Idle & Low rpm miss fire - 4AGE Bigport

[AE3SGTE]

AE82 Bigport motor only has 20,000 on it since rebuild.
TVIS is hooked up, RWD manifold from JDM biport, Installed and wired in by me ~2 months ago,
has developed a idle and low rpm miss fire in the last 2 weeks. Any thing over 4000~4500rpm is
clean and free revving, anything under 4000 will not hold even rev and misses. I changed plugs,
leads, dizzy cap and rotor but still does it. I didn't wire up the Diagnosis plug, so don't know if its
throwing a code
When driving if it has load on it then its OK, but cruise it will miss. It feels a
little bit rich, 1st to 2nd, 2nd to 3rd will pop a little back fire between gears driving sedately.
Timing is set to 0 on the marks, runs on 98 octane only, no pinging and has heaps of power, but yeah...

I noticed if I pinch the MAP sensor hose the revs clean up and the miss at idle
goes away, also it will hold clean even rev @ ~2500-3000rpm (can't see tacho).
I have the MAP and the stock FPR T pieced together off 1 brass fitting in the
back of the manifold. I was told that full vacuum signal to the FPR will make it
run choppy at low rpm? Could this be my issue?

Does anyone in Brisbane have a Bigport MAP sensor I could borrow and see if
this is my problem?

[Chrisso]

I've got a spare. PM me

[The Witzl]

vacuum leak perhaps.

[AE3SGTE]

I'm pretty sure its not a vaccum leak.
I've re installed the 3way vacuum block at the rear of the manifold instead of how I had it,
no change. I've changed fuel filter and coil also, no change. I'm getting a compression
test done Monday arvo.

I put a restrictor in the vacuum line for the MAP and disconected the TVIS, and it made it better,
it will cruise with out missing now, but try holding any rev over 2,000-4000 will miss fire.
Its really beginning to shit me off.

Chrisso, I'll ring you today!

[The Witzl]

try putting a carby-style fuel filter on the vacuum line to the MAP sensor - sounds wierd, but give it a go. It smooths out pulses in the MAP signal.

Definitely sounds like a problem with the vacuum/MAP/load signals to me.

[Chrisso]

It's odd that there would be significant pulsing in the vacuum line given it's got a standard plenum. Would longer duration cams create a problem like this?

Swapped in a spare map sensor, with no change. Tried a different ECU as well of dubious origin, no improvement.

The air intake temp sensor is not attached, but I would guess that this should only really affect warm up enrichment, and possibly the VE calcs for the fuel map, but I don't know enough about the inner workings of the stock ECU.

[The Witzl]

lack of intake air temp sensor would have no effect on this - it only does minimal adjustment to fuel mixtures.

I agree that there shouldnt be excessive pulses with a standard plenum... but its a cheap option to try ($1.20 at most servos).

Something has to be wrong here. Vacuum leak from the Tvis?

[AE3SGTE]

Fuel filter Aye, it I'll give it a shot.

Chrisso, The MAP was disconnected with your ECU yesterday when you where there,
I drove up the street before I realized something not right here!!!

I noticed that the MAT power wire has a crack in it exposing the wire right where it goes
into the end of the plug. I'll fix that and make and a separate earth wire up for it also,
but I don't think thats what is causing the issue.

I've had the injectors cleaned, nope-ah !! still doing it.
I tried a new bigger battery (from brothers car), no change....

Someone has suggested a hairline crack in the intake. I don't have mine braced, so I
might pull off the manifold and check it out.

Someone else has suggested coolant temp sender for ECU....
I try download the Diagnosis manual for 4AGE but it won't work on this POS computer...

Compression test it this avro, and valve leak test also.

[AE3SGTE]

Tried a carbi fuel filter, nah, still has miss.

Comp and valve test put back til Thursday now dognabit!

[AE3SGTE]

I re-did all the gastkets on the tvis and the intake. I checked the intake manifold for cracks,
none. Also the brace from the FWD manifold won't fit for the RWD one?
I change it from the AE82 remote thermostat to rwd water pump housing with new thermostat.
I made another earth strap to the front of the engine... started it up still doing it...

Comp test 175 all round, so not that.....
I reckon it must be the temp sensor. I'm going to change the MAT as well.

[Chrisso]

Just throwing up an idea and greater minds may shoot this down straight away.

Does timing advance change significantly due to engine load (MAP)? Is it possible that the timing, either dissy or cam(s), are slightly out and a change in load shifts the ignition advance to the "correct" timing?