Erractic hiccup at idle in 300CE
 

I have a 1990 300CE and generally when it is hot outside, the car at idle under load has an erractic hiccup and actually shakes the engine, 104.980 engine -24 valve.
This hiccup or miss comes and goes. Putting 93 octane or 89 or 83 does not make any difference.
I have replaced all the ignition components, new plugs, wires, fuel components such as distributor, injectors, fuel pumps. I also checked for all vacuum leaks and fixed all of them.
The thing I have not checked is the fuel pressure regulator, fuel pressure, throttle valve switch or the plate.
What am I not sure is it whether the electrical or fuel related?
I am quite handy, so if some experts on MB can help me, would be great.
Any advice would be great.
bshah@satx.rr.com

If this 'hiccup' is when the a/c compressor cycles on, you may need new motor mounts.

Steve

Erractic miss in 300CE
 

I had replaced the both the motor mounts.
So, I am don't think that is the case. But, I will again check it.
Again, it does not happen every time but more so
when it is hot here in South Texas.
I am still at a loss as this one is a real challenge.
i also have done this over and over again, that I when I
put new Bosch Copper plugs, the miss is gone for like 500 miles or so and then it comes back.
I do know that the spark plug wire set I bought, has the new style spark plug connector by Beru.
I also checked the resistance of each wire, and they are are between 1800-1900 ohms.
Thanks
bshah@satx.rr.com

What are the plugs' appearance when removed? Some conditions can produce a conductive glaze on the insulator.

Steve

Erractic miss in 300CE
 

The plugs look fine and dry.
Nothing out of the ordinary.
I also used the Snap on KV tester that measures the Kilo volts on each wire while the car is running and all the values are within 12-14 KV.
Thanks
bshah@satx.rr.com

Possible cause -

As MB Doc advised me, some input to the ECU is telling it to shut the idle control valve down. He recommended checking the airflow meter, decel microswitch, and throttle position sensor.

It might also be a "sneeze" in the OVP relay.

Erractic miss in 300CE
 

Ok, will check the throttle switch and the air flow meter.
Hopefully the tests are in the MB book as I have not
checked these before.
I did change the OVP relay jst a few days back as the
oen there was the OE relay.
I also had changed the MAS relay almost 2 years back as
the A/C stopped working and in this car, there is no
seperate relay for the KLIMA relay in older models.
If you have suggestions as to how to check the air
flow meter and the throttle switch, please let me know
Thanks

bshah@satx.rr.com

Before checking the throttle switches themselves:

Check the throttle linkage to make sure everything is actually returning to full 'idle state' even when you release the accelerator very gently. Sticky linkage bits seems to be a very common problem.

Can be intermittent, because it may only stick when the throttle is released gently, thus having less spring momentum to overcome the binding.

By linkage, I mean all the related parts, including the little balljoints, plastic wheels, and microswitches. The little plastic wheels seem to be particularly prone to binding.

I use a solvent, such as spray carb or brake cleaner to clean, and spray lithium grease to lube.

Board consensus says to use ATF to lube linkage, but I'm a non-conformist.

Best of luck.

Erractic Idle
 

Will check the throttle linkages again, clean them and lubricate them with lithium gease as you have suggested.
Will check to see if the throttle cable is binding or anything else.
I noticed the problem of hiccup is not noticeablly felt is when I accelerate slowly from a complete stop as suppose to sudden acceleration,
Also, the hiccup is generally felt when the
car comes to a complete stop and then when starts to hiccup at like 15 seconds after that. I do feel a suttle change in the way the car idles but the rpm is quite steady.
Thanks for your help.
bshah@satx.rr.com