Topic: What changes the timing in open loop?

[Rio_Grande]

Still working on my 97 tj, today I connected a vacume gauge to the manifold and it kept showing retarded timing. It didn't clear up while I was looking at it. So I ended the day wondering what changes the timing on these things.

[sharpxmen]

From FSM:

The PCM adjusts ignition timing based upon inputs
it receives from sensors that react to: engine
rpm, manifold absolute pressure, coolant temperature,
throttle position, transmission gear selection
(automatic transmission), vehicle speed and the
brake switch.

it is also mentioned somewhere else that the timing is adjusted based on IAT as well.

the basic operation is that you have an ignition timing table which is 2 dimensional, rpm vs map and the timing values are stored there from factory. then you have modifiers which are coolant temp, IAT, tps and according to the fsm gear selection for autos as well as brake switch (I didn't know these last 2 were part of it to be honest and i don't see the point but maybe something related to emissions, dunno).

[aw12345]

The latter two are related to idle speed, same as powersteering pressure switch.
low vacuum isn't necessarily related retarded base timing.

[sharpxmen]

The latter two are related to idle speed, same as powersteering pressure switch.

hmm, still don't know why you'd need to adjust ignition timing for that, should just be the IAC that gives a bit more air to keep the idle up and not die on you (same for P/S)

if it's retarded compared to the rpm/map table then is based on one of the input sesors (modifier varialbles) which should be coolant, iat or tps.

low vacuum isn't necessarily related retarded base timing.

not sure what you mean by that

[Rio_Grande]

The coolant and iat seem to be reading properly once the jeep is running. Although I started with a negative 40 reading on the coolant temp switch, but it straightened up right away.

Map seems to be reading about right so that puts me around to the TPS so I may give that a shot.

[mrdeath2000]

what exactly is your vacuum gauge reading/doing?    Steady, shakey, big swings, low (how low), high (how high)?

[Rio_Grande]

It is steady and snaps right back on excel. On my vac gauge it has diagnosis positions and my tj is idling right outside of normal in a diagnosis block that says timing retarded.  It may not be accurate with the newer tech.

[aw12345]

Does it give a number say 19" of vacuum at idle?
Lot of efi engines do not get up to 21" of idle vacuum which used to be the norm for a good running carb engine

[mrdeath2000]

yea, that's what I'm curious of, the actual #.

 Mine runs about 18" I think, maybe it was 19".  It wasnt steady like a tire gauge, it was a hair shakey like 17.9-18.1 kinda shakey, more of a vibration I guess.  No CEL, runs fine.

[sharpxmen]

the vac is actually based on the cam profile and altitude above sea level, don't think it matters if its carb or efi (but the cam would probably be different so that would explain it i guess).

[aw12345]

Have seen very few efi or tbi engines get over about 19" of vacuum.
But then I generally just look at map sensor voltage and a lot of them hover around 1.2 to 1.3 volts. It somewhat depends on what the idle air motor is doing to keep the thing running

[Rio_Grande]

Just below 30, my map sensor dosent seem to agree with my vac gauge. I am gonna swap it for another one I have see if it is better. I unplugged the map and it ran better. Not every problem was solved but better.

[sharpxmen]

can't run

without the map sensor 

[Rio_Grande]

Mine can and actually ran better than with it.  If I can get the time I will swap it out.

[sharpxmen]

you mean at idle or driving it around?

this is what i posted before but i didn't have spaces after/before the square brackets and it just aligned the text to the right

can't run [ right ] without the map sensor  

it would run but in limp mode