Jump to content

AE112R / 7A-FE intermittent idle surging, fault code 31


LP76

Recommended Posts

Members dont see this ad
  • Replies 60
  • Created
  • Last Reply

Top Posters In This Topic

I actually have a spare 7AFE head (and thus PCV valve) sitting around, maybe I should pull it apart to show how the internals work...

 

I had a quick look through the 4AFE/7AFE Toyota workshop manual this morning (the actual proper Toyota Australia engine manual), unfortunately I think the PCV valve gets classed as part of the Emissions system (which is chassis-specific) so wasn't covered.

Link to comment
Share on other sites

20 hours ago, LP76 said:

Also, for the manifold gasket, I assume the one between the intake manifold and the engine, not the one between the intake manifold and the intake chamber? 

actually yet the chamber gasket is probably more likely to fail than the manifold to head one.
Being big and chunky, and getting lots of oil on it from the PCV system can make it hard and brittle.
 

Link to comment
Share on other sites

Pulled the PCV valve again and checked it - seems fine. Can blow through it one way only. 

Sprayed carb cleaner all over the chamber to manifold and manifold to head interfaces - no noticeable change in revs at all.

Of course the car was idling just fine when I was checking these things again though.

I'm out of ideas.

Link to comment
Share on other sites

On 29/11/2017 at 11:11 AM, ke70dave said:

Maybe bleed that coolant system eh. 

This ^^^

This is a well known problem for late 80's and 90's Toyotas, only did this yesteray on a non turbo jz80 supra. Air gets trapped in the T/B area and tricks the ecu to believe it's running cold.

 

Link to comment
Share on other sites

On 16 December 2017 at 12:46 PM, kickn5k said:

This ^^^

This is a well known problem for late 80's and 90's Toyotas, only did this yesteray on a non turbo jz80 supra. Air gets trapped in the T/B area and tricks the ecu to believe it's running cold.

 

I did run the car with the rad cap off a few days ago, with no noticeable bubbles coming out over a period of 10-15 minutes of running, from cold to warm. I can't see how you could possibly jack the front of the car up enough to make the radiator cap higher than the throttle body though?

Link to comment
Share on other sites

25 minutes ago, LP76 said:

I can't see how you could possibly jack the front of the car up enough to make the radiator cap higher than the throttle body though?

Every little bit of help you can give the system counts.  If it isn't enough, then you can cut the bottom off a soft-drink bottle, duct-tape the neck to the radiator filler neck, and then fill it with water/coolant and run the engine.  It just makes a mess when you finally get the system bled and have to remove the bottle with some water/coolant still in it.

 

Coolant doesn't go all through the throttle body anyway, just the base of it for the ISCV.  It's not much higher than the cap

Link to comment
Share on other sites

4 hours ago, rebuilder86 said:

ive never been a believer in the idea that an air gap causes it to missread SO FAR that it thinks the engine is cold.
Hot gas is fucking HOT
But the suggestion gets thrown around so often i may be wrong.

Trust me, anyone who has ever owned a 4AGE will have experienced the air-bubble in the T/B at some stage causing a hunting idle speed.

Gas is a poorer conductor of heat (convection) than liquid (conduction) too, remember.

Link to comment
Share on other sites

1 hour ago, rebuilder86 said:

yeh i know but i struggle to see how it can stop enough heat for it to be outside of operating range.

is it located realy high up in this design, to be used as a bleed port or something?

It's just a thermo-switch (or in the case of the older 4AGEs, a wax pellet).  When there is an air-pocket around the ISCV, there is insufficient heat transfer through the air for the correct coolant temperature to be measured, simple as that.  Just google 4AGE idle air bubble and scroll through page after page of people who fix hunting idle problems by bleeding the cooling system.

Front-engine A-series engines are meant to be bled by the radiator cap.  There is no other designated bleed point in the system (like a header tank or bleed port - MR2s are obviously different due to radiator being at the other end of the car and mounted low).  It's not as bad as a VL Commodore (which had the radiator cap so low that air-pockets would form in the HEAD), but it is a recognised thing.

Link to comment
Share on other sites

i think what you see is lots and lots of people throwing the idea around, following on from the MR2 issues, where the radiator is a long way away, as you mentioned.
I think what the truth really is, is that the senders in these peoples situations are broken and replacing the sender fixes the issue. Not that a bubble of air was giving a false reading.

In fact, i do a search for "4age temperature sensor surging air bubble"
And all the resolved forums point at, manifold gaskets, IAC valves, etc, and all of them are full of people suggesting this air bubble idea but they are never the "solved" answer.
When i was 16 and beginning my car obsession, i too had this air bubble theory planted in my head from someone online. But lots of experience has proven it to be a myth.

And just explain if you can, If the ECU can't get in to hot and lean closed loop mode, due to a lack of a hot coolant signal, why would it surge. Thats the bit no ones been able to explain.
In open loop (cold and rich mode which everyone sais causes the issue), the mixture is rich, and this actually helps hunting issues wrt to vacuum leaks.

Can you show me any thread, where the poster sais the hunting issue is resolved by bleeding the system?
If so, then ill be happy to learn something. I had a nissan pulsar wen i was younger with some similar issue, and changing the sender did fix something. i can't quite remember what.

Link to comment
Share on other sites

14 hours ago, rebuilder86 said:

i think what you see is lots and lots of people throwing the idea around, following on from the MR2 issues, where the radiator is a long way away, as you mentioned.
I think what the truth really is, is that the senders in these peoples situations are broken and replacing the sender fixes the issue. Not that a bubble of air was giving a false reading.

In fact, i do a search for "4age temperature sensor surging air bubble"
And all the resolved forums point at, manifold gaskets, IAC valves, etc, and all of them are full of people suggesting this air bubble idea but they are never the "solved" answer.
When i was 16 and beginning my car obsession, i too had this air bubble theory planted in my head from someone online. But lots of experience has proven it to be a myth.

And just explain if you can, If the ECU can't get in to hot and lean closed loop mode, due to a lack of a hot coolant signal, why would it surge. Thats the bit no ones been able to explain.
In open loop (cold and rich mode which everyone sais causes the issue), the mixture is rich, and this actually helps hunting issues wrt to vacuum leaks.

Can you show me any thread, where the poster sais the hunting issue is resolved by bleeding the system?
If so, then ill be happy to learn something. I had a nissan pulsar wen i was younger with some similar issue, and changing the sender did fix something. i can't quite remember what.

For a 4AGE, there is no temp sender for the cold idle - it is literally a wax-pellet valve like the thermostat which opens and closes a bypass passage in the throttle body to up the idle.  I personally have fixed a hunting warm-idle problem in a 4AGE (a bigport in my wife's old AE82 Twincam) TWICE purely by bleeding the cooling system AND NOTHING ELSE.  Over the years of being on forums and Facebook I have seen countless other times when a hunting or high idle problem for those engines has been fixed by bleeding the cooling system (and it is really noticeable on 4AGEs due to the big difference between cold and warm idle speeds).  Remember too that an air pocket might be stationary when the engine is off, but when it is running and coolant is being pumped around things get stirred up, which can result in intermittent contact between the coolant and the wax-pellet/sender/whatever.  Usually it is a persistent high-idle that doesn't drop when the engine gets warm (also symptomatic of the wax-pellet valve dying of old age and being stuck permanently open, like a dead thermostat)  but it can cause surging if it is borderline.  It is actually a slightly different cause compared to a vacuum leak one (which is extra air causing the idle speed to rise about the ECU's preset limit causing it to pull fuel out) but can be hard to differentiate if you don't know exactly what to look for.

Yes, other issues such as a vacuum leak or faulty sender/valve can also cause similar symptoms (and I'm not discounting any of them in the slightest), but the air-bubble trapped in the throttle body causing inconsistent/high idles is something that is common across most of the transverse A-series Toyota engines (moreso in MR2s but Twinkies/SXs do suffer it too), and it is almost always suggested as the first answer because it is free, easy, doesn't require any disassembly, and is very common especially when people also have coolant leaks or have recently replaced a radiator/thermostat/hose/etc and introduced air in to the system (in my wife's case it was both - air was in the system after fixing a leaking water outlet on the end of the head and replacing a heater hose that had burst).

Link to comment
Share on other sites

15 hours ago, rebuilder86 said:

i think what you see is lots and lots of people throwing the idea around, following on from the MR2 issues, where the radiator is a long way away, as you mentioned.
I think what the truth really is, is that the senders in these peoples situations are broken and replacing the sender fixes the issue. Not that a bubble of air was giving a false reading.

In fact, i do a search for "4age temperature sensor surging air bubble"
And all the resolved forums point at, manifold gaskets, IAC valves, etc, and all of them are full of people suggesting this air bubble idea but they are never the "solved" answer.
When i was 16 and beginning my car obsession, i too had this air bubble theory planted in my head from someone online. But lots of experience has proven it to be a myth.

And just explain if you can, If the ECU can't get in to hot and lean closed loop mode, due to a lack of a hot coolant signal, why would it surge. Thats the bit no ones been able to explain.
In open loop (cold and rich mode which everyone sais causes the issue), the mixture is rich, and this actually helps hunting issues wrt to vacuum leaks.

Can you show me any thread, where the poster sais the hunting issue is resolved by bleeding the system?
If so, then ill be happy to learn something. I had a nissan pulsar wen i was younger with some similar issue, and changing the sender did fix something. i can't quite remember what.

For a 4AGE, there is no temp sender for the cold idle - it is literally a wax-pellet valve like the thermostat which opens and closes a bypass passage in the throttle body to up the idle.  I personally have fixed a hunting warm-idle problem in a 4AGE (a bigport in my wife's old AE82 Twincam) TWICE purely by bleeding the cooling system AND NOTHING ELSE.  Over the years of being on forums and Facebook I have seen countless other times when a hunting or high idle problem for those engines has been fixed by bleeding the cooling system (and it is really noticeable on 4AGEs due to the big difference between cold and warm idle speeds).  Remember too that an air pocket might be stationary when the engine is off, but when it is running and coolant is being pumped around things get stirred up, which can result in intermittent contact between the coolant and the wax-pellet/sender/whatever.  Usually it is a persistent high-idle that doesn't drop when the engine gets warm (also symptomatic of the wax-pellet valve dying of old age and being stuck permanently open, like a dead thermostat)  but it can cause surging if it is borderline.  It is actually a slightly different cause compared to a vacuum leak one (which is extra air causing the idle speed to rise about the ECU's preset limit causing it to pull fuel out) but can be hard to differentiate if you don't know exactly what to look for.

Yes, other issues such as a vacuum leak or faulty sender/valve can also cause similar symptoms (and I'm not discounting any of them in the slightest), but the air-bubble trapped in the throttle body causing inconsistent/high idles is something that is common across most of the transverse A-series Toyota engines (moreso in MR2s but Twinkies/SXs do suffer it too), and it is almost always suggested as the first answer because it is free, easy, doesn't require any disassembly, and is very common especially when people also have coolant leaks or have recently replaced a radiator/thermostat/hose/etc and introduced air in to the system (in my wife's case it was both - air was in the system after fixing a leaking water outlet on the end of the head and replacing a heater hose that had burst).

Link to comment
Share on other sites

The easiest way to determine if it is an idle-valve issue and not a vacuum leak is to take the intake pipe off to expose the throttle body, and then stick your finger over the hole for the bypass whilst the engine is warm and idling.  If the idle stops surging then the issue is in the ISCV (and either a faulty valve, sensor or air bubble), if it continues then the issue is a vacuum leak downstream of the throttle (or with the MAP sensor or TPS).

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

×
×
  • Create New...