derporsche928

Hello all,
A couple weeks ago my GT started running rough, irregular idle, down on power, hesitation, etc..... Doesn't matter whether the motor is cold or hot.....
No fault codes were stored, but found the Throttle Position Sensor was flakey and fixed it. In fact, it was intermittent in even being able to call up the fault codes with the gas pedal. I assume because the W.O.T. switch must be tied into triggering the code flashing. (It had the same problem as one on my GTS had exhibited before, except it was on the idle switch, where the solder joints inside were bad.) ..... Anyway, I digress.... After buttoning up from the intake removal to deal with this, I expected the car to be okay, but it still has the same poor running symptoms. As a mater of course, I checked the coils, caps, rotors, and they checked within spec. A couple of plug wires were suspect, so I replaced all of them with a good used set from my S4......... Still no good......
I swapped the ignition modules (drivers side in front of radiator) from the S4 as well, and still no good....... I'm currently going through the diagnosis test points in the manuals for the LH and EZK. Starting from LH Test Point 1 everything checked good until I came to the LH Test Point 5c (checking the Air Mass Sensor on page D24-16) and found that I get no voltage (spec is 1.6v) at the 6 / 7 LH connector terminals with the XXV relay removed and jumpered.
(The previous two Test Point steps a and b confirmed power supply and hot wire resistance were okay.) I confirmed I have battery voltage at the relay and the wiring from the 6 / 7 LH connector terminals to the Air Mass Sensor terminals 3 / 5 respectively checked good continuity, so I thought it must be a bad connection at the AMS, and then cleaned them, but still no luck...... I then swapped in the known good AMS from the 87, and I still get no voltage at 6 / 7....... What the heck am I missing here?
Apologies for the rambling, but trying to give what is known so far......
FWIW, the Idle Stabilizer, and AMS were replaced new last fall, and this performance problem seemed to manifest itself at the same time that my cooling fans started running continuously even after shut down for ten minutes or so, (until I disconnected the battery to kill them, but then they didn't come back on after the battery was reconnected several minutes later....)
I really tend to think these are separate issues, but thought I should mention it, as perhaps someone more brilliant than myself had seen this before and could speculate whether they may be related..... (The temp sensor 2 checks okay)
Well, I have to go to bed, (3rd shift) but am hoping when I check back tomorrow morning, someone will be able to help me understand what I could be missing regarding the aforementioned AMS troubleshooting dilemma.
Perhaps a good sleep will provide a fresh new perspective......
Any help would be most appreciated, because if I can't solve this before SITM, I'll have to cancel, and I really, really don't want to do that because it's right up there with my other most favorite thing to do.

Best,
Greg Schickel

ew928

Greg,
Do the fans still afterr-run after shutdown? Even if the engine isn't that hot.
Mine did that and I had to disconnect the temp sender on top of the intake.
The 2 wire sender next to the flappy. Temp sender should be open (infinite resistance) unless the intake is pretty darned hot. Mine was showing low resistance on a cold motor.

Was thinking MAF/S but you've got a new one in there already. Hopefully John Speake will pitch in with a real answer.

On a seperate note, did the fans kill the battery. My battery finally wouldn't let me start the car on day 3 of the running fan mystery. Voltmeter only showed 11.8V.

Good luck.

AO

Greg-
I'd love to help, but I'm out of town attending a funeral. Worst case, I think you're on the way when I go to SITM. I'm actually heading down on Thursday to run the Dragon. I could probably swing by your place and hook you up to my diagnostic tool/computer. Can't guarantee that we can get down to SITM, but maybe we can figure out the issue.

Do me a favor though, pull one plug from each bank (I think 1 & 2 will do) and compare. Report back the results.

Bill Ball

Does the car run the same with the MAF unplugged?

Your tests show you are not getting voltage from the hot wire despite continuity in the harness to the MAF plug and a known good MAF. Hard to explain, isn't it? This may be a pain, but make a set of jumper wires between the plug and the MAF to make certain you are getting contact. If that fixes it, then you know that the plug and MAF are not making good contact despite your cleaning.

John Speake

Unplug the LH ECU (remove battery grouind strap first) and then measure the resistance between pins 6 and 7 of the LH disconnected plug, with the MAF still connected. You should read a low resistance of 4 ohms.

You should also read virtually zero ohms between pin 6 LH plug and chassis.

derporsche928

Thanks for the advice fellas........ Had a long night at work and am going straight to bed this morning, but will check things per your input and a couple other things I'd thought about while trying to concentrate on work instead of solving 928 problems..... It occurred to me that my battery voltage was down around 12.4 volts instead of the normal 13.8 -ish, and I wondered if the MAF may have some kind of threshold voltage needed before it will send a signal back......
Thanks again for the input..... I'll endeavor to keep you all updated, and hope you'll follow along and continue to offer your thoughts.....

Best,
Greg

FlyingDog

I don't know about the 928 computers specifically, but most car computers will work well under 12V.

Have you tried bridging or replacing the Temp II sensor? Have you double checked every connector? Smell fuel? Getting spark on both distributors?

John Speake

MAF and ECUs will run down to at least 9v or so. The LH has pulse compensation vs battery voltage.

Randy V

Greg Schickel?! - There's a name from the past!

Any progress on gettin' this sorted out?

derporsche928

Hello again gentlemen, (or not
Okay, brought the laptop down from the house, so that I can feedback as I go thru this.......
Checked a few things per your advice, and here's what I found:
Per Andrew's request, (frame 2) checked a plug from each bank, and they both looked clean and normal.
Per Bill's advice, (frame 3) I started it with the MAF unplugged. It wouldn't start or run without giving it gas, and it ran much worse and would not idle without dying. (Bill, I am still perplexed as to why I can not get the 1.6v back from the MAF, and would like to move on with my troubleshooting, but am stuck here in my head..... Will try, per your advice, to run separate jumpers, but haven't yet......
Per John's advice, (frame 5) I checked the resistance between pins 6 and 7 of the LH disconnected plug, with the MAF still connected, and got the proper 4 ohms resistance.....
Matt, (frame 7 advice) I rechecked the temp 2 sensor, and got 1.9k ohms at 41c, which is a little outside of 1k to 1.3 k ohms at 40c spec...... (It had checked within specs the other two times I checked it.) I tried jumpering it, and the car wouldn't start, (or at least easily, and I didn't want to run the battery down any further....) I went ahead and swapped one from the S4, but before I did, I compared them in ambient air, and they checked the same. I swapped the other one in anyway, and it didn't seem to improve, (or did it? Aarghhh) Acceleration is okay, but it still seems to idle unevenly. Really need to button car up and check under load to be sure, but gut feeling by the roughness in it, is that the problem still exhists and now I am hopefully wishing......
I still keep going back in my head to this errant (non-voltage feedback) from the MAF.......
Still checking things, but soon I have to make the doughnuts. (read, go back to work)
Thanks again,
Greg

shaaark89

greg,
are you sure you got the intake back together ok? could there be false air entering somewhere beyond the mass air?
you need to be at sitm buddy!

derporsche928

Hello Tom, everyone......
Pretty dang sure the intake is all proper, but slightly concerned I may still have a Throttle Position Sensor issue, as I had re-soldered a couple cold joints on it. (It ohm'd out fine though, and still does)
However, The perplexing problem with the MAF signal back has been solved.......
Duh, (read, I'm a dumb-***) ..... Turns out I had a bad jumper wire that I was using to bridge the XXV Relay at the LH Test Point 5c. (Bill, thanks for prodding me continue to question this...)
Now I'm getting 1.8 volts back, and it does increase when I blow down the MAF....
The only concern now is Spec is 1.6v as opposed to the 1.8v I'm getting..... Should I move on......?
FWIW, Battery voltage is down to 11.55v right now, and wonder if it could be the cause of non-spec MAF signal back. Perhaps I need to do some math to determine for sure....?
I'm still also wondering whether I should even be looking at things that would normally show up as fault codes, what with me not having any......?
Now I really have to go to work.... !
Thanks again fellas, ...... Stay tuned and shoot me back any other feedback you'd be so kind as to offer.
Best,
Greg

Garth S

In the category of strange ones, read the solution in post 34 of this tale of poor running : The hall sensor can retard the timing 6 deg - but when the shield is shorted, apparently other nasty things happen.

In post #6, 12.4V was mentioned - was this on a running engine? If so, there is clearly a charging (alternator) issue, for the ~13.8V you stated is correct. 12.4V is acceptable for a battery alone with the motor off.

ew928

Greg,
Trickle charge the battery. My fan over-run issue (till I unplugged the top temp sender) drained my 12V down to 11.8 and the car would not start. Would tick like mad but not engage the starter gear.
Looking forward to your Bwaaah is restored post.

worf928

Its a long shot, but, since you have a 'donor' handy... have you tried to run the GT on the S4s LH?