4A-GZE Overboost, followed by misfire or knock.

[88AW11SC]

Hi Guys,

88 AW11 MR2 w/ 4AGZE 170Km's. Stock motor.

My little GZE has had a few moments over the last couple of days, I have no idea whats going on?

Being stock she usually runs about 0.6 bar of boost, last night she peaked out at 0.7 Bar with an air whistling sound - just like a turbo boosting. "No worries" I thought, one of the lines off the ABV has fallen off/broken therefore no boost control and the air whistle. I checked - everything looks to be fine! Hoses are good but I didn't pull the ABV off to check the spring was still tensioned. I ran an ECU diagnostic check - no fault codes, the ECU is happy.

This morning it did it again but this time I was scared! As I approached 6500RPM on WOT she went to 0.7 Bar and proceeded to either misfire or knock? I got a big puff of black smoke, lost power and instantly got off the throttle.

Now I'm worried, I have no idea whats going on. The stock ECU should be able to fuel (98 RON) up to 0.7 Bar and therefore not knock. So was it a misfire? If so why!? I replaced the plugs on it 2000Kms ago so ignition system, I assume, is good.

Finally, another symptom is being displayed that started the same time. The cat has a massive rattle, its like something is stuck in the exhaust but what? Half a valve? Would a partial blockage in the exhaust create a overboost condition - I think not? I have not had time to perform a compression test to ensure I have good cylinder pressure and that the engine is mechanically intact. It idles beautifully though so I doubt the engine has been mechanically damaged. Maybe the cat is just falling to bits?

Has anyone experienced similar symptoms? Anyone got any ideas? Anybody got any really good reference on the GZE boost control system. Anything? Im stumped?

At worst does anybody know of a true 4A guru on the Gold Coast (home town) or even Brisbane? If all else fails I may have to go looking for help.

Thanks Toyota People!
Matt.

[deviant]

The rattle could be a few things in your exhaust..

The Cat is collapsing
A heat shield is loose
The exhaust is touching the car body somewhere
A join in the exhaust is loose
The baffles in the muffler are collapsing

On my last MR2 it had a horrendous rattle / tinkle tinkle / buzzzing noise at a certain RPM range and I also had the 'check Cat' light come on a couple of times.
Pulled the exhaust off and found the cat to be in perfect condition but tipping the muffler on end caused a big pile of rusty baffles and all sorts of crud to fall out.
The noises stopped after that

[88AW11SC]

The exhaust is definitely not contacting the body anywhere, I checked and double checked that. There is a heat shield around the cat but that too is firmly secured. If I tap the cat when she isn't running I can hear something in there - i think. The cat would have around 80,000Kms on it so she would be on the way out. As soon as I get a chance I will rip the exhaust off and have a look.

The exhaust/cat rattle wouldn't cause the overboost and misfire/knock, thats what is concerning me the most.

Im now just chasing good reference on the ABV boost control system. There must be something wrong there. But still why did it misfire/knock/loose power? The stock ECU should play well with good fuel and 0.7 bar?

Scares me....I dont like engine gremlins, I just dont have enough knowledge to fix these type of issues.

[jimmy_a]

Finally, another symptom is being displayed that started the same time. The cat has a massive rattle, its like something is stuck in the exhaust but what? Half a valve? Would a partial blockage in the exhaust create a overboost condition - I think not? I have not had time to perform a compression test to ensure I have good cylinder pressure and that the engine is mechanically intact. It idles beautifully though so I doubt the engine has been mechanically damaged. Maybe the cat is just falling to bits?

Mate, I dont know a lot about superchargers, I assume they work similarly to turbos in that they use a wastegate...

logically - your wastegate could be sticking = overboosting and could be contributing to noises from the exhaust as well. That would explain some of the sounds and stuff??

[jimmy_a]

.7 bar is very low boost isnt it? isnt 1 bar = 14 or so psi? so thats only about 10psi?

Whats the fuel/boost cut on them?

[88AW11SC]

Being supercharged boost is limited mechanically. How fast the charger is spinning is how much boost she makes. The 4A-GZE has a boost control safety system called the ABV, this limits boost to about 8PSI. People that run more boost on the 4A-GZE disable this system.

My car does not have any of these mods, its stock so it should not run more boost unless there is something wrong with the ABV. No 0.7 bar is not a lot of boost but again it just shouldn't happen, this is whats worrying me. After checking all external items for boost control - ABV, VSV, vacuum lines etc there doesn't seem to be a problem - so why is it overboosting!?! Maybe the spring behind the ABV has lost tension/snapped which is allowing the overboost.

Next, the ECU should be able to handle 0.7 bar but she was not happy. It either knocked and the ECU backed off ignition or it misfired. Again WHY? These symptoms are all brand new as of yesterday. WTF is going on?

Nothing is adding up, thats why Im stumped. And a little scared, I don't want to spend cash on the AW11, I want hit Whistler and carve the mountain to shreds with my snowboard!

[jimmy_a]

It either knocked and the ECU backed off ignition or it misfired. Again WHY? These symptoms are all brand new as of yesterday. WTF is going on?

Just eliminate possible causes and keep doing your research...could be the spring, how could you test that? find someone around you who knows, buy them some beer to have a look at it

[TooF]

well a blocked cat/collapsed exhaust *can* cause an increase in manifold pressure (the increase in boost you're measuring)

this is caused by poor flow though the exhaust resutling in increased exhaust pressure resulting in poor cylinder gas evacuation, now since the supercharger is a PD pump and will keep flowing the same volume of gas regardless of whats infront of it (loosly) then you will have the inlet charge air + some left over exhaust gas resulting in the increase in manifold pressure.

if you are hearing noises from the cat that would be the first port of call. pull it out and check you can see though it, hold it upto the light and you should be able to clearly see though the honeycomb.

[feral4mr2]

The 4A-GZE has a boost control safety system called the ABV, this limits boost to about 8PSI. People that run more boost on the 4A-GZE disable this system.

even with out the ABV or ACV the s/c will only put out ~8psi limited by the OD of the crank pulley. basicaly it cannot 'overboost'.

Next, the ECU should be able to handle 0.7 bar but she was not happy.

i'm running 18psi constantly on a stock gze ecu. no probs.

that tiny bit of boost increase could of been just good ol heat soak in the i/c from too much joyful flogging around.... even a change in the weather, cold last night, denser air...

these things run prity rich in the higher rpm's, cats dont like that. it's probably got too hot and finaly cracked/shattered (hence the rattle and hearing something 'in there'). the exhaust is easy to pull off, whip it off and seperate the cat from the system and have a looksee.
wouldnt hurt just to check engine compression etc. but if the engine runs fine smoothe, no knocking sounds etc while at a standstill either at idle or raised revs (free rev that fooker man) then i wouldnt count it as a problem.

i've not long got home from night shift, is all my brain can think of at the moment.

[88AW11SC]

Feral is here! Thanks for the help upfront mate, your input is already much appreciated! TOOF!! You have answered my question from my first post:

Would a partial blockage in the exhaust create a overboost condition - I think not?

If you are right then you have explained all symptoms. The blockage is producing a back pressure which is in turn raising inlet manifold pressure, the dirty gas is creating a poor mixture which inturn causes detonation which the ECU hears and consequently backs off the ignition = power loss & black smoke (unburnt fuel). That explains all.

Feral I completely understand what your saying regarding not being able to overboost - limited mechanically unlike a turbocharger. This is whats scaring me so much - It has a stock crank pulley and has ALWAYS sat around 0.6 bar. But I swear I saw 0.7 bar on the gauge and have never seen it before - i have owned her for 4 years now.

I was wondering if a collapsed cat would produce an overboost situation - TOOF I hope you're right, if so I can explain all problems with a dead cat. Im taking the exhaust off this weekend, as Feral said its not too difficult, had it off many a time before. Maybe the honeycomb mesh has disintegrated and will fall out into my hand, if so time for a new cat. The cat aint new either, must have a good 80,000Kms on her and as Feral said they run rich which cat's dont like. Please let it just be the cat.....maybe I'll just gut the thing with a screw driver. (No I would never do such a thing Mr EPA officer)

The primary concerns are:
1) The knock/misfire at 6500RPM 0.7 bar WOT
2) Increased boost level with no modifications
3) Very sick sounding exhaust

I am taking my boost reading from the inlet runners - as close to the inlet valves as possible.

Thanks so much TOOF and Feral - love your work! Any other ideas still very much appreciated.

[jdm86gtz]

I had one of those Apexi ECV cable operated valves to quieten down the exhaust and if I closed the valve and give the car a quick blast the boost would register 0.2 BAR boost increase over the boost achieved with the valve fully open.

[Grega]

As I approached 6500RPM on WOT she went to 0.7 Bar and proceeded to either misfire or knock? I got a big puff of black smoke, lost power and instantly got off the throttle.

It definately sounds like your knock sensor (I'm assuming your GZE motor has one) caught wind of something it didn't like the sound of, dragging a bucket load of timing out of it, but your injectors were still putting fuel in which equals your black smoke cloud. I take it you were up the thing pretty hard when this happened?

Run the car, deodorant, aerostart, ect around the inlet manifold and check for leaks. May explain your whistle?

When did you last fill up? Possible you've got a bad batch of fuel?

[88AW11SC]

Looks like a confirmation of the theory from jdm88gtz! Thanks mate!

Off with the exhaust tonight.....

[88AW11SC]

Well the cat is definitely stuffed! Got the exhaust off, gave it a shake and you can hear a lump rattling around in the cat. Bits of honeycomb mesh fell out into my hand - definitely stuffed!!

Found a guy in Brisbane selling a new MagnaFlow Metal Core cat for $195 - not too bad. Gonna replace the O2 sensor as well, now that I think it has been using more fuel, I simply chalked that up to the rise in fuel prices.

So maybe the O2 sensor died, ran rich, melted the cat. Blocked the exhaust, created back pressure, created the 'overboost' then knocked with a bad mixture.

Gonna have the cat welded in Tuesday, will let everyone know how it goes...

Matt.

[jimmy_a]

Good work man. + rep for figuring out the problem in 1 page