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LP76

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

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Hey folks,

I have a '99 Corolla with the stock 1.8 7A-FE motor. 

It intermittently surges when idling (i.e. - pulling up at lights, fluctuates between ~ 500-1200 rpm), but doesn't do it all the time. Sometimes a quick press of the accelerator will stop it and it'll return to an idle of around 800 rpm or so, sometimes it doesn't. For the first few minutes after cold starting, the car will jerk a bit at low speed, but goes away within a few minutes of running.

Local mechanic has sprayed the throttle body, and it's had a recent full service (air and fuel filters, plugs and leads etc.), but it hasn't made much difference, aside from alleviating the sticky accelerator pedal.

A quick diagnostics check is returning fault code 31. 

Possible MAP sensor or IAC valve problem?

If anyone is good with these motors here, please tell!

Cheers. 

Edited by LP76

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10 minutes ago, Hiro Protagonist said:

31 should be the MAP sensor, I think 33 is the idle valve.

That doesn't necessarily mean that it's a problem with the sensor though does it?

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Map sensor code I think can indicate a vacuum leak too, as the map sensor gets a funny signal.

I'd be removing the ICV and cleaning the crap out of it, then check every vacuum hose for leaks especially the big rubber hose on the inlet manifold.

 

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9 minutes ago, ke70dave said:

Map sensor code I think can indicate a vacuum leak too, as the map sensor gets a funny signal.

I'd be removing the ICV and cleaning the crap out of it, then check every vacuum hose for leaks especially the big rubber hose on the inlet manifold.

 

I think I can hear some hissing coming from somewhere near the throttle body. 

Can you suggest a good way to find the source of the potential leak?

Thanks so much.

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Therse not much on a 7afe, i would pull off the rubber hose that goes between the airbox and the inlet and inspect.  Inspect all the vac lines.

Worst case, get some throttle body cleaner and spray it around where you think there might be leaks. 

With a map sensor you are really looking for issues after the throttle body. 

also keep in mind map sensors are dirty cheap from the wreckers, if you have a wreckers close go and grab one. Note the code on the map sensor it self and go and look at Toyotas of similar era if you cant find exactly your car, they used the same map sensor on loads of cars. 

Edited by ke70dave

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15 minutes ago, ke70dave said:

Therse not much on a 7afe, i would pull off the rubber hose that goes between the airbox and the inlet and inspect.  Inspect all the vac lines.

Worst case, get some throttle body cleaner and spray it around where you think there might be leaks. 

With a map sensor you are really looking for issues after the throttle body. 

also keep in mind map sensors are dirty cheap from the wreckers, if you have a wreckers close go and grab one. Note the code on the map sensor it self and go and look at Toyotas of similar era if you can't find exactly your car, they used the same map sensor on loads of cars. 

Only thing upstream from the throttle on a 7A is the intake air-temp sensor and that does bugger-all so it'll definitely be downstream.  In total there will be vacuum lines for the cam breather (plenum to cam cover), PCV (cam cover to manifold), MAP sensor, brake booster, fuel pressure regulator, and charcoal canister, then the idle-up circuits for the A/C and P/S.

 

I _may_ have a spare 7A MAP sensor running around from my old shell, was planning on stripping all the sensors out of it for spares but ended up selling it mostly whole.

Edited by Hiro Protagonist

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7 minutes ago, ke70dave said:

Therse not much on a 7afe, i would pull off the rubber hose that goes between the airbox and the inlet and inspect.  Inspect all the vac lines.

Worst case, get some throttle body cleaner and spray it around where you think there might be leaks. 

With a map sensor you are really looking for issues after the throttle body. 

also keep in mind map sensors are dirty cheap from the wreckers, if you have a wreckers close go and grab one. Note the code on the map sensor it self and go and look at Toyotas of similar era if you can't find exactly your car, they used the same map sensor on loads of cars. 

Okay thanks.

Also, just now, I notice there is a definite flat spot right at the start of the throttle. 

Would that help narrow down the cause of the problem?

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Anything to do with the throttle points at TPS too. Unplug the tps,  try again and report back.

Edited by ke70dave

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19 minutes ago, ke70dave said:

Anything to do with the throttle points at TPS too. Unplug the tps,  try again and report back.

Okay, will do 

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3 hours ago, ke70dave said:

Anything to do with the throttle points at TPS too. Unplug the tps,  try again and report back.

Did you mean unplug the TPS and then run the motor to compare before/after?

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So, I cleaned:

* the throttle plate & body in-situ

* PCV  valve (removed and cleaned)

* IAC valve (removed plug, removed middle hose, filled hole in throttle body with cleaner, turned valve back and forth many times, drained cleaner - moves very freely)

I checked visually for signs of vacuum leaks, but can't see anything. 

Car started fine, no hesitation. No noticeable flat spot in idle when pulling on throttle arm a few times. Did a few laps around the local area, then went back home. Sat car in neutral in carpark, intermittent idle still apparent - fluctuating between approx 700 and 1100 rpm. Seems to be when the engine is near or at correct operating temp. Any ideas? 

Should I just go ahead and replace the MAP sensor (I didn't want to remove it and mess up its adjustment)? 

Edited by LP76
coz

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Just checking, how is your coolant level?  Classic 4A/7A problem with an intermittent idle is an air-bubble in the cooling system causes the cold idle valve to open and shut even when the car is warm - might be worthwhile topping up the radiator and then bleeding the system (jack the front of the car up so the radiator cap is the highest point in the system, and then run the car until warm with the cap off and the heater tap open to try and force any bubbles out of the system).

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1 hour ago, Hiro Protagonist said:

Just checking, how is your coolant level?  Classic 4A/7A problem with an intermittent idle is an air-bubble in the cooling system causes the cold idle valve to open and shut even when the car is warm - might be worthwhile topping up the radiator and then bleeding the system (jack the front of the car up so the radiator cap is the highest point in the system, and then run the car until warm with the cap off and the heater tap open to try and force any bubbles out of the system).

Okay, will give that a go. Cheers.

Edited by LP76

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