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How to test IAC and throttle body sensors...

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Old 07-15-2014, 08:51 PM
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Default How to test IAC and throttle body sensors...

Okay, still working on my long crank/rough start issue, and I'm trying to rule out some sensor issues.

(Quick refresher for those who havent read my other thread: having a long crank/rough start issue usually first in the morning. Tested fuel pressure and found it out of spec, so replaced the fuel pump. No change. Pulled plugs and injectors, gave injectors and rail a simple cleaning and had good flow through the crossover tube. Plugs look lean on the passenger bank. Cleaned throttle body and IAC about 6 months ago when I fixed the plenum. Replaced plugs and wires at the same time. Distributor cap and rotor are 3 years/15,000 miles old. Issue is not weather dependent.)

Found this article: http://www.troubleshootmyvehicle.com...e-tps-sensor-1 on testing the TPS, and mine tests within the listed specs - but just barely. Resting voltage was 0.51 and full open was 3.63. I'm wondering if the TPS is on its last legs... bad enough to cause problems like a rough start and hesitation while running, but not bad enough to throw a code. Any thoughts here?

I also am going to test the crankshaft position sensor with the method found here: http://www.troubleshootmyvehicle.com...-sensor-test-1 , but I'm going to assume for now it is fine (or at least within spec.)

The other thing I would like to know is if anyone has a way to *test* the IAC. I would like to test these sensors before i just throw money at them, you know?
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Last edited by Twmays; 07-16-2014 at 02:04 PM.. Reason: fixed broken links
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Old 07-15-2014, 11:40 PM
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That sounds very similar to a problem I had with my 01 5.2 in the spring. It took a lot of cranking to get her going in the morning and a slightly rough/bouncy idle. I replaced the TPS and it has been running and starting fine since. Not really sure of any other ways to test the TPS, thought this might be helpful though.
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Old 07-16-2014, 03:29 AM
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You can get a TPS from rock auto for about $24.00 dollars, plus shipping.
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Old 07-16-2014, 10:38 AM
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Neither of the 'test' links seems to work for me...... Not sure what that is all about.

Test TPS with an analog meter. Digital meters don't react fast enough. Should be less than one volt at closed throttle, greater than 3.5 at WOT. Transition should be smooth all the way thru. If it bounces around, TPS is bad.

If your engine starts and idles cold around 1200 or so, drops down to around 600 or so when warm, and in gear, IAC is working fine.
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Old 07-16-2014, 02:12 PM
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Quote:
Originally Posted by HeyYou View Post
Neither of the 'test' links seems to work for me...... Not sure what that is all about.
Think I have that fixed now...

Quote:
Originally Posted by HeyYou View Post
TPS with an analog meter. Digital meters don't react fast enough. Should be less than one volt at closed throttle, greater than 3.5 at WOT. Transition should be smooth all the way thru. If it bounces around, TPS is bad.

If your engine starts and idles cold around 1200 or so, drops down to around 600 or so when warm, and in gear, IAC is working fine.
TPS: I used both an analog and a digital. Analog showed smooth movement all along the transition from closed to open. Used the digital to get my min/max readings. Should be fine, but those min/max are just within tolerance and that concerns me with the issues I'm having. (test procedures I followed said 0.5-0.9 at close, 3.5 - 4.5 when open. I was just barely above that 0.5, just barely above the 3.5)

IAC: Starts (when it wants to!) and hits right about 1200, then almost immediately drops down to 600. Slight bounce around that 600... say 590 to 620. Stays in that range when warm. Had it off when I did the plenum, and cleaned it well without moving the pintle. I'm guessing it's within spec.

They're both about $20 on rockauto, so if I have to change one I'll change both.
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Old 07-16-2014, 03:30 PM
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The PCM learns 'new' parts at initialization. When you replace a part, you MUST reset the pcm, so it 'forgets' the old parts.... otherwise, it has no clue that anything has changed, and will continue to try and run the engine off of the old data. That doesn't always work out real well.

Your idle dropping that far almost immediately isn't quite right. It shouldn't really do that... I don't think changing the IAC will make a difference either, as it just responds to PCM signals for what it should be doing. I would be interested in seeing just what the PCM thinks is going on at initial startup...... wonder if it is getting some wrong data somewhere. (coolant temp, or engine RPM....)
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Old 07-16-2014, 06:55 PM
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I would be interested in seeing just what the PCM thinks is going on at initial startup...... wonder if it is getting some wrong data somewhere. (coolant temp, or engine RPM....)
That's what I'm thinking. If the TPS doesn't resolve it, CPS is next on the list.
Any other ideas?

I need to get a good scanning tool so I can read live data... and then learn what the live data means.
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Old 07-16-2014, 07:28 PM
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I think next I would try and see what temp the PCM thinks the engine is at cold startup....
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Old 07-17-2014, 10:57 AM
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Quote:
Originally Posted by HeyYou View Post
The PCM learns 'new' parts at initialization. When you replace a part, you MUST reset the pcm, so it 'forgets' the old parts.... otherwise, it has no clue that anything has changed, and will continue to try and run the engine off of the old data. That doesn't always work out real well.

Your idle dropping that far almost immediately isn't quite right. It shouldn't really do that... I don't think changing the IAC will make a difference either, as it just responds to PCM signals for what it should be doing. I would be interested in seeing just what the PCM thinks is going on at initial startup...... wonder if it is getting some wrong data somewhere. (coolant temp, or engine RPM....)
Don't want to highjack this thread but real quick i changed my IAC when i bought the truck but i didn't reset PCM. Ho do i do that? My truck idles cold at like 900 maybe a little less in park and at 600 warm in gear so according to your earlier post cold im low.
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Old 07-17-2014, 11:33 AM
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To reset the PCM, just disconnect the battery for five or ten minutes.

If it maintains a higher idle when cold, and gradually comes down as the engine warms, or, immediately comes down if the engine is already warm, things are working as they should, and I wouldn't worry about it.

If you are stalling when putting into gear, or changing direction of travel, that is a problem..... Mine sometimes does that when it is cold out, but, now, in the warmer weather, it seems to be running just fine..... See if the problem repeats itself when it gets cold again. (not in a hurry for that....)
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