Intermittent Issue with Throttle Position Sensor…

Technical Area Engine, Transmission, Exhaust etc
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  1. My model of MX-5 is: __ Mk2 1998 1.8 RS JDM
  2. I’m based near: __ Amersham, Bucks
  3. I’m looking for technical help or recommendations on: __ Intermittent issue with Throttle Position Sensor - to advise garage…

The Throttle Position Sensor was replaced by a local general garage (not an MX5 Specialist) at the beginning of July. This was because the engine had suddenly cut out after switching on lights whilst travelling up a hill at 50mph. After switching the engine back on, my vehicle then ‘kangarooed’ for a further couple of miles and then all of a sudden, seemed to behave normally - without any reason or change of driving habit.

Since then it’s had intermittent issues with the Throttle Position Sensor and been back to the garage numerous times - the engine stalling - mainly at low speeds ie. 1st and 2nd gear.

After the TPS was initially replaced, the engine started fine from a cold start and idled nicely at 1500revs, but as soon as the accelerator was gently touched (not even fully depressed), it stalled again. I was eventually able to move off at slow speed, but had to keep the revs high so as not to stall, but then approaching a junction or traffic lights changing down to 2nd gear it would stall again, so had a constant battle juggling the accelerator, clutch and the handbrake. Once the engine had fully warmed up and travelling at higher speeds (50 - 60mph) the engine appeared to behave ‘normally’ (under power) but then surprised me - even after 6 or 7 miles - when slowing down for a roundabout and tried to stall again when changing down. I noticed a bit later, stationary in traffic, the idle speed was just below 1000 revs at 800 revs.

On Saturday, it stalled as I was actually driving around a roundabout - having set off approx. 10 minutes beforehand. Luckily it was late / early morning and no other traffic around.

Then yesterday - setting off for the garage yet again - after switching on the engine it idled for a bit at 1500 revs, then suddenly jumped to 2000 revs, without any input from me. Eventually moving away, and after carefully negotiating a roundabout on top of a steep hill, my vehicle felt very sluggish and reluctant getting to 50 - 60 mph.

The garage have been kind and patient but are totally puzzled as they really aren’t sure what to do now.

Whatever the garage has done to my vehicle hasn’t resolved the problem, and although it’s better than it was initially, the engine is still stalling intermittently - slowing down to approach a junction or a roundabout, the engine is still cutting out - or I’m having to fight with the pedals and handbrake to stop it from stalling. I’m also additionally concerned that as the damp and cold winter weather closes in it’s going to get even worse.

I was wondering if anyone else has had a similar problem and how it was resolved, so I can pass the information on to the garage…

Many thanks…

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Check the connectors are clean and not tarnished. That signal is low level and can be easily blocked by a poor connection.

There are several magic solutions claimed to be Contact Cleaners, but the one that actually works in this type of application and has a well proven track record in restoring functionality is Servisol Super 10.

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Thank you so much for your response and the link, I will certainly pass on your information to the garage.
BTW… I was editing my post whilst you sent through yours, so would be extremely grateful if you could check it and add any other information you may feel would be appropriate…
Many thanks…

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BTW… Having read the reviews on the link for Servisol Super 10, I will also recommend this to friends who regularly use electric guitars and amps! So thanks for that too…! :grin: :+1:

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I’ve been using it with them, mixers, radios, TVs , cars and other electronics since the 1960s!
:grinning:

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Back to the original problem; that sounds more like a vacuum leak or wrongly set idle control or a broken wire in the loom making intermittent contact or all of these fighting each other.

Or something else!

This sort of problem is very difficult to diagnose at a distance, but an expert with these engines will quickly track it down with the right tools.

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:grin: :+1:

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Thanks again very much for the info…
Will pass this on to the garage… :grin: :+1:
Hopefully 6th time lucky… !

Or else, may be someone could recommend an MX5 Specialist around the Amersham / Beaconsfield area in Bucks?
Thanks again…
Very much appreciated… :grin: :+1:

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I have to agree with Richard; reading your description about the problem made me think it sounds like a vacuum or air related problem if it’s something more subtle and certainly I would replace the idle control valve if the TPS didn’t fix the problem. As far as I know a faulty TPS wouldn’t cause a stall, unless you were going 5mph in 5th gear, as when your car reads the TPS as “pedal not pressed” it will pass control over to a separate idle control system, assuming the engine is not under load, by under load i mean when the car is in gear, not pressing clutch pedal and rolling. Your car shouldn’t stall/cut out unless you aren’t getting air, fuel and spark. I would assume cut out at 50mph uphill (so you would be throttling) would be an electrical fault on the ignition system or possibly the data system/ecu. In fact, I the ECUs in these cars do fail sometimes because they are in a position which allows water to get at them if there are any leaks in the body or heater matrix, or sometimes just condensation and can cause intermittent problems before failing completely!

How do you know it’s had intermittent problems with the TPS? How did anybody know that the TPS was the problem?

Are/were there any codes showing (engine light on)? Are any of these for misfire? The stall generally only happens when you are braking with the clutch pedal depressed? What is the mileage of the car and have the spark plugs ever been changed? Has the ignition system been checked including HT leads?

When you stalled on the roundabout, what pedals were you pressing, if any?

I’m sure you are aware that you absolutely SHOULD NOT be driving a car that is prone to cutting out, especially at junctions, corners and roundabouts, because your steering and braking system will behave unpredictably and it’s causing additional distraction for you as the driver, making you a danger to other road users (yes, you were lucky there was nobody on that roundabout with you). There is also the chance that you could stall and not be able to start again!

Possible things to mention to garage: Idle control valve, MAF sensor, MAP sensor, vacuum lines, ignition system. Possibly a failing fuel pump. If none of these are conclusive then possibly the sensors on the exhaust, especially the oxygen sensor (if it has one). Possibly ECU.

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Hey Ron_Damage…
Thank you so much for your comprehensive response…
It’s really VERY much appreciated… :+1:
I’m so grateful to both you and RichardFX for taking an interest and the time to respond…
I will certainly get the garage to look at a possible vacuum or air-related problem, as it is something you have both suggested, and then also take a look at the idle control valve too…
And, only because you’ve asked, will try and answer some of your questions to satisfy your curiosity…

  1. Following return from the garage the first time at the beginning of July…
    Car stalled in 1st gear when setting off (first time was during a 3-point turn on a quiet, residential road) and in 2nd gear when changing down, approaching a junction, traffic lights, roundabout etc…

  2. The ECU and alternator, and alternator output, were checked after the initial ‘stall at 50mph, going up a hill, when the lights were switched on’, in early July. No sign of the alternator seizing or evidence of any corrosion, and ECU was receiving 12 volts. The Earth tag next to the diff that earth’s onto the PPF was also checked too. But now aware (thanks to your comment) that condensation could be an issue too. I guess it would evaporate once the engine has heated up leaving no obvious evidence…

  3. It was the garage who suspected it had a TPS problem, so I was guided by them… Although not a Specialist MX5 garage, they have been extremely helpful and, unlike a previous experience elsewhere, were trying to solve the problem, rather than telling me ‘oh no, that’s too difficult to fix’ because it didn’t fit within their usual scope of ‘more obvious’, quick fix works.

  4. No, there were no code lights showing on the dashboard. I believe my car is a bit too old to have a very elaborate display for that (!) however I can honestly say that the temperature gauge and oil pressure gauge remained exactly the same throughout the experience.

  5. The stall happened when changing down from 3rd to 2nd gear, so yes, after slowing down to the right speed with the foot-brake, the clutch was depressed to change gear but then when I pressed the accelerator to continue the momentum, that’s when the stall happened. The car coasted to an almost stop, but I turned the ignition back quickly, and kept moving. I was also very aware throughout this incident there were no other cars around at all.
    To be fair it was a very wet night, and although the rain had stopped, there was a lot of surface water so I was being extra cautious and aware that the dampness could affect the performance of the engine. Have had drive belts squealing in the dampness before now, but they have both been very recently replaced too.

  6. The ‘mileage’ on the car is actually 111,750km.
    As a JDM spec, the speedo was changed to MPH, but the odometer was kept in Kilometres, because the garage we purchased it from didn’t want to be accused of ‘clocking’. Was purchased in June 1999 and 6 months old at the time. Came in via Dublin (like many JDMs did at that time to keep ‘the quota’ of imported cars down), and so arrived with Irish numbers plates, which we changed to UK plates.
    Been an absolute joy to own all these years, and determined to keep her going for as long as possible - and dreading the day when she will be ‘beyond repair’… :scream:

  7. The spark plugs were changed about 4 years ago - by a wonderful Specialist MX5 garage, who are sadly no longer in business.
    The ignition system was thoroughly checked when the HT leads were replaced on 16/07 this year after a slight engine misfire was detected.

  8. I totally get your point about the safety aspect - both to myself and other road-users - however, every time I have left the garage (it was 5 times at this stage) I have believed the ‘latest problem’ was fixed. When the ‘roundabout incident’ happened I was actually on my way home after an evening out with friends - alcohol-free, I will add - and happened approximately 10 minutes after I had set off on a very damp / wet evening. It was a 40 mile journey home and knew there were no issues travelling at higher speeds once the engine had warmed up. I am also a member of the AA and wouldn’t hesitate to call them if I got into real difficulties. I called the garage first thing Monday morning, but the soonest they could fit me in was the Wednesday, and that’s where the car is still now…

Apologies for the lengthy explanation, but I thought a thorough response could only do justice to your very thorough comments and queries, and I’m amazed at such a detailed level of interest…
Thank you.

I will certainly mention your possible list of things to check to the garage:
The Idle control valve, MAF sensor, MAP sensor, vacuum lines and the ignition system, and possibly a failing fuel pump.
I must admit, I’m not sure if the age of my car will have a sensor on the exhaust, let alone an oxygen sensor (!) but I do know the ECU has already been checked…

Once again, many, many thanks to you for your comments and interest, which are very much appreciated.

I hope my Little Car will be fixed - properly this time - so I can enjoy driving her once again instead of using a courtesy car… !

With all good wishes and best regards…
Cheers! :grin::+1:

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It’s probably worth asking them to look at the clutch pedal position switch and the neutral switch on the gearbox. Are any fault codes stored do you know? On a 1998 MX-5, the fault codes need to be read with a bent paper clip and a multimeter, do you know if the garage are familiar with this process?

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Very good shout.

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Thank you Roadster_Robbie (and again to Ron_Damage for corroboration) for also suggesting the garage look at the clutch pedal position switch and the neutral switch on the gearbox. :blush: :+1:

Have absolutely no idea whether fault codes are stored, and I doubt whether the garage know about reading the fault codes with a bent paper clip and a multimeter (!) but am sure they’ll be willing to find out…!

Pleased to report that after collecting my vehicle from the garage yesterday, all seems to be running okay… :crossed_fingers: :crossed_fingers: :crossed_fingers:
Garage increased the idle speed slightly, which we’re hoping will do the trick…
Gave them a print out of Ron_Damage’s suggestions to check:
Idle control valve, MAF sensor, MAP sensor, vacuum lines and the ignition system, although said it was definitely not a failing fuel pump, as they had checked that already…

Garage have said they will research the list in preparation for future issues, so now I can add:
the clutch pedal position switch and the neutral switch on the gearbox to the list too…

Once again, many thanks for your suggestions… Very much appreciated…
Anything to keep my Little Car on the road…!
Cheers! :beers: :beers:
(just wish they were real ones rather than virtual ones! :crazy_face:)

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You said no codes or lights on the dashboard but has a fault reader been put on the car? Although yours is an early Mk2 and doesn’t have the later OBD connector there is a way of interrogating for fault codes, using a simple LED test lamp.

A few years ago I had a fault with my NB 1.8i which some said was certain to be the coil pack. I bought a new one but it wasn’t needed - I interrogated the ECU and found it to be the TPS. Fitting a new TPS did cure that fault.

(By coincidence, the brand coil pack I bought but didn’t need back then has just been fitted to my son’s 1.8iS turbo - his HAD partly failed, causing a misfire).

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You’re most welcome, Sunny. Didn’t have time for a longer response over the weekend.

I should have thought to include the clutch switch and neutral switch in the inspection. You know I explained about the engine not being under load? It’s these two switches (in conjunction with the TPS ) that tell the car when to use its idle control system, so it’s quite an obvious thing to check in that regard! Those switches should be checked before the idle control valve is replaced.

In answer to those answers you gave me:

  1. Again, very strange to get a cutout when throttling, especially after having had the TPS replaced (if I’ve got my timeline correct), so the cutout when pulling off again suggests ignition problems to me. Although when you’re changing down approaching a junction, then it seems more like an idle control system malfunction. Both of these together could in fact point to the clutch/neutral switch.

  2. The ECU receiving 12v is not the same as actually being in good working order. To some degree, problems with the ECU’s on these cars can only be diagnosed by process of elimination, unless you have a spare conveniently lying around to plug in! I would add that it’s highly unlikely that the ECU is at fault here and it seems like a simpler problem.

  3. TPS seems like a stab in the dark, I’m sad to say. Maybe they had their logical reasons for believing it could be that, or perhaps it was just a relatively economic thing to attempt, but there are plenty of logical reasons why it can’t be the TPS and it seems a strange place to start. But who knows, until the problem’s solved!

  4. Well, if your engine light isn’t on then that eliminates many things, hopefully. I don’t know how it is for your mx5 but I have an mx3 from 1998 and it’s actually OBD2 capable… but doesn’t have an obd2 port! The process for reading obd1 codes is widely available on the internet and requires patience but not a huge understanding of mechanics. Did you ever notice the “diagnosis” box in the engine compartment? basically, you put a paperclip in between uuuh pin 10 and ground? I can’t remember which pins off the stop of my head, and this puts the car into “code reading” mode. You then use a multimeter or most prefer a 12v (?) light source of some sort to go from… one of the pins (I call it the “output pin”) to ground. It will pulse out a code on the light or multimeter which gives you a number, which you then reference and that tells you what part/system the car has detected an error in!

  5. That sounds EXACTLY like a stuck idle control valve! Did you let out the clutch again when you’d changed gear? If you did, then you should have effectively bump started the car and it should have fired up again! If you let out the clutch in gear and it failed to fire again, then that means most likely no spark, so ignition would be the problem.
    Also, you really shouldn’t have to worry about dampness affecting the engine unless your air filter desperately needs changing!

  6. Low mileage, nice!

  7. Hmmm, sounds like your spark plugs, if they were indeed changed, should have plenty of life in them. As long as they actually put quality new plugs in there. This misfire, how was that detected? This has whisperings of the ECU beginning to periodically malfunction, but don’t let me worry you, could all be coincidence. Of course the other parts to bear in mind at this point are the coil pack and the NOTORIOUS CAM ANGLE SENSOR!!! The cam angle sensor on the way out could CERTAINLY explain the cutouts, the previous misfire, the kangarooing, could explain it all! Edit: Actually, this is a mk2, so the cam angle sensor is not NOTORIOUS like the mk1, and actually the mk2 has a cam angle sensor and also a crank angle sensor. I think it is definitely worth checking those out if the idle system checks pass. Despite the ignition system being checked recently, coils can still fail suddenly. Although in my experience when coils fail they fail, they don’t have intermittent problems unless it’s just connection/ground issues.

  8. I’m not one to talk because I’m the sort of pillock who manages to drive into a body of water now and then and sometimes “endurance tests” his own repairs on the public road, so I’m certainly no saint, just didn’t want your little problem turning into a much bigger one and would hate to hear of such a rare and cool example of an MX5 being written off!

Cheers!

Edit: Just wanted to add that the MAF sensor can be quite dramatic, and a bad or no reading can actually cause the car to cut out, although I haven’t confirmed this on a mk2, I know it’s true for a mk1. They are very easy to clean, simply dismantle and use some wd40 and a cotton bud to gently massage them back into health. Also not that much to replace for peace of mind.