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Discussion Starter #1
My car's never been happy doing hot starts - it always required a bit more turning over before it would catch. Recently it has been worse. After a long drive a few days ago I got back in it, and it would turn over but not catch. I checked everything, and came back to it 30 minutes later and it started perfectly.

Today it happened again - it started perfectly this morning, I drove 20 minutes to the shops, and when I came out a short time later it would turn over but not catch. I left it for 30 minutes again but still no go. I've got 3 days of AA cover left so I thought I'd get my money's worth. He turns up after an hour, tries it and it cranks but won't catch. Checks all the stuff I already checked, and can't find anything wrong. I tell him I suspect the NTC sensor, and if he has a multimeter we can check the readings with the Bentley. He has a go at it, and gets nothing. No reading at all. Puts a new sort of attachment on the plug and gets nothing from that either.

Now I'm assuming that being an AA man he knows how to use a multimeter, because I'm not 100% sure that I do. So I'm guessing he did it right. What I'm wondering is, if an NTC sensor fails, could you get no reading at all? If so, that would presumably explain the hot start problem. And for people who have solved their hot start issues by changing the NTC sensor, where did you get one from?
 

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From what I understand, NTC sensor is simply a temperature-sensitive resistor. I'd think it should still read something, even if broken, just be out of spec range. Check your voltmeter against something you know, and try setting different ranges on your Ohmmeter in case you're measuring in the wrong range. But even if your NTC sensor is bad, I think you'd simply run rich or something like that.

I assume your throttle plate is clean, your TPS and accelerator cable are adjusted properly, etc.

For some reason, my car doesn't like hot starts as much either. It catches fast, but at first its idle dips to 600 RPM or so and you can feel it struggling a bit to adjust idle, then it's normal. Even the many things I checked and cleaned, I never did diagnose that.
 

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No reading at all suggests an open circuit {infinite resistance, off scale on a digital multimeter} or a short {0 resistance}. With the NTC, low resistance means hotter, high means colder. Open circuit could mean frozen! But, since resistance measuring might be a bridge affair, an open circuit might not be correctly interpreted.
 

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If the NTC sensor has a resistance outside the operating limits it will show as a fault code and you will have a check engine light. Any multimeter will read the correct resistance which should be around 350 ohms at running temp and 3000 ohms when cold (summer). If it's brocken it wil run on limp home and run very rich as well. Maybe if the sensor only breaks when it heats up? - this would fool the ECU into cold start mode and flood the engine when you try to do a hot start?
 

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Saab Mad
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When this next happens, remove the connector to the NTC sensor. Then bridge the two terminals inside the connector (with the ignition off). Unless the ECU gets smart with out-of-range readings, bridging the terminals should simulate a hot-engine condition with correct fuelling instead of flooding.

I have a feeling that NTC sensors can be a real nuisance to get hold of. Is it a Bosch sensor? There's a Bosch main agent in Gloucester, and I could get a sensor from there if you like.

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eEuroparts.com sells NTC sensors for like $20. I got a new one that I ordered a year ago and it's laying on my table in front of me. If you really can't get a hold of one, I could help out. The casing is stamped with "BOSCH 0 280 130 026". I bought this for my '87 16V SPG.
 

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Jeremy,

there is a Bosch Agent in Ipswich ;) look in the yellow pages mate :cheesy: :cheesy:

or pay the main dealer £££££££


Paul

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Sleeper baby, no show & all GO..................
 

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Discussion Starter #9 (Edited)
I've been out and had a look at it, and it is the standard sort of turquoise colour plug. There's no name on it - doesn't say Bosch anywhere, but there is a number: 025 906 231, which is different to that Bosch one that Deniss mentions. The EPC lists two different ones - the one that is specified as being cars with Lucas injection is part number 74 85 006, Bosch is 93 57 021. Seeing as my car is Lucas I'll try them first and see how far I get. The strange thing is, I'm not getting any CEL, although it's definitely working as it comes on at startup briefly as usual. I just can't think what else could be causing this.

Edit -

Right, it's definitely a Lucas one. They can get one in a couple of days down in Ipswich. It's only £7.

Paul, you don't happen to have a 3/4" deep-well socket over there do you? :)
 

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Hi All,

I know so little compared to you all but I replaced my NTC sensor about 2 weeks ago and it didn,t do a thing to change hot or cold starting though I didn,t have an issue with ignition...cold idle is still my bane.

However, I have noticed an improvement in fuel consumption, slight but noticeable.

S
 

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Jezzadee said:
I've been out and had a look at it, and it is the standard sort of turquoise colour plug. There's no name on it - doesn't say Bosch anywhere, but there is a number: 025 906 231, which is different to that Bosch one that Deniss mentions. The EPC lists two different ones - the one that is specified as being cars with Lucas injection is part number 74 85 006, Bosch is 93 57 021. Seeing as my car is Lucas I'll try them first and see how far I get. The strange thing is, I'm not getting any CEL, although it's definitely working as it comes on at startup briefly as usual. I just can't think what else could be causing this.

Edit -

Right, it's definitely a Lucas one. They can get one in a couple of days down in Ipswich. It's only £7.

Paul, you don't happen to have a 3/4" deep-well socket over there do you? :)
I have everything mate ;)

when are you coming around for me to work on your car again :cheesy: :cheesy: :cheesy: :cheesy:

Paul

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Discussion Starter #12
Not sure exactly - I've PM'd you.

You should have enough parts to build a couple more C900's to go with the one decorating your front lawn by now, what with all those bits I've got you off Ebay :cheesy:
 

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Hej Jezz.

Vil du fortælle mig om hvordan det kommer til at gå, og hvor du finder en ny temperatur sensor! Jeg tror også min trænker til at blive udskiftet!

Will you tell me how it goes? And there you find a new sensor. I allso think mine is bad!
Did you ever look for you ignition timing for me mate?
 

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Discussion Starter #14
Hej Lars, naturligvis jeg skal fortælle du om resultaten. Den omskifter skal komme fra den Lucas kontor i Ipswich, litt sørlig fra her, og kostet £7.99. Jeg håpe det er for hele omskift, og ikke kun stik - jeg skal telefonere dem igjen i morgen.

Beklager, men jeg har glemt til finde ut tændingen for du. Jeg skal prøv på lørdag - måske det er mulig uten tændingenspistol.

And for those who can't read Danish, or Danes who prefer not to see their language destroyed :D :

Naturally I'll let you know the results. The sensor is coming from the Lucas office in Ipswich, just to the south of here, and costs £7.99. I hope that's for the whole sensor and not just the plug - I'll ring them again in the morning.

Sorry but I completely forgot to find out the timing for you. I shall try on Saturday - maybe it is possible to see without a timing gun.
 

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Make sure the connections are clean on the sensor and plug; I had a hot start problem which was solved by scraping off the corrsion on both the plug and the sensor.
 

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Discussion Starter #18 (Edited)
The car will start fine from cold. It will also start fine when hot, for example driving until normal temperature on the gauge, then switching off to fill with petrol, then restarting with no problem. However, if the engine was off for more than about 10 minutes, it will not start again until it is completely cold, which takes at least an hour. It will crank normally, but does not catch.

We measured the NTC sensor resistance at hot temperature, roughly 80ºC, and it was 370 Ohms, which is correct on the Lucas system. The Lucas NTC sensor readings should be as follows:


Lucas
0 deg: 5700-5900 Ohm
20 deg: 2400-2600 Ohm
40 deg: 1100-1300 Ohm
60 deg: 500-700 Ohm
80 deg: 300-400 Ohm

and just for the record:

Bosch
-20 deg C: 14000 Ohms
-10 deg C: 9000 Ohms
0 deg C: 5800 Ohms
10 deg C: 3800 Ohms
15 deg C: 3000 Ohms
20 deg C: 2600 Ohms
25 deg C: 2000 Ohms
30 deg C: 1700 Ohms
80 deg C: 320 Ohms

The resistance rose as the car cooled, and when cold it was again within spec. The car also starts when warm if you disconnect the NTC sensor, so that looks a lot like the culprit here. The check engine light comes on, but it will run, and then you can plug the sensor back in, CEL goes out and it continues to run. The other thing is that the idle fluctuates wildly through certain temperature phases but at what temperature exactly needs to be confirmed. Normal running temp is 1/3. What this seems to point to is that the NTC sensor is within the correct range when cold, and when hot, but that somewhere in between the resistances are going wrong.

It should be possible to obtain a more accurate picture by measuring the exact coolant temperature and comparing with the NTC readings as it drops from hot to cold. I'm not entirely sure of the best way to do this yet - a thermometer in the expansion tank is probably most accurate.
 

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Saab Mad
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You could remove the NTC sensor, suspend the probe in a pan of cold water on the stove, multimeter across the pins. Light the gas and see what happens as the water heats up.

Water in the expansion tank will be much cooler than that in the cylinder head. Measuring sensor performance off the car is the only reliable way to thoroughly test it IMO.

If the sensors are so cheap, and there is some doubt over it, then TBH I'd just replace it.

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What a interesting thread.

I have to add that the temperature in the expansion tank is not that much lower if at all than would you find coming out of the main body of the engine block, i know as i tried to place my after market gauges temperature probe there and it gave me a good reading but my method of installation was flawed and it leaked, so i had to remove it to the expansion tank which gives me the same reading for the same engine condition.

I didnt think i was suppose to place the probe in the expansion tank but i couldnt find another suitable place for the probe that wouldnt leak, i did test it out first with a thermometer, it was just as accurate and the results i get from my gauge are very quick and sensitive.
 
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