Ok this is the strangest idle problem I have ever encountered with anything. Any input would be great.

I start the car (warm or cold it doesnt matter... as I get the same results). The car revs to 3-4000 rpm (it was 4 now it seems to like 3)

once it comes down from that it will drop to about 1k and bounce up to 1500... and it keeps doing it. up and down and up and down (you get the idea)

but even better it does it while I am driving... Up to about 3000 rpm it will keep surging with that 500rpm increase...

Anyone have any idea? or possibly had the same problem?

 

You mean while you're driving with a load on the engine (clutch locked to the engine, clutch pedal not depressed), you surge 500 rpm's?

 

Mine does the same thing! Looks in my case to be a bad vacume leak and something I have NO idea is.

Any suggestions helpful!

Mine dosent serge while driving however.

 

I dont notice how high it surges for the fact that it starts bucking...

 

Pulled any codes yet?

Search for major vac leaks- have you done anything to the intake assy lately?

Might also want to check your grounds. Have you added any "new" ones lately?

 

Unplug the TPS and see what happens.

Idle going up and down is usually a vac leak.

Bucking might be a bad spot in TPS.

hugues-

 

i am working on pulling codes tonight or tomorrow.. the bucking is caused by the surging... ran perfectly fine... then sunday I pulled in the intake off... never touched the throttel body or anything.. just checking things out basicly.. put it back on and then it started

 

Ah, the old "I pulled my UIM off and now the car doesn't idle right" thread, lol...

 

and that thread leads where?

 

Vac leaks, man...Take everything back apart exactly as you did before, and put a thin film of RTV silicone on all of the exposed mating surfaces upon reinstallation...

 

Or replace your UIM gasket..... that seems to be my problem.

 

maybe they should have used 4 bolts instead of 2... I will try that and see what that does first...

Thanks...

any ideas if that isnt it?

 

After that, if it's still bad, go the TPS route as Hugues mentioned...

 

I have an 86 mazda and it started doing the same thing as well? The only other thing i can tell you is when i turn the a.c. on the surge stops.

 

That's because turning the A/C on energizes the air bypass solenoid valve, which might "cover up" your vac leak, so to speak, by allowing lots of extra intake air to keep the A/C from killing the engine's idle...

 

My car does the same when i first start it up and when I give it throttle. Talk about embarassing, from the outside it looks like I can't shift because the idle drops so low.
What does UIM stand for?

 

Upper Intake Manifold

 

Hey guys, what about an idle that is totally fine until I brake, turn on the lights, turn the wheel, etc. Basically anything that draws power makes the idle drop and randomly slightly surge but it never dies. BUT it idles faboo when the A/C is on. What do you think? Alternator? grounds?

Ive been scouring threads for weeks and havent been able to find anything quite like this.

 

Its your BAC .

 

Do you still have the BAC on the car? Left side of the dynamic chamber (throttle body area) on the S4's, don't know about the S5's...

 

Oh yeah it's in there, Ive taken it out and checked it too. Everything is working fine.

 

So you say...

The only way to tell for sure is to watch the voltage drop at the BAC's output at the ECU, unless you can hear it clicking & doing it's thing...Your problem sounds like it's not "doing its thing", though

 

I just did what the Haynes manual said to do to check the BAC, should I attack it in a different way?

 

The BAC itself can check out great, but what if the circuit (wiring) from the ECU is bad?

Mazda is kind of ***-backwards with their FSM troubleshooting steps. Instead of checking individual components first, as the FSM instructs, we should be checking the entire circuit (when we can), most of which can be done at the ECU. Then if the circuit is bad, we can start checking the small stuff.

When the Haynes or FSM tells you to pull the BAC plug and verify an idle drop, that's checking the whole shebang, because it's obvious the BAC circuit is good, if it brings the idle up when the plug is reconnected. We know it must be getting power (which also verifies that the ground is good), and that the valve itself must be working...But if just reading out the BAC coil with a meter for resistance, that leaves 90% of the circuit untouched (or, "untroubleshot" )

Similar tests can be done at the ECU...If the voltage at the ECU output for the BAC is dropping (due to the square-wave DC voltage duty cycles becoming more frequent), then the circuit is good, from power to ground. If the BAC valve coil was bad (open), no voltage would be dropping (in fact, no voltage would be present at the ECU pin we're checking). If the ECU internal BAC circuit was bad, no voltage would be dropping. If the ground was bad, same thing. See how it works?

Let's take the TPS as another example. If we check it at the ECU, and we get our 1 volt, that's telling us that the entire circuit is good- power, TPS, and ground. No need to pull the corroded TPS plug apart on the engine and check resistances, you've just checked everything you need to with one meter reading...

This same scenario applies to the injectors, CAS inputs, all of the sensors, the solenoids, etc...

 

Thanks wayne. Speaking of TPS..... I was doing your ECU adjustment method and was able to get the correct reading on the narrow range but the full would read .46. Ive heard that you should adjust to the narrow and that something in the s5 tps is self adjusting. Is this correct? could these two problems be related? You da man wayne!