89 300SE Low Idle, etc (presearched) - Rant
 

Car: 89 300se, 150k miles

Symptoms: cold start fine first time everytime - 750 RPM. then bogs down as I have to nurse it that first half mile. Once warmed up idles just at 500 and takes two cranks to restart. But usually runs okay with some struggle up hills with AC, others in the car. Uses little to no oil,

Well I have spent the last several months searching this site and others to understand how to locate the specific source of my problems but without success. Have taken it to two local indys to have them play with mixture with little difference.

What has been done: Bosch Plugs, new plug wires, new distributor, new rotor, new air cleaner, injectors were replaced by PO, vacuum lines checked/replaced as needed, idle control valve cleaned with new hose installed on top, OVP fuse fine. no error or warning lights display something is wrong.

Haven't spent much money at all on this car except maintenance, few fuses, relays, and an AC conversion and like many here I hope to keep it for a long time. I really like my MB. I have rebuilt engines on cars and motorcycles and am a fair home mechanic but not an electronic expert for sure. Also why I do like working on the car I do not want it to be the focus of my life right now with diagnosing this problem every weekend.

I am coming to the conclusion though that there are so many variables that might cause my problems that it could be any sensor or valve or switch that it might be easier to start testing/replacing everything suspect. Sort of an annual investment plan. Parts guys are going to love to see me coming!

Or I can just drive it like it is and put up with it's "character."

Sorry for the rant. Anyone have closer feel for what I should fix I might throw some more money at it.

Keep up the good work here. It has helped me a great deal.

Ranting in Austin,

tpynes

Replace the OVP relay, it's probably dead, and replace the O2 sensor if you cannot verify that it works properly.

I would also check your temperature sensor, your car acts as if the mixture control system is stuck in open loop mode and running too lean.

You should also check mixture control operation.

If someone has left the passenger side window open in a rainstorm, ot dumped a soda on the floor, lift the mat and check the connector for the O2 sensor -- they get shorted if wet and this will cause running problems.

Peter

I would go with Fred's 1st pick -> OVP relay and I would not buy the K.A.E part offered here and at other web sites. Buy it from the dealer. Yes it costs more and yes it's better than the K.A.E part.

The OVP powers the idle control circuit, among other things. I had a very similar problem that a new OVP solved.

Follow up: 89 300SE Low Idle, etc (presearched) - Rant
 

Follow up

Replaced OVP. Bought it from the dealer. Hungary made. Two fuses/my old one was one fuse. Went ahead and used it anyway.

Results: 1. Never had a cold start problem but did have some stumbling after heading out in the morning...ony for a block or two. That has improved.
2. Warm restart is still needed sometimes. No improvement here
3. Overall performance of the car has improved. Car runs smoother and has more power. Shifts better somehow.
4. Rough idle. By rough idle I mean an idle that moves up and down like it is searching maybe 50-75 RPMs, sometimes a little more. No improvement here at all
5. Cold Idle did change from 750 to 900.
6. Warm Idle stays around 500-520 with searching sometimes around 500 575

Replacing the OVP felt like it fixed part of the problem. I suspect mixture adjustments done on this car will need to be readjusted now.

From other posts I am also wondering if the bolt under the rotor might be a culprit along with Idle Contol Valve lower hose, connector or unit itself.

Thanks to everyone. I will also check the o2 sensor and suggestions above.

Tony
Austin, Texas

Have you checked the TVS for proper operation?

Duke

Your car also had a bulletin for a rewiring of the start signal to the KE controller that adds 50rpms. Some cars really need it.

Thanks Stevebfl
ma car is from 1988, does it have the rewiring you mention;
Could you give more detail on the rewiring?

I think it applied to 86-89 and I'm not sure the models or whether a euro would have the same circuit. Probably would but whether it woiuld act the same way I don't know.

I doubt I'll get time soon to find the specifics. Basically the wire from terminal 50 on the starter goes to the KE controller. It passes through a connector in the fuse box on 124 cars (not sure on 126). One opens the connector and separates the two wires from the two pieces of the connector. Then take the wire to the starter and insulate it and stow it. Take the wire going to the controller and ground it.

It won't be nice if you don't follow direction. Grounding the wrong side will let the smoke out, when trying to start.