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1994 Toyota Previa Question: Rough idle, bad hesitation on acceleration, perfect idle with TPS disconnected

 

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David in Sydney, Aberdeen, MD, January 18, 2011, 18:28
 Rookie

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!

6 Answers
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  • Answer #1

    January 18, 2011, 21:12
    Profile_thumbnail
     Master

    Without getting too complex, it appears you're on to something with the Throttle Position Sensor. Indeed it could be a wiring issue, but the sensor itself could be faulty. Testing the wiring would be the right next step.

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  • Answer #2

    patrick mannion from Greg Solow's Engine Room, January 18, 2011, 23:01
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     Master

    I have replaced a few TPS on Previas. David in Australia a company called Alldata may have useful information an tests for you. For free Toyota theory and training material look at
    http://www.autoshop101.com/
    look at "technical article" on left of page.

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    David in Sydney, February 05, 2011, 02:52
     Rookie

    Thanks you very much Patrick - the technical articles were very useful and allowed my to investigate and test many things. I have done a lot more testing and investigation and I cannot, unfortunately, 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 airconditioning engaged, and will frequently stall the engine. Listening at the exhaust pipe, you can hear a 'phut, phut, phut' noise when the ening 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 airconditioning 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 anout 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 kow what other compensating action is taken by the ECU when it detects a fault with the TPS after it is disconneted.

    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 postion sensor.

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

    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.

    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 plently of power) right up through the rev range, even under heavey 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 patially failing. I can not test the igniter, and I have not tried a replacment. While it obvioulsy 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 opertion 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 trhottle closed could build up the fuel mixture in the manifled, 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 oether conditions. If it was stayig partially or fully open at idle, the mixture would be wroing (too lean?) causing the bad idle.

    Does anybody have any suggestions about these ideas or anthing else to try?

    UPDATE:

    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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  • Answer #3

    November 10, 2011, 12:03
     Rookie

    My 93 Previa is doing almost the same thing. Runs great for first mile or so and then runs terrible. Did you ever find out what was the problem? Tks

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    David in Sydney, December 12, 2011, 16:10
     Rookie

    Sorry for the late reply. No still have not been able to solve the problem, in fact it has got worse. Not using the car now, and only get to try new ideas to fix it every few weeks. May soon have to give up and try and sell the car in non-working order.

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    FREEHEELBOY, February 23, 2012, 16:51
     Rookie

    Hello david,

    Any luck on solving this problem? I have a 1995 supercharged Previa and the exact same symptoms. Any help would be appreciated. My check engine light came on with PO-420 code. Catalytic converter but I don't think it is the problem?

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    wcbrase, March 05, 2012, 23:04
     Rookie

    I am experiencing the same problems with my 92 Previa and have tried most of the same remedies, without success. Yesterday I tried cleaning the idle control valve without removing it from the throttle body by spraying throttle body cleaner into the inlet from the bottom and into the outlet by opening the throttle plate and spraying in from the top. This seemed to improve performance 90%, but not quite 100%. Next step will be to remove the idle control valve (after I get a new gasket from the dealer) and clean it on the bench and test it with a battery, off-line. If this works I will report back.

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    FREEHEELBOY, March 06, 2012, 09:54
     Rookie

    Thanks WC,

    looking forward to any news. I'll post what I find as well.

    Cheers

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    wcbrase, March 09, 2012, 07:20
     Rookie

    I now know a lot about cleaning the idle control valve but still don't have a clue what's wrong with my Previa. I can see that it's necessary to actually remove the valve to clean it well. The Phillips screws are torqued tight and you don't want to chew them up, so clean the screws with carb cleaner, put a dab of valve-grinding compound on the tip of your Phillips screwdriver, use a long Phillips of the right size, and push very hard while turning with the other hand, after giving each screw a sharp hammer blow before starting. Once this is done it's a piece of cake getting the valve out. You will need to plug or clamp the supply coolant hose when you remove it from the valve or it will drain your entire cooling system. When the valve is removed, you can easily remove the motor to see how easily the valve itself rotates. I lightly greased the armature (a permanent magnet that slides into the motor) with synthetic grease, and the whole device can be tested using a 9v. battery which should rotate the valve one direction or the other depending on the polarity of the battery connection to the outer two of the three terminals. One could actually remove just the motor without removing the entire valve assembly and do this same test to get an idea whether the valve moves, and how easily it moves. Having done all this, the car ran great for a few miles then the check engine light came on accompanied by a buzzer under the dash that I have never heard before! After that the car limped home hesitating and coughing at all speeds but especially when accelerating, idling fine but stalling as soon as even the slightest acceleration from a stop. I think yesterday's sudden deterioration of this problem was coincidental with cleaning the idle control valve, since my problem has been intermittent but gradually getting worse, regardless of what I have tried (the list is too long to mention!) One odd thing about my problem is that the check engine light has come on and the power has dropped and sometimes sputtered when I have turned left!! Yesterday the light and buzzer coincided with a lane change in the opposite direction, so I thought about anything (other than a wiring harness fault, which I understand is nearly impossible to track down) that might be a function of side forces. Another mystery is that, although one can purchase a fuel pump inertial cutoff valve for a 92 Previa, I can't find anything that says a Previa even HAS such an inertial valve. This has been such a good car I hate to give up, and it drives me nuts to think that it's probably one, simple thing going bad at the root of my problem. Running out of ideas and options...

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  • Medium-star Best Answer
    January 01, 2012, 21:53
     Rookie

    Is your Previa supercharged? Mine is and it is having the exact same problem that you are experiencing. Have you tried changing or adjusting the TPS with an ohm meter?

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    David in Sydney, April 08, 2012, 18:36
     Rookie

    SOLVED.

    It urned out that there was more than one problem. I had the intake manifold gasket replaced as it was leaking a small amount. However, there was still a problem that seemed to get worse. I discovered by accident that the car was easier to start if I disconnected the number one spark plug. With the spark plug disconnected the engine rolled or missed as you would expect, but it didn't do the sort of backfire while the engine was turning over under starter motor that would bring the engine to an immediate stop and prevent it from starting. After checking and replacing many things, I replaced the distributor cap, although it was very new, and that fixed the problem immediately. The cap showed no cracks, no visible internal burn tracks, no shorts between contacts, no open circuits, or any other apparent problems. I had tried to see if there were spark leakages around the exterior of the cap and spark plug leads, but nothing. I am pretty sure that the problem was that there was cross firing between number one and some other cylinder occurring within the plastic channels in the cap. The sockets in the cap for the two pairs of cylinders (1 & 3, 2 & 4) are very close together and I think that there was cross sparking between 1 and 3 within the plastic cap.

    Having fixed both items, the car is now starting first time and runs as smoothly as silk even though it has about 355,000 kms on the clock.

    Thanks for everyones' suggestions and help!

    ...David

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    gymover, June 03, 2012, 13:04
     Rookie

    I tried that on my 91, it would miss after warming up, as if the distrubitor was too hot, I pulled the vent hoses off and blew air thru them, and sprayed all the inside electronics with electronics cleaner, and dried with compressed air, finally it it ran flawlessly all the way to Aspen and back, about 100 miles. There is a service bulletin from toyota about the module in the distributor not handling the high heat from the exhaust and causing the cut-out problem, and I think they upgraded the electrics in the distributor in 1993. Anyway there are aftermarket dietributors from around $160 that are temp-rated for 302 degrees instead of the factory 254Deg. I would try cleaning the bottom of the heat shield so it is shiny again, and blow compressed air through the two vent hoses to clear them, they are there to keep the temp down inside the unit. Worked for me so far!

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  • Answer #5

    May 19, 2013, 15:22
     Rookie

    I have a 99 nissan sentra (2.0 engine). I spent many hours on internet looking for answers to "rough idle, acceleration hesitation" problem. I found many suggestions but didn't find many that said their suggestions worked. Two mechanics couldn't find the answer. I wasn't willing to pay thousands for their guesses.
    I finally arrived at this site and near the bottom I saw a post from David in Sidney in Australia. His problem was similar and after a lot of work he replaced the distributor cap.

    Thank you David in Sidney. Thank you a lot more than I can really express.

    The problem had gotten worse over the past several months and I was on the verge of parking the car because it was becoming unsafe to drive in traffic. Another reason I was interested in Sidney's answer is he had 350,000 km on his car. My Nissan has over 215,000 miles and like Sidney I hate to get rid of it because it now runs good.

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    Kidcivic, June 18, 2013, 12:21
     Rookie

    Thanks David in Sidney!!! You solved the problem I was having with my Honda Civic. I would of never guessed the cap was my problem. It acted like a clogged converter!

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  • Answer #6

    August 08, 2014, 23:24
     Rookie

    I have a 1993 Toyota Tarago. I disconnected battery & edu chip in my dash for 10 min to reset edu & the car ran beautifully with no idling for 5 days. After this it lost all power, and had every little symptom you are describing. Got it to the mechanic & he said after a whole day testing it was the air flow meter. Am grabbing car next week after part comes in & let you know if it resolved issue & how much this problem usually costs.

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