Poor Idle is driving me crazy!

Your situation sounds somewhat similar to this very perplexing vacuum
leak problem that was discussed on one of the CleanMPG,com Forums

Hope this helps.

FWIW, this thread also discusses some incredibly strong user support by
the SccanGauge folks.

 

The Prius braking system does not depend upon engine vacuum, so that should not cause your issue. The brake pressure accumulator pump is electrically powered.

DTC P0101 does point to the mass air flow meter. A few thoughts for your consideration:

1. Are you sure that the installed MAF is the correct part? The Toyota part number is 22204-22010. Any problems with the wiring harness connector?

Further, the Toyota repair manual provides info about how to diagnose a failed MAF vs. engine ECU in the discussion about DTC P0100, P0102, and P0103. Although your car doesn't register those DTC you may find the troubleshooting procedure relevant to your situation.

2. How about the O-ring, was that replaced with a new part when the MAF was replaced? The part number for that is 90099-14141. Are the two screws that attach the MAF secure?

3. What about the metal clamp that secures the air cleaner housing to the throttle body. Is that secure? Any cracks in the air cleaner housing or the intake manifold downstream from the MAF?

4. Have you attached a vacuum gauge to an engine vacuum port to see what vacuum is produced by the engine?

5. Have you checked the voltages at the engine ECU to see if they are correct? The Toyota repair manual provides info about this, for example p. 05-370 of the 2005 repair manual explains how to verify voltages at the ECM. Voltage is provided via IG2 and EFI M relays.

 

I would check carefully for vacuum leaks. Your "idle" speed (no load in P) is a lot higher than what I see with the Scangauge - I usually see 1200 - 1400 rpm. The Prius does not have an IAC (idle air control), but if extra air is leaking in that is not metered by the MAF, that would explain your high "idle" speed.

 

As the throttle body came from a salvage vehicle, is it possible that the wrong part number was installed or that the TB is defective?

I agree with Jason that you need to search very carefully for a vacuum leak. Since the original TB was damaged in the accident, is it possible that the intake manifold has a crack somewhere?

 

Patrick,

Thank you for making this crystal clear. :rockon:

In my post I was trying to suggest that the problem here might not be
in the vacuum hoses and their connections, but rather a leak in some
piece of equipment typically presumed to be airtight.

All you guys are far more knowlegable in automotive engineering in general
and Prius-specific trouble shooting than I am. Well, I'm here to learn. There
seems to be a general consensus that the symptoms point towards a vacuum
leak. As an aid to my education, can you help me/us understand more about
this?

In the OP, it is stated that there are three vacuum ports on the throttle
body, TB. To what pieces of equipment do they lead? You've said it's
not to the brake system. And while I've still got a lot to learn, I know
it's not to the windshield wipers.

Where then do they go? One to the PCV valve and...?

 

Hi Rokeby,

If you open the hood you will see a decal that shows the engine vacuum system. Note that there's one hose that connects to the vacuum switching valve and a purge port. The latter provides vacuum back to the fuel tank HC emissions control. See
Automotive Training and Resource Site
Look for the Technical Article, Hybrid13 Fuel and EVAP System, page 3-5.

Also, if you take a look, you will see a port that is covered by a rubber plug. This would be a good place to connect a vacuum gauge for testing (or your old-fashioned windshield wiper motor... )

You are right in identifying the PCV hoses as another potential place for a vacuum leak, especially the hose connected below the throttle plate. Other common places for a vacuum leak include the oil filler cap (if the rubber O-ring has failed or the cap is not fully tightened) and the oil dipstick not being fully inserted or with a bad O-ring.

If the OP cannot find the leak, then I suggest physically removing the intake manifold and carefully inspecting for a crack. Fill the manifold with water if necessary and see if any leaks out. Use all new gaskets when reinstalling.

 

On most EFI engines, idle speed is controlled by regulating bypass air (air bypassing the closed throttle plate). This is usually a function of the IAC valve, which is operated by the Engine Control computer. So, if there is excess air leaking past the throttle plate that is not metered (or accounted for) it will raise the idle speed. My understanding is that for the Prius, the IAC function is integrated into the electronic throttle body. You mentioned that you replaced the TPS. Any chance that it could be misaligned with the throttle plate? How are those two indexed?

 

When the engine is cold, what idle behavior does it demonstrate?

Do you know how to put the Prius into inspection mode so that the engine runs continuously, allowing you to assess engine idle over a long period of time?

 

I hope all-you-all won't mind me asking a connected, but tangential
question. I'm curious about the expected vacuum leak induced RPM
increase... curious minds want to know

As I understand what's been said, a vacuum leak, depending on its
volume, leads to a small but measurable RPM increase. I'm
guessing that the ICE is running just a little lean.

In the thread that I cited earlier, they sprayed small amounts of carb
cleaner on vacuum hose joints suspected of leaking. This method also
apparently produces a noticeable change in RPMs. When I read it, I
presumed it was an RPM increase.

Now however, I suspect it would be a decrease as the when the
flamable liquid is drawn in below the throttle body, the mixture is
somewhat richer than before.

Am I even close to understanding what is going on here?

 

This is a head scratcher. In the past, I've found the propane method to *very* reliably find vacuum leaks

Seems we've covered all the usual bases here. A cracked intake could very well cause these problems. Patrick and Jason have covered the relavent diagnostics, nothing I can add except some photos

 

Hi Howard,

I started my 2004 (which was warm from having been driven ~100 miles or so earlier in the day) in inspection mode. Then I tried removing the oil dipstick. Very little change in rpm.

Then I removed the oil filler cap. Engine rpm increased slightly. This illustrates that the idle speed control logic is able to compensate for a relatively large vacuum leak. Therefore I'm less confident now that a vacuum leak is causing your driveability problem.

Have you had a chance to perform the recommended tests that are shown in the repair manual regarding DTC P0100, P0102 and P0103? This would validate that correct voltages are reaching the MAF.

In particular, note that if the hand held tester thinks that the air flow rate is excessively high (which may be the case since you have DTC P0101), check the resistance between terminal EVG (E5-32) and body ground. If the resistance is above zero, then the engine ECU is at fault. See for example page 05-97 of the 2005 repair manual or the analogous page of the Toyota repair manual that you are referring to.

The manual goes on to show how to check continuity of the wiring harness that connects the MAF to the engine ECU. Since your car was in an accident, that wiring harness damage is a reasonable possibility to consider.

 

What better way to get information about leaks than to get it from the experts: Ford owners :madgrin:

help finding vacuum leak... - FFCobra.com - Factory Five Racing Discussion Forum

I believe that the second poster in that thread is correct: the propane will raise the idle slightly when sucked into the intake because it is a source of additional fuel.

Here is an article covering the different techniques, with the addition of the smoke method which may be the best way of all if you have the equipment:

Tech To Tech