Rough idle, bad hesitation on acceleration, perfect idle with TPS disconnected - Toyota Nation Forum : Toyota Car and Truck Forums


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Old 01-18-2011, 05:37 AM   #1 (permalink)
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Rough idle, bad hesitation on acceleration, perfect idle with TPS disconnected

This problem started suddenly. Very rough idle with a repetitive cycle that is smooth for a couple of seconds then misses badly, occasionally stalling. Worse problem is bad hesitation when accelerating after throttle at idle position, sometimes stalling. Worst after stationary idle, but also after moving at low to medium speed with closed throttle and then accelerating. Also very hard to start because of periodic 'backfire' while cranking that brings engine to standstill.

Idle is perfectly smooth for first 1 - 2 minutes after overnight cold start, before missing at idle starts. Missing and hesitation worse the hotter it gets. Turning on air condtioning makes ilde worse, with more frequent stalls. However, during normal driving over about 2000rpm and nomral traffic speeds is perfectly smooth - no missing, no roughness no lack of power, no excess fuel consumtption.

Spent approx $2500.00 on two trips to mechanic, without improvement. Replaced complete distributor, cap, leads, plugs, coil, PCV valve. Tried alternative (used) ECU. No faults being reported by either ECU. Removed throttle body and cleaned thoroughly. None of the above made any difference.

Checked for vacuum leaks, timing, temp sensor, air flow meter, throttle position sensor. All within spec. Idle speed control removed, cleaned and checked. Mechanic performed wet and dry compression test, and while not great (car has done 350,000 km) compression was pretty good and even for a car this age.

Discovered by accident that disconnecting the TPS causes the idle to be perfectly smooth. So smooth you could balance a coin on the engine even at low idle revs of around 650 rpm. In fact drivig with the TPS disconnected is quite acceptable, except that automatic gear change down does not happen (as expected).

Have had the car for approx 13 years and has been a great, reliable family car. Reluctant to dump the car for soemthing that should be fixable. Please help!
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Old 01-18-2011, 07:34 AM   #2 (permalink)
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What is a lead?
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Old 01-18-2011, 07:52 PM   #3 (permalink)
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By 'leads' I mean spark plug leads - distributor to spark plug and coil to distributor.

Also the mechanic used a gas analyser to test the exhaust, and while not perfect (this is an old car) readings were not too bad. In addition the cold start injector was tested electrically (ie resistance) and was in spec.

Things not specifically tested were cold start injector (actual fuel delivery), EGR, EGR vacuum modulator or VSV. However, given that the EGR components are not controlled by the ECU, the huge change to the idle resulting from disconnecting the TPS would be hard to explain.
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Old 01-18-2011, 10:07 PM   #4 (permalink)
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Sort of sounds indicative of incorrect firing order. Or incorrect cam timing. As well, smells of a loss of continuity in the wires...plug wires as it were. Of course you say the wires are new and that one day this problem just occured and never recinded. That's the problem with paying a mechanic to do your work. Many times they know no more than you. And then you pay them. The common consensus is to start with ignition, then spark, then timing, then compression, then fuel.

My second thought is water in the combustion chamber. Smell the exhaust. Does it wreak of anti-freeze? Do you see moisture at the tailpipe? Does it produce steam from the exhaust in large quantities and long periods of time upon first start-up?

Explain symptoms exactly and in detail before and after each service. Did anything change? Get worse? Get better? Details!
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Old 02-01-2011, 12:47 AM   #5 (permalink)
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Check the tps connector wiring between the computer and the tps sensor. Break in this connector (corroded wire inside the insulation) wires insulation might look okay . But don't be fooled. Hope this helps.
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Old 02-01-2011, 03:54 PM   #6 (permalink)
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Your problem could be a dirty or damaged EGR valve. Previas tend to respond like that if the EGR valve is really dirty or it's damaged. Another thing to consider is the fusebox next to your battery. This is another common place for the Previa. The box is the one on your that is screwed down next to your battery. Over time, it can gather contaminants inside and/ or rust can develop in the bottom of it..

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Old 02-05-2011, 05:14 AM   #7 (permalink)
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I have done a lot more testing and investigation and I cannot find any identifiable problems.

Symptoms are as follows:

1) When starting from dead cold (overnight), if the engine doesn't start on the first crank, then it is very hard to start. While cranking, there are frequent 'backfires' that cause the engine to stop dead. It is as though the ignition fires at the wrong time in the cycle and causes the spinning engine to freeze at that point.

2) Once started from dead cold (which often happens on the first crank, despite the above), the engine idles very smoothly for the first 60 seconds or so. After that, the engine starts to miss while idling, which gets worse as the engine warms up.

3) Once at normal operating temperature, the engine misses badly while idling. It idles smoothly for a second or two, then misses, which causes the revs to drop suddenly, and then the control system seems to increase the idle speed to counteract the drop in revs for a second, and then the cyle repeats. Turning on the air conditioning makes this cycle much worse. That is, the miss is worse with the air conditioning engaged, and will frequently stall the engine. Listening at the exhaust pipe, you can hear a 'phut, phut, phut' noise when the engine is missing.

4) After idling at intersections and the like, there is a very bad hesitation or stumble when accelerating away. The engine will nearly stall for 2 to 4 seconds, and then take off. Occasionally it does stall. This bad hesitation also happens when driving along at normal road speeds if the foot is taken off the accelerator for a few moments (throttle fully closed) and then accelerate again.

5) While driving at normal road speeds (after the hesitation) or accelerating and the throttle is not fully closed, there is no miss whatsoever, and the engine performs very smoothly. While ever the throttle is partially or full open, the engine performs very smoothly and with normal power, apart from the initial hesitation.

6) While stationary with the air conditioning off, if the accelerator is depressed a tiny amount to increase the revs to about 1000 rpm (ie 200-300 rpm above normal idle) the engine will run very smoothly with no miss. This may be because the throttle position sensor is signalling just off idle.

7) If the throttle position sensor is disconnected the engine will idle very smoothly with no miss. It will also run at slightly higher revs, but if the idel speed is adjusted down to about 700rpm again it still runs perfectly smoothly with no miss. I discovered that one of the effects of disconnecting the TPS is that the spark advance is increased significantly by the ECU. It looks as though it goes from the normal 12 degrees up to about 20 to 25 degrees BTDC. I don't know what other compensating action is taken by the ECU when it detects a fault with the TPS after it is disconnected.

Checks done:

Replaced plugs, spark plug leads, distributor cap and rotor, fitted brand new distributor. Checked for any vacuum leaks. Removed throttle body and thoroughly cleaned and checked it. Checked many other sensors for correct operation and voltages (where possible) at the ECU. This include the water temperature sensor, air flow sensor, air temperature sensor, throttle position sensor.

Checked that the spark plugs are connected to the distributor in the right order, checked that the TDC mark on the crank is actually top dead centre (measured piston position through the spark plug hole), and TDC is aligned in the distributor with spark plug for cylinder number 1.

Checked that the TPS voltages are correct at the ECU, just in case any wires broken between the ECU and the TPS - everything as expected. Thanks for that suggestion.

Checked that the exhaust gas recirculation valve is operating by disconnecting the normal vacuum hose and applying vacuum while idling. This caused the engine to idle worse, although only a bit worse than it already was. The change was noticeable, however. Again, thanks for that suggestion.

Fitted a different ECU, with no change to symptoms at all. Fuel pressure was checked by the mechanic.

I was worried that the problem may be caused by low or uneven cylinder compression. However because the engine will idle perfectly smoothly in some cases (e.g. TPS disconnected) this would not be the case if the problem was low compression. Similarly if the problem was broken leads or damaged spark plugs, again doing simple things like very slightly opening the throttle would not cause the symptoms to disappear. Also, because the engine runs so well (smooth and with plenty of power) right up through the rev range, even under heavy load, it would seem unlikely that the problem is simple spark plug miss-firing.

Some ideas about what could possibly display such symptoms:

1) Igniter partially failing. I can not test the igniter, and I have not tried a replacement. While it obviously still works to some degree, I wondered whether it is sometimes firing the coil at the wrong time. Because it is partially controlled by the ECU, sensing changes such as the TPS off idle could potentially affect the operation of the igniter. Firing the coil at the wrong time would explain the bad backfire while cranking.

2) Cold start injector. If the cold start injector continued to operate instead of shutting off, then too much fuel would be injected while idling (and throughout the rev range), but the effects would probably be worse when there is very little air flow such as when at idle. This would also explain why while driving at normal speed, a period of time coasting with the throttle closed could build up the fuel mixture in the manifold, which causes the hesitation when the throttle is opened again. It could also explain why if the engine does not start first time, it is very hard to start afterwards.

3) EGR staying partially open. This is supposed to shut off at idle, and in some other conditions. If it was staying partially or fully open at idle, the mixture would be wrong (too lean?) causing the bad idle. Even though my test of the EGR makes the engine idle worse, it wasn't that much worse so it could be partially open all the time.


Does anybody have any suggestions about these ideas or anything else to try?
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Old 02-05-2011, 05:49 AM   #8 (permalink)
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checking the tps wiring between the cpu


if you use a volt meter, this can fool you thinking that the wiring is okay

because even with the corroded connections, the volt meter will send a positive 12 volt signal. but very low amperage.

best way to check this is to disconnect the connector @ the ECU, and the TPS and power the wire (connect it to a positive) and use a test light on the TPS end to see if the test light LIGHTS UP. if it lights up with every wire (3) then the connection is solid.

try that.

because a volt meter fooled me once - never again -
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Old 02-06-2011, 04:48 AM   #9 (permalink)
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Today I removed the EGR, and checked that it was operating correctly. In particular I checked that it was normally fully closed and formed a good seal. All checked out perfectly. I gave it a thorough clean with carby cleaner anyway. So that rules out the EGR valve.

While there I also removed the cold start injector and checked that it was not spraying fuel all the time. It also was operating correctly, ruling it out as the problem.

Double checked the TPS wires and signals and it is operating perfectly as well.

So now I am back to square one, and have no idea what could be causing the problem. I went for a drive after reassembling everything, and the engine stalled at least 6 times - sometimes just because the idle was so bad, sometimes because the stumble on accelerating away from a standing start was so bad.

Pleas help if you have any ideas!
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Old 02-06-2011, 05:26 PM   #10 (permalink)
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have you checked for engine codes? -
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Old 02-07-2011, 03:09 AM   #11 (permalink)
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Yes, I have checked and the ECU reports no errors at all (unless I disconnect the TPS, of course, but I have cleared that out).
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Old 02-07-2011, 04:59 AM   #12 (permalink)
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when does it stall? when the engine is still cold. or when its warmed up?
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Old 02-07-2011, 08:03 AM   #13 (permalink)
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Have you checked for any cracks or breaks in the intake piping or around the air box?
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Old 02-07-2011, 09:57 PM   #14 (permalink)
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It stalls only when warmed up. The hotter it gets the worse the idle and hesitation when accelerating, up to the point of stalling sometimes.

I have checked for leaks all the way from the air filter to the throttle body and other entrances into the intake manifold such as around the EGR valve, cold start injector and so on. Also checked all the vacuum hoses and can't find any leaks. Also checked and replaced the PCV valve, but no improvement.


Can you think of anything else I can check?
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Old 02-07-2011, 11:29 PM   #15 (permalink)
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try checking the ENGINE COOLANT TEMP. SENSOR : if you got the proper wiring diagram you can just check it on the ECU connector - do you?

when it starts to fuck up when its warmed up, i would try and replace the front O2 SENSOR -

thats all i could think of for now. from what you've said so far.

an O2 SENSOR thats fucking up won't even signal a MIL sometime. its one of those lazy sensors - it works , then it doesn't. and when it doesn't, it doesn't LOL. but it doesn't trigger the ECU to trigger the MIL and or to store a code.

...
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