MmSadda

iTrader: (2)

I've got an S4 n/a with 20k on a rebuilt engine.

Occasionally (and more frequently as of late) the car will stop making any power, will act as if the throttle isn't being pressed no matter how much gas I give it in gear, and won't rev past 2k rpms in neutral.

The issue can last for quite a while, or can happen for just a millisecond at a time.

I discovered a poorly-attached replacement wire in place of one of the wires from the harness running to the throttle body. I thought the issue had been solved after soldering this wire, but it turns out that has not fully resolved the issue.

My mechanic told me that he believes a rust hole near the ECU (which has since been fixed) was allowing moisture to get into the car and mess with the computer.



So, I'm planning to replace the ECU, and test the resistance of this one troublesome wire.

However, I want to know - does anyone have any other suggestions as to what the problem could be? Any other tests I should try? Thanks!

-Collin

funkjaw

iTrader: (3)

I had a very similar problem, turned out it was the thermo switch that is by the alternator.

HAILERS

Unplug the TPS sensor and drive it for several miles. If the problem goes south, the TPS or the wiring to the TPS is corrupt.

MmSadda

iTrader: (2)

I forgot to mention, the car got a new TPS the first time this problem occurred. The wiring may well be bad, though. Any chance the ECU is the problem?

Funkjaw, thanks for the suggestion. How'd you diagnose that? or how would I know if that's my problem?

funkjaw

iTrader: (3)

Well I work next to Rotorsports which is a rotory/mazda repair shop, I described my symptoms to Paul the owner and he immediately off the top of his head thought the thermo sensor. So he grabbed a used old one off his work bench that he knew was good for me to try and it fixed the problem %100 after I put it in.

MmSadda

iTrader: (2)

Put in a new ECU as recommended by my mechanic, and the problem hasn't gone away. . .

By "goes south," do you mean if the issue gets worse, or if it stops (i.e. is better)? The wiring is VERY possibly an issue; one of the wires was (poorly) replaced to the ECU in the past.

Wow... Sounds like a pretty convincing argument for that being the issue. I'm going to need to look into that. Any recollection of how much the part cost?

funkjaw

iTrader: (3)

$2 for as many as you can pocket at the junk yard.... Brand new: http://mazdatrix.com/getprice.asp?partnum=18-8400-8574

calpatriot

Just soved a very similar problem on my 86. Turned out to be a broken wire between the O2 sensor and the ECU. The ECU did not always correctly recognize that the O2 sensor was not present. As long as it did, it operated in limp mode (rich) and the car ran fine (but ate up a cat). When it did not, it tried to adjust mixture based on the (non-attached) O2 sensor, and then it ran horribly, very similar to the way you describe (no power, unable to rev over 2000, bucking, idle pretty much ok).

I do not see anything in your posts that says you have checked to see that the system is operating in closed loop mode, i.e. flashes on the 'green light' pin of the test connector. It's easy to rig a test lamp for that. That is how I found mine, because even if you check codes, the ECU does not always report the sensor as out. In fact, when mine ran poorly, it threw no codes.

MmSadda

iTrader: (2)

Care to explain this test you mention a little further? I've not heard of it before. I'd like to try it soon.

I'm going to tear into the car this Saturday or Sunday after I get my basement painted. I'd like to start with things I can test (like the issue you describe here)

calpatriot

It is a check to see that the ECU is actively controlling the mixture. When the car is cold it doesn't, and that is called 'open loop', meaning no feedback. Also known as 'limp home' mode. It goes into limp home mode if either the car is not up to operating temp yet, or if it does not see a signal from the oxygen sensor.

Normally it should be in open loop mode (in which it runs rich at an arbitrary, ie. not controlled mixture setting) until it reaches operating temp, and then the ECU goes into closed loop mode and starts adjusting the duration of the fuel injection pulses to keep the mixture at approximately 14.7:1. This is for emissions control.

The test connector is just left of the battery, forward of the leading coil. It is green, and has six female connections. As you look at the connector, with the locking pin on top, the O2 sensor status (everybody calls it the 'green lamp' because that was the color of the indicator on the Mazda test set) is the bottom center pin. It goes low when the mixture goes richer than ideal. By connecting a lamp or LED (with a 510 ohm current limiting resistor) between the two center pins (the top one is +12v), the lamp will illuminate when the ECU sees a rich mixture, and go out when it sees lean.

In normal cruising operation the lamp should flash on and off about 8 times per 10 seconds, according to the FSM.

Under acceleration, it should be on steady (rich)

Under deceleration, it should be out steady (lean).

Mine is lit at idle, but the FSM does not specify if this is correct.

All of the tests must be conducted with the car warmed up to operating temp.

You can make a simple test set out of an LED, a resistor (510 ohms), a two conductor wire, and a couple of male crimp spade connectors.

I made mine long enough that I could connect it to the test connector, snake it out from under the hood and into the cabin past the door seal. Laid on top of the dash, it allows me to see the system operate on the road.

There is a test procedure in the FSM for in the garage, which involves disconnecting the neutral switch, which is too much like hard work for my taste.

I will bet that your lamp is out (just like mine was) indicating the ECU is seeing a lean condition all the time, which is what it will interpret if the O2 sensor is not connected to the ECU (0 volts is low, which to the ECU means lean.) Based on that input, it will keep trying to enrichen the mixture to correct the (false) lean indication, which may be why your car will not run well.

If you connect up the lamp (check the + and - terminals by touching them to the battery), it should be on at idle. If you rev up to 2000 rpm or so, once you are steady at that rpm the lamp should start flashing on and off, signalling that the ECU is modifying the mixture back and forth from the rich and lean conditions.

You can also check ECU error codes using the left two terminals (10s and 1s digits, and the top center again for +12v).

calpatriot

I also made a similar test cable to monitor the O2 sensor voltage on the road. One connector spliced into the O2 conductor near the right wheel well; the other fastened to ground. Brought it into the cabin by passing the cord under the rear of the hood, and through the door jamb. Both interfaces have soft weatherstripping so if you place it properly neither the hood nor the door will pinch the cable. I set up my voltmeter just forward of the gear shift lever, and went driving.

The voltage out of the O2 sensor runs from 0 to +1v, with 0.45v being supposedly ideal mixture. Mine ran -0.14v (lean) during deceleration, 0.56 - 0.82 (rich) under acceleration, and around 0.56 in cruise, with occaisional flashes down to .14 (lean).

All of this agreed with the behavior of the green light test, which I ran simultaneously.

MmSadda

iTrader: (2)

Calpatriot, thank you very much for your help!

As per my mechanic's recommendation, I'm going to try resetting the ECU and see if I can pull any codes from it (It hasn't thrown up any codes in the past, but that was on an old ECU.) I'll check ECU codes after resetting the ECU, and do the "Green light test" that you so vividly described. Thanks so much for that.

calpatriot

On my car it would only show a code for the O2 sensor some of the time, which fooled me for quite a while. Apparently the logic in the ECU does not always figure out that the sensor is 'gone'.

The intermittent nature of the drivability problem in my car was due to this 'feature.' When it correctly sensed the O2 sensor was missing, it went into limp mode, and drove very well. (Why would you check for codes when it drives great?)

Then, when it did not sense the O2 was missing, it drove terribly (much as you describe), and presented no codes. As far as the ECU knew, the sensor was working.

I noticed that my car sometimes ran great for 20 sec or so right after startup, and then started having trouble. The green light test showed that it was running rich (light on) for that time, and then went to lean (light out) with no flashes (crossovers).

I even replaced the O2 sensor (old one is probably fine). It ran great, and I thougt I had fixed it, but I checked for codes and there it was throwing code 5--with a new sensor.

That's when I checked the wiring and found the problem.

Yesterday I took it for a long ride in the mountains, and it did a great job.

calpatriot

If that doesn't work, there is a three page table in Ch 4a (4b if yours is a turbo) of the FSM that shows what the voltage should be on every pin of the ECU, for both ignition on (not running) and idle conditions. If you pull up the carpets on the right side, and remove the cover plate, you can take your voltmeter (using a small thin probe, such as a paper cip), and probe each of the pins from the back of the harness connector. If something is amiss, you may find one of those values not agreeing with the table.

calpatriot

How do you reset the ECU in an S4?

cmanns

^Remove all power to the ecu to clear it....

MmSadda

iTrader: (2)

Thank you again! I can't believe how recently you joined; you're clearly one of the more knowledgable and intelligent members here, juding by what I've seen so far. Again, thank you so much!

Remove the negative battery terminal, and hold the brake pedal for ~20 seconds (presumably to use up any residual electricity stored)
Yeah, but it's a little more than just removing the battery terminal. See above.

calpatriot

Glad to be of service. Many others have helped me when I did not understand something on this car. I have owned it over 10 yrs, but only recently began to get serious about doing my own work. I am still learning, but I try to share that which I have learned that might be valuable to someone else.

I do have one advantage... my undergraduate degree is in electrical engineering, and I have taught the ins and outs of manual mixture control to aviators for many years... only some of that transfers to ECU controlled emission systems, though.

MmSadda

iTrader: (2)

Okay, I got the car out finally and ran some very basic tests today.


First, I ran with the TPS unplugged. The car ran the same as it would when working normally. This is inconclusive, as the car occasionally ran properly sometimes with no changes. It may indicate, though, that the TPS or its wiring was in fact the problem.

Next, I ran with the AFM unplugged. It seemed to run a fair bit less stable in limp mode as it would with the AFM plugged in. It almost seemed as bad as when the car was undrivable.

Going to get supplies to rig up a test lamp tonight....