I have a new demon in the works.
The idle will surge from 1000rpm to 1600 rpm once the engine is warmed up.
At first start up the idle is steady at 1200 but once it's warm it surges up and down on about a one second interval. When it hits the 1600rpm mark the injectors stop firing (can feel the body of them and they temporarily quit clicking) and of course refire once the rpm's hit the 1000rpm mark. Almost as if the rpm's would continue to rise indefinitely if not for some threshhold being crossed, maybe in an ignition map, and so the injectors are dropped out.

I have checked the obvious. In the SharkTuner the ISV is staying around 25%, I have a new and functioning (according to the SharkTuner) Idle Position Switch ( shows in both in the EZK and LH monitors)....a refurbished MAF, a refurbished LH ECU, a new O2 sensor.

I switched the O2 sensor lead to the ECU to the analog output of the wideband sensor which is set up to mimic a narrow band sensor just to be sure...no difference. Disconnected battery then disconnected standard O2 sensor then reconnected battery running engine without the O2 sensor and there was no difference. The TempII sensor is testing correctly at high and low temps and the signal is correctly present at the LH and EZK ECU 35 pin connectors.

The temp during all these tests was 98 degrees centigrade.

Driving the car it is perfect, only at idle does the surging happen, or at least that's the only time it is perceivable.

I unplugged the MAF and restarted the car and the the surging goes away but it idles at 800 and the AF ratio bottoms out on my gauge at 9, smoke from the pipe looks like it's at least that rich! Does this mean the MAF is bad or does that symptom point you somewhere else? Hard to believe since the MAF was just refurbished less than 1000 miles back.

 

Idling at 1,200RPM is not normal. There must be an air leak somewhere in your system to cause this. If the leak is big enough, it will cause the car to surge at idle. The leak seems to be bad enough to surge the car to 1,600RPM where the injectors shut off for coasting. Normal LH 2.3 idle speed is ~825RPM for an S4 and ~775RPM for a GT. Since the idle is set automatically by LH, there must be some unmetered air finding a way into the engine to raise the idle.

Dan
'91 928GT S/C 475hp/460lb.ft

 

Well I had pulled the MAF and rubber boot below off to inspect it and the ISV hoses and vacuum lines nothing "looks" loose and prior to doing that I disconnected all vacuum lines and the "Y hose" that comes out from the front and connects to the oil fill tube and the exhaust gas solenoid...plugged all lines and hose connections to the intake..basically running the engine with all vacuum connections plugged with the exception of the ISV connections which were left intact and I connected a mighty vac to intake port that would usually feed the 4 way vacuum line connector. Under those conditions the vacuum was solid 20 inches and idle still surging. So if it is a leak it must be the ISV body and or it's hoses or a leaking gasket. Maybe my lower flappy bearings gave way completely....

 

Does it surge with the SC not connected to the intake? Try removing both boots one at a time (MAF boot; SC output boot) and observe for any change in idle behavior.

 

Also, poking around in the SharkTuner looking for symptoms I found that in the Warm Up compensation Map in the EZK, the blue cursor is on the last cell which is 77 degrees. If I retard the timing in that cell the surging stops and the idle sits at around 1000rpm's....odd since the temp was holding between 98 and 100 degrees at the time. I would have expected that map to be unused under those conditions.

That may mean nothing though, maybe if John Speake see's this he can tell me why it did that.
I just mention it in case it leads someone to think of something I don't know about in case the false air theory turns out to be wrong.

In the meantime though I'll go with the air leak theory since it makes the most sense and start to R&R the intake for the umpteenth time! Grrr!

 

Hmm, your description of searching for an intake leak is not that conclusive IMO. Do you have one of these:

https://rennlist.com/forums/928-foru...-for-sale.html

Dan
'91 928GT S/C 475hp/460lb.ft

 

Hi Bill!
I'll do that next. By SC output boot I'm not sure which you mean. Mine is a stage III so I have blower > three inch long hose > 90 degree curved metal pipe>two foot long hose>intercooler>Murph supplied MAF boot

I'm guessing you might say to open the system up before the intercooler first? Then try at the top of the MAF?

 

Yes, something very similar anyway. I've pressurized the intake a few times with little success mainly because I'll set the regulator to 15 lbs. and the air has always slowly drained out of my tank through the engine and never shows itself in spite of listening for it with a piece of fuel hose in my ear or spraying soap bubbles on everything I can...I even bought a 3 foot flexible camera thing and snaked it all around under there. Each time is inconclusive so I R&R the intake thinking I just haven't seen it.

I did get my idle under control a month ago by retarding the timing in the idle map and it had been holding steady at 675 rpm's until this sudden change.

Here's a question regarding the pressurizing that no one has ever really answered clearly for me: If I block off everthing and pressurize the intake should the whole thing hold steady at 15 psi or does the air leak out..like past the valves etc? Mine has always emptied a 15 gallon tank with the regulator at 15 psi in about 12 to 15 minutes.

 

So if you check on LH and EZK System monitor screens, is temp 2 recorded the same on each one, and does that appear to be correct, or reading lower on the EZK as the EZK warmup map appears to suggest ?



QUOTE=aggravation;6356130]Also, poking around in the SharkTuner looking for symptoms I found that in the Warm Up compensation Map in the EZK, the blue cursor is on the last cell which is 77 degrees. If I retard the timing in that cell the surging stops and the idle sits at around 1000rpm's....odd since the temp was holding between 98 and 100 degrees at the time. I would have expected that map to be unused under those conditions.

That may mean nothing though, maybe if John Speake see's this he can tell me why it did that.
I just mention it in case it leads someone to think of something I don't know about in case the false air theory turns out to be wrong.

In the meantime though I'll go with the air leak theory since it makes the most sense and start to R&R the intake for the umpteenth time! Grrr![/QUOTE]

 

Ok, I took the connection loose at the blower outlet diverting the air so the engine just drew its air through the intercooler, getting no help from the blower outlet. Basically the same thing. Start car, engine idles at 1100rpms, tap throttle pedal and the surging begins.
so I put that back together and removed the intercooler output from the rubber boot that sits on top of the MAF...so the MAF was just drawing air in through the boot. Same thing as the previous test.

 

I didn't pay real close attention to the difference, if there was one, other than to keep track of the temp in general and every time I looked it was 98 except at one point it was 100. A month or so ago I had checked that very thing though and they always stayed within 2 degrees of each other regardless of where in the range the temp was when I checked it.
As recently as yesterday after these symptoms had arisen I checked the TempII sensor with a multimeter and each side to ground was very close to each other both cold and hot and within spec for the temps.
I'll go connect the Sharktuner and get the exact details now....

OK, the EZK stays 8 to 11 degrees colder than the LH.
I made a log but can't get it uploaded to the Roadrunner site I usually use so I can't post it right now and can't attach it to this post either.

 

No, based on your warm-up map experiment, skip my test. You have the smoking gun already.

Oh, I see you did the test anyway and ruled that out...

I'm not sure what your base timing map is like, but you may want to investigate that as well as the warm-up map. The warm-up map should zero out at the top cells, shouldn't it? Anyway, we've seen some strange timing in the EZK maps with cells right next to each other having hugely different timing, leading to a lot of roughness in the idle. Smooth out the timing changes and the idle smooths out. We saw one "tuned" EZK map that had a 19 degree cell right next to 10 degrees.

 

Ok, now it appears I have this exact same problem that appeared. I searched the threads and didn't see what the solution was.

88 S4 Auto
Idle is normal or goes a bit high when started cold. Then when warm, in Park, it cycles 1000 to 1600.

When in gear, particularly with load from AC, it is relatively steady at 1000 (kind of high).

My next project is an intake refresh (just got the parts from Roger), but I haven't done anything yet to the intake. The engine and intake are stock.

I apologize if I missed this in my thread search, but can anyone point me in the right direction as to the fix?

Many thanks,

Mark

88 S4 Auto Silver Metallic

 

You've either got un-metered air coming in (leak after the Mass Air Flow sensor) or a failing Mass Air Flow sensor. It could be other issues. As you do the top-end refresh you can check every step of the way.

 

I used the Dwayne Home Depot Special on the MAF. I was unable to get any pressure to register. I also noticed a hiss letdown sound coming from the front of the engine when I stopped air flow to the MAF plumbing. The intake must be leaking like a sieve. The timing of this issue is good considering I'll be refreshing the intake.