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unstable idle rpms


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In the past 2 weeks I had experience a sudden jump on the rpms on idle and or low speed (5 mph) I can see rpm meter jumping from 750 rmps to 1000 rpms and at same time I can hear motor increasing rpms or truck pulling when I have it on drive; in both cases I turn off engine and start again and symptom is gone. Any ideas..

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Ok lets start with basics...

[*]Is there any error code present? (Use a OBDII error code reader)

[*]How is your fuel pressure? (At idle and WOT @ highway speeds)

As for the error codes just because the light not on doesn't mean there isn't any error codes.

As for fuel pressure you need to know both values because testing at idle might pass with flying colors but fail miserable at WOT at highway speed. This is really common.

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Anymore info on when this happens? Can you get it to repeat this on demand?

Just an fyi and might not relate to you but only thing I have heard about rpms going up on there own.

This is from Blue Chip Diesel

RPM GOES UP ON ITS OWN OR WAIT TO START LIGHT IS DUMB OR YOU HAVE

A 606 CODE

This is a new one as of 2010. A few callers have remarked that the RPM goes up on its own, so we have them check that the voltage on the blue with a black tracer signal wire (on Dodges) coming from the APPS, doesn’t go up or down when the RPM goes whacky, which confirms it is NOT the APPS. If you hot wire the pump as explained in No Start above and the idle is smooth, but if the idle rolls or is erratic with the VP44 plug being plugged in, and the APPS is good, then you have to blame the ECM. The other strange symptom or indicator of a bad ECM is when the truck won’t start until the wait to start light goes out, or it comes on when driving. New for 2012 is a problem where the sensors that get their supply voltage from the ECM are lower or higher than the required 5.0 volts. SOME of the time a bad ECM with any of these symptoms will set a 606 code to make you feel more confident in your diagnosis! These are the only three symptoms that I have seen or heard of, SO FAR, that necessitates replacing or repairing the ECM. The good news is that these unique symptoms have been eliminated every time by repairing or replacing the ECM. At least one of the symptoms described above has been common to all the very few bad ECMs, as in all 8 of them, that I have found to need repair or replacement. The only way to diagnose an ECM in my experience is to try a replacement, if diagnosing a Dodge. It doesn’t seem to matter what transmission, year or engine rating the test ECM comes from, with or without a crank sensor, as long as the above symptoms go away with the borrowed test unit. I have had many callers do it this way, so I feel confident you won’t hurt anything as long as you remember one thing, PLEASE. When installing any ECM, be SURE to ground it to the engine FIRST, before connecting the big plug. This prevents any problem from static electricity or a voltage spike getting into the ECM which can blow away the software and or computer inside. Yes the test ECM may set codes, but if it doesn’t have the above symptoms any more, then you know a replacement or repaired ECM is in your future. I don’t have any experience with Ford or Freightliner ECMs, which may indicate they are better quality, but they are very different and not interchangeable, as they have twice the number of wires going into them.

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Anymore info on when this happens? Can you get it to repeat this on demand? Just an fyi and might not relate to you but only thing I have heard about rpms going up on there own. This is from Blue Chip Diesel

I have check fuel pressure these are my results: @ 750 rpm's is 14 psi @ 3000 rpm's is 11 psi as far as dtc it show code 380 which is related to circuit 1 of air heaters. Regarding duplication, no I can not duplicate on demand, and so far what I have notice is rpm's going from 750 to 1000,but next time it happens I'll be more thorough on the symptoms
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I have check fuel pressure these are my results: @ 750 rpm's is 14 psi @ 3000 rpm's is 11 psi as far as dtc it show code 380 which is related to circuit 1 of air heaters. Regarding duplication, no I can not duplicate on demand, and so far what I have notice is rpm's going from 750 to 1000,but next time it happens I'll be more thorough on the symptoms

3000rpm free rev in neutral? Needs to have a load on it. Find an on ramp for the hi-way. I do lots of testing there.:whistle2: What is the volt meter say? Wonder if a bad heater grid circuit is causing the ecm to kick up the idle due to excessive load from a shorted grid circuit.
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I have check fuel pressure these are my results:

@ 750 rpm's is 14 psi

@ 3000 rpm's is 11 psi

as far as dtc it show code 380 which is related to circuit 1 of air heaters.

Regarding duplication, no I can not duplicate on demand, and so far what I have notice is rpm's going from 750 to 1000,but next time it happens I'll be more thorough on the symptoms

Don't tell me you just free rev'ed it... That doesn't work. :doh:

How fast are you traveling with your foot to the floor?In my vid this is up to 75 MPH @ WOT.

http-~~-//www.youtube.com/watch?v=grl72Up-8T0

As for the code... Double check the small wire to the relays. But fi they are good then the relay is bad just replace it.

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Don't tell me you just free rev'ed it... That doesn't work. :doh: How fast are you traveling with your foot to the floor?In my vid this is up to 75 MPH @ WOT.

http-~~-//www.youtube.com/watch?v=grl72Up-8T0

As for the code... Double check the small wire to the relays. But fi they are good then the relay is bad just replace it.
Hi, Regarding p380 looks like one of the trigger wires was loose and being hit by one of the steering pump hoses so I re-route the steering hose away from the wire (just enough not to touch it) and re-place the wire an so far the code has not show again. About the other issue with the jumping rpm's I just drive over 200 miles round trip going up hill and no symptom at all, I speed over 80 mph on flat road @ 2300 rpms and run just perfect. Another bit of information is one of my batteries was damage , I replace it but the symptom show it again like I said before i was driving like 10-15 mph. Also i have notice life span on my batteries less than 12 months, I have replace them one at the time is this a problem , do I need to replace both batteries at same time?
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Hi, Also i have notice life span on my batteries less than 12 months, I have replace them one at the time is this a problem , do I need to replace both batteries at same time?

It is best to replace both batteries at the same time. Make sure you are using the correct group battery for the rig. Check the CCA's and reserve capacity on them. The higher the numbers the better. Also, you should be getting longer than 12 monthes out of a battery. I would start checking your wiring from the alternator to make sure there is no problems present. Plus, I would have the alternator checked out as well.
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  • 3 weeks later...

It is best to replace both batteries at the same time. Make sure you are using the correct group battery for the rig. Check the CCA's and reserve capacity on them. The higher the numbers the better. Also, you should be getting longer than 12 monthes out of a battery. I would start checking your wiring from the alternator to make sure there is no problems present. Plus, I would have the alternator checked out as well.

Going back to the original issue unstable rpm's I was driving this past weekend on the baja small roads when I run into a dip so I release the gas pedal and hit the brakes and the motor stop , after a while decided to stop and see what was wrong because the engine sond like the old diesel bus motors and at that moment rpm's were moving from 750 to 450 and if I push just a bit the gas pedal just to hit 1000 rpm's the engines runs stable (So I'm wondering if this is a apps isssue) . I try to reset but no change, check also for any codes and no codes, so I continue my journey making sure I maintain my rpm's around 1000 rpm's. On my way back (today) the engine run fairly good it did duplicate the symptom a couple of times but inmediatle re-established it self to 750 rpm's. Any ideas apps, cam sensor ,...or ecm.
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