‘74 1.8L L-Jetronic Cold Start Low Idle, Mystery solved! Options

[Van B]

Ok fellas, here’s the run down Aux Air Regulator (AAR) and Cold Start Valve (CSV)/injector both work fine. Many of you have also been keeping up with my high idle issue that we figured out.

But yet, when the car is cold, i.e. room temp, I don’t get the high idle I should on start-up. Instead she cranks a bit and then lumbers to life. Idle lopes around 700-800rpm and then smooths out as it warms.

In all other aspects of operation, the engine seems to be pretty happy.

Thoughts? Experience? (IMG:style_emoticons/default/confused24.gif)

-Van

See post #419 on page 14 for a synopsis of the outcome.

[wonkipop]

here is what mike (mechanic) had to say after i showed him the cold start description.

its either starving for air to burn the fuel its getting. AAV.

or its lean and does not have enough fuel. CHT.

as a starting point.

don't you love it. (IMG:style_emoticons/default/poke.gif)

but apparently it can be either.
there is a lot of fuel being splashed in there and a lot of it is condensing on cold surfaces so the air bit is important. car needs basically 10 times the amount of fuel at cold start up than when its warmed and at operating temp. i didn't realise it was that quantum!

he reminded me, it had been laid up for 15 years.
so you run it for a few months/year and see what faults develop if any.

in my car it could be that the CHT has started out ok and has progressively begun to "decay". have to retest.

or its the AAV. not opening correctly. either not opening enough at the start and not closing smoothly and timely. have to retest. he did say that 2-3 minutes was actually about right usually unless its really cold. then the thermo strip/spring is going to take longer to heat up from the colder engine block and ambient air and go a bit slower.

the thermo and cooling flaps were discussed. they could be playing into the hiccups speculated for either of other two - ie exaggerating them when its a bit colder here.
but they weren't really impacting a year ago he agreed. (i can test that with the car doing a cold start on the hoist by simply pulling on those flaps manually and easing them off, the cable is still there - which i might do when i get around to doing the other tests).

re emorygt350 idea of taking off a hose. yes. do that but....its a closed system. start the car with all hoses on. crack off a decel or aav line between aav and plenum carefully. you want to mimic an AAV valve if thats what you are after simulating. its a small ish aperture thats opening in the valve. so just crack it off like at one edge and control it.
one thing to remember is the AAV is in circuit with the AFM, so the flap might be being slightly opened with the AAV full open. i don't know that for sure, might have to dig around in the manual to see what it says.

as to what a cold start up is for a 1.8 - his view is - probably not that dramatic.
system would be trying to hold something like a normal idle from start up and not a lot more. maybe around the 1000 +/- rpm mark subsiding to 900 +/-.
and it would take anywhere from 2 minutes to 5 minutes depending on ambient.
but in australia he thought 2-3 minutes.

i'll wait until christmas to go further.

[emerygt350]

Yeah, definitely if you are going to pull off the plenum. If you just pull off at the air cleaner (pre AAV) then nothing changes to the system. You then put your thumb over the line and see what happens. Do you feel it sucking? Then the AAV is open. You can let it warm as normal and keep checking on whether it is still sucking. When the idle dips that is when you would suspect the AAV is closing and you should feel it stop sucking. If it is still sucking when the idle dips then you know it isn't associated with the AAV (or at least not directly).

Or you could be like me and just install a valve right before the air cleaner.

[wonkipop]

Yeah, definitely if you are going to pull off the plenum. If you just pull off at the air cleaner (pre AAV) then nothing changes to the system. You then put your thumb over the line and see what happens. Do you feel it sucking? Then the AAV is open. You can let it warm as normal and keep checking on whether it is still sucking. When the idle dips that is when you would suspect the AAV is closing and you should feel it stop sucking. If it is still sucking when the idle dips then you know it isn't associated with the AAV (or at least not directly).

Or you could be like me and just install a valve right before the air cleaner.

what i am getting at emery is that in an L jet you have the air flow meter flap up in the aircleaner where air enters. not so sure what D jet does, don't know anything much about D jet at all - which is what you have.

the flap is reading air flow which is also feeding info into the ECU for fuel.
if the aav in cold start is pulling in enough air that it is moving the flap thats going to be part of info controlling fuel flow maybe (as well as engine temp from CHT etc).

i was being cautious about the effect of pulling off a hose and the effect that has on more than just the AAV in an L jet engine. maybe not as simple as simply leaving off a hose.
see Van B's experience when he tried a start with a hose off.

that tap of yours is great as a test device i think.
it might be just the thing.
plumb that in where the AAV is and do a manual simulation of an AAV and timed.
that would keep the AFM in the equation.

[emerygt350]

Yes this is way different ljet to djet but the aar pulls clean air pre flap. Pulling the hose off the air cleaner will not mess up the flap in any way, that is why I am suggesting it rather than a controlled leak off the plenum. This way you can monitor the activity without disrupting our very sensitive ljet.