Please help! Car can't run over 4000rpm.

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MUDGUTS

New Member
Messages
14
Location
UK
Is it possible that a fault with the gearbox could be the issue here? One thing that has been raised is the kick-down function and it interfering with the way the engine is operating. The car does the same whether it is in 'D' 'P' or 'N'. I can't see it myself but would be interested in any opinions...

The techs have said they have tried everything apart from the ECU, which, hopefully can be checked by swapping with another unit today.
 

MUDGUTS

New Member
Messages
14
Location
UK
Well, there is nothing wrong with that engine.

It's been with Thor Racing at Coventry, who have gone well above the call of duty and comprehensively checked out the engine and systems. Just about everything has been checked and analysed, most of the list below re-checked and also: Ignitor swapped for a known servicable unit, same with the ECU, Throttle Position Sensor, Fuel pump and control, etc.

Time was running out, so I have the car back now and will run it for a while.

One last theory is that the car seems to go into a self imposed limit of 4000 RPM. The tech at Thor and I (whilst on the way home), note that when the throttle is floored and the car reaches this 4000 RPM limit, moving the shift lever has no effect, I had it in 'D' '3' 'L' and even 'N' whilst it kept driving along. As soon as you lift off and it comes out of this region, you can move the shift lever and the box shifts accordingly.

The updated list is below of things already checked / to check:

Already checked:

Exhaust & Catalytic Converters checked
Valve timing checked
EGR valve cleaned & checked, pipes also checked
Throttle Body & Plenum Chamber cleaned
Distributor Caps, Rotor Arms & HT Leads replaced (Old Spark plugs were not suspect so replaced after new ones tried)
Engine sensor connections checked
Gearbox shift position sensor (confident this isn't at fault)
Fault code 21 cleared with new Oxy Sensor
Fault code 28 cleared with new Oxy Sensor
Throttle position sensor checked
MAF Sensor unpugged: Check engine light is on, engine runs rough and won't rev up much
Battery off for 30+ mins: Was disconnected at the dealers
Air Filter replaced
MAF Sensor (another one tried), this would be the third on the car
A temp sensor or faulty water pressure cap (I'll discount these as just about everything like this has been checked now)
Coil Packs
ECU

To Check:


Gearbox
 

fsuguy

New Member
Messages
3
Location
Tallahassee,FL US
Mudguts,
Have you tried changing you battery? My '92LS400 experienced a slew of odd problems and most went away as soon as I realized my battery was not up to snuff!
HTH
 

MUDGUTS

New Member
Messages
14
Location
UK
Thanks for the input fsuguy, I have'nt done this as the batt seems to be quite strong, however, first chance I get I'll have the condition checked.

I have been gradually replacing the trans fluid recently, draining a couple of litres off and then putting new fluid in, I managed to get nearly 10 litres of Type 4 very cheap but it's all gone now. Roughly 68-70% of the ATF is new now and that has made shifts much smoother.

The transmission shows no fault codes but if you floor the throttle it will change down but seems not to want to go past 4000, if you lift off it will change up and operate as normal. I am now convinced this is an issue with the transmission. If you drive around normally, its fine, even past 4000 provided you don't floor the throttle...
 

aekdbadgs4

New Member
Messages
1
Location
Corona, CA
Same Problem in a GS 400 - 1998 1uz

I have the same issue in my gs 400... have had it for the last 3 years... have tried quite a few things to fix but alas have not found a solution... Thanks for your post MUDGUTS...

I was curious about your final post confirming the gearbox is the issue... Did you finally find a way to trick the engine over the 4000rpm?

Also when I did a live monitor of my car driving I found that the ECU thought it was achieving redline rpm's of approx. 6400 when it was only at 4000... did your car do this aswell?

...monitored through the OBD port and a TIS cable plugged into my laptop usb with techstream software...
 

MUDGUTS

New Member
Messages
14
Location
UK
I have the same issue in my gs 400... have had it for the last 3 years... have tried quite a few things to fix but alas have not found a solution... Thanks for your post MUDGUTS...

I was curious about your final post confirming the gearbox is the issue... Did you finally find a way to trick the engine over the 4000rpm?

Also when I did a live monitor of my car driving I found that the ECU thought it was achieving redline rpm's of approx. 6400 when it was only at 4000... did your car do this aswell?

...monitored through the OBD port and a TIS cable plugged into my laptop usb with techstream software...

Well I can get it past 4000 RPM by using ECT Power mode and just gently accelerating but that is quite fast even with O/D off. If you punch the throttle pedal the RPM will almost freeze and it's like the fuel has been cut right down. The car won't slow down but reaches a limit.

I have made some enquiries to see if I can get this done with my car using techstream software or similar.

The reason I think it's the transmission is that there can be no fault found with the engine and even with another ECU the fault was the same.

I would be interested in any more info you might have and if there is a signal that cuts the RPM that can be checked, maybe there is a way of removing the rev. limiter...
 

gloverman

Well-Known Member
Messages
3,072
Location
Hamilton. New Zealand
Just another quick Idea. I have been thinking really hard over this one and have had one vehicle that had stock ecu and had a similar issue. We ended up fitting an aftermarket ECU to it but when we fired it up it was having trouble with the Cam/crank decoding. We removed the LH cam sensor ( the Aftermarket ECu only used the crank and onecam sensor) and found that the internal sheilding and insulation had deteriored and was touching. The cam sensor worked fine at low rpm but not up high. I thought it was worth a mention. Cheers
 

Nick M

New Member
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337
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Home
Well I can get it past 4000 RPM by using ECT Power mode and just gently accelerating but that is quite fast even with O/D off. If you punch the throttle pedal the RPM will almost freeze and it's like the fuel has been cut right down. The car won't slow down but reaches a limit.

This is what I was wondering. Does it go to 4000 and stay, or go up and down? The TPS being unplugged, or bad wiring will cause a fue cut condition. But after the RPM drops, the fuel will restart. This is standard Toyota practice.

If it sits at 4000, and just won't go higher, than you have a mechanical problem and not a control problem. For example, limited fuel flow due to a restriction or the throttle cable is binding from somethign. Having that shop say it isn't just shows they are not competent with Toyota strategy. Sorry to sound mean, but that is how it is.
 

Nick M

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337
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Home
How do you edit a post for spelling? I don't see anything next to quote, multiquote, and quick reply.
 

MUDGUTS

New Member
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14
Location
UK
This is what I was wondering. Does it go to 4000 and stay, or go up and down? The TPS being unplugged, or bad wiring will cause a fuel cut condition. But after the RPM drops, the fuel will restart. This is standard Toyota practice.

If it sits at 4000, and just won't go higher, than you have a mechanical problem and not a control problem. For example, limited fuel flow due to a restriction or the throttle cable is binding from something. Having that shop say it isn't just shows they are not competent with Toyota strategy. Sorry to sound mean, but that is how it is.

...The TPS has been swapped with a known servicable unit, with no change. The throttle operates normally and if you rev the engine in 'N' or 'P' it is fine and will rev to 6000 RPM, no problem but if you punch the throttle the RPM will drop to 4000 RPM if the car if stationary or on the road, so I would guess the fuel flow is different in each case

It's a mystery. What I would like to do is know why the ECU limits the RPM. aekdbadgs4 has a similar problem. My thoughts are that something tells the ECU to trim back the fuel
 

Nick M

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It the engine speed doesn't vary, it isn't the TPS. And by the TPS, I am talking about the circuit, not just the device. So I don't think you need to be looking at it.

...Well its with a 1UZ-FE specialist now, so far they have discovered a host of fault codes, most seem to have been related to stuff previous techs have been touching, these have all been cleared but one required the fitting of another O2 sensor. This hasn't made any difference. Should know more tomorrow.

So, what codes have come back?

Fault code 21 cleared with new Oxy Sensor
Fault code 28 cleared with new Oxy Sensor
MAF Sensor unpugged: Check engine light is on, engine runs rough and won't rev up much
MAF Sensor (another one tried), this would be the third on the car
A temp sensor

The oxygen sensors won't cause that problem. In fact, they almost never cause a noticable driving problem. What year is your car? Sorry if I missed it. If you have the Karman Vortex, that isn't a MAF. It is still volume. If you have resistance or another circuit problem, you can't swap meters and have it go away. You can have a problem causing your symptom, and not have a code because voltage isn't out of spec (pulled to ground, open). If there is a restriction, the meter will not generate a fast enough frequency to achieve high engine speed. And the eninge will lack power. You could still feather your way to 6000 rpm with the throttle. Because the airflow into the engine is still low. The KV signal (Hz) needs to be checked at the ECU. Based on having read your whole thread now.
 

MUDGUTS

New Member
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14
Location
UK
It the engine speed doesn't vary, it isn't the TPS. And by the TPS, I am talking about the circuit, not just the device. So I don't think you need to be looking at it.



So, what codes have come back?



The oxygen sensors won't cause that problem. In fact, they almost never cause a noticable driving problem. What year is your car? Sorry if I missed it. If you have the Karman Vortex, that isn't a MAF. It is still volume. If you have resistance or another circuit problem, you can't swap meters and have it go away. You can have a problem causing your symptom, and not have a code because voltage isn't out of spec (pulled to ground, open). If there is a restriction, the meter will not generate a fast enough frequency to achieve high engine speed. And the eninge will lack power. You could still feather your way to 6000 rpm with the throttle. Because the airflow into the engine is still low. The KV signal (Hz) needs to be checked at the ECU. Based on having read your whole thread now.

The codes that came back were related to the Tech unplugging various bits with the ignition on, these were cleared and months later the car shows no faults.

The car is 1997
 

Nick M

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So it really feels like volume of fuel is limited, and not spark timing or PCM limiting the speed. It is hard to say without putting my hands on it. If there isn't a control problem, then something is restricting engine airflow, and as such, the PCM just isn't delivering the volume of fuel needed for that higher engine speed. The other piece of evidence is you stated you could lightly take your way to 6000 rpm. A vacuum gauge would show you low vacuum, compared to WOT, and vacuum at zero.

If the hotwired mass air flow sensor was contaminated, it would kick out 170 codes for lean conditions. That is part of why I don't think it is a control problem. The PCM is in total control. It is just hard to say without your hands on it.
 

sideshow

Active Member
Messages
2,404
Location
Qld
early 1uzs are dumb they only have afew dtcs
the air flow meter will only have one code for it and one code for air temp
if the car doesnt have obd2 then u wont get any lean this or rich that codes

if the ecu hits 4000 rpm on the dot then shuts back down its the ecu getting some funny signal from somewhere
just goto local car sales yard who might have an sc400 for sale and road test it for an hour
go home and swap afew things over to test heheheheheh
 

sideshow

Active Member
Messages
2,404
Location
Qld
like gloverman said might also be cam sensors or cam signals

try unplugging left cam sensopr and see if problem stays same then replug
then unplg right cam sensor and see what it does

if problem get worse on one sensor when u unplug it then u know that circuit i working
 

jasonagustin

New Member
Messages
1
Location
san diego ca
just a thought

Maybe bad engine, chassis, or ecu ground? When under high load it loses ground causing power loss. Just a thought, can't hurt to add extra grounding points.
 
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