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Ivan Alvarez
Ivan Alvarez,
Category: Dodge
Satisfied Customers: 126
Experience:  lead technician at Performance Tire and Automotive
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I am trying to diagnose/solve a high idle issue. I recently

Customer Question

I am trying to diagnose/solve a high idle issue. I recently dropped a new block and heads into my truck and ever since it has had a high idle problem. It is a 2001 Ram 1500 with 5.2 manual 4wd. I have replaced the IAC and TPS. Recently set the fuel sync and replaced O2 sensor (it happened before all these replacements). The truck will run fine for the first 5 starts. After it relearns the mixture, it will high idle at just about 1500 RPM, but only while moving. If I am going any speed and toss it into neutral, the RPMs will drop but only to 1500 RPM and will only drop back down to idle if I come to a stop, or if I "lug" the engine by putting it in a high enough gear to force it below 1500.. then it will drop to a normal RPM. Any ideas?
JA: Have you located all of the vehicle's oxygen sensors? Are the wires leading to and from each intact and in good shape?
Customer: Yes. And replaced the upstream sensor recently.
JA: Are you fixing your Ram 1500 yourself? What have you tried so far?
Customer: Yes fix it myself. Replaced IAC, TPS, O2 sensor, cap, rotor, wires, IAT, temp sensor, fuel pump, and had the injectors cleaned.
JA: Anything else you want the mechanic to know before I connect you?
Customer: I can't think of anything else.
Submitted: 7 months ago.
Category: Dodge
Expert:  Ivan Alvarez replied 7 months ago.

Do you have a scanner to view data? Have you checked for vacuum leaks?

Customer: replied 7 months ago.
I have an SCT tuner and an OBD scanner dongle connected to the android app "torque" so I can see some data. I have searched high and low for a vacuum leak but have found none. Hooked up a vacuum gauge and it pulls a normal vacuum and responds normally to revving the engine. It also runs normally for the first ~5 starts but only begins the high idle after the computer runs thru all the smog checks. I have also set IAC steps manually several times with no change.
Expert:  Ivan Alvarez replied 7 months ago.

If you can see what the st. fuel and lt fuel trims are at send me the readings when idle is high. Also if it shows you the counts on the iac those reading would be helpful in trying to diagnose.

Customer: replied 6 months ago.
I drove the truck the other day and did a screen recording of some parameters from the Torque app while driving. The first part of the video, I am at a stop light. Then I accelerate and toss it into neutral just about the 0:55 mark in the video (the screen moves side to side as a marker for where this happened). I don't understand fuel trims, but you see the throttle % goes back down to its lowest value of ~12% indicating I've let off the gas. However, the RPM stays high (~1300 RPM this time) and so does the timing advance. It does not show IAC counts. I have manually set the IAC counts a number of times (by starting the engine, disconnecting the brake vacuum for a second for the IAC to close completely, disconnect the IAC, and then adjust idle to ~100 RPM below target idle (I assumed target idle for the manual is 600 RPM, so I set the closed IAC idle at 500 RPM)). One other thing I haven't mentioned: the problem only happens when the engine is fully warmed up. Just being closed loop but "cool" (the temp gauge isn't fully to operating temp) won't produce the high idle. However, on a warm day or after I drive it for long enough when the temp hits full operating temp, then it is much more likely to get stuck at 1500 RPM while moving. I forgot to mention, I have also installed a K&N cold air intake and a Hughes F1 Airgap manifold. Also removed the cooling fan and replaced with a dual electric fan setup with a thermostatic control.Video link:
Expert:  Ivan Alvarez replied 6 months ago.

I'm going to look over some specs. will get back to you soon.

Expert:  Ivan Alvarez replied 6 months ago.

Are you still using the original throttle body?

Customer: replied 6 months ago.
I have cleaned it once. Previous discussions with tech forums suggested the throttle body might just be old enough to cause a vacuum leak due to wear on the throttle shaft/butterfly plates as the motor warms up. I did not follow up with this as I did not know how to test it.
Expert:  Ivan Alvarez replied 6 months ago.

At idle the tps should be around .25 to .35. +-.05. I noticed its around 12.9 at idle. This can give you a high idle issue. The idle stop screw does not usually need adjustment. Try and set the screw back where it was originally. Once you have done this double check that tps reading. If the tps reading is still high unbolt it from throttle body and see if you can manually get the voltage to drop by manipulating sensor yourself. Let me know what your results are after doing this.

Customer: replied 6 months ago.
The 12.9 is a percentage. I have replaced the TPS and switched them a number of times with no success. I have checked the idle voltage and it has been normal. I have back probed the sensor and opened and closed the throttle plates and found the voltage increases and decreases in a linear, controlled fashion. I've replaced it with aftermarket and mopar TPSs and seen no improvement. I am convinced it is not the TPS unit. In the video, you see the high idle happens even when the throttle position returns to 12.9%. So in my mind, the TPS has not been the culprit. If it were, wouldn't TPS give higher numbers during high idle? It does not. Based on the fact the truck does fine during the first 5 starts, wouldn't this possibly indicate the PCM is the culprit? Or the wires to/from the TPS?
Expert:  Ivan Alvarez replied 6 months ago.

If it was wiring to pcm you would not be getting a tps reading on your scanner. When you back probed the tps voltage where are the voltages at closed throttle and at wide open throttle?

Customer: replied 6 months ago.
I just tested it. When cold, I back probed the middle wire with the ignition on. It reads 0.68 volts closed to 3.9 volts wide open.
Expert:  Ivan Alvarez replied 6 months ago.

Ok those voltages are not within specs. Remove the tps from the throttle body and do the same tes by back probing and looking where the voltages are. you will have to move the tps by hand. Closed voltage should be about .25 to .35. Wide open it should be around 4.5 to 4.8.

Customer: replied 6 months ago.
0.31 closed and 5.09 wide open. I see that the cold air intake prevents the throttle opening to full voltage. Could I have incorrectly set the set screw for the throttle plates too far in to cause the high voltage when closed?
Expert:  Ivan Alvarez replied 6 months ago.

Yes that is possible. Try to set idle screw back in its original position. We know for sure tps is capable of proper voltages so that now leaves the throttle in question. after resetting screw recheck voltages.

Customer: replied 6 months ago.
I moved the idle screw completely out of the way of the throttle plates. Even with the throttle plates closed it does not go lower than .6 volts. I have tried two TPS units and installed them while back probing. They both started out in range but with the slight twist required to line up the mounting bolt holes, the voltage always rises to 0.6. Am I installing the TPS incorrectly? There is only one correct way to slide the TPS teeth over the throttle tab and that requires a slight counter clockwise twist to line up the holes. Putting the tab on the other side of the TPS teeth doesn't work correctly at all.
Expert:  Ivan Alvarez replied 6 months ago.

No your ok there is only one way it goes in. I'm going to look into a couple other things that would cause your issue.

Expert:  Ivan Alvarez replied 6 months ago.

How did you go about checking for vacuum leaks?

Customer: replied 6 months ago.
Used carb cleaner around the base of the intake and throttle body and watched for RPM changes. Also checked vacuum at idle and made sure it responded properly to revving the engine.
Customer: replied 6 months ago.
I've also tried disconnecting vacuum accessories and capping that port to see if the idle issue is resolved. Did not help at all.
Expert:  Ivan Alvarez replied 6 months ago.

Ok well its not vacuum or tps. So that leaves the iac and possible wiring. Is your volt meter capable of testing continuity for wiring?

Expert:  Ivan Alvarez replied 6 months ago.

You have to rule out the iac even though its a new part. Then we have to rule out the wiring and at the same time test pcm command for the iac

Customer: replied 6 months ago.
I believe my volt meter can test continuity, though I have never used that function.
Expert:  Ivan Alvarez replied 6 months ago.

With a regular test light back probe the iac wires one at a time. With the truck running go from wire to wire and your test light should be flashing on each wire. If you run into one wire that doesn't make the test light flash give it some gas and see if it starts to flash then. If you have activity on all four wire we can move on to the next step.

Expert:  Ivan Alvarez replied 6 months ago.

If you don't get the light flashing even with giving it gas you are going to test that wire further. So once your able to do these tests let me know what you find. The best thing would be when the idle goes high but I know it would be hard to catch it since the idle drops back done when your at a complete stop.