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

[LP76]

So, after all the mucking around, the car has settled into a nice idle, no hiccups, surging, or hunting, and most importantly, no dead spot in the throttle. I didn't end up bleeding the coolant system, so I assuming that it was one or more of the components that I cleaned that were causing the problem. Thanks very much to all who offered their opinions and time. Now, to replace the rear shocks....

[LP76]

Turns out the dreaded problem I described never actually went away. I thought it did, but is still occurring intermittently. 

Most noticeable when car is cold and starting to drive. Small dead spot at the beginning of the throttle, and jerkiness when driving at low rpm until the motor warms up a little.

[rebuilder86]

I'm almost certain u have a large vac leak and it will be the intake manifold gasket itself. Usually a few mm of it gets sucked in from deterioration from oil and gasoline.

If u had said u had code 31 with no noticable symptoms at idle i would have told u it was the MAP sensor itself.

but the surging idle of up to 1500 rpm is in 95 % of cases a large vac leak.

cet up close at idle and listen around the manifold connection.

[ke70dave]

Maybe bleed that coolant system eh. 

[LP76]

On 26/11/2017 at 3:24 PM, rebuilder86 said:

I'm almost certain u have a large vac leak and it will be the intake manifold gasket itself. Usually a few mm of it gets sucked in from deterioration from oil and gasoline.

If u had said u had code 31 with no noticable symptoms at idle i would have told u it was the MAP sensor itself.

but the surging idle of up to 1500 rpm is in 95 % of cases a large vac leak.

cet up close at idle and listen around the manifold connection.

The car had some other work done at the mechanic a few weeka back and I got him to check the throttle problem as described. He removed and thoroughly cleaned the throttle body and the IAC valve. There was no noticeable deterioration of the gasket between the throttle body and the intake. I got him to check the error code 31 I had previously detected, but it was no longer present. I assume it had been cleared after my previous attempts at cleaning the throttle and various valves, which I described earlier.

When the problem arises, which is intermittently as I mentioned above, the idle fluctuates slightly between ~600 and 1100rpm. It never stalls or goes much higher than 1000. The most annoying part is the small dead spot at the start of the throttle. 

I inspected all the vac lines last time but couldn't hear/see/find a leak. 

 

 

[rebuilder86]

yes thats the throttle gasket, I'm talking about the manifold to head one. 

For a small to medium leak this happens intermittently and more often when hot as the ECU goes into lean "closed loop" mode, and the lean mixture is more sensitive to a vacuum leak. So in most cases its the opposite to what u are describing about it happening more when cold.

However if the leak is big enough it can transmit this varying vacuum signal to the fuel pressure regulator which causes that to start varying pressure aswell, which plays with the mixture a very large amount regardless of ecu open or closed loop.

Then when it warms up, the injectors are open for less time, and the surging fuel pressure regulator is less pronounced.

 

[rebuilder86]

Try not to become fixated on what I am saying, but ensure u really thoroughly check that gasket. then

PCV valve, shakes freely and blowing through it should be heavily restricted in one direction.

Brake booster, (see if it happens more when u push the brakes, if so new booster needed)

fuel pressure regulator vacuum hose ( a small leak here would have more effect on fuel pressure than the same size leak elsewhere, true fact)

check those and get back to us.

 

 

 

Edited December 1, 2017 by rebuilder86

[LP76]

On 2 December 2017 at 10:55 AM, rebuilder86 said:

Try not to become fixated on what I am saying, but ensure u really thoroughly check that gasket. then

PCV valve, shakes freely and blowing through it should be heavily restricted in one direction.

Brake booster, (see if it happens more when u push the brakes, if so new booster needed)

fuel pressure regulator vacuum hose ( a small leak here would have more effect on fuel pressure than the same size leak elsewhere, true fact)

check those and get back to us.

 

 

 

Am I able to detect a leak at the intake gasket with the motor running by spraying with carb cleaner or soapy water? I've never removed an intake manifold before, and am a little hesitant to do so. 

Pushing the brake makes no difference. I had the PCV out a month ago; inspected and cleaned it, it didn't appear restricted. 

The surging (revving up to 1100 rpm or so) happens immediately after the idle drops down to 400 or so. It doesn't all of a sudden sit at correct idle and then go straight to ~1100; it's always a noticeable drop down first, THEN a spike up. 

And there is a small, but definite, flat spot right at the beginning of the throttle.

Thanks for your help. 

Edited December 11, 2017 by LP76

[rebuilder86]

2 hours ago, LP76 said:

Pushing the brake makes no difference. I had the PCV out a month ago; inspected and cleaned it, it didn't appear restricted. 

don't look for restriction, look for the opposite. it should be restrictive. if u can blow through it (fast and hard) in the direction towards the intake manifold, then its leaking too much and will be a large vac leak.

as for the intake manifold gasket, ive always been able to hear the leak, but not until i had worked out that there was indeed a leak haha.

i have had limited success with soapy water, but good success spraying "start yah bastard" (spray ether, or starting fluid) all around the manifold. that stuff will make it revv if theres an air leak.

 

and yes, the drop to a rough stumble almost stall then overcompensation high rpm is typical of a large vacuum leak

Edited December 11, 2017 by rebuilder86

[Hiro Protagonist]

15 hours ago, rebuilder86 said:

don't look for restriction, look for the opposite. it should be restrictive. if u can blow through it (fast and hard) in the direction towards the intake manifold, then its leaking too much and will be a large vac leak.

Other way around - you should be able to blow air from cam cover to intake manifold but not the other way (the valve "opens" when there is vacuum in the intake which sucks the vapours through).

[LP76]

Sorry, I should've been more succinct. I meant that the PCV didn't appear restricted in that I could definitely hear the "rattle" when shaking it, and from memory I could only blow through it in one direction. It's easy enough to access though I suppose, so will double check.

I checked the coolant level yesterday. Still as good as gold, and running the car with the cap off didn't produce any noticeable bubbles.

The thing I don't understand is if it is indeed a vacuum leak, why would the symptoms not be present all of the time?

Anyway, it's a bit too dark to muck around now, but will try pointing some carb cleaner at the intake manifold tomorrow evening and see what happens. 

Will be reporting back. 

Cheers again folks.

[LP76]

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? 

Edited December 12, 2017 by LP76

[rebuilder86]

8 hours ago, Hiro Protagonist said:

Other way around - you should be able to blow air from cam cover to intake manifold but not the other way (the valve "opens" when there is vacuum in the intake which sucks the vapours through).

nope sorry.

thats what u read on the net. not what happens in practice. note i said "blow hard and fast" which mimmics a high vacuum situation like idle.

it should be restricted, or else it is not restricting the flow of air at idle and low power.

the blowing from the other way as u describe only tests the back seat face, which is really only practically used in the even of a backfire.

[rebuilder86]

2 hours ago, LP76 said:

Sorry, I should've been more succinct. I meant that the PCV didn't appear restricted in that I could definitely hear the "rattle" when shaking it, and from memory I could only blow through it in one direction. It's easy enough to access though I suppose, so will double check.

ok the rattle tells u nothing much.

that just proves it isnt stuck. u really need to block the pcv port in the manifold to to test if that makes the hunting stop. if so, then thats the problem. pcv valves fail a lot and more often they get dirty and leak vacuum rather than get dirt and block flow.

2 hours ago, LP76 said:

The thing I don't understand is if it is indeed a vacuum leak, why would the symptoms not be present all of the time

ok so when warming up, the ECU is ignoring the O2 sensor and fuel mix ratio (fuel trim) is locked at a preprogrammed value and this value is quite rich. during this time your surging on idle wont be too noticeable because the rich condition burns well. but when it gets hot, it starts to make adjustments to try to get the o2 sensor to read closer to the leaner stoichiometric mixture. this leaner operating condition amplifies any ignition or "fuel supply related issues", and this includes vacuum leaks.

when the engine really warms up, like after 20 minites of driving, it becomes even better at burning lean mixtures, so sometimes a leak can get better with heat.

your leak could be anywhere. u said u could hear a hissing, try to get ur ears closer to the source and see if u can pinpoint it.

i once had to bight the bullet and just rip my mums mazda 323 astina manifold off because i couldnt find what was leaking, and when it came off i found a chunk of manifold gasket missing. replaced the gasket and the issue was resolved. the aymptoms for that one where 100% identical to urs.

it was a pain in the ass for about 3 montgs before i juat bit the bullet and ripped it off and found the leak.

gaskets do just dissapear in pieces.

Edited December 12, 2017 by rebuilder86

[Hiro Protagonist]

8 hours ago, rebuilder86 said:

nope sorry.

thats what u read on the net. not what happens in practice. note i said "blow hard and fast" which mimmics a high vacuum situation like idle.

it should be restricted, or else it is not restricting the flow of air at idle and low power.

the blowing from the other way as u describe only tests the back seat face, which is really only practically used in the even of a backfire.

Sorry I thought you were implying that it would block the air flow completely if you blew from cover to intake.