For those of us that have problem regarding high idle in our beloved Benzes, given below i...
For those of us that have problem regarding high idle in our beloved Benzes, given below is a list of things to check. This info is not mine and I have not tried it myself. I have gotten this from the mbcoupe mailing list. Just thought fellow mb owners would find this info helpful, So heres what the guy has to say......
I developed a slightly high idle bug on my 1988 560SEC a few weeks back. At the same time I got the high idle bug on my 300SE. I managed to solve both and I thought Iâ€™d share my findings, just so it may help someone else one day.
For the 126 V8â€™s, at operating temp, with selector in â€˜Pâ€™ , idle should be 550 +/- 50rpm.
On cold start-up, idle will be around 850rpm for about 90seconds, since cold start injector is giving the extra shot of fuel. Once coolant temp reaches 15degC, the thermo time switch (or coolant sensor) cuts-out and breaks the circuit to the cold start injector. Then rpm should immediately drop to 550rpmâ€¦.and stay there.
However, with my 560SEC, on cold start, rpm would be at 850rpm and drop to 550 within 90 seconds â€“ so all well there. BUT as temp reached operating level of 90deg C, idle would rise to 850-900rpm.
So I then performed the following checks :
AIR METERING PLATE â€“ ensure that this was closing fully. With the engine switched off, if the plate is pressed down it should rise back up smoothly and fully, without any contact whatsoever with the intake body. In any case, if there is contact, it will ont shut fully and would also cause very very hard starting. No problems here.
OVP RELAY â€“ bad relay, or blown fuse, would give poor cold starting and an ABS light. None of these symptoms in place and the relay had good contacts so all well here.
THERMO TIME SWITCH â€“ since idle dropped to 550 within 90seconds, it was evident that coolant sensor was working fine and cutting out.
COLD START INJECTOR â€“ a leaky injector can cause high idle of course, but again since idle dropped to 550 rpm within 90 seconds this was not the problem. Also, fact that car returns 16.8mpg confirms this â€“ a leaky cold start injector would not enable even 10mpg.
VACUUM LEAKS â€“ checked vacuum after throttle body â€“ 23inHg so no problem there. Also exprimented by spraying WD40 around each injector guide whilst engine running to assess any minor vacuum leaks here, but none whatsoever since idle didnâ€™t drop with spray pattern around any of the injector guides.
THROTTLE VALVE SWITCH â€“ The three pin adaptor is situated on the inlet manifold, near the cold start injector. Checked that the central earth wire was good earth and with engine off but ignition in position II, checked the â€˜IDLEâ€™ and â€˜FULL THROTTLEâ€™ contacts â€“ at idle left pin was earthed and moving linkage to full opening, right pin was earthed. SO all working well here, thankfully, since bad throttle valve switch means removing inlet manifold and matching up the throttle valve switch with the throttle body.
THROTTLE LINKAGE â€“ checked that linkages, including linkage with cruise control actuator was not too tight and that throttle linkage was coming back to full rest when released from full. All was well here too.
IDLE AIR VALVE (000 141 16 25 - post 85 only) â€“ inlet and outlet hoses were fine and bypass pipe was good too. On removing the 2 pin connector, idle jumped to 1800rpm. I thought this was a good sign, but Satish pointed out that just because itâ€™s opening fully, doesnâ€™t mean itâ€™s closing fully as well. This meant either this idle air valve was bad or it was not receiving the correct voltage from the idle control unit to close it fully. I could not swap over my idle air valve from my 1985 500SEC, which idles perfectly at 550rpm, since although they look the same, the inlet and outlet ports on the earlier models SECâ€™s have smaller diameters, and they would not have an air-tight fit in the rubber hoses on the later models, whose idle air valves have larger diameter inlet/outlet ports. I had already thorougly cleaned the valve with carburettor cleaner, but cleaning will NOT fix a bad valve. It helps clear it up and keep it smooth, but essentially the idle air slide valve is operated by 2 solenoid resistors and no amount of cleaning is going to rectify those if they are bad.
IDLE CONTROL UNIT (006 545 84 32, on 560SEC) â€“ this idle control unit (held in a plastic bracket behind the big plastic plate behind the carpet in the passenger footwell) if faulty cannot be opened without special equipment, since they are solid state sealed units, very much like relays (the idle control unit is a relay in essence). I could not swap one over from my 500SEC again, since that has a different idle control unit, which can be easily opened and checked for bad solder joints, and is situated in the engine bay, near the ignition ECU.
Since I had eliminated all other possibilities, I was sure that it was the Idle Control Unit or the Idle Air Valve. Both are far too expensive to buy just for testing - $320 and $270 to buy new respectively from MB. So I got in touch with one of my Merc breaker suppliers. They never ever give stuff for testing â€“ if anyone does, theyâ€™ll charge $20 for the privilege. BUT, good contacts are invaluable and my man sent me an idle control unit AND idle air valve from a known working 560 which heâ€™s breaking, for testing â€“ for free !
I first tried the idle control unit down in the footwell. Before swapping, I let the car reach operating temp and idle was at 850-900rpm. I then unplugged my idle control unit and plugged in the replacement â€“ idle still at 850rpm.
I was a little worried now, as I really doubted it would be the idle air valveâ€¦.but I removed my one and tried the replacement. BANG !!!!!! IDLE STRAIGHT DOWN TO 550RPM !!!!! WELL CHUFFED !!
Phoned my breaker and asked how much for the idle air valve - $50 ! MORE chuffed !
I had hardly found time to address this slightly high idle issue â€“ even more so because my 300SE developed a very high idle at the same time !!! The 300 would idle at 1500rpm ALL the time, and the 103 straight-6 should idle at 650rpm ! Since this was more serious I concentrated on this and I did so much testing, swapping over all sorts of compenents from my 260E, which has exactly the same 103 engine management system and fuel compnents. In the end, as mentioned earlier, it was the injector seals â€“ spraying WD40 around each injector guide would drop the idle for the duration that the spray pattern was there. This was my fault, as just a week before the problem began, I had changed all 6 injectors (since one of them was clogged causing a miss at idle and I thought I may as well change all of them)â€¦but I didnâ€™t change the guides ! I donâ€™t know why, I just rushed but it was pretty silly â€“ changed 28 injectors on my other cars and always replaced the guides. This time I skipped it!!! $4 of injector guides were the cause of a severe headache !
Now both 300 and 560 idling absolutely perfectly.
Hope this helps
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