L-Jet going full Rich on Deceleration Options

[Brian Fuerbach]

1974 1.8 with L- jet. Been playing with a wide band air fuel meter and noticed that when I lift the throttle the AFR goes to max value on the gauge. I thought it was supposed to go lean. I checked the throttle position switch and it checks out fine. Going to check the wiring harness next.

What else should I check? Decel valve?

[Brian Fuerbach]

Wonkipop,
I did read the thread on on the cap. I came to the conclusion that the cap must function as a crankcase valve of some sort years ago while changing the seal and o-rings. I had tested my cap to see if the membrane was operational and it checked out.

emerygt350,
The crankcase vents into the rubber intake tube in between the Air box/AFM and throttle body. The AAR valve and Decel valves also enter in that area.

[wonkipop]

Wonkipop,
I did read the thread on on the cap. I came to the conclusion that the cap must function as a crankcase valve of some sort years ago while changing the seal and o-rings. I had tested my cap to see if the membrane was operational and it checked out.

emerygt350,
The crankcase vents into the rubber intake tube in between the Air box/AFM and throttle body. The AAR valve and Decel valves also enter in that area.

emery, here is a photo of mine.
brian's will be the same.

i was having the thought because of the location where the decel hose connects.
its between the crankcase vent hose and the afm. it could sort of want to draw air from either direction.

but brian's report of the condition of his pcv valve/cap kind of puts paid to my theory.
so unless his cap is misbehaving since he tested it, its not a valid thought.



i did come across descriptions of momentary full rich as an afr reading.
happening immediately after closing the throttle.
if fuel has condensed on the inlet manifold and port areas.
closing the throttle and high vac moment causes condensed fuel to suddenly boil off the surfaces and be drawn into the combustion chamber and expelled. it doesn't last for long and is transitory.
depends a bit on the the design of the inlet system.

then there is always the question of the gauge and probe behaving correctly.
came across people describing some gauges actually measuring full lean, but having maxed out indicating this as full rich. ???? ie unreliable indications!

[emerygt350]

interesting. I assume that PCV system is like the 73 where it is vacuum sensitive. At high vacuum it actually closes up (well, it has a little metered breathing available), opening under low vacuum when you actually need it.


That initial rich is definitely a thing, but the going full lean on full rich is obvious on my autometer at least. The gauge actually kind of says "well, if we are going to be like that, screw you" and pegs itself to full lean and stays there for maybe a couple minutes. Mine is analog so there is normally movement in the arm at all times. When the computer gives you the finger, you can tell because it stops moving at full lean. It only does that when it gets too rich of a signal. Stupid. My digital AEM in the mustang doesn't do any sort of stupid thing like that.

[wonkipop]

interesting. I assume that PCV system is like the 73 where it is vacuum sensitive. At high vacuum it actually closes up (well, it has a little metered breathing available), opening under low vacuum when you actually need it.


That initial rich is definitely a thing, but the going full lean on full rich is obvious on my autometer at least. The gauge actually kind of says "well, if we are going to be like that, screw you" and pegs itself to full lean and stays there for maybe a couple minutes. Mine is analog so there is normally movement in the arm at all times. When the computer gives you the finger, you can tell because it stops moving at full lean. It only does that when it gets too rich of a signal. Stupid. My digital AEM in the mustang doesn't do any sort of stupid thing like that.

yes emery - am sure thats how it works on the 74 L jet too.
might also do a little balancing act between crankcase and inlet pressure differences at other times.

guess i was trying to think of the next thing to look at to be sure the info telling brian this was happening was 100% reliable.

is this a new thing brian? never done it before with the probe hooked up?

[Brian Fuerbach]

interesting. I assume that PCV system is like the 73 where it is vacuum sensitive. At high vacuum it actually closes up (well, it has a little metered breathing available), opening under low vacuum when you actually need it.


That initial rich is definitely a thing, but the going full lean on full rich is obvious on my autometer at least. The gauge actually kind of says "well, if we are going to be like that, screw you" and pegs itself to full lean and stays there for maybe a couple minutes. Mine is analog so there is normally movement in the arm at all times. When the computer gives you the finger, you can tell because it stops moving at full lean. It only does that when it gets too rich of a signal. Stupid. My digital AEM in the mustang doesn't do any sort of stupid thing like that.

yes emery - am sure thats how it works on the 74 L jet too.
might also do a little balancing act between crankcase and inlet pressure differences at other times.

guess i was trying to think of the next thing to look at to be sure the info telling brian this was happening was 100% reliable.

is this a new thing brian? never done it before with the probe hooked up?

The lower idle has always happened after a hill. Only noticed the correlation between afr and this after the probe was installed. Not sure where I read it but I was under the impression that fuel was to be cut off with the throttle closed. When this was proved wrong by the probe I wanted to find the issue. May not even be an issue. Maybe what I read about fuel cutoff was pertaining to D-jet?

[wonkipop]

interesting. I assume that PCV system is like the 73 where it is vacuum sensitive. At high vacuum it actually closes up (well, it has a little metered breathing available), opening under low vacuum when you actually need it.


That initial rich is definitely a thing, but the going full lean on full rich is obvious on my autometer at least. The gauge actually kind of says "well, if we are going to be like that, screw you" and pegs itself to full lean and stays there for maybe a couple minutes. Mine is analog so there is normally movement in the arm at all times. When the computer gives you the finger, you can tell because it stops moving at full lean. It only does that when it gets too rich of a signal. Stupid. My digital AEM in the mustang doesn't do any sort of stupid thing like that.

yes emery - am sure thats how it works on the 74 L jet too.
might also do a little balancing act between crankcase and inlet pressure differences at other times.

guess i was trying to think of the next thing to look at to be sure the info telling brian this was happening was 100% reliable.

is this a new thing brian? never done it before with the probe hooked up?

The lower idle has always happened after a hill. Only noticed the correlation between afr and this after the probe was installed. Not sure where I read it but I was under the impression that fuel was to be cut off with the throttle closed. When this was proved wrong by the probe I wanted to find the issue. May not even be an issue. Maybe what I read about fuel cutoff was pertaining to D-jet?

yes, its hard getting to the bottom of it all, once you start drilling down.(IMG:style_emoticons/default/laugh.gif)

what i read says it is only very early D jet cars that have fuel cut off.
i try not to read anything on D jet. why go there? L jet serves as "amusement" enough with occasional migraine.

can't find anything definitive in the tech manuals about how ours behave exactly.
how does the ECU do it exactly. more gloss over descriptions. input this, input that and presto.

stuff i have read says do not cut the fuel off at all.
but they are meant to go lean.
back off and coasting were known as almost the dirtiest phase in an engine and the epa was out to mop it up.

so in the case of ours.
far as i understand it.
the distributor is sending the signals to the ECU and its sending them to the injectors to fire. twice for each combustion cycle. other inputs from the AFM and the temp sensors let the ECU decide how long for when it fires. the lowest setting it can get to is rock bottom. which would be the throttle switch on idle and possibly the AFM flap bouncing back further than the idle position it gets opened to when you have tuned it at idle. which likely explains the 1 second death my car gets cold when i rev it and it "dies" then resurrects all on its own. the decel is supposed to stop that retreat of the flapper right back to rest and below the position its opened to at idle. and it is open a little bit at idle.

to get extra fuel through those injectors on decel and coast, something would have to be telling it to keep the injectors open longer. it won't be the throttle switch because its telling the ecu the throttle is closed. and you have tested your TPS. it won't be the decel valve because its making the flapper behave exactly to the air demand its allowing. and you have tested and tuned your decel so its not that.

it won't be a temp sensor in the head because that would screw you up all over the shop.

it won't be the incoming air temp. same deal as cht.

it won't be the pcv valve. you got a good one.

if the probe is 100% right and info is good.
then the injectors are staying open too long and letting too much fuel in?
they will open and close by default regardless.
the degree of enrichment is by how long they stay open.

there is one other possibility.
which is a leaking cold start injector.
a lot of times you can tune around it and not notice it.

but in a situation like this where you have say tuned it at idle etc its dealing with it and you have tuned it out. but when it gets down to minimal fuel demand suddenly that bit of extra fuel is popping up.

the test for the cold start injector leaking is to pull it out.
get a bucket. get ready for a mess. go through the start sequence and crank.
watch it go off and finish its spray and the leave it sitting there and see if it drips.

PS - the californian 75 1.8s did have fuel cut off.
but i think that was for over-rev - this was in place of the speed limited rotor in the distributor. and that was to do with having cats.
the fuel cut off was a big switch that was mounted off the lower battery tray support.
kind of looked about the size of the normal efi relays we have mounted there.
but is not there on all the other 1.8s of 74 and 75.