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Discussion Starter · #1 ·
Hi guys,

Cold fast idle has been acting up again. Need some help with diagnosis please.

At start, the cold fast idle is at about 2200 or so, pops down slightly after a few seconds. As the coolant warms up, the idle reduces slow and steady. Then, at around 1800 it starts to dive 500 to 800 rpm and then right back up. It does this down/up, down/up, down/up process through it's warming process. The down/up time interval is about 2 seconds each, goes for sometimes 60 down/up cycles. When it reaches a certain temperature, the rpm down/up process stops and it levels out at around 1400 or so and carries on with normal warming/idle process.

Let's see if I have this correct. As the coolant warms up, the ect sensor works together with the ecm to determine coolant temperature. With this information, the ecm sends out signal to the vsv which works with vacuum to the throttle opener diaphragm, which controls idle during warm up.

I can see the diaphragm working, the vsv either works or doesn't when I unplug vacuum hose. The bottom hose from the vsv goes to the manifold and the upper hose goes to the throttle opener diaphragm on mine.

Maybe the ect sensor (thermistor) is bad? Part way through the warming process maybe the thermistor isn't sensing correctly, providing incorrect info to the ecm? Anyone else have this issue?

Thanks,
Curt
 

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That throttle opener diaphragm, which is open by default,. should close when the ECU comes on line - that's what creates that 'pop down' after a few seconds, and your idle should drop about 500 rpm or so, when the diaphragm shuts. Of course, you need to make sure that the diaphragm is seated correctly - however, once the ECU is online, that diaphragm should not open again, until the next time you cold start the car.
Normally, idle 'hunts' because the ECU is losing control of the idle. After the throttle opener closes, it is the thermal idle air valve and the idle speed solenoid that controls idle. On your car, you can pinch the air hose from the air pipe at the air filter, that leads to the idle speed solenoid - the idle should drop - but the question is, does the hunting stop as well??
 

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Discussion Starter · #3 · (Edited)
That throttle opener diaphragm, which is open by default,. should close when the ECU comes on line - that's what creates that 'pop down' after a few seconds, and your idle should drop about 500 rpm or so, when the diaphragm shuts. Of course, you need to make sure that the diaphragm is seated correctly - however, once the ECU is online, that diaphragm should not open again, until the next time you cold start the car.
Normally, idle 'hunts' because the ECU is losing control of the idle. After the throttle opener closes, it is the thermal idle air valve and the idle speed solenoid that controls idle. On your car, you can pinch the air hose from the air pipe at the air filter, that leads to the idle speed solenoid - the idle should drop - but the question is, does the hunting stop as well??
Hi Bex,

I don't think mine has the air line from the air filter, my idle speed solenoid is electric. I took some of these things off in the summer and cleaned them up. One of them vibrates like crazy, the other had a round, wedge shaped plug. Why do you think the idle hunting only happens for a certain part of the warming up process?

I should also mention that if I unplug a vac hose from the vsv or the ect electrical connection when the idle problem is happening, the idle problem stops. It revs high, but the idle down/up scenario stops.

Thanks,
Curt
 

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Yeah, sorry, you are a 1990 - I have to pay better attention to the 'fine print'....:eek:
Although Rhino will know better, when you disconnect some of the connections to the computer, it forces the computer to use a predetermined table for setting the air/fuel ratio, rather than actually using the information its getting from the sensors.
What are the things that you took off this summer, and what is vibrating? If you don't know the names, try uploading a photo and pointing the the parts you are talking about.
 

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Discussion Starter · #5 ·
Yeah, sorry, you are a 1990 - I have to pay better attention to the 'fine print'....:eek:
Although Rhino will know better, when you disconnect some of the connections to the computer, it forces the computer to use a predetermined table for setting the air/fuel ratio, rather than actually using the information its getting from the sensors.
What are the things that you took off this summer, and what is vibrating? If you don't know the names, try uploading a photo and pointing the the parts you are talking about.
One thing I took off this past summer and cleaned up was the isc, what I would maybe call the plunger was vibrating nicely. The other thing I took off to see if the valve closed properly was the air valve, making sure it was open when cool and closed when hot. Just checking things out to see if they were gummed up while cleaning up the throttle body. The isc was a bit dirty, but nothing else seemed out of sorts.

Thanks,
Curt
 

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What 'air valve' are you talking about? The wax pellet air valve that is open during warm up is on the throttle body itself, and can't be removed. It is on the passenger front of the throttle body, behind a metal plate, just to the side of the throttle position sensor. What is your warm idle, when the car is completely warmed up??
 

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Discussion Starter · #7 ·
What 'air valve' are you talking about? The wax pellet air valve that is open during warm up is on the throttle body itself, and can't be removed. It is on the passenger front of the throttle body, behind a metal plate, just to the side of the throttle position sensor. What is your warm idle, when the car is completely warmed up??
That's the one, I took the metal plate off to see what shape the rubber wedge was in and to see if it moved from cool to hot. When the car is completely warmed up, no electrical load, it idles at about 800 rpm. At least it did last time I checked timing and adjusted valves. I'll check it again this weekend.

Thanks,
Curt
 

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Discussion Starter · #8 ·
What is your warm idle, when the car is completely warmed up??
I checked it with all electrical load off and fully warmed up. Not sure if a cold winter's day makes a difference, shouldn't, but the idle is no longer at about 800 rpm. It's sitting at just over 1000.
 

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Discussion Starter · #9 ·
Update:

I've noticed that the idle problem goes away under certain conditions.

If I start the car from cold, with electrical load off, the idle issue does not always happen. If I start the car from cold, with electrical load on, the idle problem happens part way through the engine warming process.

If I turn electrical load off (lights, blower, etc) during the idle down/up issue, the idle issue goes away. I can turn the idle problem off and on this way.

Some time ago I noticed the rpm used to go up when electrical load was first turned on, then it would come down again. Now, the rpm goes up and stays up.
 

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The idle speed solenoid normally will regulate the idle when there is a load on the car - normally, if you turn the headlights and heater blower on high, the idle will remain the same, or drop down slightly. If you have A/C and turn it on, the idle should rise a bit. If you are saying that you basically get your idle hunting with an electrical load on, it sounds as though this is an ISC issue - either wiring, plunger sticking or ISC getting a bit tired.
 

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Discussion Starter · #11 ·
The idle speed solenoid normally will regulate the idle when there is a load on the car - normally, if you turn the headlights and heater blower on high, the idle will remain the same, or drop down slightly. If you have A/C and turn it on, the idle should rise a bit. If you are saying that you basically get your idle hunting with an electrical load on, it sounds as though this is an ISC issue - either wiring, plunger sticking or ISC getting a bit tired.
Thank you Bex, much appreciated. With electrical load, the idle rises around 500 rpm and stays there. So, without load, the rpm sits at about 1000 (instead of 800 like where it used to be). With lights on or blower on it jumps to around 1500 and stays there. I'll check out my ISC.

This is in addition to the idle down/up problem through part of the warming up process. Which doesn't occur if I let the car warm up without electrical load.

Cheers!
 

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Discussion Starter · #12 ·
Update

New ISC = problem solved!

Thanks guys,
Curt
 

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Thanks for the update, and glad you got this sorted. Enjoy! ;)
 

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This is some great information, my 89 is doing the exact same thing half way through warm up, but not all the time I'm going to try turning the load on and off to see if that changes things up. I'll keep you guys posted to the results.

Also can you guys elaborate on what isc stands for and is it difficult to replace with a new one?

Thank you for all your help.
Mark
 

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ISC is the idle speed controller - a solenoid. It is easy to replace it, but pretty expensive. They can be checked and cleaned, although a bit more difficult if yours is an 89, as yours would be electrical only. However, you should start your own post with your specs, problem, maintenance and testing you have done. While problems may seem similar on the surface, they can end up being entirely different....;)
 
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