inconsistent idle and lurching at constant speed

[GreasyJoe]

Hi all, I'm after some guidance on where to start looking.

Firstly inconsistent idle on occasions. Normally the car will behave itself sitting nice and evenly on tick over but other times, either initially starting the car or a little time down the road, I can get either idle bobbing quickly around 950 and just below. Other times the engine will idle at about 1500 or on the very odd occasion near 2000 rpm, like its still compensating for being cold, though its just about warm? Eventually it may settle but not always. Sometime turning the engine off resets things?

The other issue I've noticed is the car is going along at constant 30mph not under acceleration, and will noticeably lurch/jerk a few times, then continues normally,
The engine temp can be from cold to running temp (though the gauge in the car hardly moves to near quarter unless in last summer stuck in traffic). Though less likely to do it when up to full running temp.

The car can behave itself completely on other occasions. It can be quite random.

It has a new air filter
Haven't changed the fuel filter or plugs so not sure how old they are. Though as said when it runs and accelerates it seems fine.

Any suggestions what is the cause, where to start looking? (Order by obvious to the obscure please)
Any sensors/electronics that could be playing up?
If suggests vacuum hoses leaking can someone point me where they are and the diameter inner/outer size and length of the hoses I'd need to replace, as not been able so far to find any details.

Owned the car for nearly a year. Was previously SOARN for a couple of years before and got MOT'd prior to putting it up for sale. Was serviced but not really maintained for its age.

The car is 1992 SE turbo, approx. 85k miles (under restoration when the weather gets better. Most rubber items may need replacing hence the mention of the vacuum hoses).

If you want to see the car, it was used to review the model on youtube the other year. Not sure why he picked mine as a 'good' example? lol

[url=https://www.youtube.com/watch?v=EaaopxfJMu4&t=98s]

[texas2201]

I've got a similar problem..... well the lurching/stalling bit .... I'm still trying to solve it - I do one thing and it gets better - then it comes back...... going into garage for a smoke test for vacuum leaks and to check the base timing. Now it seems to be over fuelling - flooding - so suspecting one of the injectors is sticking - so they will get checked as well

i've fitted new plugs, coils, VSS, throttle cable, injectors,

[GreasyJoe]

I've got a similar problem..... well the lurching/stalling bit .... I'm still trying to solve it - I do one thing and it gets better - then it comes back...... going into garage for a smoke test for vacuum leaks and to check the base timing. Now it seems to be over fuelling - flooding - so suspecting one of the injectors is sticking - so they will get checked as well

i've fitted new plugs, coils, VSS, throttle cable, injectors,

Interesting you mentioned overfuelling. have had a smell of petrol since i got the car. never sure where it is coming from. a lotus specialist didn't pick anything up. not sure if its around the tank or the engine. never really noticed it in the engine bay though always thought the exhaust was a bit sooty (not sweep Just another thing to investigate.
Strange you mention you replaced the injectors but you think it may still be one sticking?
Be good to know how you get on texas2201

[texas2201]

Will do.... see what Tuesday results in

[texas2201]

Garage was very helpful - but it was out of their depth. Did find that the new VSS was loose ..... removed, refitted and tightened. No change to the problem.

Piggy backed a new Temp Sensor and as soon as engine was warm EML came on with a code 21 TPS ..... seems like the bad CTS was "hiding" this before.

Ordered a new TPS from Rockauto and a proper CTS - allegedly arriving Wednesday .... in Portugal from the US ..... we will see

And of course one of the screws on the TPS has been mullered by a previous visitor!!!! New screws and Throttle body gasket (the one fitted is in perfect shape/condition) will be ordered from SJS if the transport price is not offensive. Lost a jubilee clip in the engine bay - will keep looking for it. Bottom hose on TB was spawn of the Devil snap on Clip..... I F**kin hate those - came off in two pieces.... 40 mins of my life I wont get back!!!! so another Jubilee needed.

[dapinky]

Martin,

When I changed the TPS on the old red car, the screws were bu66ered - they are a simple M4 setscrew (I cant remember if they were M4, or M5 - but they are a standard thread which I had in a box!), so i just replaced them with 'normal' hex-head ones from the garage shelf and used a small spanner to nip them up.

[texas2201]

Martin,

When I changed the TPS on the old red car, the screws were bu66ered - they are a simple M4 setscrew (I cant remember if they were M4, or M5 - but they are a standard thread which I had in a box!), so i just replaced them with 'normal' hex-head ones from the garage shelf and used a small spanner to nip them up.

thanks Dave

[texas2201]

Gasket and screws for the TPS ..... the transport price and the import tax were horrendous, so I didn't order them. A friend who lives in Portugal found he had some of them in his Man Cave. Also the Gasket was all in one piece and not damaged. Saga of the TPS from Rockauto - summary - 2.5 days from the Mid West US to Lisbon. 5 days from Lisbon to Azenhas ..... it is a 40 minute drive! then the delivery guy turns up - Cash or cheque only for the 39% import tax!!!! Went to calendar to check which century we were in..........

Fitted the TPS all went together but would tick over at 2,000 ???? Looked at the CTS and stripped off the tape to find one wire broken and the other hanging by a thread....... arghhhh. Fortunately my local friend was able to come over with his soldering iron and we got it sorted. Also discovered that the connections in the plug on the Hall sensor I fitted last year were not the best - so we dismantled that and soldered the connections. Now ticking over at 1100 or slightly lower. It is coming down with use ,,,, does the ecu "learn" from use?

So eventually problem (s) solved ..... running sweet again

[chrism]

That's good.

Yes, re idle speed. - I had an MGTF135 before the m100 and, after playing with/cleaning the TPS, the high idle settled later the same day. Later when I upgraded and resync'd its ecu prof advice was that it would take ~200ml to 'settle into a new driving pattern'.

Also with my m100SE, after addition of 02 sensor and upgrade to 1499, I had similar high idle until the ecu 'repositioned' itself a day or so later - so I reckon it'll settle down of it's own accord quite soon.

The TF had a stepper motor that gradually homed in to its optimum idle speed position. I presume the m100 has something similar that gradually adjusts...

[GreasyJoe]

Garage was very helpful - but it was out of their depth. Did find that the new VSS was loose ..... removed, refitted and tightened. No change to the problem.

Piggy backed a new Temp Sensor and as soon as engine was warm EML came on with a code 21 TPS ..... seems like the bad CTS was "hiding" this before.

Ordered a new TPS from Rockauto and a proper CTS - allegedly arriving Wednesday .... in Portugal from the US ..... we will see

And of course one of the screws on the TPS has been mullered by a previous visitor!!!! New screws and Throttle body gasket (the one fitted is in perfect shape/condition) will be ordered from SJS if the transport price is not offensive. Lost a jubilee clip in the engine bay - will keep looking for it. Bottom hose on TB was spawn of the Devil snap on Clip..... I F**kin hate those - came off in two pieces.... 40 mins of my life I wont get back!!!! so another Jubilee needed.

Lots of interesting info guys.

I'm a bit lost I have to admit to some of the terminology and what these items do, common issues that happen with them, and usual resolutions.
Could you let me know what VSS and CTS are? What they do? TPS is the throttle position sensor, I seen mention in posts but not what the common issue is/what does it affect?

Also had a new development which can be read here.
forum.lotuselancentral.com/viewtopic.php?f=31&t=29492

[texas2201]

VSS is the vehicle speed sensor which goes on the back of the Speedo and converts the cable rotation to an electronic signal for the ecu to determine the cars speed.
TPS is the throttle position sensor which tells the ecu were the throttle is so it can provide the right amount of fuel

CTS coolant temp to the ecu.... which also lets the ecu know how much fuel is required

[Fredjohn]

VARIABLE speed sensor.

[texas2201]

VARIABLE speed sensor.

But it is an optical sensor

[Fredjohn]

Turns out we are BOTH wrong!!!

It's VEHICLE.

See Section 17.06a Page 2 Item 60 of the parts manual.

[dapinky]

I was just going to shout that out at you!..... but before I did, I had to go and check, because I'd look even more stupid if I wasn't correct (and I had no percentage of "rightness" to apply to my argument - just that I always understood it to be 'vehicle')......

....... still, lets all just call it the VSS and accept that it is a magic box that does clever stuff I don't understand.

[Tuga2112]

sorry for the complete hijack, but i seem to be in trouble with mine (VSS)

and i checked SJ and they only cost about 20 quid. making me wonder if this is a comon GM part that is actualyl available for £8 somewhere else. ?

ive checked wiki, but no entry for the VSS there.

[dapinky]

sorry for the complete hijack, but i seem to be in trouble with mine (VSS)

and i checked SJ and they only cost about 20 quid. making me wonder if this is a common GM part that is actually available for £8 somewhere else. ?

ive checked wiki, but no entry for the VSS there.

Joao,

I'm in no position to go and dig out my spares at the moment, but I can tell you that the whole instrument panel is from an Astra/Cavalier/Calibra of the era (with different coloured dial faces, different warning lamp masks, and a different set of gears to drive the odometer, dependent on what vehicle it is intended for).

I bought an Astra F version many years ago just for some spare bits, and the VSS is identical to the Elan unit.... I bought it because it was a fiver so seemed worth it! - there were 2 versions used in the Astra - those with and without a tachometer, but the VSS should be the same.

I assume that the VSS can be bought from Vauxhall places, and it probably shares the same part number as the one you have fitted.

[Tuga2112]

seems the calibra instrument panel is a very close match to the elan

https://www.ebay.co.uk/itm/284847215592 ... R4iY95PPYQ

althought a little pricey

[dapinky]

seems the calibra instrument panel is a very close match to the elan

https://www.ebay.co.uk/itm/284847215592 ... R4iY95PPYQ

althought a little pricey

I think I paid about £10-£15 for mine - but as it was listed for an Astra Van it probably attracted a different audience!!!! - the perspex cover is different, but the actual panel is the same. (The cover and it's frame simply unclip from the panel).

[Tuga2112]

When i looked, i found some Astra clusters but none of them had RPMs
the astra van does have RPMs and the calibra clusters also have them.
the good thing is that i dont think i need a VSS after all. my RPMs were not reading. and i assumed the VSS could be the reason. but after close inspection i found the RPMs circuit has nothing to do with the VSS. and turned out my printed circuit had a cut . which also explained why the low oil pressure light and alternator light didnt come on before i started the car.

all fixed now with a blob of solder in the circuit board. next time i will be using a solder ribbon instead which will give it a cleaner finish.