WHY does my 3S ignition cut?

[Wildsupra]

Just took my 87 MR2 for its first test drive, it seems to ignition cut for no apparant reason eith when it get anywhere near starting to make boost or if i give it more than about 1/4 throttle. Its a gen 1 3S-GTE engine

Could it be a weak igniter or something like that?
Ive taken it round the block maybe 10 times and maybe for about 5 seconds it will come onto boost hard and start to pull/smoke tires but then after about 5 seconds of fun it goes back to cutting again. It also only gives me that 5 seconds the first time i boot it, it wont do it again until i switch the car off for about a minute or so then try again.

It feels like ignition cut rather than fuel becoz it is so sudden and doesnt cough in the slightest, just cuts out completely until i back the throttle right off.

Anyone had a similar problem maybe with their GT4? As its a GT4 engine om using.

Cheers
Matty

[edited by ed: Y is a letter, WHY is a word - fool!]

[Merudo]

Have you tried pulling any codes?

[Wildsupra]

Nah not yet, need to wire up the engine check light.

This is weird though, just took it round the block again and it didnt cut once, boosted and revved sweet as, full throttle and boost no probs. Got me puzzled.

No doubt it'll play up again though.

Might go wire up the eng check light ay.

Matty

[MR22ZZ]

sounds like a faulty MAF or MAP sensor, not sure which the GT4 uses... but it's showing those symptoms

[Wildsupra]

sounds like a faulty MAF or MAP sensor, not sure which the GT4 uses... but it's showing those symptoms

hmm it sure does, checked the error codes and the only one its reading is 54 which is low intercooler water or intercooler pump not working, which is cos i ran my own pump.

Here's a silly proposition.........i dont think im running a turbo pressure sensor at all its not in there anywhere and i dont remember getting one with the motor.........but why does it sometimes run sweet and boost well??

Matty

[JustCallMeOrlando]

Cos the ECU is dumb. Seriously, it should know if you don't have one plugged in.

[JustCallMeOrlando]

Actually, was the Gen1 3S AFM? If so, there won't be a turbo pressure sensos

[3sgte]

Based on a Canadian spec ST165:
The ECM uses a "turbo pressure sensor" (map sensor) mounted on the firewall to monitor boost. If overboost is detected, Whammo, fuel cut. After it happens the first time in a drve cycle, this fuel cut will occur whenever the manifold pressure rises above a much lower trigger (atmospheric pressure?).

This map sensor is in addition to the vane type AFM.

A boost leak can also result in a similar feeling.

[JustCallMeOrlando]

So the ECU has a map sensor in addition to the vane AFM? The 1G ECU just has the vane and triggers boost cut by the AFM voltage. Strange.

[-=DV=-]

Just took my 87 MR2 for its first test drive, it seems to ignition cut for no apparant reason eith when it get anywhere near starting to make boost or if i give it more than about 1/4 throttle. Its a gen 1 3S-GTE engine

Could it be a weak igniter or something like that?
Ive taken it round the block maybe 10 times and maybe for about 5 seconds it will come onto boost hard and start to pull/smoke tires but then after about 5 seconds of fun it goes back to cutting again. It also only gives me that 5 seconds the first time i boot it, it wont do it again until i switch the car off for about a minute or so then try again.

It feels like ignition cut rather than fuel becoz it is so sudden and doesnt cough in the slightest, just cuts out completely until i back the throttle right off.

Anyone had a similar problem maybe with their GT4? As its a GT4 engine om using.

Cheers
Matty

mate thats not ignition cut..it is fuel cut from overboosting, my gt4 does the EXACT same thing when u over boost..it doesnt cough or splutter at all, it feels like both fuel and spark have been cut at the same time, until u get back off the throttle, then it will do it every time it even STARTS to see +ve pressure until u stop the car..switch it off..then switch it back on.... u need to get urself a boost guage and have a look at whats going on..cheers mate

[hamgatan]

yeap im with DV...

hook up a FCD to it and see if it improves the situation... i think john (importbitz) might have one for around 80-100ish.. ask him

[-=DV=-]

just thaught id add some more, if its a gen1 chances are u dont have a map type sensor at all, the us and uk gen1 3sgtes had map sensors aswell as afms, the map was used for fuel cut purposes, the jdm 3sgtes on the other hand dont,they use a bunch of calculations involving the volume of air measured by the afm, the known volumetric capacity of the intake tract and manifold, throttle position and revs to determine when there is too much pressure in ur intake manifold...im guesing just a way to stop people simply hooking up fuel cut defenders and blowing motors, the uk and us 3sgtes will go to doing it this way if you unplug the wire from the map sensor.

[hamgatan]

with a flapper type AFM? i thought only Karmann Vortex types did that?

[JustCallMeOrlando]

Nope, the 1G does that too. It was so bullshit rich that I was able to tighten the AFM spring a couple of notches to move the boost cut that much further away, but my AFR's are still under 10:1.

[Wildsupra]

Cheers for all the info guys.

Yeah DV i have been getting very confused because quite a few 165's seem to have the turbo pressure sensor as well as the AFM. but then i cant find a plug for one to even plug into. I think you have confirmed that my engine simply shouldnt have one.
I think it is fuel cut, i said the wrong thing in the first place, i meant i dont think its leaning out.
The thing that confused me was the fact it wont even boost in the slightest once it does it the first time, all my other turbo cars would still boost after hitting fuel cut.
Off to get a boost gauge then, maybe i should try backing the boost controller right off too I knew that 3" dump pipe was gunna cause it to cut

Matty