1988 560sl high idle in park and neutral

[alteredx]

I am having a problem with my newly acquired 1988 560sl. The symptoms are a high idle at 1200RPM and sometimes minor idle surging when the car is in park or neutral. The car idles normal in gear and runs good. The economy pegs to the left when the engine is running idle but sometimes moves slightly making me suspect a small vacuum leak. When driving the economy guage often moves into the red with only subtle throttle increases. I pulled the idle control valve and noticed that it is very fouled inside with carbon and barely makes any noise when I shake it. I replaced the idle control valve but the symptoms remain the same. When replacing the idle control valve I notice that the 2 hoses that it connects to seem to be very hard and loose fitting. I have ordered replacements for them. I also notice that valve cover to idle air distributor hose is very hard and extremely loose and oil has leaked out all over the idle air distributor. I ordered a replacement for that too. I also pulled and inspected the idle speed controller module but I do not see any visible signs of failure. I ordered a replacement for that just in case. I still have not checked the over voltage protection relay because the fuse is so tightly seated that it will not pull out. I think that I will have to take the kick panel off to get a better reach since I can barely get my hand to it from the glove box. All other functions of the car work excellent.

Can anyone offer me any additional advice on correcting this idle issue?

[meltedpanda]

correct the vacuum leaks as you have found them and symptoms may subside, these cars are very vacuum dependent , especially at idle

[abiby]

It also wouldn't hurt to clean the idle control valve with carb cleaner.

Frankly most of the rubber vacuum fittings and plastic lines are probably due for replacement. Most of the vacuum system is easily replaced if you take care to do it one line at a time.

[alteredx]

I have replaced all suspect rubber hoses and inspected all vacuum lines and joints and can find no leaks. My economy guage is now behaving normal again after replacing the hoses but my idle problem persists. I have thoroughly cleaned the idle control valve and even installed a replacement valve but no change. I checked the voltage to the valve when idling and it reads 4 volts so I think that would mean the over voltage relay is working. I replaced the idle speed controller but that did not fix the problem either. I sprayed carb cleaner over everything obvious while idling but saw no change. I am thinking that I might have to remove the intake because I heard that there are 8 seals under there and they could be the problem. I also heard that the problem could be the throttle switch having failed or needing adjustment. How does the throttle switch adjust? Does anyone have any additional advice? The car idles at 1200 in park but idles perfectly normal in drive with no other symptoms.

[KCM]

Originally Posted by alteredx

I also heard that the problem could be the throttle switch having failed or needing adjustment. How does the throttle switch adjust?

The throttle position switch is located on the end of the throttle butterfly shaft buried in the front center V of the engine. It simply is a switch that turns on at full idle (butterfly closed) and at full throttle (butterfly wide open). If this switch doesn't make contact at idle, it can cause high idle. It should not need adjustment unless it is replaced or has come loose.

First thing to check is to make sure you have no linkage binding that would prevent the throttle from closing completely against it's stop at the butterfly. To check the switch, disconnect the connector on the upper left side of the engine next to the valve cover. The connector has three wires. One is a common, one is for idle, and the third is for full throttle. At the connector leading to the switch, simply use an ohmmeter to check between the common and the idle wire with the butterfly closed (idle position). You should have continuity between two of the wires (one common, the other the idle side of the switch). You can do the same test with the butterfly wide open (footfeed down) between the common and the third wire. If you don't have continuity between two wires in both positions, it is bad. Remember, one is the common and is used for both idle and full throttle.

Had one go bad and caused the high idle. It is buried in the high heat area of the engine, so no wonder they go bad sometimes.

[1988560sl]

Hi,
I have a 1989 560 sl and it recently began riving with a high idel and starting intermit. However, now it will not start at all and acts as if it is getting no fire. Any suggestions?

[meltedpanda]

I have a good article on the tps cleaning procedure if you PM me your email I can send it

[CamelotShadow]

Originally Posted by alteredx

I am having a problem with my newly acquired 1988 560sl. The symptoms are a high idle at 1200RPM and sometimes minor idle surging when the car is in park or neutral. The car idles normal in gear and runs good. The economy pegs to the left when the engine is running idle but sometimes moves slightly making me suspect a small vacuum leak. When driving the economy guage often moves into the red with only subtle throttle increases. I pulled the idle control valve and noticed that it is very fouled inside with carbon and barely makes any noise when I shake it. I replaced the idle control valve but the symptoms remain the same. When replacing the idle control valve I notice that the 2 hoses that it connects to seem to be very hard and loose fitting. I have ordered replacements for them. I also notice that valve cover to idle air distributor hose is very hard and extremely loose and oil has leaked out all over the idle air distributor. I ordered a replacement for that too. I also pulled and inspected the idle speed controller module but I do not see any visible signs of failure. I ordered a replacement for that just in case. I still have not checked the over voltage protection relay because the fuse is so tightly seated that it will not pull out. I think that I will have to take the kick panel off to get a better reach since I can barely get my hand to it from the glove box. All other functions of the car work excellent.

Can anyone offer me any additional advice on correcting this idle issue?

Hope you got it worked out.
Seems I too have old hard hoses but my economy does not peg at idle.

On the hunt for vacumn issues...before I would suspect idle control mnod but I'm not sure if I have one as my 500 sec ia a 83

Arrgh

[JohnnyT]

Have you replaced the Idle Control Module which is located passenger side front floor board? Had the same problem with my 85 380 SL. The module is in a position where as if you have a very minor windshield leak or your top windshield gasket leaks in a heavy downpour it will kill the module. Fought the same problem for almost a year, broke down and replaced the module and haven't had a recurrence since.