Hunting idle when cold

[Gilbertd]

I've just spent ages searching the forums for an answer to this problem and not yet found the answer. It seems most threads refer to a problem when hot and are down to the waxstat AAV or air locks in the cooling system. This isn't, it's when cold.

It's my daughters car and I'm trying to get it sorted properly. 1989 Mk1, normally aspirated with 180,000 miles on the clock. Start it from cold and the idle rises to 3,000 rpm or so but hunts as the ECU cuts the injectors so the revs fluctuate between 3,000 and about 800 rpm. Once warm it's OK. I've had the throttle body off and cleaned it all out. Checked and adjusted the TPS so it is operating as the book says. Put it all back together and the problem is the same.

BUT, if I disconnect the plug to the vacuum sensor, the revs drop and stabilise and can be set using the idle speed screw. The vacuum hoses are all OK with no splits and if I pull the one off that goes to the sensor and plug it, again the revs are stable. Having searched for the part number for this thing (89420-17020) I find that it is a stupid price.

The book is no use at all as it doesn't seem to even acknowledge that this thing exists. Can someone explain to me what it is actually doing? I suppose what I'm asking is can I leave it disconnected or, better still, remove the whole lot, including the 3 port bit (with only 2 ports used) and plug the hole in the manifold with a bolt?

[Gilbertd]

OK, been doing a bit more playing with this thing. If I unplug the electrical plug from the vacuum sensor, I get a nice smooth idle hot or cold. Slightly higher when cold due to the waxstat AAV as it should be. Seemed to be perfect, BUT, that brings on the check engine lamp. If I leave it connected and plug the two pipes going into the unit screwed into the plenum chamber, the idle speed is stable but the idle is lumpy. Didn't try taking the bit out and putting a bolt in the hole but I suspect it would be the same as plugging the two pipes. There's obviously something else involved that I haven't found yet.

So, does anyone have any idea what this thing is doing, what it is controlling (as there is obviously something else) and why I've got a 3,000 rpm idle when cold?

[TigerTebb]

I have the same problem with my 1988 mk1, it sits at 3000rpm to begin with then seems to fluctuate around the 2300-2600rpm, which gets lower as it heats up until its hot and sits on idle. When I find out what's wrong with mine I'll PM you and see if that helps you any

[jimi]

That vacuum sensor is your MAP (Manifold Absolute Pressure) sensor, it's a crucial part of the engine management system, the ECU uses the data from the MAP sensor to control the fuelling. If you disconnect it then all you will do is is screw up the fuel injection and the car won't run worth a "censored" without it.
I don't know which "book" you are referring to but there' is a section on the MAP sensor in Toyota's 4A-GE engine repair manual (RM063E) If your referring to the Haynes book of lies then you won't find anything about it ... because the Haynes manual is based on U.S. spec cars which use a AFM (Air Flow Meter) not a MAP sensor.
Hunting idle is normally due to either` air in the cooling system or a vacuum leak allowing excess air to get drawn into the intake after the throttle butterfly. A major cause of problems is the AAV (or waxstat) More info on that here
http://www.mr2oc.co.uk/know-your-2-know ... kbartid=54

http://www.mr2oc.co.uk/know-your-2-know ... kbartid=55

http://www.mr2oc.co.uk/know-your-2-know ... kbartid=56
Bear in mind that a MK1 will idle at 2000rpm and upwards when cold, it's meant to.

[Gilbertd]

Thanks for that, it's all starting to make sense now. The manual I have is a Toyota manual in PDF format but appears to be a US one as it talks about the AFM so that explains why there's no mention of the MAP sensor. To get a normal idle when hot, the idle screw is screwed fully home. There's no intake air leaks (I've had a smoke tester on it) so it sounds like the Waxstat is working but allowing too much air through all the time. Not closing off fully when hot and opening too much when cold. The idle is hunting as the ECU is detecting high revs but with the TPS showing throttle closed it is shutting off the injectors.

Looks like I need to do the manual choke mod but I'll probably take a signal off the temperature sensor so it is automatic. If it works, I'll write it up so anyone else who wants to do it can.

[jimi]

I haven't seen a BGB (Big Green Book aka Toyota MR2 Repair manual) that covers the MAP engine, but the BBB (Big Blue Book aka Toyota 4A-GE Engine repair manual) does. ... Just for you here's a scanned copy http://www.mr2oc.co.uk/ocdownloads/Zip% ... Manual.pdf