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Diagnostic Starting Point: High Idle
Posted to Technical Theory Forum on 8/16/2013 14 Replies

There was a car with a very puzzling high idle concern in my shop recently that got me interested in the problem so I have been working on developing a diagnostic process for dealing with it in the future and doing research on the issue in general, by no means am I claiming to know it all, but I'm presenting the things that I have found here for feedback and input.


For my purposes here I'm defining a "High Idle" as a vehicle that starts, warms up, and runs relatively smoothly at a speed of greater than 1000rpm. Some cars will idle higher when cold to help reach op temp, but generally idle speed should be in the 600-900rpm range... Agreed?

Some of you will notice I mainly focus on systems with bypass valves, feel free to chime in about electronic throttle systems.

Possible Causes:

1. False "learned" or "adaptive" data

2. Faulty bypass air controls / valve

3. Vacuum Leaks

4. Bad Sensor Input

1. Probably the most common occurrence of a high idle concern is after cleaning the throttle body. It seems extremely common that the PCM of a car will learn the amount of airflow it gets through the closed throttle and will adapt to coking of the throttle body over time by increasing the amount of bypass airflow, then when a technician goes and cleans the TB and restores the normal airflow, the computer is bypassing way too much air for the (now) less restricted throttle. In most cases disconnecting the battery for an hour to reset the adaptive data will fix this, many manufacturers have an idle relearn procedure which will take care of it. Some cars may need the factory scantool to relearn the idle properly. Of course this applies to vehicles with bypass air valves, not electronic throttles.

2. Another common fault that causes a high idle is a sticking bypass air control valve. Some people report that tapping on the IAC valve may temporarily relieve the condition. As could cleaning. If the bypass valve clogs it may stick in a position where too much air bypasses, but the clogging itself is unlikely to cause a high idle as it will tend to choke airflow through the valve. That is, a clogged IAC valve would tend to cause stalling or rough idling and hard starting? The idle air valve mechanism might have electrical faults internally, or faults in the wiring leading to the controlling computer. Testing for those problems seems to be more application specific and depends on the type of valve.

For systems with an electronically controlled throttle, sticking due to carbon deposits seems to be the most common failure.

3. Vacuum leaks. In order to raise the idle speed substantially a vacuum leak will have to be pretty darn bad or combined with a sticking IAC. There are a lot of different ways unmetered air can be drawn into the engine and smoke testing seems to be the best way to locate and detect them. The harder to spot vacuum leaks include a PCV valve that is flowing too much, a leaking valley gasket on a pushrod V engine, a throttle stop screw that's been tampered with, leaks at the throttle shaft, a binding cruise or throttle cable, excessive wear of the throttle blade, or a leaking brake booster. Some claim a leaking EGR valve can cause a high Idle but I would expect it to be very rough because of the dilution with exhaust gases.

4. Bad sensor input. This is the one that really whipped me on the car that made me want to learn more about this issue. The idle airflow is increased in response to increased loads placed on the engine, so things like charging system loads, power steering load, AC compressor load, all cause the PCM to change idle strategy. Problems can occur when a load is perceived but not actually present, so a car may idle high because of a shorted power steering pressure switch, or an AC ON command with an open circuit on the compressor clutch (?). Hondas I believe have an "electrical load detector" that goes bad and signals high alternator load and raises idle. A bad ECT could theoretically do it since the PCM commands more air for a cold engine (I guess to improve idle quality and hasten warmup?).

But the most confusing and for me unanticipated input that could cause idle speed problems is the TPS. Here's what a document I found online says:

Throttle Follower The throttle follower parameters are used to adjust decay and delay rates for the IAC motor during throttle closure to slow the return to idle during a sudden throttle closure.

If you go back to my previous thread about dashpots you'll see why I was interested in them. Apparently engine management systems will increase the bypass airflow to offset the closure of the throttle to keep the engine operation smooth as airflow changes. Which means if you have a malfunctioning TPS signal that drops out intermittently, the PCM may interpret it as the throttle slamming shut and increase the idle air. The idle system is designed to compensate for changes in the vehicle so the problem may lead to the idle speed becoming permanently elevated in order to stabilize it.

On the vehicle I met that seemed to be what was happening. The idle could be controlled with the scan tool and would stay correct after being reset, but within a day it would be back at 2000 rpms, and the TPS signal was fluctuating ever so slightly. Replacing the TPS, Pigtail, and eventually PCM seems to have cured that one.

As a side note there are some nuances to the TPS signal, like some TPS sensors have integral switches for WOT and idle (fully closed) positions, additionally the engine management system reads the lowest voltage as the closed throttle position in most cases.

The TPS signal could be erratic if the sensor itself is physically worn, or the mounting screws are loose, but could also be erratic because of a bad PCM that isn't putting out a stable 5v reference, or because of connections that intermittently cause the signal to dropout, like loose connector, intermittent sensor short to ground pulling the 5v low, or broken wiring. The best tool here I think is a scope.

Okay so now that I have a clear description of a problem and a list of most of the probable causes I just need to find a process that will help me quickly and accurately diagnose it. I'm a bit weaker on this part, still trying to sort out the best strategy.

I think the first thing to do is to connect a scan tool and read the idle command whether counts or percentage or angle or grams/sec, I first want to know how the PCM is reacting to the idle speed. If the PCM is trying to close off all bypass airflow then most likely there is a vacuum leak or a sticking IAC pintle, in other words a physical problem. If the PCM is commanding the bypass open then we need to figure out why. Closed off bypass air would mean proceed with #s 2&3 - sticking bypass or vacuum leaks. Increased bypass air would mean proceed with #s 1&4 - bad adaptive learning values or bad sensors. This would be a good time to look at scan data about charging system load, AC, P/S and to check for any DTCs.

I think I would try operating the vehicle loads while watching the MAF or calculated load - if I see that the amount of air flowing into the engine changes when I turn loads like AC and P/S on and off consistently then I can be pretty sure the PCM has control of the idle air bypass and is actually changing it. May have to watch fuel trims at this time as well. If the PCM is able to effectively command changes in airflow then I would think the valve is working.

At this point I might try an adaptive reset or idle relearn or battery disconnect to see if that improves the condition and for how long. The next step would be to put a scope on the TPS signal voltage and wiggle wires, tap the sensor, and drive around looking for fluctuation / glitches?

The previous step of changing the vehicle load around should tell me if the IAC is sticking or not, if the engine airflow doesn't change when the commanded airflow is changed then I would probably remove the IAC, clean it, reinstall and recheck. If it still doesn't work then I'd look at the schematic and check the wiring.

If it seems like a vacuum leak is likely I would pinch the PCV line and check RPM drop, pinch brake booster hose and recheck, check for slack in throttle cable, listen for vacuum leaks, check any grommets in the intake plumbing, inspect the throttle bore, perform a sealed cranking vacuum test (low vacuum points to possible EGR pintle or valley gasket leak). Smoke the intake system and check for leaks.

Okay that's about it, I think I'm at my wit's end here. If I didn't find the cause of the high idle concern I'm really not sure what else I would do.

Looking forward to a discussion of the issue and best diagnostic process, -Andrew

Andrew Libert
General Motors
Ann Arbor, Michigan, USA

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