2000 DF40 High Idle

[AmmoGuy]

Recently purchased a 2000 DF40. Motor seems to be well kept, and in very nice shape. Just under 400 hours.

Engine fires up, seems to run great, but idles around 1100-1200 rpm. If I pull the IAC breather line and pinch it (per the manual), idle drops to normal 800-900, but jumps back up as soon as I release it. Hosed the IAC valve out with throttle body cleaner, but no change.

Is it likely that the IAC is bad? Or is there a chance another sensor is just never telling the IAC to close?

[AmmoGuy]

Anyone with any thoughts?

[Murray]

One would think if a sensor was causing you grief you would be getting an alarm/code. That being said, sounds like a potential IAC issue. IAC issues are fairly common on this forum. Have you checked for extraneous air leaking into the intake manifold or possibly a fuel blockage/restriction which would increase your air to fuel ratio? Have you inspected the spark plugs for a lean condition? S e l f f i x e r d o t c o m (all on e word) has a genuine Suzuki service manual for $12US in pdf format that will give you some insight, well worth the money.

[AmmoGuy]

One would think if a sensor was causing you grief you would be getting an alarm/code. That being said, sounds like a potential IAC issue. IAC issues are fairly common on this forum. Have you checked for extraneous air leaking into the intake manifold or possibly a fuel blockage/restriction which would increase your air to fuel ratio? Have you inspected the spark plugs for a lean condition? S e l f f i x e r d o t c o m (all on e word) has a genuine Suzuki service manual for $12US in pdf format that will give you some insight, well worth the money.

Thanks for the response.

I thought about about an air leak somewhere, but if that were the case, pinching the IAC breather tube wouldn't cause the RPMs to drop to normal range. Same with fuel ratio. I'd assume that if there was a fuel delivery issue it would show up, even when I pinched off the line? The IAC seems to be open all the time. I can blow air through the valve even when I remove it from the motor.

I sprayed some throttle body cleaner into the IAC and let it sit for a little while yesterday. When I blew it out, I could no longer blow air through the IAC valve?
When I re-installed the IAC valve, the motor didn't want to crank. After giving it a touch of throttle and getting it started, it seemed to idle just a little lower, but still not down to the 800-900 range. Weird. I don't know if I should try and clean it some more, or if it is toast.

Thanks for the manual recommendation. I already have a copy and have studied it in detail.

[Murray]

Not sure what your idle RPM should be but 800-900RPM range seems high, I would think you’re after a 650 RPM idle range. With my engine (2009 DF90A) I can only test the IAC, to ensure it’s working correctly(duty cycle) I need the SDS software. I’m ASSUMING yours is the same, basically I have no option to adjust the idle RPM, the ECM controls the idle based on the input from the following sensors CKP, MAP, TPS, Cylinder temp. sensor, IAT and the Shift position sensor. I have read on my service manual to check the valve clearances for improper idle issues also. Should you decide to replace the IAC search this form for after market sources much cheaper than buying one from Suzuki.

It’s a 22 year old engine, I’m wondering if perhaps corrosion has set in on one of the mentioned sensors and not sending a correct/valid signal to the ECM?

Sorry I can’t be more help, perhaps someone else will chine in with with a smaller list of sensors to check.

[AmmoGuy]

Not sure what your idle RPM should be but 800-900RPM range seems high, I would think you’re after a 650 RPM idle range. With my engine (2009 DF90A) I can only test the IAC, to ensure it’s working correctly(duty cycle) I need the SDS software. I’m ASSUMING yours is the same, basically I have no option to adjust the idle RPM, the ECM controls the idle based on the input from the following sensors CKP, MAP, TPS, Cylinder temp. sensor, IAT and the Shift position sensor. I have read on my service manual to check the valve clearances for improper idle issues also. Should you decide to replace the IAC search this form for after market sources much cheaper than buying one from Suzuki.

It’s a 22 year old engine, I’m wondering if perhaps corrosion has set in on one of the mentioned sensors and not sending a correct/valid signal to the ECM?

Sorry I can’t be more help, perhaps someone else will chine in with with a smaller list of sensors to check.

800-900 RPM is the spec'd idle for this motor.
Idle adjustment is made by pinching off the IAC hose, and adjusting the throttle body air screw to the desired RPM. When the IAC hose is released, it SHOULD maintain that setting (given it's input from sensors gives it the correct info). Meaning, when the engine is warm and idling, the IAC is almost closed (or should be) at nearly 0% duty cycle. The fixed air bypass screw allows most of the needed air through the throttle body. When I release the pinch on mine, the IAC is basically wide open and the engine goes back to the higher idle.

The IAC on this motor receives input from CKP, MAP, CTP, and Cylinder temp sensors. I've confirmed that the CTP is working correctly (simple continuity test). But yes, it could theoretically be one of the other three feeding bad information to the IAC. I haven't ruled that out.

I do not believe there is an aftermarket IAC valve option for this motor. The pre 2010 DF40/DF50 IAC valves were a bit different than most of the others.
However, I ordered a $30 used IAC from ebay that has a "working" guarantee, so I'll see if that causes any changes.

Thanks again. I'm all ears to any suggestions.

[AmmoGuy]

Follow up.

It was the IAC. Replaced it and motor idles/runs properly now.