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Thank you all for your input. Had ECU rebuilt; ran fine for a week. No longer misses and runs fine at cruise.
New or maybe related issue: when coming to a normal stop (not from excessive speed; RPMs 1500-3000), put in clutch, RPMs drop momentarily to 250.
Then one of two things happen. 1) engine dies, 2) engine recovers.
If 1) then engine easily restarts and idles normally.
If 2) engine recovers and idles normally.
Thinking of some transitory event when engine RPM dropping.
WOT or Idle switches ??
A GT should idle at 775rpm when engine is up to temperature - at lower water temps the idle speed will be up to 250 rpm higher.
The ISV may be sticking, you could try the WD40 trick - search will show it.
Make sure the closed throttle switch (TPS) is set up correctly so that it clicks reliably when the throttle is cracked open and also when it closes back to the foot off position.
However it may be that the switch is intermittent. They can suffer from cracked internal solder joints.
After running Worf's diagnostics to determine the cause of my "idle die" problem the problem seems to have gone away. It's been a few days but I'm leaning towards the culprit being a sticky ISV.
The ISV diagnostics call for bridging LH pins 17 and 21 and then grounding pin 33 to ground. This forces the ISV to audibly exercise. I did this operation several times and it was clear that the ISV was actuating. My gut says this loosened up the ISV so now my idle is stable. A bunch of carbon blotches exited from the exhaust after the diags but are now gone.
I reserve the right to completely retract this if my idle goes to hell tomorrow.
Tried the WD-40 or carburetor cleaner route. Upon start up the next morning, car idled a bit roughly.
As soon as I again disconnected the hose aft of the Y-fitting on drivers side of engine, to reapply a 2nd shot, the idle smoothed out. Took car for a drive with the hose disconnected, everything runs fine. Idle stale a little above 750, with AC on or off. No engine dying issues when clutch depressed coming to a stop sign.
So, is this a successful isolation of the problem to the ISV, and if so, how needed is the ISV?
Or, is there some blockage upstream on this hose that is alleviated by unplugging the hose?
Or, ??
Checked Idle and WOT switches; both checked good:
- on LH and EZK - idle switch closed until throttle pushed slightly then switch opens (ohms go to max).
- on LH and EZK - WOT switch open until throttle pushed generously then closes (ohms drop to near zero).
Checked ISV - heard click from engine bay as ground disconnected from pin 33.
Picture attached of the hose disconnected:
Engine runs fine with hose disconnected. When reconnected, have idle issues; engines dies half of the time when spooling down from cruise.
Hook ISV hose back up but disconnect MAF electrical connector to see if idle holds albeit rough. Connector could be corroded if idle holds when disconnected but stalls when connected. Try cleaning and using Deoxit on mating connector surfaces.
If this checks out ok I'd look at CPS connector for possible disintegration. Per the forum, CPS rarely fails but connectors get brittle and fail.
Engine runs fine with hose disconnected. When reconnected, have idle issues; engines dies half of the time when spooling down from cruise.
By disconnecting this hose you have created an intake leak. A more precise phrase is that you have created a source of unmeasured air a.k.a. "False Air."
The only reason your 928 runs better with this hose disconnected is that the unmeasured air is being matched with "false fuel."
If everything was perfect with your GT, disconnecting this hose should cause the idle to go way up.
So, you need to find the source of this extra fuel. In fact, I'm going to bet that you have two problems that are introducing extra fuel.
There are three likely possibilities:
1) Mass-Air Sensor. I posted this in post #2 to this thread. We haven't heard from you about the age or condition of your MAS (assuming I didn't miss anything.)
You can send your MAS to Kevin at Injection Labs. He can test it and tell you if it is out-of-spec. I bet that it is.
2) Temp-II sensor. If the sensor is faulty or if there is high-resistance in the circuit it will read cold and cause the LH computer to richen the mixture (more fuel.)
Follow my diagnostic guide and check the resistance values at the ECU pins at two temperatures: shop cold and engine warm. The resistance values should correspond to the values in the table in my guide for the sensor. If they don't then check at the pins of the sender. It's two circuits. The positive for each circuit is one of the pins the ground is a ground point in the engine bay or, most of the time, the cross-brace bolts will do.
3) The manifold vacuum system is sucking-in fuel from one or more broken diaphragms in the fuel pressure regulator or the pressure dampeners.
Pull the vacuum lines from each of these and see if the ends smell like fuel or drip fuel or if fuel comes out of the vacuum connectors to the regulator or dampeners.
Do these easy tests and report back before you go to less-likely culprits like the CPS.
Temp II sensor new; checked good after installation.
Checked how? By resistance values at the ECU pins? You can replace that sensor as many times as you want and if the electrical connection is poor you will still get the wrong temperature reading to the ECUs. Wires go bad too. So do pins in connectors.
MAS inspected, appeared in good condition: electrical wiring redone; however, not re-calibrated.
You can't inspect a hot-wire Mass-Air Sensor and determine it's 'goodness' unless you have a flow-bench and a reference.
Does your mechanic have a flow bench and reference?
The MASs are good for 20k-90k miles depending on a great many factors. If you have no records for a new MAS or a rebuilt MAS in the last ~20k miles then yours is probably way out of spec.
Maybe someone in the area with a known-fresh MAS could 'lend' it to you for 5 minutes to see if it cures your problem?
If none of the above...
I'm going to write this one last time, as the third time may be the charm:
Send your MAS to injection labs for testing. It will cost postage. If it is indeed within spec Kevin will tell you and send it back to you for postage. If it's out-of-spec he'll quote you the rebuild cost.
I performed some additional tests with the hoses; please see very busy picture below.
Checked how? By resistance values at the ECU pins? You can replace that sensor as many times as you want and if the electrical connection is poor you will still get the wrong temperature reading to the ECUs. Wires go bad too. So do pins in connectors.
You can't inspect a hot-wire Mass-Air Sensor and determine it's 'goodness' unless you have a flow-bench and a reference.
Does your mechanic have a flow bench and reference?
The MASs are good for 20k-90k miles depending on a great many factors. If you have no records for a new MAS or a rebuilt MAS in the last ~20k miles then yours is probably way out of spec.
Maybe someone in the area with a known-fresh MAS could 'lend' it to you for 5 minutes to see if it cures your problem?
If none of the above...
I'm going to write this one last time, as the third time may be the charm:
Send your MAS to injection labs for testing. It will cost postage. If it is indeed within spec Kevin will tell you and send it back to you for postage. If it's out-of-spec he'll quote you the rebuild cost.
Everything is as it should be.
+1 I also wondered how a mechanic could "inspect" a MAF and determine that it "appeared" good without specific testing, but I'm glad you beat me posting it.
+1 I also wondered how a mechanic could "inspect" a MAF and determine that it "appeared" good without specific testing, but I'm glad you beat me posting it.
Unplug the MAF electrical connector. Does it hold a rough idle? If it does it's the MAF or the electrical connector. If connector contact points are not corroded it's the MAF as everyone is suggesting.
My idle problem cleared up by cleaning contacts surfaces with lightly sanding contacts with fiberglass pencil sander and deoxit.
09-29-2015, 12:17 PM
