I think my map sensor is playing up?

[Evan]

I have a 100kw 4age in my ae86.. and the idle is screwy, as it sits there and revs to 2000 then back down to 1000 and again..
Also i can't hold any lower rpm than 3000 standing still with only the least amount of throttle possible.. car still drives fine.. but its anoying..
Could it be a leak behind the butterfly or a map sensor prob?
cheers

[Quinn]

A friend had a similar problem, he replaced the map sensor how ever the issue was still present. In the end the problem was an extremely worn butterfly. You could always try swapping out the MAP sensor with a known working from someone else's car, and then work your way from there?

[Evan]

Cool thanks

[The Witzl]

I would suggest the following are worth looking at before blaming an otherwise faultless device....

1. Vacuum leaks - check / replace all intake manifold gaskets from throttle body to head, put thread sealant on items threaded into plenum

2. PCV hose - where is this hooked up to? Have a read of my "FWD 4age into RWD car" thread in the FAQ section of tech & conversions.... there is some information there you should take head of.

3. Aux. Air Valve - are the water lines plumbed up to it? If not, have you blocked the air passages off?

4. Power steering Idle up ports - have you blocked these off?

5. Idle screw - is it correctly adjusted?

6. TPS - is it correctly set?


Enjoy.....

[psychofox]

I had this problem happen on my old AE93 SX Seca.

The stock 4AGE computer does wierd things when it thinks there is an air leak in the intake system when it is idling. It erratically idles between 1000-2500 rpm as if you were blipping the throttle. very annoying.

Mine started happening when a hose fell off the throttle body. I reconnected it - all sweet.

It also happened when I put the engine into my AE86 - I had removed all the plumbing off the bottom of the T/B and blocked everything up. It ended up that the idle bleed screw (the vertical screw on the T/B) was all the way out???? I screwed it in a bit and the idle problems stopped.

If it was a MAP sensor problem the car wouldn't drive well at all, as that is the main load input for the computer. For an example of how bad it would be if the MAP was broken, try disconnecting vacuum hose from the MAP sensor and see how well the car goes.....

[4agte]

could also be that there is an air pocket in the cooling system somewhere that can make the engine hunt but maybee not as much as you are suggesting i would also be looking for a vacume leak

[roadsailing]

search the old forums, i had exactly this problem and fixed it by plateing the air bypass valve, in any case there is heaps of good info on the old forum.

[takai]

Sounds like a vac leak, certainly the rising and falling of revs does. Try unplugging a vac hose to see if it makes any difference.