Idle Speed and Air Temp/Humidity Options

[StarBear]

As getting ready for the 2022 Northeast Gathering, I'm encountering a semi-recurring idle issue on my 1974 1.8L LJet FI. When outside air is cool and dry, idles just fine at 950-1000 rpm. When outside air is warm and humid, after warmup the idle falls off to barely running. I've just been adjusting the air flow screw (maybe 1.5-2 full turns) to open it. Of course, the reverse happens - if I preadjust the screw it idles near 1800 then settles down to 1100 after the engine gets totally warm (15 mins or so).
I presume (?) there SHOULD be some regulation, perhaps with the ECU or AFM, to monitor air temp and humidity and make adjustments automatically? I didn't have to manually do this adjustment in the past.
The AAR works and tests out fine. Just annoying to have to pull over after running a while and adjust the screw. (IMG:style_emoticons/default/dry.gif)
Your thoughts @Van @Van_B and @Wonkipop ?

[StarBear]

CHT Sensor? Rpm seems to stay steady if air stays cool and dry, though (not often here in NJ this time of year).

[wonkipop]

only thing i can think of is the other temp sensor.
temp sensor 2 i think it is called in factory manuals.
this is inbuilt into the AFM unit.
i don't think you can do anything about that one if it starts to play up.

that sensor measures incoming air temp.

its possible with high humidity perhaps the electrical connections are affected by moisture? i seem to remember that living in chicago it got monstrously humid unlike anything i experience in australia. perhaps try cleaning the electrical plug connection to the AFM with electrical cleaning fluid. also the plug to the ECU. nothing to lose. check all the engine grounds for being clean and corrosion free. thinking you had it standing stored for your horrible winter and you have only set the car free recently?

@Van B might have better ideas.

[StarBear]

Will try cleaning those plugs. Grounds are A+ and it doesn’t sit for more than a few weeks at a time during the winter. Will explore that temp2 sensor. Thanks, @wonkipop !

[Van B]

Steve, sorry for the delay, I’ve been traveling for work and forced to do things like socializing after hours lol…
Anyway, next time it’s all hot and steamy, you should grab hold of the throttle body and try to force the butterfly closed. If the idle drops back to where it’s supposed to be then I think you may have a worn TB that’s not seating and reveals itself when the metal heats and expands just enough to not seat properly.

[Van B]

On these cars idle is far more likely to climb from metered air than any fueling change. When you mess with the AFM flapper while the car is running you can better understand how small the fuel window is for idle. A bit too little fuel, idle drops, a bit too much, idle drops.

[wonkipop]

Steve, sorry for the delay, I’ve been traveling for work and forced to do things like socializing after hours lol…
Anyway, next time it’s all hot and steamy, you should grab hold of the throttle body and try to force the butterfly closed. If the idle drops back to where it’s supposed to be then I think you may have a worn TB that’s not seating and reveals itself when the metal heats and expands just enough to not seat properly.

oh thats a very good suggestion.

[StarBear]

Steve, sorry for the delay, I’ve been traveling for work and forced to do things like socializing after hours lol…
Anyway, next time it’s all hot and steamy, you should grab hold of the throttle body and try to force the butterfly closed. If the idle drops back to where it’s supposed to be then I think you may have a worn TB that’s not seating and reveals itself when the metal heats and expands just enough to not seat properly.

I’m actually having the reverse problem - when hot and humid the idle drops. Sort of as if the less dense air and added moisture yields less O2. I checked the LJet guide and 74s don’t have the air temp sensor in the AFM.

[Van B]

Check it all the same.

But, it seems to me that you need to set idle for hot and humid.

With my new AAR, the heated wire alone is not enough to completely close the AAR in cold conditions. The cold engine bay sucks the heat right out of the aluminum housing. But after I drive a while, everything is warm enough that the AAR closes up completely.

I’m thinking you had your idle set for a season and engine bay temp when the AAR was open just a fraction.

[wonkipop]

Steve, sorry for the delay, I’ve been traveling for work and forced to do things like socializing after hours lol…
Anyway, next time it’s all hot and steamy, you should grab hold of the throttle body and try to force the butterfly closed. If the idle drops back to where it’s supposed to be then I think you may have a worn TB that’s not seating and reveals itself when the metal heats and expands just enough to not seat properly.

I’m actually having the reverse problem - when hot and humid the idle drops. Sort of as if the less dense air and added moisture yields less O2. I checked the LJet guide and 74s don’t have the air temp sensor in the AFM.

thats interesting about the sensor I in the AFC.
what L jet guide is that steve?

i have the service guide to the 74 model i downloaded from mr b's website - guide says its there. and it tells you how to check it - adding that it cannot be replaced.

factory workshop manual also says its there in the EFI section.

i mistakenly called it sensor 2 in posts above. sensor 2 is the cht.

hmmmmm?


not a bad theory of Van's that the AAV is playing up.
bloody things. a bit like a half century old pop up toaster when you think about it? (IMG:style_emoticons/default/smile.gif)

[StarBear]

@Van B and @wonkipop Will check them out. I have the LJet 912E guide from mr B, which says it’s there but I have a note that it for later models. Will check again! Wonder What one does if it can’t be replaced - a work around or just reset the idle screw and live with it? (IMG:style_emoticons/default/mad.gif)

[Van B]

The Porsche manual does a terrible job of breaking out year to year changes. I know the 75 has a 7-pin and the 74 a 6-pin. And the manual only says to test resistance between two sets of pins for a pass fail.
Either way, I still think there's no issue with your car other than seasonal allergies!

[StarBear]

The Porsche manual does a terrible job of breaking out year to year changes. I know the 75 has a 7-pin and the 74 a 6-pin. And the manual only says to test resistance between two sets of pins for a pass fail.
Either way, I still think there's no issue with your car other than seasonal allergies!

Hehehehe. To my best (?) reading, the D-Jet and 912E LJet have an air intake temp sensor but the 1.8 doesn't as it only has 6 pins - until maybe later with the 7-pin.
@wonkipop I checked Mr. B's website and can't find your reference; there are a few LJet manuals but none seem specific to the 1.8. My guess is that the 1.8 has Temp Sensor II (CHT sensor) but not the intake one?
Meanwhile, will try to figure out testing the butterfly as Van suggested.
Have cleaned the AFM and ECU contacts with electrical cleaner spray followed by air blast.

[StarBear]

Ok, looking at Haynes page 177 on fig 9.87 current flow 1974 USA part 3: Shows G19 as air flow meter but no temp sensor. Temp 2 CHT shows as G18.
Going to 1975 on page 185 it shows G19 as afm, G17 as temp sensor 1 and G18 as temp sensor II.
No temp sensor I in our 1.8s that I can determine. Will head to butterfly test.
Also found the oil fill cap a bit loose, so that does funny things, too. Tight now.

[wonkipop]

Ok, looking at Haynes page 177 on fig 9.87 current flow 1974 USA part 3: Shows G19 as air flow meter but no temp sensor. Temp 2 CHT shows as G18.
Going to 1975 on page 185 it shows G19 as afm, G17 as temp sensor 1 and G18 as temp sensor II.
No temp sensor I in our 1.8s that I can determine. Will head to butterfly test.
Also found the oil fill cap a bit loose, so that does funny things, too. Tight now.

you could well be right as the plugs are different and i have always wondered about that.
and thats a good explanation for that plug difference.

the stuff from mr. b' site is here.

https://bowlsby.net/914/Classic/zMan_1974_1...iningManual.pdf

its in factory tech manuals section.
it is a service training manual for the introduction of the 1974 models and covers the 1.8.
however it does not mean it is correct.

i will now look into this as you are on to something.
possibly a good thing its not there. one less thing to screw up.
entirely possible they held off until 75 on the temp sensor even if they intended it to be there in 74 just to keep it simple. would just mean its slightly less refined in terms of mixture fine tuning and wasn't needed for emissions until 75 when the standards tightened?

good stuff pointing this out. the mysteries of the first L jets hey.

[wonkipop]

oh yeah steve. a loose oil cap will make your idle stumble and die, even a tiny bit loose.

[StarBear]

next time it’s all hot and steamy, you should grab hold of the throttle body and try to force the butterfly closed.

Van; could you explain just a bit how to do this? I get the concept but not the action. Thanks!!!

[Van B]

next time it’s all hot and steamy, you should grab hold of the throttle body and try to force the butterfly closed.

Van; could you explain just a bit how to do this? I get the concept but not the action. Thanks!!!

When the car is idling just grad the arm where the cable attaches and try to twist it further closed. Sometimes you won’t really feel movement but your idle will drop. That’s how I learned my TB was worn out.

[StarBear]

next time it’s all hot and steamy, you should grab hold of the throttle body and try to force the butterfly closed.

Van; could you explain just a bit how to do this? I get the concept but not the action. Thanks!!!

When the car is idling just grad the arm where the cable attaches and try to twist it further closed. Sometimes you won’t really feel movement but your idle will drop. That’s how I learned my TB was worn out.

Got it. Thanks. Will test with oil filler cap now tighter first, then if still happens will do this test. (IMG:style_emoticons/default/biggrin.gif)

[wonkipop]

@StarBear

i looked through the electrical schematics in the factory manual.
additional current flow diagram AFC engine.
you are absolutely right.
only list temp sensor II for 74.
temp sensor I and II are listed for 75.

thats a relief.
74 1.8 = a classic mate. the old keep it simple stupid principle and one less thing to go the way of a toaster.