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You dont need to drain the coolant to change it, As long as the system isnt pressureised you wont loose any coolant. Just unscrew the expansion tank cap and then screw it back on (pref when the engine is cold obviously). Then pull out the clip and then the sensor. - easy!

sweet! thanks!
will i need a new o-ring tho... or does one come with the fresh sensor

the readings i got were annoyingly on-spec (maybe it was a loose clip!).... starting out at 2500 ohm at 25 celcius ("cold"=ambient temp), dropping steadily, to low 600s as it warmed up and i gave up.

disconnecting the clip caused it to idle slower (double-checked with revcounter) and made it harder to rev up - choking/chugging/spluttering and showing what could count as severe hesitation, though nothing like how it behaved before. perhaps i should risk "diagnostic mode" and turn on ignition/run starter with the sensor already disconnected? after all if it's not giving any signal for some reason when cold, that would have the same effect.

i also tried connecting the clip onto the dash guage sensor, after a curiosity test showed it to be reading resistances about 2/3 that of the ECU sender. results were similar to completely disconnecting the clip, but slightly milder, including when running at normal temperature.

be worth £8 for the problem to be solved..

Yeah the sensor should come with a new seal. If you start the engine without it youll need to disconnect the ECU for a while so it will forget the fault code generated by having no sensor

im willing to try disconnecting it at startup and seeing what happens, even if it means having to unplug the battery later on to clear the fault codes.
when it got nicely warm today (having a little run-around "playing" with the new revometer ), about 80-85c - blame it on the weather! - it was still idling wierdly unevenly (you could see the interior light flickering in time, when it was on) and occasionally hesitating. dont know about the mpg figure yet but i'll be testing that too next couple days. never had under 29 before - in a case of extreme and repeated cross-city hammerage - so anything significantly less will show a prob!

disconnected the sensor when it was at that temperature and it had no effect on the running (and multimeter showed a reasonable 200ohms) .... so p'raps it's something else.

i guess the number of times i'd just pinged it off the limiter might have been a factor in slightly rough running whats the damage possible with running for very long over 6000rpm again?

no wonder i'd been having problems pulling away though, with whatever the fault is... on a "normal" start the needle stayed around 1500 rpm or less ... on a "fast" one it still didnt avoid dropping under 2500 at full throttle!!
a measured start where i revved it up to 3500 and tried to keep it there or above felt rocket-like - until the tyres ran out of grip.

hmmm if i hop down to GSF for a new sensor, wonder if they'll be pissed if i insist on testing it with the meter before accepting the purchase

A broken wire from the blue water coolant temparature sensor loom, would cause misfiring.

By disconnecting the blue temp sensor you are putting the ecu into service mode using default sensor settings. Engine revs will increase slightly.


Sounds very hall sender-ish from your description.

Also check your alternator. It should read a voltage drop of over 12.2v between the battery earth and alternator live. As a lower voltage causes the ecu to play silly buggers with the injector pulses and fuelling goes all over the place.

egads, more stuff to think about hehe thanks....

right so if i disconnect the hall sender, will that let the engine run in a possibly nobbed-up mode that i can check whether it's similar to my symptoms, or will it just stop dead as it doesnt know when to send the sparks?

and will it be safe with my flimsy Aldi £3.99 digital multimeter to check the voltage between alternator and battery?! i suppose there is a fair chance it's that - metally flakes from the kaput fanbelt may have got inside it or still be fouling up the pulley.. or the wire gone bad... or even, after seeing the mess a simple loose wire in my indicator switches caused, a short elsewhere?

just in case there is any confusion (btw it's 6.30am, i had to deliver some stuff to stafford after work - so im a bit confused ), it wasn't a problem of misfiring.... more like you say, the fuelling being all over the place.

i dunno about disconnecting the sensor making the revs rise and the engine misfire - from what i can see, it actually idled a good 100rpm or so (or more!) lower when i unplugged it, especially when cold (dropping from maybe 900+rpm, to a flat 750... could see the ISV hard at work from the movements of the tacho needle tho!), and although the engine response was decidedly muted until it reached the lower mid 2000s, it wasnt misfiring - it was smooth and merely acting as if the pedal wasnt pushed anywhere near as far down. taking, say, 2 or 3 seconds to reach 2000rpm from idle on a snapped-open full throttle... ppthhhhhrrrrrrrrRRRRRRRMMMMMNNNGGG, whereas when everything was running fine it'd be only a single second or less... pthrrrRRRMMMMMNNGGG (then past that point, not much difference being evident)

If you disconnect the hall sender the engine will stop dead because its a vital ignition component.

fair dos

yep, that happened

havent yet figured out exactly where to plug the multimeter prongs to test the alternator voltage, and like hell i'm touching the battery with them!

a little worried about doing it as its small, cheap, and doesn't seem to have a very high amperage tolerance at all when compared to the alternator specs... whats a healthy one likely to be kicking out at a 800-900rpm idle, both volts and amps wise?


BTW i worked out my consumption for the last 85 miles or so....... it included some bits of caning about (country roads and about 5 miles on a less evil bit of the m42), some idling while testing, and some in-town stuff, but the great majority was pootling at 35-50 on A/B roads, and that trip up & down the foggy motorway at around 55 average (which in itself was about 40 miles)....

30.9mpg.... with the previous 169 miles, which included the point at which i swapped the clocks, at a much healthier 45mpg under what seemed like roughly similar conditions. ouchie. It's hardly 17mpg, but then it's an SPi 1-litre been driven fairly gentle rather than a 75hp beast of an MPi GT driven more "normally", so it could be really pointing at a fault there

i'll have to find some time to do another "quick" 45mph or 56mph run on the route i used before to see if thats actually as bad as it looks, or if i've just been manhandling the car (and the throttle!) far, far more than i thought!

Put the positive tester wire on the big thick alternator wire and the negative wire to an earthing point (or the battery) Dont worry about blowing ur tester up. As long as you do it properly i e in parallel and not in series with what ur testing youll be ok.

Tahrey1043 wrote: 30.9mpg.... with the previous 169 miles, which included the point at which i swapped the clocks, at a much healthier 45mpg under what seemed like roughly similar conditions. ouchie. It's hardly 17mpg, but then it's an SPi 1-litre been driven fairly gentle rather than a 75hp beast of an MPi GT driven more "normally", so it could be really pointing at a fault there

Christ! Im lucky to get 25mpg on my G!!!

Thanks gaz... couldn't see anything obvious coming out the top of the alternator last time i had a look so i'm guessing that's underneath?

G40racer.... yeah but how do you drive it?

That 30.9 is my (checks records)... oh, i mean, that 30.2 mpg (typo!) is my third worst recorded economy figure from the last 2 years (beaten only by a 30.0 and a 29.2) - the records a bit patchy in places, but it does include some examples of extreme hammering cross-town etc. In more everyday driving - a bit of cruising here, a bit of screaming there, some keeping with the traffic in other places - i'd be expecting something between 34 and 47 averaged depending on the balance... eg average for the tank so far is a bit over 45. Compared to 34, 30 may not sound like much difference, but law of diminishing returns and all that - the balance of holding the throttle to the floor in 2nd must get ever higher to counter out any efficient cruising. And i had a good half of the distance between fills somewhere in the 55mpg zone at least.

Then again it was only 85 miles, so it cant really be a representative sample. Maybe if i get chance to visit a show or something i'll be better able to keep a tab on it.

Tahrey1043 wrote: G40racer.... yeah but how do you drive it?

25mpg is me cruising and taking it easily.

When on circuit, its down to 7-10mpg!!!

Im also getting through a hell of alot of methanol and water, since the water/methanol injection is boost activated and I love the power it gives so I go over the boost threshold all the time

I do miss my GT though and its wicked fuel economy!


Incidentally, referring to your problem... if it is your hall sender - you can't really diagnose/comfirm that its faulty (if anyone knows if you can, please let us know). You just have to know the symptoms. A new hall sender is about £38 for Bosch. And either strip the distributor down yourself and replace it yourself or do as much as you can then get a garage/electrician to replace it to cut costs.

With your alternator, I cant remember if it produces more voltage/amp as rpm increases, which if it does, then your alternator might be reading just below the threshold, fooking up your ecu-injector timing pulsing, but when over 2K rpm, its reads just enough voltage to allow the ecu to correctly control them. Just a thought.

polog40racer wrote: 25mpg is me cruising and taking it easily.

ouch... and there i thought the G was still supposed to pull in similar economy to, say, a 1.6 Golf...

Im also getting through a hell of alot of methanol and water, since the water/methanol injection is boost activated and I love the power it gives so I go over the boost threshold all the time

Hmmm do you think that might be your problem?
(the WHAT injection system?!!!)

Incidentally, referring to your problem... if it is your hall sender - you can't really diagnose/comfirm that its faulty (if anyone knows if you can, please let us know). You just have to know the symptoms.

What are those then?
This is all getting a little annoying, because - possibly because of the recent wannabe-heatwave - the problem hasnt really come back with any anger. Non-reproducible results = "what the hell was going wrong?", as well as "is it going to come back?"
The engine does seem to be slightly detuned to what I remember, and theres that tentative thirstiness - but it's good to go from 1000rpm at part throttle if needs be, and seems rapid enough from the high 1000s in 1st gear at full whack negotiating the parked-car strewn streets round my house... So the original defect that pricked up my ears is missing.

A new hall sender is about £38 for Bosch.

That's not music to my ears when I was previously looking at an £8 temp sender! ... what did you say a dicky one might cause again? Because I still don't think the motor is/was misfiring.. had to put up with enough of it before!
(that cost on top of a potentially knackered front left damper, changing the brake fluid and getting the drums sorted if i cant get the handbrake to adjust right... damn! )

And either strip the distributor down yourself and replace it yourself or do as much as you can then get a garage/electrician to replace it to cut costs.

I'm guessing you don't mean "replace the cap and rotor" here?? Something that involves taking the cam cover off?

With your alternator, I cant remember if it produces more voltage/amp as rpm increases

Well, it does go up as the engine speed rises, to a point, where the regulator kicks in and does whatever it does to keep the volts and amps down to a "safe" level. But I don't think it's the alternator so much any more..... testing it (after a lot of dicking about trying to find "clean" contact spots - including piercing some wire insulation) showed engine-off, 12.5v.... idling around 950rpm, 13.8v.... seems healthy enough to me, seeing as a part-charged battery should give out around 12.5v (which really should be all mine is fit for any more!), and regulated charging voltage is "over 13 but almost always under 14 volts".

Hmmmm though!!! Didn't try revving it - perhaps the regulator isnt working and the volts are too high at idle and rise much to high at increased speeds which has done something to ECU voltage compensator routines.. what with it "learning" the engine condition and all, but with the limits of 1990 compact/robust/cheap tech, i'd think that learning capability to be very limited.
Would also explain the somewhat electric racket that comes from the engine past 5000rpm!

((mind you i think a similar thing about both the alternator and the temp sensor - could be a "loose wire" type fault that only manifests on alternate thursdays, something like that, so the device could look to be perfectly fine until i next notice a fault... maybe even in the external wiring itself. Argh))


Think I'll just have to lay this subject to bed until the patient has a relapse or the tests come back positive for a fuel tapeworm

Thanks for the help so far guys, watch for this one reawakening.

hey dude.
I put an earth wire from the Lambda sensor earth to the battery the weekend and it's sorted all of the rubbish I had like this out.

Give it a go, can't hurt.