The Official Mongo's 928 Exorcism Thread (Begins on Page 4)
05-17-2010, 12:32 PM
#106
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When the MAF output volts drop below about 2.2v at idle the LH goes into limp home mode.
Below about 2700rpm there's a fixed pulse width of about 3mS, it's only modifier is Temp 2 value, so you can start a cold car in limp home.
Above about 2700 rpm is another fixed injector pulse region, about 5mS IIRC.
HTH
05-17-2010, 12:34 PM
#107
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I am just glad I found the problem FINALLY. 
BUT - I'm still going to rip those fuel lines, FPR and damper out the back of the car and clean those grounds! I have two spiffy new ring terminals I bought this weekend just dying to go in there.
BUT - I'm still going to rip those fuel lines, FPR and damper out the back of the car and clean those grounds! I have two spiffy new ring terminals I bought this weekend just dying to go in there.
05-17-2010, 02:33 PM
#110
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They don't go Alpha-N because there's no throttle position pot on the 928.
When the MAF output volts drop below about 2.2v at idle the LH goes into limp home mode.
Below about 2700rpm there's a fixed pulse width of about 3mS, it's only modifier is Temp 2 value, so you can start a cold car in limp home.
Above about 2700 rpm is another fixed injector pulse region, about 5mS IIRC.
HTH
When the MAF output volts drop below about 2.2v at idle the LH goes into limp home mode.
Below about 2700rpm there's a fixed pulse width of about 3mS, it's only modifier is Temp 2 value, so you can start a cold car in limp home.
Above about 2700 rpm is another fixed injector pulse region, about 5mS IIRC.
HTH
05-17-2010, 02:40 PM
#111
Found that same issue on a '91 last week while doing the fuel lines. Replaced the connector and good to go.
05-17-2010, 04:17 PM
#112
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Hey Sean,
I left Roger a voicemail for a request to order. Do you two work in the same office together? I am sure you guys are busy down there...
I left Roger a voicemail for a request to order. Do you two work in the same office together? I am sure you guys are busy down there...
Last edited by Mongo; 05-17-2010 at 04:46 PM.
05-17-2010, 06:23 PM
#113
Under the Lift
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After fiddling for a while with Andy's car yesterday, we're now betting on the harness/plug for the MAF, as Andy noted. Initially the car started and idled fairly well. Warmed up and still ran well, with very slight occasional drops in idle, but no pronounced hunting. All plugs firing, no bad stuff coming out the tailpipe, nothing clearly abnormal EXCEPT a marked hesitation on quick throttle blipping. Didn't die and didn't have bad return to idle, just serious hesitation, but then nice VROOM. Test drive was about the same, but after a couple of WOT runs, it began to misbehave, with the idle dropping to almost nothing and smoke out the rear. That's when I disconnected the the MAF and found the car ran the same or perhaps better with it disconnected. The wires near the plug had been taped over and were brittle but there were no obvious breaks or crossed wires. Andy's replaced almost every component, including the MAF, so that MAF plug wiring is the next item. Sure, it's possible the new MAF has a heat-related fault, but the wiring needs repair regardless. Also, I think Andy had done MAF swapping already and we found Temp II right on the money at the ECUs, so I think we're on the right track now.
05-17-2010, 06:30 PM
#114
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Its good to reflect on the symptoms a bit. Our maps may change some. Jadz had a "no-start when hot" S4 last week. CPS wires badly frayed in harness. But even with no-start, the tach aways bounced.
05-17-2010, 06:43 PM
#115
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Right. Andy's isn't absolutely classic for MAF but close enough, depending on the wire fault. The hesitation fits and the final behavior definitely fits. His tach takes and odd single bounce to 2K on initial cranking. Then the car starts and runs fine for quite a while (other than the throttle hesitation) before acting up like a bad MAF or MAF wiring.
05-17-2010, 07:02 PM
#116
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Hey Bill,
Once again thank you for coming all that way to help me out yesterday. When I am back on the road problem free I would like to get a run together for the backroads of Nicasio, Inverness, and/or Occidental. We can all eat at that Puerto Rican restaurant for lunch I took you to.
Once again thank you for coming all that way to help me out yesterday. When I am back on the road problem free I would like to get a run together for the backroads of Nicasio, Inverness, and/or Occidental. We can all eat at that Puerto Rican restaurant for lunch I took you to.
05-17-2010, 11:31 PM
#118
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They don't go Alpha-N because there's no throttle position pot on the 928.
When the MAF output volts drop below about 2.2v at idle the LH goes into limp home mode.
Below about 2700rpm there's a fixed pulse width of about 3mS, it's only modifier is Temp 2 value, so you can start a cold car in limp home.
Above about 2700 rpm is another fixed injector pulse region, about 5mS IIRC.
HTH
When the MAF output volts drop below about 2.2v at idle the LH goes into limp home mode.
Below about 2700rpm there's a fixed pulse width of about 3mS, it's only modifier is Temp 2 value, so you can start a cold car in limp home.
Above about 2700 rpm is another fixed injector pulse region, about 5mS IIRC.
HTH
This may explain why doing a relearn can help so many cars.
Does the car have to be reset to come out of limp home?
05-18-2010, 05:15 AM
#120
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The car might idle, but rich, the fixed pulse is a bit too long. My experience has been almost entirely with non-cat cars in limp home (cats were not mandatory fitment here until about 1993 IIRC).
So US cars may behave somewhat different, because the O2 loop may not have enough range to cope with an injector pulse so far off optimum.
It also means that the adaptation will also be well out after a limp home session, so yes, you have a good recomendation to do a battery reset.
As soon as the MAF volts are in the expected range, the LH will automatically revert to "normal" mode without a battery reset, but because of potential adaptation issues I would recommend that a reset is carried out.
Something Andy should do when the MAF wiring has been sorted out.