Why does this 4AC run on 3 cylinders only when its cold..?

[Aust162]

Hey everyone, my friend has an ae71 with a 4ac. the car was sitting idle for almost 1.5years. its now back on the rd and has since done 800kms of driving.

When you start it in the morning or after it has been sitting for more than 2hrs it idles on 3cyl for the first 2-5mins. its cyl #1. (closest to timing belt) thats not working. BUT it drives fine once it has load on it over 1500rpm even while the motor is cold. so its just at idle this problem.

when you pull the lead off this cyl it makes no difference to how it runs. you can see the spark jumping as well.

i have fitted new spark plugs, rotor button (cap looked new already), f/filter and cleaned the carby + oil change.

Any ideas?

[TERRA Operative]

Replace the leads.

[Talasas]

It will most likely be ignition related, you generally shouldn't drop a single cylinder on a carby engine like that. If replacing the leads doesn't do it look for anything else that can cause weak spark, check the plugs for fouling. Also check your circuit opening relay/ignition relay by replacing it with another one, I've had that happed before.

[20v_KE25]

The reason for running rough is the Auxilary Accelarator Pump diaphram has a hole in it and it is sucking raw fuel into no 1 cylinder. There is a vacum port on no 1 inlet port tract which goes to TVSV which is water temp controlled. When the engine is cold and in warm up stage it holds the diaphram against small spring presure and when you accelarate suddenly vacum reduces and it squirts extra fuel to prevent flat spots. If you get a pair of pointy nose pliers and clamp the small vacum hose while the motor is running, within a few seconds the motor will run smooth then if you release it it will run rough. You must do this test while the engine is cold. The pump diaphram is under a small cover which is held by three screws and has a single vacum hose going to it.

[Aust162]

TERRA - swapped the lead with a known working spare i had and it didn't fix it. Wished it was that! would be too easy of a fix

Talasas - I pulled the plug out of Cyl 1# after 1min of cold running and it was covered in fuel. all other plugs are nice and clean and don't have a strong fuel smell.
A 4AC doesn't have a COR like a 4age engine though, would it? its a points ignition system, different to the fwd ae82 4ac.

20V_KE25 - very interesting. i think your on the money. i always hear of these Pump diaphrams failing on the old 4ac but never understood exactly how it works

20v ke25- how do you go about fixing the Pump diaphram?i'd image almost all of them in the wreckers would be no good by now. can you buy from toyota or do you know if there is a way it be blocked it off instead and maybe adjusting the idle screw manually? it will probably run ok minus the choke? system since its not too cold in melb suburbs..

thanks for the help. much appreciated.

[pandaah]

this doesnt sound ignition related to me at all.. 20vke25 sounds like he knows whats going on..

[SilverRA23]

I recall reading a post by River many many moons ago where he replaced said diaphragm on an AE71.. I am sure he bought the part for cheap from Toyota.

[SilverRA23]

Found it... http://www.toymods.org.au/forums/tec...-c-engine.html

I will take back the cheap part of my reply! I have a recollection of seeing them for sale on ebay but can't find them now.

[Aust162]

^ thanks very much. i was trying to search the forum for that thread but this site is running so freakin slow atm! i also found in the old forum archives a user called "170bhp" replied to basically the same question and he said you can block off the hose coming from the diaphragm to the manifold with a ballbearing or something similar and this should work in some way too

I found they have them for sale on US websites for 35ish.

[20v_KE25]

Easy job, just three screws hold the little cover, there is a small spring under the cover and the diaphram. The diaphram is also the gasket when you remove the diaphram you might get a little bit of fuel to leak out but that is about it.

[Aust162]

Today i had a play with this 4ac. i blocked the hose coming from the diaphragm pump going to cyl #1 on the intake runner. car is now idling sweet, hot or cold and it seems to drive just as usual. (exhilarating 4ac performance )

If this doesn't end up being a permanent fix i'll buy a diaphragm from the states. $35ish. (i called MR.T in Heidelberg and he wanted $140 for the diaphragm and approx 4week wait from japan..)

[20v_KE25]

Just reading that you called MR. T in Heidelberg for a price on diaphram, well I worked there for 31 years.