Toyota Repair: 95 Toyota Celica bucking on throttle change, crankshaft position sensor, infinite resistance

Question
Hi Tim,

I bought a set of feeler gauges and adjusted the TPS to within the spec you mentioned.  Unfortunately though after having driven the car for a couple of days, I think the change has had little overall effect.  I did try the trick of shorting the idle contact and in my car the fuel cutout was kicking in at 2200 rpm. (That's a neat trick :) )  If nothing else, this change might save me a bit of fuel if it wasn't cutting out properly before.

Something I find a bit strange is that the TPS being connected or not doesn't seem to make any difference to the drive, but also with the TPS disconnected the engine fault light doesn't stay on.  I'd have thought it would.  At some stage in the past I also disconnected my O2 sensor to see what would happen, and there was also no significant change to the feel of driving and the light didn't come on then either.

Back to the bucking problem, can you think of anything else in the way of sensors or mechanical components I should check? A friend of mine suggested I should test my crankshaft position sensor, but I couldn't find any info on where it is or how to test it.  Do you think this is worth checking?

Thanks again,

James.


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Hi Tim,

Well, your suggestion proved to be very interesting.  I checked the idle contacts in the TPS and sure enough, they were not closing with a full release of the throttle (they were infinite resistance at any position).  So I adjusted the mount position slightly and fixed that so they open at around 0.5mm or so. I just estimated this by eye.  I drove a few times around the block and this change has improved things but not completely removed the problem.  I think I should probably drive the car for at least a week though before I make any firm statement as to what effect this fix has had - but it was definitely a step in the right direction.

By the way I tried driving the car with the TPS completely disconnected and the car felt the same to drive as it did a week ago which I really did not expect - I assumed the ecu needed this info, but I guess it relies mostly on the MAP sensor.

Just to answer your question about the precise nature of this problem...

Basically, any time I change the position of the throttle either by pushing down or releasing, by either a little bit or a lot (nudging the pedal or flooring it) this problem occurs.  It does it in all gears, but is most noticeable in lower gears such as second.  Regarding the example of approaching a red light, imagine you are in second gear at 1800 rpm, decelerating by being in gear and not touching any pedal.  Then as you get to around 1600 rpm say, you realise you need just a little more throttle to actually get to the lights... still red so you don't want to accelerate hard.  When I put my foot very lightly on the throttle to accelerate a small amount, the car jerks quite a lot.  This was just one example though and in fact it does the same thing if I'm driving in 5th gear at 80km/h and I suddenly take my foot completely off the throttle to coast for a bit... the car will oscillate a little (less than in the previous example because I'm in a higher gear).  In all cases, the oscillation does decrease steadily, so the jerking lasts for 2 seconds or so and then the car settles and behaves properly according to the new throttle position.

In terms of pressing the pedal harder to see if it goes away... it basically does the same thing all over again.  If I press lightly and the oscillation begins, then I press harder, the oscillation kind of starts again.  The only way to avoid it is to change throtle positions very very lightly, almost so lightly that it is impossible.

Do you think there could be some other issue that the TPS problem was exaggerating?

Thanks again for your help. You have been much more helpful than at least two local mechanics and an auto electrician :)

James.

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Thanks for the speedy reply.

In this particular model of celica it uses a MAP sensor for engine load sensing and I don't think it has an air flow meter.  There is an air filter box, then the tube goes from there to the top of the engine.  The map sensor is connected directly to the top of the engine down stream of the rubber air tube.  I checked the tube all the same and it does have a small crack in it.  I wrapped the cracked area with high quality duct tape and it made no difference (i'd have expected some difference?... although I know this isn't a good solution)  Also, I'm thinking that because the map sensor is downstream, this small crack wouldn't cause my problem.

What do you think?

Cheers again,
James.
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Hi Tim,

I have a 95 toyota celica (manual, 5sfe engine, 160,000K on the clock) The car basically works very well, but for one irritating problem.  When I change the position of the throttle while driving, the engine rpm seems to oscilate a bit before settling down again.  It doesn't matter if I put my foot down or take it off - or whether I change position by a lot or a little, it almost always does it.  An example is coasting up to a red light, I put my foot on the gas very lightly to accelerate a very small amount, and the whole car jerks faster/slower until it settles.

I have had a lot of work done on the car so I'll mention that to eliminate some possibilities. I have replaced the clutch and thrust bearing, I have 4 new shock absorbers.  I have had an 'in car injector flush' done (i've also tried a bottle of injector cleaner in the fuel) I have a new near-side drive shaft and the road side shaft looks in good condition.  I've had the car tested at an auto electrician who used a scanner and found nothing.

It's driving me nuts because it feels like it should be a minor problem, but it makes the car somewhat unpleasant to drive.

Any help is much appreciated.
Cheers,
James.
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James,

Remove the air intake tube between the throttle body and air flow meter. Inspect it carefully for cracks when you flex it.  These car are famous for it. When the crack opens up it allows un metered air into the engine and cause it too starve for fuel. Any time the engine torgues up it will pull it open.
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James,

Toyota used many different engine combinations in the Celica over the years so it is hard to remember which ones do & don't use air flow meters. If yours uses a map sensor then you are correct in that a cracked air tube should not affect it.  

This type of problem can be difficult to diagnose without driving the car myself, but I am not giving up just yet..:)

You mentioned that coming up to a light (assume the light had just gone green, but you didn't need to stop) and you press the accelerator to part throttle, then it starts to buck. Did you downshift to a lower gear or staying in high gear?  If you press the pedal farther does it go away?

Another thing I would check is your TPS sensor. It has a set of idle contacts that should be open at part throttle. If they don't open it goes into "fuel cut" around 1,200rpm.

Let me know, I am curious to see what you find.
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James,

I am glad we are possibly getting somewhere. I was looking through my old Toyota school book and found some good information for you. Lets check your TPS adjustment using a feeler gauge. First, make sure that if it has a vacuum throttle opener that you retract it and allow the linkage to contact the throttle stop screw.  Now hook your meter up between the IDL contact and ground or common terminal. Toyota says that a 5S-FE in a Celica should have continuity between IDL & ground with a .020inch  feeler gauge inserted between the stop screw and linkage. Now with a .034inch feeler gauge it should become an open circuit.

Now lets talk about what role this TPS plays. When the IDL contacts are closed and engine RPM is above a set limit (usually 1,200RPM) The computer shuts off the fuel injectors known as "fuel cut". So when you are coasting to the light in gear and the engine is above 1,200 RPM your injectors are off. Only the transmission is turning the engine over. Now, as you press on the gas pedal and the IDL contact opens the injectors turn back on and the engine accelerates. You can try this to find out at what RPM your Celica actually goes into fuel cut. Unplug the TPS sensor and jump IDL to ground in the harness. Now start the engine and rev it up until it cuts out and starts to surge. That is your "fuel cut" RPM.  I have seen several cars with a vacuum leak constantly surge up and down at idle because of the fuel cut kicking in.

Pretty Cool Huh? So when you decel down a hill your not wasting any gas at all.

So anyway, adjust the TPS best you can and let me know how she drives.

Good Luck.  

Answer
James,

The crankshaft position sensor is inside the distributor. I doubt that is your problem. I would try hooking a vacuum gauge up to the intake next and see if it flutters any when the car bucks. This would indicate a burnt valve allowing combustion to leak into the intake manifold.