T1 Misfire/Boost Cut Shennanigans

[daffyflyer]

Okay, so just bought a T1 from down on the south island to use as a company car for our new employee (seriously!). Looks like it's forum member "QikLude"'s old car?

It's got a bigger turbo, which I believe is the Isuzu/Holden Rodeo based hybrid thingy and a 2.5" exhaust.

When we first got it the wastegate was seized shut, and it'd spike straight to 15psi and stutter/cut out. This seems to be the ECU boost cut, that'd make sense right?


Now we've just had the exhaust housing media blasted and generally cleaned up, and the wastegate opens properly. It's still a bit small though, and between about 5000 - 6000rpm it starts to creep and gets to about 11psi.


The weird thing though, is now when it gets around 10 - 11psi, it starts backfiring/stuttering, as if it's hitting the boost cut, however the boost cut shouldn't be that low should it? It certainly wasn't before.

Leads and plugs are fresh, Coil is some big fancy external unit. Unsure of condition of Distributor though. Apparently has a bosch "910" fuel pump too.

Any hints where to start with diagnosing this one?

[bitterberry]

T1 boost cut is 14psi as far as I'm aware. I've got a similar problem on my T1 especially when it's cold, stuttering/jumping under high boost. In my mind it's either a damaged turbo (play in the shaft), spark plugs are worn or have the wrong gap OR needs a new coil. See if KingCity replies here, he's got a lot of knowledge and may be able to pinpoint more accurately what it is.. Also, where do i sign up for a T1 as my company car? Hahaha

[daffyflyer]

Ok, yeah, definately cutting around 10 - 11psi, and was 14psi or so before, seems like a problem that is getting worse.

Plugs seem fresh, but not sure if gapped right. Leads are brand new today, coil is fancy but possibly old. Turbo has just been taken apart by a turbo specialist and they said it looked fine besides the stuck wastegate.


I run a company that makes a game about cars (www.automationgame.com), and our new programmer needed transport to work, and loves old kei cars and similar. Thus the T1

[CIV2NV]

Not 100% sure on the characteristics of the Rodeo turbo but the standard T1 turbo will act similar to what you have got if the waste gate port is not modified when the bigger exhaust is installed.

[bitterberry]

ive got a rodeo turbo not yet in the car and the wastegate is bigger than t1 wastegate, to me it looks big enough to prevent this but maybe it could need to be opened a little more. Rodeo turbo is meant for 2" pipe I think

[daffyflyer]

Yeah, the boost creep/over boosting makes sense, and I have plans to fix that, so that's fine.

What is confusing me, and what I need help diagnosing is the missfire/stutter around 11psi. It made sense when it was happening around 14psi, as that'd be the boost cut. But now its started at 11psi, and I'm running out of things to check..

[bitterberry]

The water lines on your turbo will have been blocked up, correct? Potentially the turbo's been damaged since it hasn't been cooled as intended?

[daffyflyer]

The turbo has taken apart just a few days ago by a specialist, and appears to be in entirely good health.

I would suspect heat damage would manifest as turbo bearing problems, and unless I'm failing to understand something, I doubt that'd give the symptoms I'm currently having, which seem much more like something fuel or ignition related (misfiring, backfiring suddenly at a specific boost level)

Are there any folks around here who are intimately familiar with the T1 ECU/Fuel System?

[bitterberry]

Just throwing stuff out there that it could be, I'd like to know the cause of this issue too. Hopefully one of the more experienced guys comes on to help but they don't come on all that often. Maybe run some injector cleaner through your next tank of fuel? Could be partially blocked so it can't dish out all the fuel needed at high boost levels? Have you tried turning the boost down to 10 psi? Does it have an electronic boost controller or bleed valve?

[daffyflyer]

Can't turn the boost down, as the wastegate isn't small enough to keep it down that low.

Fueling issues seem like a good plan to investigate next, maybe I'll pop the injectors off and test em..

[Hyperblade]

Okay, so just bought a T1 from down on the south island to use as a company car for our new employee (seriously!). Looks like it's forum member "QikLude"'s old car?

It's got a bigger turbo, which I believe is the Isuzu/Holden Rodeo based hybrid thingy and a 2.5" exhaust.

When we first got it the wastegate was seized shut, and it'd spike straight to 15psi and stutter/cut out. This seems to be the ECU boost cut, that'd make sense right?


Now we've just had the exhaust housing media blasted and generally cleaned up, and the wastegate opens properly. It's still a bit small though, and between about 5000 - 6000rpm it starts to creep and gets to about 11psi.


The weird thing though, is now when it gets around 10 - 11psi, it starts backfiring/stuttering, as if it's hitting the boost cut, however the boost cut shouldn't be that low should it? It certainly wasn't before.

Leads and plugs are fresh, Coil is some big fancy external unit. Unsure of condition of Distributor though. Apparently has a bosch "910" fuel pump too.

Any hints where to start with diagnosing this one?

Hi welcome to the forums.

So a couple of things to clear up and to make sure we are all talking about the same thing. Please note this is all based on what the unmodified versions feel like, as your's is heavily modified I don't know what impact that will have in terms of symptoms.

Boost cut feels like your accelerating amazingly than you hit a massive immovable brick wall. It's not a stutter or a miss, it's a rock hard your engine cuts out. You can not miss it when it happens, and it doesn't move around in the boost range. FYI stock boost for T2 is 12psi and it cuts at 16-18. T1 has stock boost of 8psi not sure when it cuts.

So if we are talking stutter or miss, than you have to go through the normal diagnostics for any car. which are electrics, fuel, and compression.

So things to check for electrics are.
Leads (never ever had a problem with stock ones, over 200,000k's on mine), coil (again stock coil is rock solid), spark plus (gaps), distributor (definitely can cause issues, check points for wear).

Fuel
fuel pressure, fuel filter, injectors, age of fuel in tank(it's been sitting a while so we don't know what crap it has got in there)

Compression
Do a compression check.

Something else worth doing is checking the timing.

Now in saying all that, it sounds like your idling fine and lower in the rev range is fine, so i think we can eliminate the ecu from being a problem and I don't think compression is the issue or the turbo.

However your running higher boost than stock so it's hard to say how the fueling is at that boost or if the spark is right.

But it appears to be under high load, so that's when the most spark/fuel are required so those would be my first guesses.

I'm also assuming it was originally running right, but there's always a chance it wasn't...

[KingCity]

Can't turn the boost down, as the wastegate isn't small enough to keep it down that low.

Fueling issues seem like a good plan to investigate next, maybe I'll pop the injectors off and test em..

Its not the wastegate size that's stopping it from staying under 11psi, 11psi is the rate for the standard actuator (spring pressure), if the boost is creeping above 11psi through the rev range then that's when you know the wastegate isn't coping & is to small for the job but is only likely to do that around 5000rpm & above. Fuel cut is just under 1bar (around 14psi) on both T1 & T2 City's so yeah it does sound like it could be a fuel delivery or ignition issue so i'd start with that coil by replacing it with an original item as they do the job fine & are basically faultless, you can use one from a N/A city for testing but they aren't quite the same as turbo ones so try to get a original turbo one if possible.

[daffyflyer]

Hi welcome to the forums.

So a couple of things to clear up and to make sure we are all talking about the same thing. Please note this is all based on what the unmodified versions feel like, as your's is heavily modified I don't know what impact that will have in terms of symptoms.

Boost cut feels like your accelerating amazingly than you hit a massive immovable brick wall. It's not a stutter or a miss, it's a rock hard your engine cuts out. You can not miss it when it happens, and it doesn't move around in the boost range. FYI stock boost for T2 is 12psi and it cuts at 16-18. T1 has stock boost of 8psi not sure when it cuts.

So if we are talking stutter or miss, than you have to go through the normal diagnostics for any car. which are electrics, fuel, and compression.

So things to check for electrics are.
Leads (never ever had a problem with stock ones, over 200,000k's on mine), coil (again stock coil is rock solid), spark plus (gaps), distributor (definitely can cause issues, check points for wear).

Fuel
fuel pressure, fuel filter, injectors, age of fuel in tank(it's been sitting a while so we don't know what crap it has got in there)

Compression
Do a compression check.

Something else worth doing is checking the timing.

Now in saying all that, it sounds like your idling fine and lower in the rev range is fine, so i think we can eliminate the ecu from being a problem and I don't think compression is the issue or the turbo.

However your running higher boost than stock so it's hard to say how the fueling is at that boost or if the spark is right.

But it appears to be under high load, so that's when the most spark/fuel are required so those would be my first guesses.

I'm also assuming it was originally running right, but there's always a chance it wasn't...

Yeah, not sure if this is boost cut or misfiring, if it's a misfire it's a very bad one, almost feels like a rev limiter. But considering it's not always at the same boost and is happening at 11psi lately, It really can't be the boost cut.


Leads are brand new and fresh, plugs look close to new, but I'll check gaps. Coil is unknown, that'll be the next thing to check (good call!). Distributor cap might be old, will order a fresh one.


Fuel system I have no idea, all I know is it has fresh fuel, so I'll do a proper check of all that


I forgot to mention, Idle is not perfect, it idles OK, but once warm it does the occasional little "parp" splutter noise every 5 seconds or so. Other than that it drives perfectly until about 5000rpm/11psi.

[daffyflyer]

Can't turn the boost down, as the wastegate isn't small enough to keep it down that low.

Fueling issues seem like a good plan to investigate next, maybe I'll pop the injectors off and test em..

Its not the wastegate size that's stopping it from staying under 11psi, 11psi is the rate for the standard actuator (spring pressure), if the boost is creeping above 11psi

It only creeps up to 11psi above about 5000rpm, and before the misfire got worse, was creeping to 15psi near the redline. Best I can tell actuator pressure is 7 -8psi or so on this (aftermarket) turbo

[KingCity]

Haha true, I didn't even take into account of the different turbo, im not sure of actuator spring pressure for it. If your are only making 11psi boost at 5000rpm then theres something not right, Bitters had the same issue on his so yeah maybe he can help more. You should be hitting 11psi by 3000rpm at the latest with stock City turbos & I would expect not much higher for your turbo. If the turbo & wastegate are fine & there are not boost leaks then it sounds like a timing issue, go & check the timing belt is set right then make sure your dizzy is correctly set, this is going to either fix the problem or eliminate that being the issue but if it starts missing at different boost levels then its likely to be another issue but like Hyperblade explained just check everything but timing would be an easy place to start.