StarionX

I've been having a rough idle for a month or so. I know I need to get a bit of a tune, but what's strange, is that when the car is idling, the revs drop when the brakes are applied. The engine almost cuts out (in fact it has a few times). It's almost as if I'm getting some sort of power drain (but the lights don't flicker or go low). If I take my foot off the brake, the idle goes back up (it's still rough, but at least the engine doesn't want to die).

I have an 87 TII, 126K kms. Replaced the fuel injectors last year (with new ones).

Any help would be appreciated.

Samps

vacuum leak, or electrical short in the brake light system.

cymfc3s

the brake system is boosted and the brake booster runs off engine vacuum. If you have vacuum(s) leaks the revs will drop some. Its not very nocticable if your pulling a lot of vacuum at idle.

StarionX

Ok... that's starting to make a little sense now. I just found out that my front brake discs are badly warped and rusted, and need remachining. Doing that tonight. I'll see if they find any vacuum leaks.

Thanks people.

StarionX

Ok, it does not seem to be brake related. If car is idling, either pressing brake peddle, using electric windows, turning on heater (basically anything electrical in nature), seems to cause the revs to drop very low. I've had the alternater and battery checked, and they seem to be good.
Here's the other thing. If I press the brake pedal and keep it pressed, the revs drop ( to almost 0) for a few seconds, but then they come back up. The same with the window and heater. The revs drop initially, but go back up. When I'm driving, I don't notice any problems, only at idle.

HELP??

Bambam7

Sounds electrical....

Do two things-

1-Run a heavy gauge wire from the battery neg to the chassis.
2-Run a heavy gauge wire from the battery pos to the big output nut on the alternator.

Good chance of fixing that.

Oh- also make sure your alternator belt is tight... who knows.


Also- re-ground the engine-firewall behind the intake manifold while you're at it!

Do you have A/C? what happens when you put that on? (BAC valve...) What about the blower motor for the heater?

Flyby116

Same thing happened to me. I ran a few grounds and nothing fixed it. But it blew so i never found the right fix lol. Im still thinking its the ground or someshort in soem part of the wiring. Itll will prob take time to find but you will. Good luck

Nick

StarionX

Hehehe, you assume that I know what the hell you're talking about
I'll try the wiring, that sounds simple enough.
Alternator belt was tightened today.
When you say re-ground, do you basically mean to attach some wire to the engine block, then to to the chassis??
A/C has never worked, it does turn on though ( so theoretically, it works, just doesn't blow cold air, probably needs re-gassing).
It also does it when you turn the heater on.
Sorry about the stupid questions, but I'm not too mechanically minded.

Bambam7

Sorry- My fault.... I wouldn't have know what the hell I was saying a few years ago...

Yes- regrounding the firewall is just adding another wire from the engine block to the chassis.
There is already a ground on the passenger side- right behind the intake manifold, that bolts onto the engine, and plugs into a spade plug on the firewall. This is doubtfully the source of your problem... but it's just for good measure. It's a crappy ground and can lead to problems.

Definately do the other wires from the battery though.

Take a short peice (1 foot or so ) of 8 gauge wire, and 2 ring terminals. Clamp the terminals to either end of the wire.
Attach the ring terminal on one end to the negative terminal on the battery. Unbolt any close bolt/nut on the chassis near the battery- and attach the other end to that.

Make another peice of (8 ga) wire, about 3 feet long or so, and put ring terminals on either end again.
attach one end to the postitive terminal on the battery, and the other and to the big nut on top of the alternator, that already has a wire on it.

This should take about 10 minutes!

StarionX

Thanks BamBam, I'll try all that when I get a chance.
Too bad I don't live closer to you, I could just bring my car around to you, throw lots of money at you and let you tinker away with it.

Josepi

Soldering would be the better thing to do.

jparsiphal

Looks like a BAC valve problem

The BAC valve should rev up the engine to compensate for extra loads, like AC, etc.

Do a search for 'BAC valve' on this forum. You should find plenty of info. A simple test is to disconnect the BAC valve connector and see if engine RPM drops. If does not, something is wrong and further testing is needed (could be BAC itself or connection to the emission computer). The BAC is located on intake manifold right above the oil dipstick.

John -

sunshine

iTrader: (3)

read this:


http://www.teamfc3s.org/info/articles/idle.html

StarionX

Thanx people, I will look into the BAC as well.

hugues

Starion X

Have you fixed your problem ??

I am quite interested since I am having the very exact same problem.

I can tell you that it is not alt related (if alt seems to be producing appropriate voltage) since I have replaced mine with a new one, and same problem.

It's probably not the BAC valve either since I don't think the ECU knows about when you apply the brakes.

Any updates ???

Thanks,
hugues -

88 vert

i have a similar problem but mine comes and goes i htink its a vaccum leak but not sure and cant find it.it will idle fine at bout 1000 rpms one day then the next bout 600 rpms and i cant adjust it any higher no matter what i do then when i hit the brakes real hard it stalls out.but some days it does just fine with idle ???????????

Angel Guard Racing Team

It is normal for the revs to drop a little but not that much...

Fikshun

I had the same problem... Every time I stopped at a light, I would have to use the e-brake so I could keep my foot on the gas to keep the idle up.

The fix for mine........

I adjusted the TPS. I haven't the slightest clue why it was doing that, but it was... I had to turn my TPS several turns counter-clockwise to get it to keep it running while stopping.

I dunno if this helps at all.. but it worked on mine.

Fikshun

hugues

Fikshun:

I have same problem so I am interested to hear the TPS adjust fixed the problem.

Did you adjust the TPS according to manual ? or did you adjust differently ?

What is your current rpm at idle (750 or higher ?) ?

When you apply brakes,
do the rpm still drop significantly ?

Thanks,
hugues -

cblake3

iTrader: (2)

Any answer to this problem. I am having a similar problem. I added the additional wire from the alt. to the + battery post. This helped with the idle drop when I turn the headlights on.
I already have the additional ground wires (to fix the 3800 rpm hesitation) and I have adjusted the TPS(test lamp procedure). The BAC has been checked and it is OK.
The idle is dead on at 750, but still drops when the brakes are applied.

hugues

cblake:

Do the rpm drop and go back up ?
If they don't go back up, your BAC is not functioning right.

I assume your problem is that they drop a lot (almost stall) and then go back up (but not immediately)
when you put anything electrical on

In my opinion,
first thing to look at is your charging system (Alt and battery) and the wires. In my case, believe it or not, a weak battery was the problem.

Second is engine compression. If engine is tired, you are gonna have a hard time with your electricals because of low-end torque.

Also,
make sure your idle speed is set at 750 rpms with initial set coupler jumpered and that the TPS is properly adjusted, but you already took care of that.

For the brake part, you could try to use LED lamps (4 1157 and 1 1156), or you could remove 2 of the 1157 and see if that helps. If you go the LED route, make sure you get bright ones, otherwise you run the risk of pple behind you not seeing when you brake (kinda dangerous for everybody).

Hope this helps,
Hugues -

cblake3

iTrader: (2)

RPM drops and then goes right back to normal. If I pump the brakes at idle, I can stall the the motor.
What do you mean by running LEDs? Is this instead of the stock bulbs. My guess is less electrical draw.

I don't understand how the problem could be related to low compression. Compression should not be a problem anyway. I had the thing tested about 500 miles ago and it had good numbers all around.

hugues

As you probably know, when you turn on something electrical, the alt has to work harder to provide for extra current needed. So, the alt pulley is harder to turn, so an engine with low compression will have a harder time than a healthy one. Since rotaries have low low-end torque to begin with, it certainly doesn't help if the compression is lower than normal.

If you say your compression is good, the BAC is good, the idle speed is good, the TPS is good, then I can see only 1 thing remaining:

bad or defficient charging system (alt, battery, and/or juice wires). I would have it checked under load at autozone and I would possibly replace the battery wires and/or add grounds.

I know you said your TPS and idle speed were spot on .... but ...
Please double check the TPS. If the tps is off, the ecu is misinformed about the plates position at idle. You should have about 1V at the TPS (engine running at idle and hot).
Double check your idle speed. It must be set with initial set coupler jumpered. If idle speed is off, the ecu may be waiting too long to send signal to BAC.

Yes, the stock brake lights draw like 10-15 amps. That's a lot if you think about it. If you go LED, the draw is negligible but they are not cheap. It is obviously a band-aid solution as well but it may give you some piece of mind when in stop-and-go traffic.

Another band-aid solution on S4 is to put a relay that activates the power steering switch when you turn on something with high current draw like the headlights. By activating the power steering switch, the ecu will jack the rpm up (1,000+ rpms).

Note that on s5, there is an headlight switch input to the ecu, something you don't have on the s4, so I believe the mazda engineers must have seen that something was not up to par with the s4 electrical setup (that's just my opinion here).

I think I would still double check that the BAC gets correct signal from the ECU when things get turned on.
With no loads on at idle, the voltage at BAC should be around 10V (that's dc voltage). When you turn on the headlights, the voltage should drop a bit and stay there until you turn off the lights (around 9.5V). Of course, the more accessories you put on, the more the voltage will drop. Note that those voltages are relative to each other, what's important is that you a see a voltage drop at the BAC when you turn an electrical on. When voltage at the BAC drops, its duty cycle increases and more air (and fuel via ECU) passes through. Check for posts by HAILERS on the subject for more real life data on this.

Hope this helps somewhat,
Hugues -

RETed

Stock alternator sucks!


-Ted