it on a scan tool and read ALL the live data while the engine is running. what is the fuel pressure with key on/engine off, and key on/engine running ? check it at the rail on regulator, back of engine, top of intake.

what is the condition of the fuel filter at back of car near tank ? a fuel pressure/volume test must be done.

what is condition of the catalytic converters ? pull the o2 sensors, insert a backpressure gauge, read the pressure. if it's above 3 psi, you need new converters. it should be zero, but I see new converters with 3 psi.

what's the condition of the mufflers, any chance they're blocked ? there are vacuum hoses that go down behind engine at back of intake on top, very hard to find a leak there. also hard to find a leak where vacuum hoses run under windshield wiper area cowl cover.

are both sides of motor staying in closed loop when warmed up, with o2 sensor output voltage varying from 50 to 900 mv ?? if it's staying near the high side, or staying near the low side, there is a mixture problem, or the o2 sensors are bad, or there is a wiring problem.

pull the hoses off both sides of gas tank vapor system, located on top of each throttle body. block it with caps on throttles. does the problem go away ? if so there's a leak in vapor system.

pull the hoses off engine crankcase feed, on BOTTOM of each throttle body. block at throttle body. does problem go away ? if so the crank evacuation lines under intake are leaking.

fwiw you can block the egr valves at the intake as well, remove the metal lines under the the throttle bodies, insert a thin .030" piece of sheetmetal cut to proper size, retighten flange- it will block the egr valve permanently, from any leaks in the metal feed lines.

keep in mind a hole or leak in the air injection lines, that run on both sides of engine, and under the engine, will suck fresh air INTO the exhaust, and lean the o2 sensors out as well. you may or may not even hear it. the only test if block it somehow see if problem goes away.

last it may be egas or lh injection modules acting up. but I'd look at all the easy cheap stuff first.

I've noticed these engines tend to act up when they get hot, rather than cold.


Air pump, EGRs, air filters, are ok. No leackage at exhaust system, muffers are in prefect condtion, nothing is blocked. O2 sensors - replaced, works properly.
Tempeature sensors - brand new, proper resistances. No problems with vaccum - everything looks ok.
Rotor caps etc - no problems. I've changed them from side to side and no effect. Always problem at right cylinder row.
No fault codes in computer.
Only problem is just one injector. Not even it, but a cotrol signal for it.

It really seems strange...
 

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Discussion Starter #12
unplug the MAF located on driver's side, if it goes away, it may be the MAF sensor, remember the MAF and intake air temp sensor and throttle body, etc. on LEFT side, is what feeds the RIGHT side bank of cylinders. so if you have a leak in the vapor cannister or crank ventilation system on the left side, it leans out the right side cylinders too. and vice versa. EGR valves by contrast, affect the side they are located on.
MAF works properly. A problem is even I'll unplug both MAF's.
 

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the factory brand new caps and rotors for that year, may give you a drive-ability problem, due to excessive gaps at the rotor to cap interfaces. I've measured .090" at the terminals, and .040" at the center posts, that is way too much, it'll cause a rich problem, and the computer will compensate lean- causing the problem you have now.
the only way to know for sure is read the ignition secondary voltage with a scan tool. mine was high over 42kv, with a new late model cap/rotor, it dropped down into spec 32-35kv.
mercedes changed the cap/rotor design in mid-1993, and intuition tells me, this is why.
in 1996 they went to coil on plug, and same reason.
distributors used on a late model obd2 system, causes strange side effects when the rotor/cap gaps get large, because the ecm is always trying to compensate.
 

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Discussion Starter #14
ok that means an entire BANK of cylinders is lean, which is usually what happens with these cars, being it's 2 different systems controlling each side. now you have narrowed it down. it's not a single injection plug, it's a sensor or leak or pressure affecting the entire bank of cylinders, or an air filter blocked ? pull both air filters out, replace the lids, see if it goes away. if it does you have a blocked air filter. with my snap on 2500 I can actually see each cylinder separately 1-12 and locate a misfire exactly on the ignition. it will also show each injector firing or not. you need that capability. if you pull the plugs you can read them, are they all white, or just one white ?
Air filters are new. I've checked all injectors by osciloscope, and just one is a problem when hot, Somethig is shutting off this injector when hot. It seems a "computer".
There are no misfires.
 

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the factory brand new caps and rotors for that year, may give you a drive-ability problem, due to excessive gaps at the rotor to cap interfaces. I've measured .090" at the terminals, and .040" at the center posts, that is way too much, it'll cause a rich problem, and the computer will compensate lean- causing the problem you have now.
the only way to know for sure is read the ignition secondary voltage with a scan tool. mine was high over 42kv, with a new late model cap/rotor, it dropped down into spec 32-35kv.
mercedes changed the cap/rotor design in mid-1993, and intuition tells me, this is why.
in 1996 they went to coil on plug, and same reason.
distributors used on a late model obd2 system, causes strange side effects when the rotor/cap gaps get large, because the ecm is always trying to compensate.
r
I think it's not a problem with rotos or caps -as I've written - I've changed sides from right to left and no results. Still right side prblems.
 

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Air filters are new. I've checked all injectors by osciloscope, and just one is a problem when hot, Somethig is shutting off this injector when hot. It seems a "computer".
There are no misfires.
ok I see. perhaps computer, or connector as JAL stated, or wiring to it...
I've also see single cylinders foul out from crankcase fumes being directed at that cylinder,
via the crankcase vent tube. it was strategically located on intake manifold that it oil fouled one cylinder.
it was a bitch of a problem to find but so simple too. I thought it was a stuck piston ring at first !
that's why first thing I always disconnect/block all peripheral systems to see it it goes away first, it often is the add-ons emissions, not the engine itself.
I'm pretty sure these cars have the capability, the if one cylinder misfires long enough, the computer then shuts off the fuel to that cylinder as well eventually. but I'm not 100% sure.
 

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ok I see. perhaps computer, or connector as JAL stated, or wiring to it...
I've also see single cylinders foul out from crankcase fumes being directed at that cylinder,
via the crankcase vent tube. it was strategically located on intake manifold that it oil fouled one cylinder.
it was a bitch of a problem to find but so simple too. I thought it was a stuck piston ring at first !
that's why first thing I always disconnect/block all peripheral systems to see it it goes away first, it often is the add-ons emissions, not the engine itself.
I'm pretty sure these cars have the capability, the if one cylinder misfires long enough, the computer then shuts off the fuel to that cylinder as well eventually. but I'm not 100% sure.
I'm also disconnecting everythig, O2 sensors, exhaust temp. sensors, MAFs and always motor works similar- cold -OK, hot - shaking...
 

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Check the pins in the connector where the fuel injection harness mates with the main harness in front of the coffin box. Maybe you have a corroded or pushed pin.

Jon
 

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As I know. an injector termial has two pins - 12V (continiously) and a ground given by "computer". I think, that I still need to search.
There are a number of possibilities and the best approach to solve the issue is to check everything that can affect fueling on one bank only, i.e., check ALL pins on the respective LH module, both cold and warm. StarTek gives the following information on this subject:

 
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