ISM guidance

[glockworks21]

Maybe someone can direct me to thread. Im sure this must be explained somewhere I just cant find it.

cold start (40-50 degrees sitting in garage) the car always fires up immediately. then idle spikes up and down 500rpm rapidly every second ( this is a new problem) I can hold gas and keep it smooth. after 30 sec or so its stable but idles around 600 rpm until it warms up and then idle sits at 750. It never stalls..
its a newer dgo ism.. maybe a year old. Ive cleaned it out and oiled it..

today I started it up cold and it started doing same thing. I quickly pulled the plug on ism and the idle stabalized at 750 and it ran smooth no issues.. Im going to test this a couple more times.. I just want to understand this system better. Maybe its going bad already. My other question is do I even need it if the car idles smooth without? I was thinking of keeping it unplugged a few days and seeing how it goes..

any advice or re direction to another thread is appreciated..

Dave..

[Bitsyncmaster]

It's good you have cleaned and oiled your idle motor. Try to remove the vacuum advance. It should not have any when cold anyway but just want to make sure. What advance would do it increase the RPM of the engine (require less fuel/air) and that would make the idle motor run closed or very near closed. The idle motors have very little torque running near their extrema open or closed.

If that advance is not on, my next guess would be your mixture is wrong. That could be due to the WUR or the base setting is not adjusted to the book value.

A vacuum leak can also cause the same problem (results in ISM running to close to fully closed).

[David T]

The biggest cause for idle problems is vacuum leaks. Like Bitmaster says, a problem with the vacuum advance can also affect the idle. So can dirty fuel injectors with bad patterns. Other possibilities include worn spark plugs, bad ignition wires, valves needing adjustment, in other words, just about everything can affect the idle. When diagnosing this problem I like to start with a compression test. If all of the cylinders are not close (within 10%) you will never gt a good idle. The motor should never idle below 770 unless you shut off the idle motor system. The idle system compensates for loads so when you run the A/C or the alternator is working hard the idle stays at 775.

[glockworks21]

I reinstalled the old ism today. Its the original bosch. The car fired up and idles perfectly. No surges at all and it had been sitting for 4 days without being started..
I originally changed that one out about a year ago when i was throwing money at problems.
Lesson learned..


Thanks again...

[82DMC12]

I reinstalled the old ism today. Its the original bosch. The car fired up and idles perfectly. No surges at all and it had been sitting for 4 days without being started..
I originally changed that one out about a year ago when i was throwing money at problems.
Lesson learned..


Thanks again...

I've heard nothing good about those crap smaller ISM, some of them come from China on amazon or aliexpress. Very high failure rate.

[r00b]

I've heard nothing good about those crap smaller ISM, some of them come from China on amazon or aliexpress. Very high failure rate.

The new smaller ones? There seems to be only one, same part the vendors sell. Unfortunately it's our only choice if you need a replacement.

[82DMC12]

The new smaller ones? There seems to be only one, same part the vendors sell. Unfortunately it's our only choice if you need a replacement.

Yeah they are junk. Fortunately there are plenty of good used ones out there. I have two, one of them NOS. Time to start hoarding stuff like this because it's disappearing.